A Guidebook to Mechanism in Organic Chemistry

The Neopentyl Rearrangement

attack by EtOH can be at Ct or C 3 and a mixture of the two possible
ethers is indeed obtained:

Me • CH (OEt) • CH=CH 2

EtOH. + H®

Me • CH=CH • CH 2 • OEt

If, however, the reaction is carried out in ethanol with ethoxide ions
present as p'owerful nucleophilic reagents, the reaction proceeds as a
straightforward SN 2 displacement reaction, ®OEt displacing CI®,and
only the one product, Me-CH(OEt)CH==CH 2 , is obtained. Allylic
rearrangements have been observed, however, in the course of dis­
placement reactions that are undoubtedly proceeding by a bimolecular
process. Such reactions are designated as S^' and are believed to
proceed:

R R

Y^CH^CH^CH-^Cl -> Y—CH 2 —CH=CH + C1®

S;v2' reactions tend to occur more particularly^vhen there are«lftilky

substituents on the a-carbon atom for these markedly reduce the rate

of the competing, direct displacement reaction by the normal S,v2

mode. Allylic rearrangements, by whichever mechanism they may

actually be proceeding, are relatively common.

MeCH-*-CH^=CH 2

Me • CH= CH—CH 2

(ii) With change in carbon skeleton
(a) The neopentyl rearrangement: A good example is the hydro­
lysis of neopentyl chloride (VI) under conditions favouring the SNl
mechanism (it may be remembered that the S^2 hydrolysis of these
halides is highly hindered in any case, p. 65); this might be expected to
yield neopentyl alcohol (VIII):

Me Me Me

Me—C—CH 2 Cfc-^> Me—C—CH 2 Me—C—CH 2 OH + Ha

II I

Me Me Me

(VI) (VII) (VIII)