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alkanes - SUMMARY General Information{Alkanes...

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Unformatted text preview: SUMMARY General Information {Alkanes} Conformational Isomers of Alkanes General Information (Cycloalkanes) Conformational Isomers of Cyclobexane Synthesis of Alkanes 0 Preparation of Alkanes via Catalytic Hydrogenation | Preparation of Alkanes via Catalytic Hydrogenation 0 Preparation of Alkanes via the Wolff-Kishner Reaction {or Clemmensen Reduction) 0 Preparation of Alkanes From Alkyl Halides Reactions of Alkanes ‘ Synthesis of Alkyl Halides From Alkanes via Free Radical Halogenation Alkanes 1 Alkanes 2 Alkanes 3 Alkcmes 4 (see Aikenes 6) (see Alkynes 6, Keys, item 3) (see Aldehydes/Ketones 7} (see Alkyl Halide; 4) {see Alley! Halides 2} Alkanes GENERAL INFORMATION Alkaues 1 Alkanes (C—C) Introduction: Aikanes, otherwise known as saturated hydrocarbons, aliphatic compounds, or paraffins, have the chemical formula CnH2n+2' They are found in animals, plants, natural gas, and petroleum deposits. The number of isomers of a particular alkane increases with the number of carbons. Physical Properties: 0 The melting and boiling points of unbranched alkanes increase as the carbon chain lengthens because the strength of the attractive van der Waals intermolecular force increases with molecular size. 0 The melting and boiling points of alkanes with the same number of carbons decrease as the amount of branching increases. This occurs because the strength of the van der Waals intermolecular force decreases with branching. 0 Alkanes are soluble only in nonpolar solvents (e.g., hexane, toluene}. ° All alkanes are less dense than water. Chemical Properties: 0 Carbon atoms are sp3 hybridized, have tetrahedral geometry, and have a bond angle of about 109.5 °. 0 There is fairly free rotation around the C—C bonds. 0 In general, alkanes are energetically stable. 0 A complete combustion reaction between an alkane and 02 yields C02, H20, and energy. 0 The PKa of alkanes is extremely high. Alkanes have (1) negligible acidity or basicity and (2) low oxidizing or reducing power. They are therefore very unreactive. Nomenclature: . In naming an alkane, four rules apply: “a. 1. The names of alkanes end with the suffix -ane. 2. The prefix of an alkane name depends on the longest continuous carbon chain. (In the following example, it is seven carbons long, and so the prefix for the molecule name is hept-.) When there are two chains of the same length, the chain with the most substituents is considered the main chain. 3. If there are any substituents attached to the carbon chain, they are assigned the lowest number possible numbering from the end of the chain. [In the example below, the methyl is attached to C~2 (rather than 06), while the ethyl is attached to C—4.] 4. The substituent names and numbers precede the prefix and are ordered alphabetically (e.g., the complete name of the alkane below is 4-ethyl-2-methylheptane). Carbon number -* 1 2 3 4 5 6 7 CH3CHCH2CHCH2CH2CH3 l | CH3 CHzCH3 Ewan H NMR: The proton attached to saturated carbons is characteristic at 8 = 0—1.5 ppm. C NMR: Characteristic peak at 3 = 20 ppm. IR: The C—H bond has a characteristic absorption at 2850—2960 cm‘l. The C—C bond has a characteristic absorption at 800—1400 cm‘l. CONFORMATIONAL ISOMERS OF ALKANES Alkanes 2 The conformational isomers of a particular alkane are produced by rotation about the C—C bond and are rep- resented by Newman projections. In a Newman projection, the molecule is viewed as if one were looking down the axis of a C—C bond (see Figure below). The lines radiating from the center of the circle denote the bonds between the carbon closest to the viewer and its attached atoms, whereas those radiating from the out- side denote the bonds between the carbon furthest from the viewer and its attached atoms. Newman projec- tions and energy diagrams for the various conformations of ethane, propane, and butane are shown below. It is important to appreciate the energy differences between many conformations of the same molecule. The ratio of two conformational isomers of a particular alkane in equilibrium (i.e., with equilibrium constant Keq) at a certain temperature T can be calculated using the formula AG = -RT In Kc (where AG is the Gibbs free energy difference between the two conformational isomers and R is the universal? gas constant). Line-wedge Newman representation pmjection viewer 1 3 \ §,5 4 1 5 <§ :>2\\~\“°_°\ a 6 2 a 3 En 'h d l Ethane ( CH 3—CH3) ergy vs (11 e ra angle (DA) plot Conformations (Newman projections) H" H H H Q H H H H HH H Eclipsed (E) Staggered (SJ Eclipsed (E)—-—occurs at Staggered (SJ—occurs at 0" so“ 120° won 240“ 390° 350° DA of 0°, 120°, and 240° DA of'60", 180°, and 300° Dihedral Angle Propane (CH3—CH2—CH3) Conformations (Newman projections) H H CH3 H CH3 H‘ ‘ H H H H H H Eclipsed (E) Staggered (S) Eclipsed (El—occurs Staggered (S)—occurs at dihedral DA of 0", 120°, at DA of 60°, 180°, and 240° and 300° Butane (CHrCHZ—CHZ—CHQ Conformations (Newman projections) cua °“= Eclipsed (Em—less stable than EM because of CH3/CH3 ii H H interaction; occurs only at DA of 0° Eclipsed (EL) C”; H CH H HQ i Q g H H OH, H H CH3 Eclipsed (EM) Anti (A) Eclipsed (EM)—more stable Anti (A)—occurs only than EL because of CH3IH at DA of 180°. interaction. EM occurs at DA of 120° and 240“. Energy vs dihedral angle (DA) plot 0° 60° 1 20° 180“ 240" 300° 360° Dihedral Angle OH: H CH; Gauche (G)—occurs at DA of 60° and 300° H H H Gauche (G) Energy vs dihedral angle (DA) plot EL EL 3.6 kcallmnl 3.8K l mll'mol G * 0.9 kcalfmol Energy _.. 