Saturday, January 26, 2008

IIT JEE Revision Benzene

Effect of o-, m- and p- directing groups in mono-substituted benzenes

In planning syntheses based on substitution reactions of mono-substituted benzenes, you must be able to predict in advance which of the available positions of the ring are most likely to be substituted.

Basically, three problems are involved in the substitution reactions of aromatic compounds: (a) proof of the structures of the possible isomers, o, m, p, that are formed; (b) the percentage of each isomer formed, if the product is a mixture; and (c) the reactivity of the compound being substituted relative to some standard substance, usually benzene.

the Pattern of Orientation in Aromatic Substitution

The reaction most studied in connection with the orientation problem is nitration, but the principles established also apply for the msot part ot the related reactions of halogenation, sulfonation, alkylation and acylation.

The group present on the benzene ring affect the incoming attacking groups.

Two effects: orientation, reactivity

Orientation effect: The three possible disubstituted products -ortho, meta and para are not formed in equal amounts.

some substituents groups are ortho directors and some others or meta directors or para directors

Reactivity:some activate the ring and make it more reactive. Some deactivate and make it (benzene) less reactive.

Three groups are classified

1. Ortho and para directing activators: Groups release electrons and activate the benzene ring

-OH, -OCH3, NH2, -NHCOCH3, -CH3

2. Meta directing deactivating groups: They withdraw electrons from the benzene ring and deactivate it

-NO2, -CN, -CHO

3. Ortho and para directing deactivating groups

Halogens -F, Cl, Br, I

No comments: