Strength of Organic Acids

An acid is a substance which donates a proton. By the sheer fact that a proton is a charged particle, you would expect that a polar bond is more susceptible to be broken to release a proton than a non-polar bond. There are two main factors that will affect how readily a substance donates a proton.

1. Strength of the bond broken to release the proton.
2. Stability of the anion that is formed when the proton is released.

In organic chemistry, the organic acids break the O-H bond. Hence, with a common bond broken, to discuss about the differing acid strength in terms of bond strength will be meaningless. Hence, we will be discussing about the strength of acid through the stability of the anion formed from the dissociation. Since, the more stable the anion, the less incline for it to accept the proton, the more willing for the substance to remain in its dissociated form.

Since organic acids are weak acids, we would expect partial dissociation of the acid. Hence, the magnitude of the acid dissociation constant will provide good information about the strength of the acid. Therefore, a acid that produces a stable anion (conjugate base) would imply that the acid is stronger than another acid which does not form a stable anion.

There are two factors that affects the stability of the anion.

1. Delocalisation of charge by resonance
2. Induction effect.

In the delocalisation of charge by resonance, we requires the atom that has the negative charge to be attached to another atom which is part of a double bond. The ethanoate ion is a suitable example to illustrate this pattern.

In this case, the p orbital of C and O can overlap with the p orbital of the O that carries the negative charge. Hence, the electron can flow and therefore causing the anion to be stabilised.

While, in the induction effect, the substitutents can cause an overall electron donating or electron withdrawing effect. A substituent which is electron withdrawing, will pulls the electron density away from the atom that carries the negative charge. This effect spreads out the negative charge and hence creating a delocalisation.

When we compare the effect of the phenyl substitutents. There are two effects:

If the phenyl ring is attached directly to an atom that carries a negative charge, it will be able to delocalise the charge via resonance. If the phenyl ring is attached indirectly to the atom with the negative charge, it will act as an electron withdrawing substituent.

Although the phenyl ring is capable of delocalising the charge via resonance, into its ring of C atoms. It is important to realise that the extent of this delocalisation is smaller than the delocalising of the charges which we see in the case of the carboxylate ion (i.e. ethanoate's case).  This is becaue the latter delocalises the charge to O, a more electronegative atom than C.

In addition, electron donating substituents on benzene ring will diminish the electron withdrawing effect of the ring. Converse is true for electron withdrawing substituents.

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^{Article written by Kwok YL 2010.}

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