Dehydration of secondary and tertiary alcohols involves the formation of a carbocation
intermediate, so be sure to check the structure of the carbocation for the possibility of
rearrangement. Remember that a carbocation will rearrange if rearrangement produces a
more stable carbocation (Section 4.6). For example, the intitially formed secondary
carbocation in the following reaction rearranges to a more stable tertiary carbocation:The following is an example of a ring-expansion rearrangement. Both the
initially formed carbocation and the carbocation to which it rearranges are secondary
carbocations, but the initially formed carbocation is less stable because of the strain in
its four-membered ring (Section 2.11). Rearrangement relieves this strain. The
rearranged secondary carbocation can rearrange by a 1,2-hydride shift to an even more
stable tertiary carbocation. ( is any base present in the solution.)PROBLEM 7What product would be formed if the preceding alcohol were heated with an equivalent
amount of HBr rather than with a catalytic amount ofPROBLEM 8List the following alcohols in order of decreasing rate of dehydration in the presence
of acid:OHOHCH 2 OH CH^3 CH 3H 2 SO 4?B≠RCOH >>RRCHOH RCH 2 OHRRrelative ease of dehydrationa tertiary alcohol a secondary alcohol a primary alcoholincreasing ease of dehydrationSection 12.5 Elimination Reactions of Alcohols: Dehydration 447Tutorial:
Carbocation rearrangementssecondary carbocationH 3 PO 4
CH 3 C CHCH 3CH 3CH 3CH 3 C CHCH 3CH 3CH 3 + H 2 OCH 3 CCH CH 2CH 3CH 3CH 3 C CCH 3CH 3CH 3CH 2 CCHCH 3CH 3CH 3OH∆
+tertiary carbocationCH 3 C CHCH 3CH 3CH 3++ + +3,3-dimethyl-2-butanol1,2-methyl
shift3,3-dimethyl-1-butene
3%2,3-dimethyl-2-butene
64%2,3-dimethyl-1-butene
33%H+ H+H 2 SO 4 ring-expansion
∆ rearrangement rearrangementsecondary
carbocationtertiary
carbocationCHCH 3
CH 3 BCHH 3 CH 3HB+H 2 OCHOH +CH 3+++
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