8Chapter 04

Not much difference in reactivity of different

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Unformatted text preview: alent to one another CH3CH3 + Cl2 420°C CH3CH2Cl + HCl (78%) + Cll2 C hν Cl + HCl HCl (73%) Dr. Wolf's CHM 201 & 202 4- 130 Chlorination of Alkanes Chlorination of Alkanes Major limitation: Chlorination gives every possible Chlorination monochloride derived from original carbon monochloride skeleton. Not much difference in reactivity of different hydrogens in molecule. Dr. Wolf's CHM 201 & 202 4- 131 Example Example Chlorination of butane gives a mixture of 1-chlorobutane and 2-chlorobutane. (28%) CH3CH2CH2CH3 Cll2 C hν CH3CHCH2CH3 (72%) Dr. Wolf's CHM 201 & 202 CH3CH2CH2CH2Cl Cll C 4- 132 Percentage of product that results from Percentage substitution of indicated hydrogen if every collision with chlorine atoms is productive productive 10% 10% 10% 10% 10% 10% 10% 10% 10% 10% Dr. Wolf's CHM 201 & 202 4- 133 Percentage of product that actually results Percentage from replacement of indicated hydrogen from 18% 18% 4.6% 4.6% 4.6% 4.6% 4.6% 4.6% 18% 18% Dr. Wolf's CHM 201 & 202 4- 134 Relative rates of hydrogen atom abstraction Relative rates of hydrogen atom abstraction divide by 4.6 18% 4.6% 4.6 =1 4.6 18 4.6 = 3.9 A secondary hydrogen is abstracted 3.9 times faster than a primary hydrogen by a chlorine primary atom. atom. Dr. Wolf's CHM 201 & 202 4- 135 Similarly, chlorination of 2-methylproane Similarly, gives a mixture of isobutyl chloride and tert-butyl chloride tert-butyl CH3 CH (63%) CH3CCH2Cl Cl CH3 CH H Cl2 CH3CCH3 H hν CH3 CH CH3CCH3 (37%) Dr. Wolf's CHM 201 & 202 Cl 4- 136 Percentage of product that results from Percentage replacement of indicated hydrogen replacement 7.0% 37% Dr. Wolf's CHM 201 & 202 4- 137 Relative rates of hydrogen atom abstraction Relative rates of hydrogen atom abstraction Relative divide by 7 7.0 7 =1 37 7 = 5.3 A tertiary hydrogen is abstracted 5.3 times faster than a primary hydrogen by a chlorine atom. primary Dr. Wolf's CHM 201 & 202 4- 138 Selectivity of free-radical halogenation Selectivity of free-radical halogenation Selectivity R3CH > R2CH2 > RCH3 chlorination: chlorination: 5 4 1 bromination: 1640 82 1 Chlorination of an alkane gives a mixture of Chlorination every possible isomer having the same skeleton every as the starting alkane. Useful for synthesis only as when all hydrogens in a molecule are equivalent. Bromination is highly regioselective for substitution of tertiary hydrogens. Major synthetic Dr. Wolf's CHM 201 & 202 4- 139 Synthetic application of chlorination of an Synthetic application of chlorination of an Synthetic alkane alkane alkane alkane Cll C Cl2 hν (64%) (64%) Chlorination is useful for synthesis only when Chlorination all of the hydrogens in a molecule are equivalent. Dr. Wolf's CHM 201 & 202 4- 140 Synthetic application of bromination of an alkane Synthetic application of bromination of an alkane H CH3CCH2CH2CH3 CH3 Br2 hν Br CH3CCH2CH2CH3 CH3 (76%) (76%) Bromination is highly selective for substitution Bromination of tertiary hydrogens. of Major synthetic application is in synthesis of tertiary alkyl bromides. Dr. Wolf's CHM 201 & 202 4- 141 End of Chapter 4 Dr. Wolf's CHM 201 & 202 4- 142...
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This document was uploaded on 01/03/2012.

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