Page 612 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

594 An older technique is to measure the rate of halogenation of the carbonyl compound.
Ketones and aldehydes in their carbonyl forms do not react rapidly with the halogens
CHAPTER 6 but the enolate is rapidly attacked. The rate of halogenation is therefore a measure of
Carbanions and Other the rate of deprotonation.
Carbon Nucleophiles
O O –
slow
2
R 2 CHCR' + B – R C CR' + BH
O – O
fast
R C CR' + X2 R 2 CCR' + X –
2
X
Table6.8givesdataontheratesofdeuterationofsomealkylketones.Fromthesedata,
the order of reactivity toward deprotonation is CH > RCH > R CH. Steric hindrance
3
2
2
to the approach of the base is the major factor in establishing this order. The impor-
tance of steric effects can be seen by comparing the CH group in 2-butanone with the
2
more hindered CH group in 4,4-dimethyl-2-pentanone. The two added methyl groups
2
on the adjacent carbon decrease the rate of proton removal by a factor of about 100. The
rather slow rate of exchange at the CH group of 4,4-dimethyl-2-pentanone must also
3
reflect a steric factor arising from the bulky nature of the neopentyl group. If bulky groups
Table 6.8. Relative Rates and E for Base-Catalyzed
a
Deuteration of Some Ketones
Ketone Relative Rate a E a b
O
CCH H 100 11.9
CH 3 2
O
CH CCHCH 3 41.5 12.1
3
H
O
H CH CCH CH 3 45
2
2
O
CC(CH ) <0.1
CH 3 3 2
H
O
H CH CCH(CH ) 45 12.3
2
3 2
O
CH CCHC(CH ) 0.45
3 3
3
H
O
H CH CCH C(CH ) 5.1
2
2
3 3
a. In aqueous solution with sodium carbonate as the base. The data of
C. Rappe and W. H. Sachs, J. Org. Chem., 32, 4127 (1967), given on a
per-group basis have been converted to a per-hydrogen basis.
b. CH 3 O -catalyzed exchange in CH 3 OD. T. Niya, M. Yukawa,
−
H. Morishita, H. Ikeda, and Y. Goto, Chem. Pharm. Bull., 39, 2475
(1991).