hello . can some one give me examples of electron withdrawing groups and electrons releasing groups?

Answer 1

Activating Substituents (electron releasing groups)
ortho & para-Orienting*

–O(–)
–OH
–OR
–OC6H5
–OCOCH3 –NH2
–NR2
–NHCOCH3
–R
–C6H5

Deactivating Substituents (electrong withdrawing groups)
meta-Orienting*

–NO2
–NR3(+)
–PR3(+)
–SR2(+)
–SO3H
–SO2R –CO2H
–CO2R
–CONH2
–CHO
–COR
–CN

Deactivating Substituents (electron withdrawing groups)
ortho & para-Orienting*
–F
–Cl
–Br
–I
–CH2Cl
–CH=CHNO2

*refers to what position in the ring will a functional group add to if a functional group is already present.

Answer 2

The list given above is correct. Now, so you can understand why, the following analogy can be used:

If you look at the atom directly attached to the C atom, if it is a compound which has one or more lone pairs of electrons, it is an "electron donating" group through resonance. This means a resonance structure can be drawn which has the electron pair donating itself to the C atom, enhancing the electron density in this area. Examples are -OH of -NH2.

If there are lone pairs ONE ATOM away from the C atom, resonance structures can be drawn which pull electron density from the C atom, decreasing the electron density. These are "electron withdrawing" through resonance. Examples are -COOH and -CN groups.

The exceptions to these rules are alkyl groups. While there are no lone pairs of electrons present, alkyl groups are considered to be EDG's through induction. This means that they are able to shuttle electron density through the sigma bond.

Another exception are the halides (-Cl, -F) which are considered to be EWG's through induction. These electronegative atoms are able to remove electron density from the C atom.