Key concepts - l Kg Concepts—Structure and Bonding...

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Unformatted text preview: l Kg! Concepts—Structure and Bonding Important Facts - The general rule of bonding. Atoms “strive” to attain a com electrons Second-row elements “want” eight electrons. Formal charge is the difference between the number of valence electrons of (Section 1.30). See Sample Problem 1.4 for a stepwise example. 0 Curved arrow notation shows the movement of an electron pair. The tail of the arrow always begins at an electron a‘ 'th bond or a lone pair. The head points to where the electron pair “moves” (Section 1.5). p In El er In a Electrostatic potential plots are color-coded maps of electron density (Section 1.11). I plete outer shell of valence electrons (Section 1.2). H "wants" two an atom and the number of electrons it “owns” indicating electron—rich and electron-deficient regions . he Importance of Lewis Structures (Sections 1.3, 1.4) A properly drawn Lewis structure shows the number of bonds and lone pairs present around each atom in a molecule. In a valid LeWIs structure, each H has two electrons, and each second-row element has no more than eight. This is the first step needed to determine many properties of a molecule. l—“T’ Geometry ‘ [linean trigonal planar, ortetrahedral] (Section 1.6) Lewis structure a a 4'7—- —> Hybridization ‘ [Spy SP2, 0" 5/33] (Section 1-8) l l ‘—--+ Types of bonds [single, double, or triple] (Sections 1.3. 1.9) Resonance (Section 1.5) The basuc principles: - Resonance exists when a compound cannot be represented by a single Lewis structure. - Resonance structures differ only in the position of nonbonded electrons and 7t bonds, not atoms. - The resonance hybrid is the only accurate representation for a resonancestabilized compound. A hybrid represents the compound better than any single resonance structure because electron density is delocalized. The difference between resonance structures and isomers: - Two isomers differ in the arrangement of both atoms and electrons. - Resonance structures differ only in the arrangement of electrons. 6: ('6: :b: // // / orig—q CHSCHZ—C\ ._+ CHQCHz-C\\ :QTCHB ‘ QLH IQ_H L isomers 41 L— resonance structures 4T Geometry and Hybridization The number of groups around an atom determines both its geometry (Section 16) and hybridization (Section 1.8). Number of groups Geometry Bond angle (°) Hybridization linear 180 sp 3 trigcnal planar 120 sp2 4 tetrahedral 109.5 sp3 Drawing Organic Molecules (Section 1.7) - shorthand methods are used to abbreviate the structure of organic molecules. CM 9H2 : CH37(‘37(‘Z*C—CH3 = (CH3)2CHCH20(CH3)3 l H HCH3 skeletal structure isooctane condensed structure 0 A carbon bonded to four atoms is tetrahedral. The best way to represent a tetrahedron is to draw two bonds in the plane, one bond in front, and one bond behind. Bond Length - Bond length decreases across a row and increases down a column of the periodic table (Section 1.6A). I Bond length decreases as the number of electrons between two nuclei increases (Section 1.1OA). I Bond length decreases as the percent s—character increases (Section 1.108). - Bond length and bond strength are inversely related. In general, shorter bonds are stronger bonds (Section 1.10). I Sigma (0') bonds are generally stronger than it bonds (Section 19). Electronegativity and Polarity (Sections 1.11, 1.12) - A polar bond results when two atoms of differe nt electr n ' ' n h h n H r y I it i l n O egatIVIty values are be ded toget er, W 6 ever C or is bonded to N O o A m h ‘ h n m V Y polar olecule as on er 0 e polar bond, or two or Dre bond dipoles that reinforce Key Concepts—Acids and Bases A Comparison of Bronsted—Lowry and Lewis Acids and Bases Type Definition Structural feature Examples Bronsted—Lowry acid (2.1) proton donor a proton HCI, H2804, H20, CHSCOOH, TsOH Bronsted—Lowry base (2.1) proton acceptor a lone pair or a 1-: bond ‘OH, 'OCHS, H’, 'NHZ, NHS, CH2:CH2 Lewis acid (2.8) electron pair acceptor a proton, or an unfilled BF3, AICI3, HCI, CHECOOH, H20 valence shell, or a partial (+) charge Lewis base (2.8) electron pair donor a lone pair or a It bond ‘OH, _OCH3, H', 'NHZ, NH3. CH2=CH2 Acid—Base Reactions [1] A BronstedeLowry acid donates a proton to a Bronsted—Loww base (2.2). ., //.\ i‘ HiQSH + H: H E; HiQ= + “44’” acid base conjugate base conjugate acid proton donor proton acceptor [2] A Lewis base donates an electron pair to a Lewis acid (2.8). ‘9”2/‘\ 9H2 CHrg + —» CHrc~ajrz CH3 CH3 Lewis acid Lewis base electrophile nucleophile - Electron-rich species react with electron-poor ones. - Nucleophiles react with electrophiles. Important Facts I Definition: pK = -log Ks. The lower the pKa, the stronger the acid (2.3). - The stronger the acid, the weaker the conjugate base (2.3). 0 In proton transfer reactions, equilibrium favors the weaker acid and weaker base (2.4). - An acid can be deprotonated by the conjugate base of any acid having a higher pKa (2.4). Periodic Trends in Acidity and Basicity (2.5A) Periodic trends in acidity Periodic trends in hasicity Increasing acidity » " Increasing basicin i i iciH iNiH iO—H H’F gs 70* ~N’ ~os F‘ | i 3 a i I g > —s—H H—CI 3 .