Isobutyraldehyde, hydration

The protonated intermediate in Eq. (5) is very reactive and could not be observed spectroscopically under the reaction circumstances. Fast hydration to isobutyraldehyde and the secondary amine occurred (75). This mechanism is exactly analogous to that of the hydrolysis of enolate anions (79), as is to be expected. 

Data for aliphatic aldehyde enolisation are very scarce, probably because the enolisation process is often complicated by oxidation and hydration. Nevertheless, the rate constants for base- and acid-catalysed iodination of R R2CHCHO were determined in aqueous chloroacetic acid-chloroacetate ion buffers (Talvik and Hiidmaa, 1968). The results in Table 4 show that alkyl groups R1 and R2 increase the acid-catalysed reactivity in agreement with hyperconjugative and/or inductive effects. This contrasts with aliphatic ketones for which steric interactions are important and even sometimes dominant. Data for base-catalysis are more difficult to interpret since a second a methyl group, from propionaldehyde to isobutyraldehyde, increases the chloroacetate-catalysed rate constant. This might result from a decrease of the a(C—H) bond-promoted hyperconjugative stabilisation of the carbonyl compound... 

Other functional groups may be present in the molecule containing the double bond. Methallyl alcohol, H2C = C(CHjX HjOH, is hydrated by a mixture of 25% sulfuric acid in the presence of isobutyraldehyde to give the cyclic acetal of isobutylene glycol with the aldehyde. Hydrolysis of the acetal by dilute mineral acid gives isobutylene glycol (94%). Hydration of the double bond by aqueous sulfuric acid has been used to make chloro-i-butyl alcohol from methallyl chloride and /S-hydroxybutyric acid from crotonic acid. ... 

Methallyl alcohol, CH2=C(CH3)—CH2OH, is hydrated by a mixture of 25% sulfuric acid and isobutyraldehyde with formation of a cyclic acetal of 2-methyl-1,2-propane-diol hydrolysis of the acetal by dilute mineral acid gives a 94% yield of the glycol.68... 

In Chapter 6 we showed you that water adds reversibly to the carbonyl group of an aldehyde the aldehyde and the hydrate are in equilibrium. Here s the example with isobutyraldehyde (2-methylpropanal). The equilibrium constant is the concentration of hydrate at equilibrium divided by the concentration of aldehyde, also at equilibrium. 

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