0° 60° 120° 180° 240” 300° 360° Dihedral Angle GENERAL INFORMATION Alkanes 3 Introduction; Cycloalkanes, otherwise known as alicyclic compounds, are alkanes with a ring structure and have a chemical formula of CnHZn' They are found in biologically important molecules such as steroid hormones, terpenes, and prostaglandins. Physical Properg'es: ' As with alkanes, the melting and boiling points of cycloalkanes generally increase as the number of carbons increases. 0 Like alkanes, cycloalkanes are soluble only in nonpolar solvents. 0 Like alkanes, cycloalkanes are less dense than water. Chemigl Properties: I As in alkanes, the carbon atoms are sp3 hybridized, have tetrahedral geometry, and have a 109.5 ° bond angle. 0 As in alkanes, cycloalkanes are unreactive for the most part. However, cycloalkanes with increased ring strain (mainly due to angle strain) are somewhat reactive. 0 Unlike the rotation in alkanes, rotation about the C—C bond is restricted because of the ring structure. 0 Cyclohexane is the only cycloalkane with zero ring strain {no angle, steric, or torsional strain). Nomenclature: 1. Cycloalkanes are named by adding the prefix cyclo— to the corresponding alkane name (e.g., in the follow- ing structure, the six-carbon alkane ring is cyclohexane). 2. Substituents attached to the cycioalkane are assigned the lowest possible number. If there is more than one group present on the ring, they are .numbered alphabetically (e.g., the cycloalkane below is 1-ethyl-3- methylcyclohexane). a... 3. The term cis is used to describe two substituents located on the same side of the ring, whereas the term trans is reserved for those positioned on opposite sides of the ring (e.g., the complete name of the molecule below is cis-l-ethyl-3-methylcyclohexane). en's-1-efl1yl-3-methylcyclohexane Spectroscopy: H NMR: The hydrogens attached to saturated carbons have a peak at 8 = 0—1.5 ppm. IR: The C—-H bond has a characteristic absorption at 2850—2960 (sin—1. The C—C bond hasa characteristic absorption at 800-13000111—1. CONFORMATIONAL ISOMERS OF CYCLOHEXANE Alkanes 4 1. Cyclohexane exists in four defined conformations—~chair, half-chair, twist-boat, and boat. Of all the con- formations, the chair (ground state, E = 0) is energetically the most stable because it has the least amount of steric strain (i.e., bulky groups are not close enough to interfere with each other) and no torsional strain {i.e., there is no eclipsing of bulky groups). The diagrams below further illustrate the energy differences between the various conformations of cyclohexane. Note: The ratio of two conformations of cyclohexane in equilibrium (i.e., with equilibrium constant Keq) at a certain temperature T can be calculated using the formula AG = —RT aneq (where AG is the Gibbs free energy difference between the two conformational isomers and R is the universal gas constant). _ _ Twist-boat (T B) Boat (B) Chair (C) Half-chair (HC) E=®5.5 kcallmol E=®6.5 kcallmol ground state. E=O E=+1().1 kcallmol 2. The transition from one chair conformation to another proceeds through the four conformations in sequence: chair, half-chair, twist-boat, boat, then back through twist-boat, half-chairhand finally the second chair conformation. Hall-Chllr Half-Chair Energy —-> Twist-Boat Chair Chair Conformation 3. In the chair conformation of cyclohexane, substituents can be either axial or equatorial; each carbon has one axial and one equatorial group. The axial hydrogens bonded to C-1, (3-3, and (3-5 in the diagram below point straight up, while those bonded to C-2, C—4, and C—6 point straight down. Furthermore, the equatorial hydrogens bonded to C-1, C-3, and (3-5 in the diagram below point slightly' down, while those bonded to C—2, C-4, and C-6 point up. (4) © © Axial hydrogens Equatorial hydrogens 4. The chair conformation can undergo a ring flip, causing substituents that were previously axial to become equatorial, and vice versa. If a bulky substituent is present on a cyclohexane ring, the substituent will be in the equatorial position because steric hindrance with neighboring groups (e.g., hydrogens} is minimized. H @ H ,» \ (£355) £53) 2 —CH3 groups axial 2 -CH3 groups equatorial (less stable) (more stable) .4. ...
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