§ 75* or a E '5 ° 3 'fi H—Br : N Br— 3 ‘“ _ c .1: H71 i 1- Factors That Determine Acidity (2.5) [1] Element effects (2.5A) The acidity of H~A increases both lefteto-right across a row and down a column of the periodic table. [2] Inductive effects (2.58) The acidity of H—A increases with the presence of electron—withdrawing groups in A. [3] Resonance effects (2.50) The acidity of H—A increases when the conjugate base A:' is resonance stabilized. [4] Hybridization effects (2.50) The acidity of H—A increases as the percent s-character of the A:‘ increases. _l Key CgriceptS—lntroduction to Organic Molecules and Functional Groups Types of Intermolecular Forces (3.3A) Type of force Cause E van der Waals Caused by the interaction of temporary dipoles Ea - Larger surface area, stronger forces g - Larger, more polarizable atoms, stronger forces 3, dipole—dipole Caused by the interaction of permanent dipoles é, hydrogen bonding Caused by the electrostatic interaction of a H atom in an O— H, N— H, or H— F bond with the lone g pair of another N, O, or F atom E ion—ion Caused by the charge attraction of two ions Physical Properties Property Observation Bailing point (3AA) - For compounds of comparable molecular weight, the stronger the intermolecular forces the higher the pp. CHacHZCHchZCH3 CHaCHZCHZCHO CHECHZCHZCHQOH VDW VDW, DD VDW, DD, HB bp=36°C bp:76°C bp=118°C Increasing strength of intermolecular forces Increasing boiling point - For compounds with similar functional groups, the larger the surface area, the higher the hp. CHSCHQCHZCHS CHGCHZCHZCchHa bp:0°C bp:36°C Increasing surface area Increasing boiling point - For compounds with similar functional groups, the more polarizable the atoms, the higher the hp. CHaF CHal bp = 778 ac hp : 42 so Increasing polarizability Increasing boiling point Melting point (3.48) 0 For compounds of comparable molecular weight, the stronger the intermolecular forces the higher the mp. CH3CHZCH20HZCH3 CHGCHZCHZCHO CHBCHZCHZCHQOH VDW VDW, DD VDW, DD, HB mp=7130 °C mp:—96 °C mp :—90 °C Increasing strength of intermolecular forces Increasing melting point - For compounds with similar functional groups, the more symmetrical the compound, the higher the mp. CH30H20H(CH3)2 (CHalac mp=7160°C mp=e17°C Increasing symmetry Increasing melting point Solubility (3.40) Types of H20 soluble compounds: - Ionic compounds - Organic compounds having 5 5 Cs, and an O or N atom for hydrogen bonding (for a compound with one functional group). Types of compounds soluble in organic solvents: - Organic compounds regardless of size or functional group. Key: VDW = van der Waals, DD : dipole—dipole, HB = hydrogen bonding Reactivity (3.8) - Nucleophiles react with electrophiles. I I - Electronegative heteroatoms create electrophilrc carbon atoms, which tend to react wrth nucleophiles. - Lone pairs and 1: bonds are nucleophilic sites that tend to react with electrophiles. _l Key Concepts—Alkanes General Facts About Alkanes (4.1-4.3) - Alkanes are composed of tetrahedral, sp3 hybridized C atoms. - There are two types of alkanes: acyclic aikanes having molecular formula 0,,H2" + 2, and cycloalkanes having molecular formula CnHZn. - Alkanes have only nonpolar 0—0 and C— H bonds and no functional group, so they undergo few reactions. - Alkanes are named with the suffix -ane. Classifying C Atoms and H Atoms (4.1 A) 0 Carbon atoms are classified by the number of carbon atoms bonded to them; a 1" carbon is bonded to one other carbon, am so forth. - Hydrogen atoms are classified by the type of carbon atom to which they are bonded; a 1° H is bonded to a 1" carbon, and so forth. Names of Alkyl Groups (4.4A) CHa— : 7E CHsCHZCH2CH2— = M; methyl butyl CHQCHQ— = /\§ CHSCHzfliHCHa = AK ethyl sec-butyl NW CHSCHZCHze = /\/E (CH3)2CHCH27 = propyl isobutyl (CHSbCHe = /l\ (CH3)SC— = g isopropyl E ten-butyl Conformations in Acyclic Alkanes (4.9, 4.10) 0 Alkane conformations can be classified as eclipsed, staggered, anti, or gauche depending on the relative orientation of the groups on adjacent carbons. eclipsed staggered anti gauche H H T CH3 H H H H H H H H H H H H \H H H H CH3 H CH3 CH3 - dihedral angle = 0° - dihedral angle = 60° - dihedral angle of two - dihedral angle of two CH3 groups = 180° CH3 groups : 60° I A staggered conformation is lower in energy than an eclipsed conformation. - An anti conformation is lower in energy than a gauche conformation. Types of Strain - Torsional strain—an increase in energy caused by eclipsing interactions (4.9). - Steric strain—an increase in energy when atoms are forced too close to each other (4.10). I Angle strain—an increase in energy when bond angles deviate from 109.5a (4.11). Two Types of Isomers [1] Constitutional isomers—isomers that differ in the way the atoms are connected to each other (4.1A). [2] Stereoisomers—isomers that differ only in the way the atoms are oriented in space (4.138). cis trans CH3 0% Q Q CH3 CH3 CH3 (“CH3 L constitutional J L . J stereorsomers isomers Conformations in Cyclohexane (4.12, 4.13) - Cyciohexane exists as two chair conformations in rapid equilibrium at room temperature. I Each carbon atom on a cyclohexane ring has one axial and one equatorial hydrogen. Ringeflipping converts axial H’s to equatorial H’s, and vice versa. An axial H flips equalorial. An equatorial