Final answer:
Formaldehyde forms the largest percentage of hydrate among carbonyl compounds due to its high reactivity and the presence of hydrogen atoms attached to the carbonyl carbon, which makes it highly soluble in water.
Step-by-step explanation:
The carbonyl compound that forms the largest percentage of hydrate is formaldehyde. In some cases, carbonyl compounds form hydrates through the reaction of the carbonyl group with water, a process facilitated by hydrogen bonding. While most carbonyl hydrates are unstable and exist in small proportions, there are notable exceptions. Formaldehyde, for example, can form a stable hydrate due to its simple structure and high reactivity. As a result, formaldehyde is often sold as an aqueous solution known as formalin, which is generally 37% to 40% formaldehyde by weight.
Different carbonyl compounds can form hydrates, but the proportion of hydrate formed varies, with formaldehyde and acetaldehyde showing a greater tendency to hydrate due to the influence of the hydrogen atoms attached to the carbonyl carbon. This high tendency to form hydrates in formaldehyde is reflected in its common use in aqueous solutions.