Acetaldehyde, hyperconjugation

The Hydrate and Enol Form. In aqueous solutions, acetaldehyde exists in equihbrium with the acetaldehyde hydrate [4433-56-17, (CH2CH(0H)2). The degree of hydration can be computed from an equation derived by BeU and Clunie (31). Hydration, the mean heat of which is —21.34 kJ/mol (—89.29 kcal/mol), has been attributed to hyperconjugation (32). The enol form, vinyl alcohol [557-75-5] (CH2=CHOH) exists in equihbrium with acetaldehyde to the extent of approximately 1 molecule per 30,000. Acetaldehyde enol has been acetylated with ketene [463-51-4] to form vinyl acetate [108-05-4] (33). 

Inuzuka, K. The calculation of the energy levels of acetaldehyde by a semi-empirical molecular orbital method including the hyperconjugation effect. Bull. Chem. Soc. Japan 36, 1045 (1963). 

Certainly, the polar nature of the hydroxyl groups affects the rate of hydrolysis. Kreevoy and Taft investigated the effect of polar groups on the hydrolysis of acetals. They noted that the rates of hydrolysis of diethyl acetals of chloroacetaldehyde and hydroxyacetaldehyde differ from that of acetaldehyde diethyl acetal by factors of 4.1 X 10 and 3.4 X 10, respectively. They attributed this decrease to the polar effect of the substituent and to the number of alpha hydrogen atoms (which tend to stabilize the transition states by hyperconjugation). It was also shown that the polar effects of the substituents on the carbon chain are additive. 

In ground-state acetaldehyde (2), the principal delocalizations involve vicinal hyperconjugation from the p-type oxygen lone pair Uq" into the adjacent ch and cc antibonds... 

The equiiibrium constants for addition of alcohols to carbonyl compounds to give hemiacetals show the same response to structural features as the hydration reaction. Equilibrium constants for addition of methanol to acetaldehyde in both water and chloroform solution are near 0.8 The structural effects of the alcohol group have been examined. " Steric effects result in an order of CH3 C2H5 > (CH3)2CH > (013)30 for acetaldehyde hemiacetals. EWG substituents in the alcohol disfavor hemiacetal formation and this trend is believed to reflect the decreasing n tt hyperconjugation (anomeric effect, see Topic 1.2) as the substituents become more electron withdrawing. 

Evidently, the C-H bond should hyperconjugate more strongly in acetaldehyde than it does in propene, and the C-H bond length difference as we go from 0° to 90° in torsion angle should increase more. And that s what we find. The calculations show that in this case the C-H bond length increase in acetaldehyde is 0.0047 A, vs. 0.0030 A in propene. This particular type of bond stretching has also been referred to in the literature as the carbonyl effect ... 

It is noteworthy that the Cme-Hop NBO energy change represents an important barrier forming contribution for acetaldehyde, suggesting that the relaxation independent AF (jr) component must largely arise from hyperconjugative interaction. Further discussion is reserved for Section 6. 

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