H flips axial. - In substituted cyclohexanes. groups larger than hydrogen are more stable in the roomier equatorial position. - Disubstituted cyclohexanes with substituents on different atoms exist as two possible stereoisomers. - The cis isomer has two groups on the same side of the ring, either both up or both down. C The trans isomer has two groups on opposite sides of the ring, one up and one down. Oxidation—Reduction Reactions (4.14) ‘ Oxidation results in an increase in the number of 6—2 bonds or a decrease in the number of c— H bonds. - Reduction results in a decrease in the number of 0—2 bonds or an increase in the number of C— H bonds. — Key Qoncepts—Stereochemistry Isomers Are Different Compounds with the Same Molecular Formula (5.2, 5.11) [1] Constitutional isomersgisomers that differ in the way the atoms are connected to each other. They have: different IUPAC names; - the same or different functional groups; and - different physical and chemical properties. [2] Stereoisomers—isomers that differ only in the way atoms are oriented in space. They have the same functional group and the same IUPAC name except for prefixes such as cis. trans, R, and S. - Enankiomersistereoisomers that are nonsuperimposable mirror images of each other (5.4). 0 Diastereomers—istereoisomers that are not mirror images of each other (5.7). Some Basic Principles When a compound and its mirror image are superimposable, they are identical achiral compounds. When a compound has a plane of symmetry in one conformation. the compound is achiral (5.3). When a compound and its mirror image are not superimposable, they are different chiral compounds called enantiomers. A chiral compound has no plane of symmetiy in any conformation (5.3). A tetrahedral stereogenic center is a carbon atom bonded to four different groups (5.4, 5.5). For n stereogenic centers, the maximum number of stereoisomers is 2" (5.7). plane of plane of SymmeW [‘ : stereogenic center] symmetry CH3 /CH3 (IIHa CH3 /CH3 \ . .\ . HWC— Y H /C\""H Cl Voicfi" H V Cl , H . , , H H CHSC 2 CI H Cl H no stereogenic centers 1 stereogenic center 2 stereogenic centers 2 stereogenic centers i i Chiral compounds contain stereogenic centers. A plane of symmetry makes these compounds achiral. Optical Activity Is the Ability of a Compound to Rotate Plane-Polarized Light (5.12) 0 An optically active solution contains a chiral compound - An optically inactive solution contains one of the following: - an achiral compound with no stereogenic centers - a meso compoundian achiral compound with two or more stereogenic centers - a racemic mixture—an equal amount of two enantiomers The Prefixes R and 5 Compared with d and l The prefixes R and S are labels used in nomenclature. Rules on assigning R,S are found in Section 5.6. - An enantiomer has every stereogenic center opposite in configuration. If a compound with two stereogenic centers has the RR configuration, its enantiomer has the 8,8 configuration. - A diastereomer of this same compound has either the as or S,H configuration; one stereogenic center has the same configuration and one IS opposite. The prefixes d (or +) and I (or —) tell the direction a compound rotates plane-polarized light (5.12). - Dextrorotatory (d or +) compounds rotate polarized light clockwise. - Levorotatory (I or —) compounds rotate polarized light counterclockwise. 0 There is no relation between whether a compound is R or S and whether it is d or I. The Physical Properties of Isomers Compared (5.12) Type of isomer Constitutional isomers Enantiomers Diastereomers Racemic mixture Equations ( Physical properties Different Identical except for the direction polarized light is rotated Different Possibly different from either enantiomer observed rotation (°) ength of sample tube (dm) dm = dedmete" - Specific rotation (5.120): SPSCiiiC _ 0i 6 = concentration (g/mL) 1 ii”) :10 cm - Enantiomeric excess (5,12D): 99 % of one enantiomer — % of the other enantiomer [ [a] mixture ‘ [a]pureenantiomer X 100% use __| Key Concepts—Understanding Organic Reactions Writing Equations for Organic Reactions (6.1) - Use curved arrows to show the movement of electrons. Full-headed arrows are used for electron pairs and half-headed arrows are used for single electrons. V 0 Reagents can be drawn either on the left side of an equation or over the reaction arrow. Catalysts are drawn over or under the reaction arrow. Types of Reactions (6.2) i v = H heteroatom [1] Substitution "f—Z + Y —' (I: y + Z Z or a Y replaces Z | | \ , / , [2] Elimination agac- + reasem —* /C’C\ T X Y rm l Two 0 bonds are broken. 1: bond \ / kée [3] Addition /C:C\ + X’V ' . . r 0 n This it bond is broken. Two is bonds are iormed. Important Trends Values compared Trend Bond dissociation energy The higher the bond dissociation energy, the stronger the bond (6.4). and bond strength Energy and stability The higher the energy, the less stable the species (6.5A). Ea and reaction rate The larger the energy of activation, the slower the reaction (6.9A). E3 and rate constant The larger the energy of activation, the smaller the rate constant (6.9B). Reactive Intermediates (6.3) ' Breaking bonds generates reactive intermediates. ' Homolysis generates radicals with unpaired electrons. I Heterolysis generates ions. Reactive General Reactive intermediate structure feature Reactivity . l , . . Radical 70 Unpaired electron Electrophilic | Carbocation —(‘;4 Positive charge; Electrophilic i only six electrons around C Carbanion Net negative charge; Nucleophilic lone electron pair on C Energy Diagrams (6.7, 6.8) transition state Ea determines the rate. >. E” o : L|.l AH° is the difference in bonding energy between the reactants and products. products > Fleaction coordinate Conditions Favoring Product Formation (6.5, 6.6) Variable ‘ Value ‘ Meaning KEu Keg > 1 More products than reactants are present at equilibrium. AG“ AG“ < O The free energy of the products is lower than the energy of the reactants. AH“ AH° < O Bonds in the products are stronger than bonds in the reactants. A8“ A8“ > 0 The products are more disordered than the reactants. Equations (6.5, 6.6) AG" = *2.303HTlog K3,:l AG" : AT — TATS° lree energy change in change in Keq depends on the energy ditierence change bonding energy disorder between reactants and products. i T: Kelvin temperature (K) i F! : 1.987 cal/(K-mol), the gas constant T = Kelvin temperature (K) Factors Affecting Reaction Rate (6.9) Factor ( Effect Energy of activation Larger Ea - — a + slower reaction Concentration Higher concentration - ~7-r faster reaction Temperature Higher temperatur * — s + faster reaction Key ancepts—Alkyl Halides and Nucleophilic Substitution General Facts about Alkyl Halides - Alkyl halides contain a halogen atom X bonded to an 5;:3 hybridized carbon (71). - Alkyl halides are named as halo alkanes, with the halogen as a substituent (7.2). - Alkyl halides have a polar me bond, so they exhibit dipole—dipole interactions but are incapable of intermolecular hydrogen bonding (7.3). - The polar C- X bond containing an electrophilic carbon makes alkyl halides reactive towards nucleophiles and bases (7.5). The Central Theme (7.6) - Nucleophilic substitution is one of the two main reactions of alkyl halides. A nucleophile replaces a leaVing group on an sp3 hybridized carbon. FFX + iNu' —> HmNu + x? nucleophile leavmg group The electron pair in the CmNu bond comes from the nucleophile. I One 6 bond is broken and one 6 bond is formed. - There are two possible mechanisms: 8N1 and 8N2. 5N1 and 5N2 Mechanisms Compared 8N2 mechanism 5N1 mechanism [1] Mechanism - One step (7.118) - Two steps (7.138) [2] Alkyl halide 0 Order of reactivity: CHSX > FiCHgX > ' Order of reactivity: HgCX > RQCHX > RCH2X > RQCHX > Racx (7.1 1 D) CH3X (7.130) [8] Rate equation 0 Rate = k[FiX][:Nu’] - Rate = kiRX] - Second-order kinetics (7.11A) - First-order kinetics (7.13A) [4] Stereochemistry I Backside attack of the nucleophile (7.110) I Trigonal planar carbocation intermediate (7.130) - Inversion of configuration at a stereogenic - Racemization at a single stereogenlc center center [5] Nucleophlle - Favored by stronger nucleophiles (7.178) - Favored by weaker nucleophiles (7.178) [6] Leaving group 0 Better leaving group 77+ faster reaction 0 Better leaving group 77+ faster reaction (7.170) (7.170) [7] Solvent - Favored by polar aprotic solvents (7.17D) - Favored by polar protic solvents (7.17D) Important Trends 0 The best leaving group is the weakest base. Leaving group ability increases leftitoeright across a row and down a column of the periodic table (7.7). - Nucleophilicity decreases lefteto-right across a row of the periodic table (7.8A). - Nucleophilicity decreases down a column of the periodic table in polar aprotic solvents (7.8C). - Nucleophilicity increases down a column of the periodic table in polar protic solvents (7.80). - The stability of a carbocation increases as the number of 8 groups bonded to the positively charged carbon increases (7.14). important Principles Principle Example - Electron»donating groups (such as Ft groups) stabilize a - 3° Carbocations (830') are more stable than 2° posrtive charge (7.14A). carbocations (RECH‘), which are more stable than 1° carbocations (RCHf). - Sterio hindrance decreases nucleophilicity but not - (CH3)SCO’ is a stronger base but a weaker nucleophile basrcity (7.88). than CHgCHZO'. - Hammond postulate: In an endothermic reaction, the - 8N1 reactions are faster when more stable (more more stable product is formed faster. In an exothermic substituted) carbocations are formed, because the reaction, this is not necessarily true (7.15). rate-determining step is endothermic. - Planar, spz hybridized atoms react with reagents from - A trigonal planar carbocation reacts with nucleophiles both sides of the plane (7.13C). from both sides of the plane. l Key ancepts—Alkyl Halides and Elimination Reactions AComparison Between Nucleophilic Substitution and [3 Elimination Nucleophilic substitution—A nucleophile attacks a carbon atom (7.6). l 70-07 + X:‘ 7020_ l (3 l l substitution good product leaving group llElimination—A base attacks a proton (8.1). BA H n l \ / ‘C’C— ‘—> C:C + H—B‘ + X:“ | )‘(D / \ elimination good product leaving group Similarities Differences I ln both reactions RX acts as an electrophile, reacting with I In substitution, a nucleophile attacks a single carbon atom. an electron-rich reagent. - Both reactions require a good leaving group X:' that can I In elimination, a Bronsted—Lowry base