SolutionsWater and Alcohols
1. Stable hydrates
Formaldehyde in water is almost completely hydrated and chloral (trichloroacetaldehyde) forms a stable crystalline hydrate, whereas acetone hardly hydrates. Explain.
Alkyl groups stabilize the C=O by inductive effect and hyperconjugation and are crowded in the hydrate, where the carbon becomes sp3 with smaller angles. Formaldehyde has no alkyl groups. In chloral the strongly electron-withdrawing CCl3 group destabilizes the carbonyl and shifts the equilibrium towards the hydrate.
2. Hemiacetal or acetal
What is obtained when benzaldehyde is treated with methanol: a) without catalyst; b) with excess methanol and a catalytic amount of acid, removing the water formed?
- An equilibrium lying far towards benzaldehyde, with a small proportion of the hemiacetal.
- The dimethyl acetal of benzaldehyde (dimethoxymethylbenzene). Removing the water shifts the equilibrium.