Examples of electron withdrawing groups relative to hydrogen are

Based on the ability of a substituent to withdraw or donate electron density, the substituents can be classified as electron-withdrawing or electron donating groups relative to hydrogen. Halogens and many other group such as nitro $\left(-{\mathrm{NO}}_2\right),\text{ cyano }(-\mathrm{CN})\text{, carboxy }(-\mathrm{COOH})$,

ester $(-\mathrm{COOR})\text{, aryloxy }\left(-\mathrm{OAr},\text{ e.g. }-{\mathrm{OC}}_6{\mathrm{H}}_5\right)\text{, etc., }$ are electron-withdrawing groups. On the other hand, the alkyl groups like methyl $\left(-{\mathrm{CH}}_3\right)\text{ and ethyl }\left(-{\mathrm{CH}}_2-{\mathrm{CH}}_3\right)$ are usually considered as electron donating groups.