Are Aldehydes Acidic: 3 Facts You Should Know

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Aldehydes basically consider being a neutral molecule as it does not dissociates into water, it is only soluble in water. In this article we will get to know about are aldehydes acidic

Aldehydes have a functional group in its molecule i.e. carbonyl (C=O) group which is attached with a hydrogen atom with a single covalent bond. Due to presence of that single hydrogen atom in –CHO group of aldehyde, they are considering to be quite acidic in nature.

Is aldehyde a weak acid?

Yes, aldehyde shows some acidic nature and behaves as a weak acid.  If we see the structure of aldehydes, the alkyl group ‘R’ is attached to a –CHO group in its molecule. The aldehydic –CHO group is at the end of its structure and consists of a carbonyl group and a hydrogen atom.

The carbonyl (C=O) group in aldehydes have more electronegative carbon atom and less electronegative oxygen atom which creates dipole moment on them. The hydrogen atom in –CHO group of aldehyde molecule is quite acidic which makes aldehyde acidic in nature.

The hydrogen atom attached to carbonyl group of aldehyde molecule is not acidic hydrogen. But the hydrogen atom attached carbon atom neighbouring with the carbonyl carbon of –CHO group of aldehyde is an acidic hydrogen atom.

are aldehydes acidic
Structure of propanal showing alpha hydrogen atom and alpha carbon atom

Also the aldehydes are weak acids as the hydrogen attached to carbonyl group is ionisable. The carbonyl group containing compounds are quite more acidic which is described by the conjugate base resonance stabilization of carbonyl group, means by the stability of anion made by deprotonation. This anion is known as enolate ion.

The negative charge is shared mainly by the carbonyl oxygen and the alpha carbon atom of aldehyde with resonance which creates stability on an highly keyed up carbanion. So, due to the conjugate base resonance stabilization, the alpha hydrogen atom of aldehyde is weakly acidic in nature making the aldehyde molecule a weak acid.

Why is aldehyde weak acid?

Aldehydes are weak acidic in nature due to the presence of alpha hydrogen atom in it. The –CHO functional group is present in aldehydes containing carbonyl group and a hydrogen atom. The alpha hydrogen is the acidic hydrogen atom attached to the carbon atom neighbouring to carbonyl group carbon atom of –CHO group of aldehyde as shown in the above image.

The acidic nature of any compound is depends upon the following points:

  • As we already know the acids are the compounds which give out H+ ions so aldehyde forms.
  • As the H+ ion get eliminated the left over ion is an anion. For that we have to check the stability of anion. Stability can be check by various ways but we have to first check by its resonance structure.  If there is any negative charge is present in the anion then it is more stable in nature. If there is no resonance structure is of that molecule then we can check the stability by carbanion with +I and –I inductive effect.
  • Finally, if the anion is highly stable then the compound is more acidic in nature. Because it becomes more stable after losing H+ ion and thus to maintain stability it can easily loose H+ ion and become more acidic.

If we see the first aldehyde member i.e. formaldehyde, so the anion of formaldehyde is more stable in nature. Hence, aldehyde is quite more acidic than ketones. If we see the gas phase acidity of aldehydes then aldehydes has 1640 kJ/mol. As hydrogen gas has 1700 kJ/mol and ammonia has 1680 kJ/mol values. So aldehyde is more acidic in nature than hydrogen and ammonia gas. But aldehyde is less acidic than water as water has gas phase value of 1600 kJ/mol. Even the hydrogen atoms in aldehydes are not having any electronic effect, as they do not donate or accept electrons from other atoms.

As the alkyl group have weak electron donating nature as they have a tendency to destabilize the anions, they can stabilize the carbocations. This is due to the alkyl group is attracting electrons towards a negative charged atoms which is electrostatically not favorable. So, the ketone anion where are more alkyl group present is less stable as compared to aldehyde anion and hence it aldehyde is more acidic than ketone.

What is the pKa of aldehydes?

Remarkably aldehydes have low pKa values comes between the ranges of 15 to 20. The aldehyde has exact pKa value of seventeen. So they can act as Bronsted acid in any acid and base reaction with a strong base.

The acidic hydrogen is an alpha hydrogen atom which is linked to the carbon atom attached to the carbonyl carbon of aldehyde group. There is a low electronegativity difference between carbon atom and hydrogen atom. So, the carbon hydrogen (C-H) bond of alkanes are barely polarised in nature. Hence, hydrogen atoms of alkane molecules are not acidic in nature. So the pKa values of alkane molecules could be nearly fifty which is very high than aldehydes.

So, aldehydes having low pKa value because of less electron donating capacity of proton rather than alkyl group of ketons are more acidic than ketones too. This all makes the aldehydes a weak acid in nature.