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Solutions for Carboxylic Acids
(b) The spectrum of the students' samples shows the carboxylic acid present. Contact with oxygen from 
the air oxidized the sensitive aldehyde group to the acid.
(c) Storing the aldehyde in an inert atmosphere like nitrogen or argon prevents oxidation. Freshly prepared 
unknowns will avoid the problem.
44 Cyanide substitution is an SN2 reaction and requires a 1° or 2° carbon with a leaving group. The 
Grignard reaction is less particular about the type of halide, but is sensitive to, and incompatible with, 
acidic functional groups and other reactive groups.
CH2Br
Br
CH2Br
Br
HO
HO
Br
CH2BrHO
(a) Both methods will work.
Mg
ether
CO2 OR NaCN
∆
CH2COOH
(b) Only Grignard will work. The SN2 reaction does not work on sp2-hybridized carbons, and 
nucleophilic aromatic substitution does not work well on unactivated benzene rings.
Mg
ether
CO2
COOH
(c) Grignard will fail because of the OH group. The cyanide reaction will work, although an excess of 
cyanide will need to be added because the first equivalent will deprotonate the phenol.*
NaCN
∆
CH2COOHHO
(d) Grignard will fail because of the OH group. The cyanide reaction will fail because SN2 does not 
work on unactivated sp2 carbons. In this case, NEITHER method will work.
(e) Both methods will work, although cyanide substitution on secondary C will be accompanied by 
elimination.
Mg
ether
CO2 OR NaCN
∆
COOH
(f) Grignard will fail because of the OH group. The cyanide reaction will work. Since alcohols are 
much less acidic than phenols, there is no problem with cyanide deprotonating the alcohol.*
NaCN
∆
CH2COOHHO
* The pKa of HCN is 9.1, and the pKa of phenol is 10.0. Thus cyanide is strong enough to pull off some of 
the H from the phenol, although the equilibrium would favor cyanide ion and phenol. The pKa of secondary 
alcohols is 16–18, so there is no chance that cyanide would deprotonate an alcohol.
OH CNHO
pKa 10.0
pKa 9.1The side with the
weaker acid is favored.
H3O+
H3O+
H3O+
H3O+ H3O+
H3O+
H3O+
CN
H
O O
OH
stock bottle
(a)
students' samples
45
505