removes a proton to accept the electron density in the C—X bond. form a 1: bond, and two carbons are involved in the reaction. The Importance of the Base in E2 and E1 Reactions (8.9) The strength of the base determines the mechanism of elimination. I Strong bases favor E2 reactions. I Weak bases favor E1 reactions. strong base CH3 ‘OH \ T» /C:CH2 + H20 + Br- CH CH3 3 CH ,(l:_CH \ Sameproduct, 3 l 3 / differentmechanism 8r CH3 H20 \ ’0:on + H30+ + Br’ weakbase CH3 E1 E1 and E2 Mechanisms Compared E2 mechanism E1 mechanism Mechanism I One step (8.48) I Two steps (8.68) Alkyl halide - Rate: chx > RzCHX > RCHZX (8.4C) - Rate: R30X > FlgCHX > RCHZX (8.60) Rate equation I Rate : k[RX][8:] I Rate = kIRX] I Second-order kinetics (8AA) I First~order kinetics (85A) Stereochemistry I Anti periplanar arrangement of H and X (8.8) I Trigonal planar carbocation intermediate (8.68) Base I Favored by strong bases (8.48) I Favored by weak bases (8.6C) Leaving group I Better leaving group in» faster reaction I Better leaving group —~+ faster reaction (8.48) (Table as) Solvent I Favored by polar aprotic solvents (8.48) I Favored by polar protic solvents (Table 8.3) Product I More substituted alkene favored (Zaitsev rule, 8.5) I More substituted alkene favored (Zaitsev rule. 8.60) Summary Chart on the Four Mechanisms: 5N1, 5N2, E1, or E2 Alkyl halide type Conditions Mechanism 1" RCHZX strong nucleophile e 7+ 5N2 strong bulky base 77+ E2 2" RZCHX strong base and nucleophile »—-> 8N2 + E2 strong bulky base ~ + 52 weak base and nucleophile 77» 5N1 + E1 3" RSCX weak base and nucleophile -—-> 5N1 + E1 strong base - — -> E2 Key Concepts—Alcohols, Ethers, and Epoxides _l General Facts about ROH, ROR, and Epoxides ' ' a ' ' d and tetrahedral (9.2). Ith e com ounds contain an O atom that is sp hybridize I > ‘ . fill this comgounds have polar C— 0 bonds. but only alcohols have an O- H bond for intermolecular hydrogen bonding (9.4) Alcohols and ethers do not contain a good leaving group. Nucleophilic substitution can occur only after the OH (or 08) groupis erted to a better leavmg group (9.7A). > ‘ > ~ I (8:2:ides have a leaving group located in a strained three-membered ring. making them reactive to strong nucleophiles and aim HZ that contain a nucleophilic atom Z (9.15). A New Reaction of Carbocations (9.9) I Less stable carbocations rearrange to more stable carbocations by the shift of a hydrogen atom or an alkyl group. l l l 1,2Ashift ’?’9* ’9’?’ \j R Fl (or H) (or H) I Besides rearranging, a carbocation can also react With a nucleophile (7.13) and a base (8.6). Preparation of Alcohols, Ethers, and Epoxides (9.6) [1] Preparation of alcohols R—X + ‘OHj—> RAOHi + x, 0 The mechanism is 8N2. - The reaction works best for CH3X and 1° RX. [2] Preparation of alkoxides—A Bronsted—Lown/ acid—base reaction RioiH + Na‘H’ _. RiO'I Na* + H2 alkoxide [3] Preparation of ethers (Williamson ether synthesis) FliX + Fi—OFi') + X' - The mechanism is 8N2. 0 The reaction works best for CHgX and 1° RX. [4] Preparation of epoxidesilntramolecular 3N2 reaction - A two-step reaction sequence: [1] The removal of a proton with base forms an alkoxide. [2] An intramolecular 3N2 reaction forms the epoxide. halohydrin Reactions of Alcohols [1] Dehydration to form alkenes a. Usmg strong acid (9.8, 9.9) l | Hso \ /. —ceci 2 4 0:0 i + H20 ‘ i | Or / \ i H 0” TsOH rrrrrrrr «l , b. Using Pool; and pyridine (9.10) POCIa \ / , C:C i + H20 ' pyridine / \ I . Order of reactivity: FigCOH > RZCHOH > RCHZOH. The mechanism for 2° and 3° ROH is E1—carbocations are intermediates and rearrangements occur. The mechanism for 1° ROH is E2. The Zaitsev rule is followed. The mechanism is E2. No carbocation rearrangements occur. [2] Reaction with HX to form RX (9.11) HioH + H‘X —> Rixl + H20 Order of reactivity: R300H > RQCHOH > RCHZOH. The mechanism for 2° and 3° ROH is SNi—carbocations are intermediates and rearrangements occur. The mechanism for CHGOH and 1" ROH is 8N2. [3] Reaction with other reagents to form RX (9.12) R—OH + $002 ‘ > R~C| ‘ pyridine - R~0H + Par3 R—Bri Reactions occur with CHSOH and 1° and 2° ROH. The reactions follow an 8N2 mechanism, [4] Reaction With tosyl chloride to form tosylates (9.13A) O _ 0 ii ii - The 0—0 bond is not broken, so the configuration at a stereogenic center is retained. FiiOH+C| E \ / CHSDH Reactions of Tosylates Tosylates undergo either substitution or elimination. depending on the reagent (9.138). :Nu’ i i 7949—3 + ’OTs ‘ ‘ H Nu i 797?, 7 H OTs . / B" a \c:c 1 + TsOH / \ i - Substitution is carried out With a strong :Nu’, so the mechanism is 8N2. - Elimination is carried out with a strong base, so the mechanisr is E2. Reactions of Ethers Only one reaction is useful: cleavage with strong acids (9.14). R—OiR‘ + Hex 4» nextl + R'—xi + H20 [X=Bror|] ' " Reactions of Epoxides - With 2" and 3° R groups, the mechanism is 8N1. 0 With CH3 and 1° R groups, the mechanism is 8N2. Epoxrde rings are opened with nucleophiles :Nu’ and acids HZ (9.15). [1] :Nu' HZ [2] Hz0 fie 0 The reaction occurs with backsmle attack, resulting in trans or anti products. - With :Nu', the mechanism is 8N2, and nucleophilic attack occurs at the less substituted C, 0 With HZ, the mechanism is between 8N1 and 8N2, and attack of 2’ occurs at the more substituted C. _| Key Concepts—Alkenes General Facts About Alkenes - Alkenes contain a carbon—carbon double bond consisting of a stronger 6 bond and a weaker 1[ bond. Each carbon is 5/32 hybridized and trigonal planar (10.1). Alkenes are named using the suffix -ene (10.3). Alkenes with different groups on each end of the double bond exist as a pair of diastereomers, identified by the prefixes E and Z (10.3B). Alkenes have weak intermolecular forces, giving them low mp’s and bp’s, and making them water insoluble. A cis alkene is more polar than a trans alkene, giving it a slightly higher boiling point (10.4). Because a 1t bond Is electron rich and much weaker than a a bond, alkenes undergo addition reactions with electrophiles (10.8). Stereochemistry of Alkene Addition Reactions (10.8) Areagent XY adds to a double bond in one of three different ways: - Syn addition—X and V add from the same side. C_C O Syn addition occurs in hydroboration. - Anti addition—X and Y add from opposite sides. "mo:cw X2 - Anti addition occurs in halogenation and halohydrin I \ or \ f rm f x2, H20 x(0H) o a Ion. - Bath syn and anti addition occur when carbocations are intermediates. H _ x H\ /X(OH) H . _ _ \ 7 ,C—C\ or and \ H20, H’ X(OH) on \r, Syn and anti addition occur in hydrohalogenation and hydration. Addition Reactions of Alkenes [1] Hvdrohalogenation—Addition of HX (X : Cl, Br, 1) (10940.11) - The mechanism has two steps. - Carbocations are formed as intermediates. O Carbocation rearrangements are possible. 0 Markovnikov’s rule is followed. H bonds to the less substituted C to form the more stable carbocation. 0 Syn and anti addition occur. HCH:CH2 + HAX —> Fi—(IEHAOHZ X H alkyl halide [2] Hydration and related reactions (Addition of H20 or ROH) (10.12) H2804 For both reactions: RCH:CH2 + H’OH R’?H_?H2 0 The mechanism has three steps. OH H - Carbocations are formed as intermediates. alcohol - Carbocation rearrangements are possible. - Markovnikov’s rule is followed. H bonds to the less substituted C to form the more stable carbocation. H2504 - Syn and anti addition occur. RiOH 7cH2 OR H ether RCH:CH2 + H —OR [3] Haiogenation (Addition of X2; X = CI or Br) (1 11.13—10.14) I The mechanism has two steps. HCH:CH2 + xix R79H_(I:H2 - Bridged halonium ions are formed as intermediates. X X I No rearrangements can occur. ViCina' dihalide - Anti addition occurs. [4] Halohydrin formation (Addition of OH and X; X : Cl, Br) (10.15) H20 - The mechanism has three steps. I HCH:CH2 T X_X —' RTQHTC‘HZ ) - Bridged halonium ions are formed as intermediates. OH X 1 - No rearrangements can occur. halohydrin ; - X bonds to the less substituted C. ~ Anti addition occurs. - NBS in DMSO and H20 adds Br and OH In the same fashion. [5] Hydroboration—oxidation (Addition of H20) (10.16) - Hydroboration has a oneistep mechanism. RCH :CH2 M R7CH—CH2 - No rearrangements can occur. [2] H20? Ho— H OH . - OH bonds to the less substituted C. i - Syn addition of H20 results. _l Key Corgcepts—Alkynes General Facts About Alkynes ' Alkynes contain a carbon—carbon triple bond consisting of a strong 0 bond and two weak 1: bonds. Each carbon is sp hybridized and linear (11.1). - Alkynes are named using the suffix -yne (11.2). I Alkynes have weak intermolecular forces, giving them low mp’s and low bp's, and making them water insoluble (11.3). ' Because its weaker 7! bonds make an alkyne electron rich, alkynes undergo addition reactions with electrophiles (11.6). Addition Reactions of Alkynes [1] Hydrohalogenation—Addition ot HX (X : Cl, Br, 1) (11.7) X H _ I I i HiczciH R—c—ciH l - Markovnikov‘s rule is followed. H bonds to the less (2 Wu”) >‘( ll“ 1 substituted C to form the more stable carbocationt geminal dihalide ; [2] Halogenation—Addition of X2 (X : Cl or Br) (11.8) X X RscgciH R7é_é_H - Bridged halonium ions are formed as intermediates. (2 eqU'V) >‘( )‘( ( - Anti addition of X2 occurs. l tetrahalide : is] Hydration—Addition of H20 (11-9) R H 0 - Markovnikov‘s rule is followed. H bonds to the less _ H20 \ ¥ / ii ' substituted C to form the more stable carbocation. R7C=C7H (.1250A FTC ‘ — R CH3 f - An unstable enol is first formed, which rearranges to a HgSO4 HO H Wketone l carbonyl group. enol [4] Hydroboration—oxidation—Addition of H20 (11.10) ‘i? R H 7 [1] BH3 \ / FR /C\ i . . . Fli CzciH 7 020 r C H i - The unstable enol. first formed after OXIdation, [2] H202, HO / \ / \ . H OH H H ; rearranges to a carbonyl group. aldehyde '1 enol . ‘ Reactions Involving Acetylide Anions [1] Formation of acetylide anions from terminal alkynes (11.68) - Typical bases used for the reaction are NaNH2 and Rwach + :a .:-> FFCECZ’ + HB‘ NaH [2] Reaction of acetylide anions with alkyl halides (11.11A) A - The reaction follows an S 2 mechanism. H*CEC:‘ Fl 7x —» H~CEC*R x N + L; + - The reaction works best with CH3X and RCHZX. [3] Reaction of acetylide anions with epoxides (11,118) [1] O - The reaction follows an 8N2 mechanism. HsCEC,CHZCH20H Opening of the ring occurs from the back side at the less substituted end of the epoxide. 