The Hammett substituent constant σσ A

quick glance at the pKas of some substituted benzoic acids in the table below will show how well they correlate electron donation with pKa. The substituents at the top of the table are electron donating and the anions of the benzoic acids are correspondingly less stable so these are the weakest acids. At the bottom of the table we have the electron-withdrawing groups, which stabilize the anion and make the acid stronger. The whole range is not that great, only one pH unit or so, because the carboxylate anion is not conjugated with the ring. Hammett decided not to use the pKas themselves for his correlation but defined a new parameter, which he called σ. This σ shows how electron donating or withdrawing a group is relative to H as a difference between the pKas of a benzoic acid derivative with the substituent and benzoic acid itself. If the acid required to determine σ for a new substituent was not avail able, σ could be determined by correlation with other reactions. Here are the equations and the table of σ values for the most important substituents. A different value of σ for any given substituent was needed for the meta and the para positions and these are called σm and σp, respectively.

There is no point learning the precise fi gures in this table, but it will help if you form a general idea of what a σ value means. If σ = 0 the substituent has no effect: it is electronically the same as H. If σ is positive, the substituent is electron withdrawing. This is unfortunate perhaps, but just remember that the comparison is with acid strength, and acids with electron-withdrawing substituents are stronger. Positive σ means a stronger acid so the substituent is electron withdrawing. The more positive the charge induced on the ring by a substituent, the larger its σ value. Negative σ means weaker acid and electron donation. Inductive effects from polarization of σ bonds are greater for σm than for σp because the sub stituent is nearer.

Conjugation is generally more effective in the para position so σp > σm for conjugating substituents. Indeed, the NH2 group has a large negative σp and a zero σm. The NH₂ group donates electrons strongly to the carbonyl group of benzoic acid from the para position but does not conjugate in the meta position where its donation happens just to balance the effect of electronegative nitrogen. The OMe group has a negative σp but a positive σm because a weaker electron donation from the lone pairs is more important in the para position but the effect of very electronegative oxygen on the σ framework of the ring in the meta position is more important than lone pair donation that doesn’t reach the carbonyl group. You do not need to learn any σ values but you should be able to work out the sign of σ for well-known substituents and estimate a rough value.

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مواضيع ذات صلة

Non-linear Hammett plots

A complete picture of the transition state from Hammett plots

Using the Hammett ρ values to uncover mechanisms

Reactions with small Hammett ρ values

Reactions with negative Hammett ρ values

Reactions with positive Hammett ρ values

Equilibria with positive Hammett ρ values

The Hammett relationship

Systematic structural variation

Crossover experiments

Labelling experiments reveal the fate of individual atoms

Be sure of the structure of the product