Exercise 12: - Nucleophilic substitution reactions of alkyl halides
1.
a. Select the member of each pair of compounds that will react faster by an S N2
mechanism:
i.
CH3CH2CH2CH2F or CH3CH2CH2CH2OCH3
CH3CH2CH2CH2F
ii.
CH3CH2CH2CH2Cl or CH3CH2CH2CH2F
CH3CH2CH2CH2Cl
b. Which SN2 reaction of each pair would you expect to take place more readily in a
protic solvent?
i.
CH3CH2CH2Cl + (C6H5)3N  CH3CH2CH2N(C6H5)3+ + Clor
CH3CH2CH2Cl + (C6H5)3P  CH3CH2CH2P(C6H5)3+ + ClCH3CH2CH2Cl + (C6H5)3P  CH3CH2CH2P(C6H5)3+ + Cl-
ii.
CH3CH2CH2Cl + CH3CH2S-  CH3CH2CH2SCH2CH3 + Clor
CH3CH2CH2Cl + CH3CH2SH  CH3CH2CH2SCH2CH3 + HCl
CH3CH2CH2Cl + CH3CH2S-  CH3CH2CH2SCH2CH3 + Cl-
c. Which SN1 reaction of each pair would you expect to take place more rapidly?
i.
(CH3)3CBr + H2O  (CH3)3COH + HBr
or
CH3CHBrCH3 + H2O  CH3CHOHCH3 + HBr
(CH3)3CBr + H2O  (CH3)3COH + HBr
ii.
(CH3)3CBr + H2O  (CH3)3COH + HBr
or
(CH3)3CCl + H2O  (CH3)3COH + HCl
(CH3)3CBr + H2O  (CH3)3COH + HBr
d. 1-Chlorobutane reacts with 0.01 M NaCN in ethanol to give predominantly
CH3CH2CH2CH2CN whereas under the same conditions (CH3)3CCl reacts to
give (CH3)3COCH2CH3. Propose mechanisms to explain these reactions.
2. When 2-propanol is heated in the presence of H2SO4, a possible product is
diisopropyl ether, (CH3)2CH-O-CH(CH3)2.
a. To what type of reaction does this belong?
Nucleophilic substitution
b. Propose a possible mechanism(s).
3. The SN2 reaction can occur intramolecularly. What product would you expect
from treatment of 4-bromo-1-butanol with sodium methoxide?
4. (S)-2-butanol slowly racemizes in dilute acid solution. Propose a mechanism.
5. SN2 reactions take place with inversion of configuration whereas racemization
occurs in SN1 reactions. Explain why retention of configuration occurs in the
following reaction:
6. Choline, a constituent of phospholipids, has the formula C5H15O2N. It dissolves
in water to form a strongly basic solution. It can be prepared by the reaction of
oxacyclopropane with trimethylamine, (CH3)3N, in the presence of water. What is
its structure?