Benzene: Substituent Effects

Question

In regards to benzene reactions, would it be fair to generalize that atoms connect to a carbon on the benzene ring that are more electronegative than carbon (or are C themselves) will be activating substituents and those that are less electronegative than C are deactivating substituents?

This is of course taking into account the fact that substituents with multiple bonds (carboxylic acids, etc) are deactivating substituents regardless.

Answer 1

Rofl...you tell 'em, Sara!

Yes as a general statement, you seem to have it down fairly well.

As an example, the -CH3 in toluene activates the ring. Moreover it activates specific positions...para first then the two orthos (possibly some steric effect here). That's why it's so easy to make trinitrotoluene.

Conversely, the nitro group on nitrobenzene deactivates the ring...difficult to add a second -NO2...and renders the metas more subject to substitution. But when this -NO2 is reduced, to make aniline, the ring is greatly activated and again o and p substitution is is favored.

(For further study: Is this a general rule, that activaters steer subs to o and p and deactivators to m?  If so, why?)

This can all be explained by resonance effects/stability of the intermediates, and is much too time-consuming for this poor typer (especially since this site makes it impossible to draw structures). Moreover it has been a loonng time since I've been confronted by org reaction mechanisms.

You seem to be interpreting your text well.  Pose questions such as this to yourself and seek the answers there...best way to increase your understanding of this complicated subject. Good luck, and keep up the good work.

Answer 2

Why don't you read an O-Chem book on the matter?   Ring activation and addition location direction are related matters that are usually covered.  

Answer 3

Rates of Reaction of  substituted benzenes to halogenation-

-N(CH3)2   5x10^18

-OH   1x10^11

-OCH3 1x10^9

-NHCOCH3 2x10^8

-CH3 3x10^2

-N=NC6H5  4.6

-F 1.2

-H 1

-Cl or -Br or -I 0.1

These groups are ortho-para directing and they are activating (relative reaction rates > the rate for no group, i.e. -H)

The following groups are meta directing and they are deactivating

-H 1

-COC6H5 2x10^-3

-COOC2H5 2x10^-3

-COOH  8x10^4

-CN 2x10^-5

-NO2 2x10^-6