alkenes - SUMMARY Alkenes General Information Alkenes 1...

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Unformatted text preview: SUMMARY ' Alkenes General Information Alkenes 1 Synthesis of Alkenes ' From Alkyl Halides via Dehydrohalogenation Alkenes 2 0 Preparation of Alkenes via Acid-Catalyzed Dehydration (see Alcohols 8) I Preparation of Alkenes via the Wittig Reaction (see Aldehydes/Ketones 9) Reactions ofAlkenes 0 Preparation of Alkyl Halides via Electrophilic Addition of Hydrogen Halides Alkenes 3 0 Preparation of Dihalides via Electrophilic Addition Alkenes 4 0 Preparation of Halohydrins via Electrophilic Addition Alkenes 5 ' Preparation of Alkanes via Catalytic Hydrogenation Alkenes 6 0 Preparation of Alkyl Bromides via Free Radical Addition Allaenes 7 - Preparation of Aldehydeleetones via Oxidative Cleavage with O3 Alkenes '8 ' Preparation of Cyclopropanes via Carbene Addition Allaenes 9 0 Preparation of Alcohols From Alkenes via Oxymercuration-Demercuration {see Alcohols 2) 0 Preparation of Alcohols From Alkenes via Hydroboration—Oxidation (see Alcohols 3} 0 Preparation of Alcohols From Alkenes via Hydroxylation with 0504 (see Alcohols 4) (or KMnO4) GENERAL INFORMATION memes 1 Alkenes (C=C) Introduction: Alkenes are unsaturated organic compounds with a carbon—carbon double bond (C=C) and have the chemical formula CnHZn‘ They are found throughout nature (e.g., rhodopsin in vision) and are some of the most important chemicals produced synthetically (e.g., ethylene}. Physical Properties: 0 Alkenes have increased melting and boiling points as their molecular weights increase. ' Alkenes with fewer than five carbons are gases at room temperature. 0 Alkenes are soluble in nonpolar solvents. 0 Alkenes are less dense than water. Chemical Properties: ' The carbons of the double bond are sp2 hybridized, have planar geometry, and have bond angles of about 120°. ' Because the double bond consists of one sigma and one pi bond, rotation around it, which does not occur at ordinary temperatures, requires a large amount of energy (65 kcallmol). 0 Increasing the alkyl substitution of the carbons of the double bond (i.e., replacing hydrogens with alkyl groups) increases alkene stability because of a phenomenon called hyperconjugation (overlap of a pi" orbital involved in the double bond with a nearby sigma bond) and to the increased bond strength between 51::2 and sp3 hybridized carbon atoms. 0 Some alkenes can exist as two isomers (cis and trans}; a comparison of the chemical properties of these two forms is presented below: a. cis-Alkenes are less stable than trans—alkenes. Reason: Steric strain between alkyl groups on the same side of the double bond causes the cis isomer to be less stable. .,..-. b. If alkenes are hydrogenated into alkanes, the enthalpy of this reaction (AHOhydrogenation) is more nega- tive for the C15 than for the trans isomer. Nomenclature- General rule - Replace the -ane of an alkane with -ene (e.g., propane —) propene). Carbons involved in the double bond are assigned the lowest possible number. For isomers 0 Cis (same side of the double bond) and trans (opposite sides of the double bond) designations can be used A\ /A A\ /c for alkenes of the type c=c or c=c , where A is either hydrogen or an alkyl group, and B and C /\/\ a c B A are different from A and each other, with a maximum of one identical substituent on each carbon of the CH; fH; ' CH CH CH CH cH CH K 3 \s / I - - \3 / double bond. For example, c=c and c=c are both as alkenes, as 18 etc / \ / \ / \ H H oH2 CH H H | / \ 0 The E, Z system is a more general way of naming alkenes with multiple substituents (other than hydrogen) attached to the carbons of the double bond. Simply put, the E, Z system assigns relative priorities to each substituent based on the atomic number of the atom (or atoms, depending on the circumstance) directly attached to one carbon of the double bond. The substituent containing the atom with the higher atomic number receives a high priority, while that containing the atom with the lower atomic number receives a low priority. This process is repeated for the substituents bonded to the other carbon of the double bond. If the high priority substituents are on opposite sides, the alkene is designated E. Otherwise, it is designated Z. q Spectroscopy: .1. H NMR: The hydrogen attached to a carbon of the double bond has a characteristic peak at 5 = 4.5—6.5 ppm. . This is due to a deshielding effect caused by the electrons in the double bond. C NMR: Characteristic peak at 8 = 1401313111. IR: The C=C bond has a characteristic absorption at 165 0—1670 cm'l. The =C—H bond has a characteristic absorption at 3020—3100cm—1. SYNTHESIS OF ALKENES Alkenes 2 FROM ALKYL HALIDES VIA DEHYDROHALOGEMTI ON I CH ”6 /l H\ /CHS s a \E—cqa fl, c=c + H20 + m1 / '5 ”cu, / \ H H CH; H 2-Iodobutane rmns-Z-Butene (alkyl halide) (alkene) Keys: 1. This is a major method used to generate alkenes in high yield. 2. This reaction proceeds through an E2 (bimolecular elimination) mechanism (see Alkyl Halides 8 for more details on E2) 3. A strong base {e.g., NaOH or alkoxide) is needed to carry out the reaction. N_otes: 1. It 15 important to keep in mind when synthesizing alkenes by this method, that SN 2 side reactions {see Alkyl Halides 6 for more information) can aiso occur simultaneously to yield undesired substitution products This 15 why a strong base IS crucial for the dehydrohalogenation reaction. 2. In the sample reaction above, cis—Z-butene is also formed. Mechanism: 1. A strong base (B:) begins to abstract the hydrogen of the alkyl halide, initiating the formation of the double bond and the departure of the leaving group (note the transition state in which the dotted lines represent the bonds that are being partially broken and formed). This transition state has periplanar geometry (i.e., B, H, C, C, and X lie in the same plane). Reason: The (3—H and C—X sigma bonds that disappear are trans— formed into the new p orbitals of the pi bond. This process occurs more easily when they are antiparallel. 2. Loss of HX leads to the alkene product. Note that the remaining groups maintain their relative positions. 3' 73 aged B“ H 5.. ‘H 3.. R\ /R.. :1? Rm \\ S‘- Hm \N‘lc—C‘) i} ““1029" i} /C=C + BH + X- n‘ R‘ K \ H' \ij 3- x8. R' R'" Alkyl halide Transition Alkene state where X = Cl, Br, I REACTIONS OF ALKENES Aikenes 3 PREPARATION OF ALKYL HALIDES VIA ELECTROPHILIC ADDITION OF HYDROGEN HALIDES CH: H CH3 H \ / HCl | | 0:!) —} H—C—C—H / \ ether I H H Cl H Propane 2-Chloropropane (alkene) (alkyl halide) Keys: 1. Historically, the addition of HX to a double bond has been described as a Markovnjkov addition reaction: The halogen (—X) group is attached to the more substituted carbon, while the —H group is attached to the less substituted carbon. ' 2. The mechanism of this reaction proceeds through an achiral carbocation intermediate. Because the most stable carbocation (tertiary > secondary > primary) forms, the halogen becomes attached to the more highly substituted carbon of the double bond. 3. A racemic mixture of alkyl halide product may be obtained if the positively charged carbon in the carbo- cation intermediate becomes a stereocenter in the product (see Mechanism, step 20). 4. A mixture of products can result if the carbocation undergoes a type of rearrangement called a hydride shift to form a more stable carbocarion. CH3CH2 H H CHSCHz H H FBWECHIEHI GchHZ H H I | l HBr I l OfH 2 | (I: CHg—‘f—C—c—H W CHa—‘f—g—(Iz—H f Cfla—g—Cf—l—H 1H,] . it'll . H 111' H 2“ Cation 3° Cation lBr lBr' CHSCTIZ lil II-l CHICHz H til °"‘“T“i“f‘" °“"“f“i‘?"" {H} Br H Br (ti) H 2° Bromide 3° Bromide m A mixture of products can also be obtained if the ends of the double bond have different substituents and can form two equally stable carbocation intermediates. CchflchgCH=CHCHa gag—r} CH30HZCHZCIL'ICH26H3 CI 1- CHscHZCHzDHzthCHg Cl MLhanim 1. A pair of electrons from the double bond attacks the electrophilic HX to generate an achiral, trigonal pla— nar carbocation intermediate. It should be noted that the H is attached to the less substituted carbon atom. 2. The halide ion (X‘) then adds to either face of the positively charged carbon, thereby forming the alkyl halide product. If the positively charged carbon becomes a stereocenter in the product, a racemic mixture may be obtained. It should be noted that X is attached to the more substituted carbon atom. ( bottom side ) addition of X R\ /R" c=c L / \ - . H' H ( top Side .. ‘ca addition of X ”*6 Carbocation RNII \‘W’H intermediate R H Alkene Alkyl halide A where X = Cl, Br. I a... REACTIONS OF ALKENES Alkenes 4 PREPARATION OF DHMLIDES VIA ELECTROPHILI C ADDITION H H Cl \H H, cu \ / \ CH: CH: / c=c 3% “c—c’ + \c—c” / \ Gel. “a“! \ / \I’I’H CH3 CH3 CH3 Cl C1 0H3 cis—Z—Butene (2R, 3R)—2.3—Dichlorobutane (ZS, 3S)-2,3-Dichlorobutane (alkene) (dihalide) (dihalide) Keys: 1. This reaction produces a vicinal dihalide (i.e., the two halogen groups are attached to neighboring carbons). 2. This reaction uses anti addition to produce only trans products (see Mechanism). 3. The reaction proceeds through a three-membered cyclic halonium ion intermediate. 4. In most cases, a racemic mixture of products is obtained if either carbon atom of the three-membered ring becomes a stereocenter in the product (see Mechanism, step 2). For symmetric alkenes (i.e., those with the same substituents on both carbons of the double bond), cis-alkenes give a racemic mixture; trans-alkenes give a meso dihalide. 5. In general, organic solvents (e.g., CCl4) can be utilized in this reaction. The presence of water in the sol- vents frequently leads to the formation of halohydrins instead (see Alkanes 5 for more information). 6. Both chlorine (C12) and bromine (Brz) can be used in the reaction. Note: Fluorine (F2) is not used in this reactipn because of its dangerously high reactivity (explosive), while iodine (12) is simply not reactive enough. a... Mechanism; 1. The pi electrons of the double bond attack a halogen molecule (X2 ), leading to the formation of a three- membered cyclic halonium 10!: intermediate (e ..,g bromonium' 1on when X: Br). 2. The halide 1011 (X‘) )attacks the backside of either carbon of the cyclic halonium 1on intermediate (anti addition). This produces the vicinal dihalide product (as a racemic mixture if R, R’, R", and R'” are all different) and explains why only trans addition products are formed. .‘ a, x /H" xy" 3 ’2, / product from =c —®+ “o c——c a, L K 0— on X“ attackin E“ Ice) ?\0[ an /@ @ ”’H" E F>c'[\n n. n... x carbon a R “if; t + 6+ 6 :x: . . R" Cyclic halomum x\c R. .. product from ion intermediate \\ §’ X‘ attacking ) R‘;/%— ®\x carbon b where X = Cl, Br, I trans-Dihalide products REACTIONS OF ALKENES Aikenes 5 PREPARATION OF HALOHYDRINS VIA ELECTROPHILIC ADDITION H H CI\ ‘H H, /CI \ / c g en, en, 2,, c=c i> “c—c’ + \c—cfl / \ "20 H“? \ / \””H CH3 CH3 CH3 OH Ho CH3 cis-Z—Butene (2R. 3R)-3-Chloro—2—buta.nol (ZS, 3S)-3-Chloro-2-butanol (alkene) (halohydrin) (halohydrin) Keys: 1. This reaction uses anti addition to produce trans products (see Mechanism). 2. The reaction proceeds through a three-membered cyclic halonium ion intermediate (see Notes, item 3). 3. A racemic mixture of halohydrin product is obtained if either carbon atom of the three-membered ring becomes a stereocenter in the product (see Mechanism). 4. Both chlorine (C12) and bromine (Brz) can be used in the reaction. Notes: 1. N—Bromosuccinimide (NBS) is usually substituted for Br2 in the synthesis of bromohydrins. 2. Halohydrins are important in the preparation of epoxides {see Hikers 6). 3. When a stable (3°, benzylic) carbocation can be formed (e.g., from 1-phenylpropene), H20 adds to the benzylic carbocation center to give a mixture of steroisomers. Mechanism: In a nutshell, the synthesis of halohydrins is mechanistically similar to that of dihalides. The main difference is that instead of X‘ 32H 0 IS the nucleophile used to attack the cyclic 1011 intermediate. 