Page 380 - Mechanism and Theory in Organic Chemistry

1,2-Elimination Reactions 367

Table 7.12 ORIENTATION OF THE DOUBLE BOND IN THE PRODUCTS OF REACTION 7.37
2-Hexenel
X 1-Hexene

--
SOURCE: R. A. Bartsch and J. F. Bunnett, J. Amer. Chem. Soc., 90,408 (1968). Reprinted by permission
of the American Chemical Society.

Table 7.13 PRODUCTS OF REACTION 7.38 WITH VARIOUS BASES
Percent Percent
Base Solvent (CHS)~C==CHCH~ (CH3),CHCH=CH2
N(Bu) ,Br Acetone 99.7
N(Bu) ,CI Acetone 99.2
NaOEt Ethanol 82
KOtBu t-Butanol ,23.1

SOURCE: G. Biale, D. Cook, D. J. Lloyd, A. J. Parker, I. D. R. Stevens, J. Takahashi, and S. Win-
stein, J. Amer. Chem. Soc., 93, 4735 (1971). Reprinted by permission of the American Chemical
Society.

hydrogens is of overriding importance to product determination. Since the
acidity of the proton, the reactivity of the leaving group, and the strength of the
base all help determine where the transition state lies in the spectrum, all of these
affect the ratio of Hofmann to Saytzeff product.
The strong electron-withdrawing ability of fluorine, which renders the
protons acidic, the low reactivity of this halogen as a leaving group, and the
strength of the base toward hydrogen assures that Reaction 7.37, when X = F,
lies well toward the E2H end of the spectrum. The data in Table 7.12 show that,
CH3(CH2)3CH-CH3 2 CH3(CH2)2CH=CHCH3 +
- OCH
I CH3(CH2),CH=CH2 (7.37)
X
as expected, the more acidic primary protons are lost preferentially to the less
acidic secondary ones, giving predominantly Hofmann-type product. In the
series of increasing atomic weight, the halogens become simultaneously less
electron-withdrawing and better as leaving groups; therefore as the fluorine is
substituted in turn by C1, Br, and I, Reaction 7.37 moves more toward the E,C
end of the spectrum and Saytzeff products become more important.94
The importance of the base in determining the nature of the transition state
and thereby the product can be seen from Table 7.13. When Reaction 7.38 is
carried out with KO-tBu in t-butanol (E2H conditions), 76.9 percent Hofmann
olefin is obtained. However, when the same reaction is carried out with
+N(Bu)~ Br- (E2C conditions), 97.3 percent Saytzeff product is obtained. The

84 R. A. Bartsch and J. F. Bunnett, J. Amer. Chem. Soc., 90, 408 (1968).