1 Key Concepts—Oxidation and Reduction Summary: Terms that Describe Reaction Selectivity - A regioselective reaction forms predominantly or exclusively one constitutional isomer (Section 8.5). - A stereoselective reaction forms predominantly or exclusively one stere0isomer (Section 85). - An enantioselective reaction forms predominantly or exclusively one enantiomer (Section 12.15). Definitions of Oxidation and Reduction (12.1) Oxidation reactions result in: Reduction reactions result in: - an increase in the number of C 72 bonds, or - a decrease in the number of C~Z bonds, or 0 a decrease in the number of C — H bonds - an increase in the number of C? H bonds Reduction Reactions [1] Reduction of alkenes—Catalytic hydrogenation (12.3) H2 “4 li1 - Syn addition of H2 occurs. RicH:CH_R Pd Pl or Ni Rifikfiz’R - increasing alkyl substitution on the 0:0 decreases the rateol ‘ ' H H 'j reaction. alkane [2] Reduction of alkynes a. PVCECnR H2 ——> Lindlar catalyst b. H’CECnFi Na .-> c. FlicEC-Fl NHS trans alkene 1 Two equivalents of H2 are added and four new 0— H bonds are formed (12.5A). Syn addition of H2 occurs, forming a cis alkene (12.55). The Lindlar catalyst is deactivated; reaction stops after one equivalent of H2 has added. Anti addition of H2 occurs. forming a trans alkene (12.50). [31 Reduction of alkyl halides (12.6) [1] LiAIH4 [2] H20 H'X RnH alkane j The reaction follows an 3N2 mechanism. CH3X and RCHQX react faster than a more substituted RX. [4] Reduction of epoxides (12.6) /0\ [1]LiA|H4 9H1 woncw [2]H o FTCM / 2 H \ 1 alcohol ! Oxidation Reactions [1} Oxidation of alkenes a. Epoxidation (12.8) \ / 0 1 {3:0 + ncoaH —+ ....6—\c..,,l / \ ‘ \’ l epoxide 1 The reaction follows an $42 mechanism. In unsymmetrical epoxides, HT (from LiAlH4) attacks at the less substituted carbon. 0 The mechanism has one step. - Syn addition of an O atom occurs. The reaction is stereospecific. b. Anti dihydroxylation (12.9A) \C_C/ [1] RCOaH / T \ [2] H20 (H+ or HO’) - Opening of an epoxide ring intermediate with 'OH or H20 forms a 1,2-diol with two OH groups added in an anti fashion. c. Syn dihydroxylation (12.93) HO OH . \ / 1 050; 2 NaHSO l c=c r—Hl ] 4 if] 3 “Tc—cf” [1]OsO4, NMO; [21NaHso3 I \ . l l 0' 1.2-diol KMnOA, H20, 140- s as. .. 0 Each reagent adds two new C—O bonds to the 0:0 in a syn fashion. d. Oxndative cleavage (12.10) “hie/R [1103 / T \ [2] Zn, H20 or R H CHgsCHa [2] Oxidative cleavage of alkynes (12.11) Fl Ft‘ [1] O FFCECiR‘ ,—>[2] H30 :c:o + 0:0: internal alkyne 2 HO OH carboxylic aCIds R*C:C*H [1103 R\c-o + T [2] H20 / T 002 terminal alkyne HO Both the o and 1: bonds of the alkene are cleaved to form two carbonyl groups. - The (S bond and both 7! bonds of the alkyne are cleaved _' Key 3C0ncepts—Mass Spectrometry and Infrared Spectroscopy Mass Spectrometry (MS; 13.1 —1 3.3) I Mass spectrometn/ measures the molecular weight of a compound (13.1A), - The mass of the molecular ion (M) = the molecular weight of a compound, Except for isotope peaks at M + 1 and M + 2, the molecular ion has the highest mass in a mass spectrum (13.1A). I The base peak is the tallest peak in a mass spectrum (13.1A). - Acompound with an odd number of N atoms gives an odd molecular ion. A compound with an even number of N atoms (including zero) gives an even molecular ion (1 3.1 B). I Organic monochlorides show two peaks for the molecular ion (M and M + 2) in a 3:1 ratio (13.2). I Organic monobromides show two peaks for the molecular ion (M and M + 2) in a 1:1 ratio (132). - High-resolution mass spectrometry gives the molecular formula of a compound (13.3A). Electromagnetic Radiation (13.4) - The wavelength and frequency of electromagnetic radiation are inversely related by the following equations: A c/v or v = CI)» (13.4). I The energy of a photon is proportional to its frequency; the higher the frequency the higher the energy: = hv(13.4). Infrared Spectroscopy (IR; 13.5 and 13.6) I Infrared spectroscopy identifies functional groups. I IR absorptions are reported in wavenumbers, V = 1])». I The functional group region from 4000—1500 cm'1 is the most useful region of an IR spectrum. - C—H, O— H, and N— H bonds absorb at high frequency, 2 2500 cm". I As bond strength increases, the V of absorption increases; thus, triple bonds absorb at higher V than double bonds. c:c 020 ~ 1650 cm“ _ 2250 our1 Increasing bond strength Increasing V I The higher the percent s-character, the stronger the bond, and the higher the V of an IR absorption. | / iciH =c EC’H l \ H 0593*H Csp27H CwiH 25% s-character 33% s-character 50% s-character 3000-2850 cm“ 3150—3000 cm" 3300 cm" Increasing percents -character Increasing \7 i Ke25C0nceptS—Nuclear Magnetic Resonance Spectroscopy ‘HNMR Spectroscopy jij The number of signals equals the number of different types of protons (14.2). [2] The position of a signal (its chemical shift) is determined by shielding and deshielding effects. - Shielding shiits an absorption upfield; deshielding shifts an absorption downfield. - Electronegative atoms withdraw electron density, deshield a nucleus, and shift an absorption downfield (14.3). i This proton l5 shielded. | This proton is deshielded. ~07H h Its absorption is upfield, 7(3in Its absorption is farther downfield, 0.972 ppm. )‘( 2.5—4 ppm. I Loosely held it electrons can either shield or deshield a nucleus. Protons on benzene rings and double bonds are deshielded and absorb downfield, whereas protons on triple bonds are shielded and absorb upfield (14.4). / (H 7C:C i deshielded H shielded H downfield absorption upfield absorption [3] The area under an NMR signal is proportional to the number of absorbing protons (145). I4) Spin-spin splitting tells about nearby nonequivalent protons (14.6—14.8). I Equivalent protons do not split each other’s signals. I A set of n nonequivalent protons on the same carbon or adjacent carbons splits an NMR signal into n + 1 peaks. - OH and NH protons do not cause splitting (14.9). I When an absorbing proton has two sets of nearby nonequivalent protons that are equivalent to each other, use the n + 1 rule to determine splitting. I When an absorbing proton has two sets of nearby nonequivalent protons that are not equivalent to each other, the number of peaks in the NMR signal : (n + i)(m +1). uC NMR Spectroscopy (14.1 1) [i] The number of signals equals the number of different types of carbon atoms. All signals are single peaks. (2] The relative position of ’30 signals is determined by shielding and deshielding effects, I Carbons that are sp3 hybridized are shielded and absorb upfield. I Electronegative elements (N, O. and halogen) shift absorptions downfield. I The carbons of alkenes and benzene rings absorb downfield. I Carbonyl carbons are highly deshielded. and absorb farther downfield than other carbon types. I Key Concepts—Radical Reactions General Features of Radicals - Aradical is a reactive intermediate with a single unpaired electron (15.1). - Acarbon radical is sp2 hybridized and trigonal planar (15.1). r The stability of a radical increases as the number of C atoms bonded to the radical carbon increases (15.1). - Allyllc radicals are stabilized by resonance, making them more stable than 3° radicals (15.10). Radical Reactions [1] Halogenation of alkanes (15.4) FI—H x2 - The reaction follows a radical chain mechanism. M or A Ft—X i - The weaker the C— H bond, the more readily the hydrogen is replaced by X. Chlorination is faster and less selective than bromination (15.6). I Radical substitution at a stereogenic center results in racemization (15.8). alk lhal'd X = Cl or Br y I e [ZJAllylic halcgenation (15.10) CH =CH70H N—BS, _ . 2 a [W or ROOR CHrCHCHzBr r 0 The reaction follows a radical chain mechanism allylic halide (3] Radical addition of HBr to an alkene (15.13) HBr F‘i if - A radical addition mechanism is followed. FlCHeCH2 W A‘ or Ft—C—C—H - Br bonds to the less substituted carbon atom to form the more substituted, ROOR H 3,- more stable radical. alkyl bromide _| Key Concepts—Conjugation, Resonance, and Dienes Conjugation and Delocalization of Electron Density - The overlap of p orbitals on three or more adjacent atoms allows electron density to delocalize. thus adding stability (16.1). 0 An allyl carbocation (CH2=CHCH2”) is more stable than a 1" carbocation because of p orbital overlap (16.2). - In any system X=Y—Z:, Z is sp2 hybridized to allow the lone pair to occupy ap orbital, making the system conjugated (165). Four Common Examples of Resonance (1 6.3) [1] The three—atom "allyl" system: X=Y—; <—> ésY:Z * = +.—. '. or " [2] Conjugated double bonds: " 4—D or .——» s_. [3] Cations having a positive charge .9, s, + adjacent to a lone pair: X_Y x:Y [4] Double bonds involving one atom {‘0 + — _ _ : .—. _ - more electronegative than the other: X Y X Y' [elewonegat'v'ty 0” > X] Rules on Evaluating the Relative "Stability" of Resonance Structures (16.4) [1 Structures with more bonds and fewer charges are more stable. [2 Structures in which every atom has an octet are more stable. [3 Structures that place a negative charge on a more electronegative atom are more stable. The Unusual Properties of Conjugated Dienes [1 The 0—0 6 bond joining the two double bonds is unusually short (16.8). [2 Conjugated dienes are more stable than the corresponding isolated dienes. AH“ of hydrogenation is smaller for a conjugated diam than for an isolated diene converted to the same product (16.9). [3] The reactions are unusual: - Electrophilic addition affords products of 1,2-addition and 1,4—addition (16.10, 16.11). - Conjugated dienes undergo the Diels—Alder reaction, a reaction that does not occur with isolated dienes (16.12—16.14), [4 Conjugated dienes absorb UV light in the 200—400 nm region. As the number of conjugated n bonds increases, the absorption shifts to longer wavelength (16.15). Reactions of Conjugated Dienes [1 Electrophilic addition of HX (X = halogen) (16.10—16.11) HX (1 equiv) EchH CH—CHQI :le2 CH—CH CH2 CH2=CH—CH:CH2 X H X 1,2-producl 1 ,d-product kinetic product thermodynamic product - The mechanism has two steps. - Markovnikov‘s rule is followed. Addition of H+ forms the more stable allylic carbocation. - The 1.2-product is the kinetic product. When H+ adds to the double bond, X‘ adds to the end of the allylic carbocation towhini it is closer (CZ not CA). The kinetic product is formed faster at low temperature. I The thermodynamic product has the more substituted, more stable double bond. The thermodynamic product predominatesat equilibrium. With 1,3—butadiene, the thermodynamic product is the 1,4-product. ...
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Key concepts - l Kg Concepts—Structure and Bonding...

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