1. The alkene reacts with the halogen molecule (X2 or XOH) to form a three- membered cyclic halonium ion intermediate 2. Nucleophilic attack by water on the backside of either carbon of the cyclic ion intermediate (the side oppo- site the halogen) opens the ring. A racemic mixture is produced if R, R', R", and R’” are all different. 3 A base (e. g., X“, H 20) abstracts a proton from the newly added water molecule to generate a trans halo» hydrin and HX (01: 21130+ if H20 was the base). $43 F; x R, X a. g R' / R' / ”He—cf; —®m+ Nae—ca,” 7Q" \c—cum ® Keven,” w, H... n / 1\In' / \‘w' / ’R" 'XQ H' 1" H'" "—7 Rm H0 Rm 8+ 8' :5—H a I ”95 H =.X= {- Cyclic halonium x\ *‘Rn x\ :1" a? H'" :9 R'" where X = Cl. Br, 1 ion intermediate “(3—5) i’ “C —C ) an“ \ no“ \ I of" I OH R' {4 R' H -x trans-Halohydnn products REACTIONS OF ALKENES Alkenes 6 PREPARATION OF ALKANES VIA CATALYTIC HYDROGENATION H H' (H; n: \c (3/ H2 \c c/ = —> _ / \ PdlC in ethanol H “u“ ”I”, H CH: CH3 CH3 CH3 cis-Z-Butene Butane (alkene) (alkane) Keys: 1. Syn addition of hydrogen atoms is frequently observed (i.e., hydrogen atoms are added to the same side of the double bond). 2. A source of hydrogen and a metal catalyst [e.g., platinum (Pt), palladium (Pd), or nickel (Ni)] are needed to reduce the double bond. ' 3. This reaction takes place on the surface of the solid metal catalyst. 4. Because of steric hindrance effects, the hydrogens add to the side of the double bond with the least bulky substituents. In the example below, the hydrogens add to the side opposite the gem~din1ethyl group. CH3 CH3 CH3 CH3 ‘ cu3 H2 CH3 —> PdfC in ethanol H OH I CH3 H 5. The enthalpy of hydrogenation (AHOhydroie-nationl can be used to estimate alkem stability. Aikenes with a lower heat of hydrogenation are more sta 1e. ‘ Note: Under mild conditions (e.g., room temperature and H2 gas at latrn), only the alkene portion will be selec- tively reduced if the alkene contains other potential reducible functional groups such as aldehydes, ketones, esters, benzene rings, or nitriles. Mechanism [exggt details of the mechanism are quite complexlz 1. The hydrogen (H2) adsorbs onto the surface of the catalyst and dissociates to atomic hydrogen. 2. The alkene also adsorbs and forms a complex with the metal catalyst. 3. The hydrogens attach to the alkene, one at a time, to produce an alkane. 4. The alkane is released from the catalyst. ' R\ /n" H—H G) H Ill R /FI" ® lll ill R.’CTC\R... + _ mImJJIn—’I:I:Il1mnm+n,°— R," ’Emmumfimmm] metal l® catalyst H R" Fl' ’2’ 5? Ru: ‘ n \E—é) “\(l: /H' .L a G) H w I [mm mm REACTIONS OF ALKENES Alkenes 7 PREPARATION OF ALKYL BROMIDES VIA FREE RADICAL ADDITT ON CH3\ 7 /H cl" ri c=c fl» H—C—C—H / \ peroxides I 1 H H H Br Propene l -Bromopropane (alkene) (alkyl halide) Keys: 1. The regiochemistty of the free radical addition of HBr is opposite that of electrophilic addition (1.6., anti- 2. Markovnikov) and the less substituted alkyl bromide forms (1° > 2“ > 3°). This is a free radical chain reaction: A bromine radical adds to the less substituted carbon atom of the dou- ble bond to produce the more substituted alkyl radical. Alkyl radicals are more stable if 'they have more alkyl substituents. Alkyl Radical: Methyl < 1° < 2° < 3” Stability: Least => => Most . This reaction is catalyzed by peroxides, which can be added to the reaction mixture or produced by a reac- tion between the alkene and atmospheric oxygen. Notes: In summary, an alkene and HBr can undergo two possible types of halogenation reaction: ( 1) an electrophilic, Markovnikov halogenation reaction (see Alkenes 3 for more details) and (2) a free radical, anti-Markovnikov halogenation reaction. An alkene and‘HCl or HI, on the other hand, can undergo only the former reaction. Mechanism (can be divided into three phases): 1. Initiation . 1. UV light or heat-induced homolytic cleavage of a peroxide molecule (obtained by reaction of the alkene with atmospheric oxygen or R—m—H (BIL—V» 2n—o- . . . . (or am) intentionally added to the reaction mixture) generates two alkoxy Peroxide Alkoxy radicals radicals (R00). 2. The alkoxy radical abstracts a hydrogen atom from HBr to form an alcohol and a bromine radical. R—oamr in» R—O—H + Br' Bromine radical II. Propagation 3. The bromine radical adds to the less substituted carbon of the alkene to form a bromoalkyl radical. R n" R n" 4. The bromoalkyl radical then abstracts a hydrogen atom \mfi Br @ .\C_CI:_H from another HBr molecule to form an alkyl bromide and ./ T \ / | another bromine radical. 5. Repetition of steps 3 and 4 generates more radicals Bromoalkyl radical needed to sustain the reaction cycle. R n" 111. Termination l I >C7C7H+HBr—>H7C7CH+Br‘ 6. This phase occurs when two radicals combine in a l I - - R' Br chance C0lllSlon to form a stable molecule. R" R“ R R R" a m H H" “\th @ l i l I \. I d I I CC— (Ii—H + )C— C— —H —> H—C—C—C—C—H or C—C—H + Br' —> Br—C—C—H + HBr | | l | l / | | | R' Blr H' Br i Br Fl' R' Br R Br R' Br Alkyl bromide REACTIONS OF ALKENES Alkenes 8 PREPARATION OF ALDEHYDES/IGETONES VIA OXIDATIVE CLEAVAGE WITH 03 CH3 CH3 0 O \ / c=c M g + g / \ 2) Zn In cmcoorlmzo / \ / \ H CH, CH: H CH: CH, Z-Methyl-Z-butene Ethanol Propanone (alkene) (carbonyl compounds) Keys: 1. Ozonolysis of alkenes can generate various types of carbonyl groups depending on the types of substituents present on the alkene molecule. The accompanying table summarizes the possible reactants and products. Number of Alkyl Groups on Alkene Number and Type of Carbonyl Compounds Produced None Two formaldehydes One One aldehyde and one formaldehyde Two (one on each carbon of the alkene) Two aldehydes Two (both on one carbon of the alkene} One ketone and one formaldehyde Three One ketone and one aldehyde Four Two ketones 2. The ozone (O3) acts as an oxidizing agent to cleave the alkene double bond. 3. This reaction proceeds through a molozonide and an ozonide intermediate, both of which are highly unstable. Notes: - Q a . . u . . Ozonolysrs can also be used for alkynes, but it generates carboxyllc ac1ds lustead. ngjnolySIS of internal alkynes generates two carboxylic acids, whereas ozonolysis of terminal alkynes generates one carboxylic acid and one formic acid (HCOOH) molecule. 0 D O O 03 || ll 03 II II R—CEC—FI' —)- C + C R—CEC—H —> C + C / \ / \ / \ / \ R OH H“ OH H OH H OH Internal alkyne Carboxylic acids Terminal alkyne Carboxylic Formic acid acid Mechanism: 1. Oxygen gas is exposed to high voltage to form ozone (03). 2. Ozone reacts with the alkeue to form a molozonide intermediate. 3. The molozonide intermediate then breaks down into two fragments. 4. These two fragments then rearrange to ferm an ozonide intermediate. 5. Reduction of the ozonide with a reducing agent (e.g., zinc metal in acetic acid or dimethyl Sulfide) then results in cleavage of the ozonide into two carbonyl compounds. 30: —>® 20: Av 9:? O \ 101/ :9:6 a? .3 .- H Ru /.. .- /OI:- ' ' \ / ® :0. :0: © 0 .o. c_ —> ~—+ l | / \ FI—C C—FI" R—C C—R" FI' R'“ | | l | R' H'" H' n'" Alkene Molozonide lg) O 0 — ... c”; (ll «A Fl\ [0 o\ /':| + . ~ C C / \ / \ Zulu CH3C00HJ’H20 Fl R' Fl" Fl"' 01' Fr/ \0/ \R“ Di [h 1 [rd Carbonyl compounds me y su I e Ozonide REACTIONS OF ALKENES Alkenes 9 PREPARATION OF CYCLOPROPANES VIA CARBENE ADDITION OJ CI H\_c c/H CHCb ‘ec ——~> / \c KOH / \ CH3 CH, "five—cum,” CH; CH; cis-Z-Butene cis-Dichlorocyclopropane (alkene) (cyclopropane) Keys: 1. This reaction is stereospecific (in the example above, cis-alkene —) cis-cyclopropane). 2. This reaction occurs in two steps: generation of dichlorocarbene and reaction of dichlorocarbene with the alkene (see Mechanism). 3. Carbenes (R20) have the following properties: a. They are neutrai molecules with six electrons in the valence shell of the central carbon. b. Despite being neutral, they are great electro...
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