Question

Why is aldehyde hydrogen not acidic?

Answer 1

An aldehyde is a chemical molecule that contains a functional group of the formula $-CHO$ , which consists of a carbonyl centre linked to hydrogen as well as any generic alkyl or side chain R group. An aldehyde or formal group is the functional group itself.
Aldehydes, which are formed by removing a hydrogen from an alcohol, are prevalent in organic chemistry; formaldehyde being the most well-known. Many perfumes are or include aldehydes because they are usually intensely fragrant.

Complete answer:
Aldehydes have a planar, $s{{p}^{2}}$-hybridized carbon core linked to oxygen by a double bond and hydrogen by a single bond. The $CH$ bond is normally not acidic. An -hydrogen in an aldehyde is considerably more acidic than a normal alkane, with a $p{{K}_{a}}$ approaching 17. This is due to resonance stabilisation of the conjugate base ($p{{K}_{a}}$ about 50). This acidification is due to I the formyl center's electron-withdrawing properties and (ii) the conjugate base, an enolate anion, delocalizing its negative charge. The aldehyde group is fairly polar, similar to I. Deprotonation of the formyl proton is difficult to do.
The acyl anion, which is formed by deprotonation of an aldehyde proton, is a highly unstable anionic species that must be maintained at low temperatures. Direct deprotonation is not a viable method for synthesis of acyl anions, with the exception of some hindered dialkyl formamide, since the deprotonated species will almost instantly add to the extremely reactive carbonyl of the starting material to form an acyloin compound.
The formyl $CH$ bond is weaker than a normal bond between hydrogen and a $s{{p}^{2}}$-hybridized carbon, with about 360 kJ/mol. As a result, aldehydes are prone to hydrogen-atom abstraction in the presence of free radicals, which explains why aldehydes are so easy to auto oxidize.

Note:
Many aldehydes, such as cinnamaldehyde, cilantro, and vanillin, are present in trace amounts in essential oils and contribute to their pleasant aromas. Aldehydes are uncommon in numerous natural building blocks, including amino acids, nucleic acids, and lipids, possibly due to the strong reactivity of the formyl group. The majority of sugars, on the other hand, are aldehyde derivatives. These aldoses exist in the form of hemiacetals, which are disguised versions of the parent aldehyde. In aqueous solution, for example, only a small portion of glucose persists as aldehyde.