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260 Chapter 6 Ionic Reactions: Nucleophilic Substitution and ­Elimination Reactions of Alkyl Halides
H
H
H
O�
H
O
CH2CH2CH3
H3C
H3CH2C
CH2CH2CH3
CH2CH2CH3
H
∆
O
H
H
H
Additional solvent molecules (water)
deprotonate the alkyloxonium ion.
�
O
H
H3C
H3CH2C
O
H
�
CH2CH2CH3
H
�
H
50%
H3C
H3CH2C
H
H3C
H3CH2C
O
50%
O
The product is a racemic mixture.
The SN1 reaction of (S)-3-bromo-3-methylhexane proceeds with racemization because the intermediate carbocation is
­achiral and attack by the nucleophile can occur from either side.
•••
practice Problem 6.8
Keeping in mind that carbocations have a trigonal planar structure, (a) write a structure
for the carbocation intermediate and (b) write structures for the alcohol (or alcohols)
that you would expect from reaction of iodocyclohexane in water:
CH3
I
(CH3)3C
H2O
SN1
6.12B Solvolysis
●
A solvolysis reaction is a nucleophilic substitution in which the nucleophile is a
molecule of the solvent (solvent + lysis: cleavage by the solvent). The SN1 reaction of
an alkyl halide with water is an example of solvolysis.
When the solvent is water, we could also call the reaction a hydrolysis. If the reaction had
taken place in methanol, we would call it a methanolysis.
Examples of Solvolysis
(CH3)3C i Br +
H2O
9: (CH3)3C i OH
+ HBr
(CH3)3C i Cl + CH3OH 9: (CH3)3C i OCH3 + HCl
•••
S o lv e d Pr o b l e m 6 . 5
What product(s) would you expect from the following solvolysis?
Br
CH3OH
Strategy and Answer: We observe that this cyclohexyl bromide is tertiary, and therefore in methanol it should
lose a bromide ion to form a tertiary carbocation. Because the carbocation is trigonal planar at the positive carbon, it can
react with a solvent ­molecule (methanol) to form two products.