The Hammett relationship

What we would ideally like to do is find a way to quantify the effects that electron-donating or -withdrawing groups have on the transition state or intermediate during the course of a reaction. This will then give us an idea of what the transition state is really like. The first ques tion is: can we defi ne exactly how efficient a given group is at donating or withdrawing electrons? Hammett took the arbitrary decision to use the pKa of an acid as a guide. For example, the rate of hydrolysis of esters might well correlate with the pKa of the corresponding acid.

When Hammett plotted the rates of ethyl ester hydrolyses (as log k since pKa has a log scale) against the pKas of the correspond ing acids, the initial results were not very encouraging as there was a random scatter of points over the whole graph.

Hammett had used some aliphatic acids (substituted acetic acids) and some aromatic acids (substituted benzoic acids) and he noticed that many of the points towards the top of the graph belonged to the substituted acetic acids. Removing them (brown points) made the graph a lot better. He then noticed that the remaining aromatic compounds were in two classes: the ortho-substituted esters reacted more slowly than their meta- and para-isomers and came towards the bottom of the graph (orange points). Removing them made the graph quite good (remaining green points).

It was not a perfect correlation but Hammett had removed the examples where steric hindrance was important. Aliphatic com pounds can adopt a variety of conformations and the substituent in some of them will interfere with the reaction. Similarly, in ortho-substituted aromatic compounds the nearby substituent might exert steric hindrance on the reaction. Only with meta- and para-substituted compounds was the substituent held out of the way, on a rigid framework, and in electronic communication with the reaction site through the flat but conju gated benzene ring.

Notice that the straight line is not perfect. This graph is an invention of the human mind. It is a correlation between things that are not directly related. If you determine a rate constant by plotting the right function of concentration against time and get an imperfect straight line, that is your fault because you haven’t done your measurements carefully enough. If you make a Hammett plot and the points are not on a straight line (and they won’t be) then that is not your fault. The points really don’t fit on a perfectly straight line. As you will see soon, this doesn’t actually matter.

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مواضيع ذات صلة

Systematic structural variation

Crossover experiments

Labelling experiments reveal the fate of individual atoms

Be sure of the structure of the product

Variation in the structure of the aldehyde

formation of an ester intermediate

formation of an intermediate dimeric adduct

Be sure of the structure of the product

Variation in the structure of the aldehyde

Proposed mechanism C: formation of an ester intermediate

Proposed mechanism B: formation of an intermediate dimeric adduct

Cross metathesis