[Goala]

Hi,

Please look at the file attached. I saw it while I was googling about EAS (Electrophilic Aromatic Substitution) on the internet.

I understand why meta substitution is preferable as the carbomethoxy group is electron withdrawing and nitration at the meta-position prevents through resonance that there be a carbocation adjacent to the carbomethoxy carbon.

However... I don't understand why the ortho isomer is prefered over the para isomer. If someone could answer this it would be great.

Thanks,
Goala

[fledarmus]

Possibly because there are two ortho positions and only one para position? Or to put it in mass balance terms, the concentration of ortho positions in the reaction mixture is twice as high as the concentration of para positions?

[Goala]

Yes, I thought of that.

But it seems that there is 0.0025 concentration at ortho and only 0.001 concentration at para. This isn't equivalent to a "times two" concentration.

I got this image from a fairly reputable site: http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/benzrx1.htm

The picture is about 3/5th down. Control find "ethyl benzoate" and you should see the above mentioned picture.

[opsomath]

It isn't always. For instance, iodination of aniline (aminobenzene) with iodine happens almost exclusively at the para position.

[Goala]

Ok.

There isn't a reasoning based on carbacationic intermediates?

[fledarmus]

Not really - the electronics between the para and ortho positions are very similar. What really drives the difference is typically either a steric effect or a directing effect. Very large substituents on the benzene ring make it difficult for other groups to approach the ortho position and tend to favor para substitution, while other substitutents can interact with either the approaching electrophile or through non-resonance interactions with an adjacent charge  to favor an ortho substitution.

[orgopete]

The aromatic substitution reactions are a little more complicated than indicated. For example, if the nucleophilicity was the only determining factor in the ratio of products, then all electrophilic aromatic substitution reactions should give the same ratio, that is bromination, nitration, sulfonation, etc. and it should not matter what the reagents or conditions are. However, we know that the conditions and the reactants can and do result in different ratios. Therefore we should conclude that while the addition is a slow part of the reaction, the rate or ratio is not determined only by that attack. We might also imagine that the addition is reversible to some degree. Then the loss of a proton from the arenium ion can also play a role in the final product. For example, if the loss of a proton from the ortho position is greater than from the para position, then the final ratio of ortho to para can be biased toward the ortho product.

[opsomath]

The most important factor is the ortho-para or meta-directing tendency, but steric hindrance can easily bias it to be completely ortho or para. If there are no strong influences, often you get a mixture of ortho and para product.