When substituted benzene molecules undergo electrophilic substitution reactions,
substituents on a benzene ring can influence the reactivity.
substituents that activate the benzene ring toward electrophilic
attack can alter the reaction rate or products by
electronically or sterically affecting the interaction of the two reactants.
deactivating substituents removes electron density from the benzene ring, making
substitution reactions slower and more difficult than benzene itself. For example, a hydroxy or methoxy substituent in
phenol and anisole increases the rate of
electrophilic substitution, while a nitro
substituent decreases the ring's reactivity. Electron donating
substituents activate the benzene ring toward electrophilic
attack, and electron withdrawing substituents deactivate the ring, making it less reactive to electrophilic attack.
The strongest activating substituents are the amino
(-NH2) and hydroxyl (-OH) groups.
Toluene, aniline and phenol
are activated aromatic compounds. Examples of deactivated aromatic compounds
are nitrobenzene, benzaldehyde and halogenated benzenes.
-COOH, -COOR, -COH, -COR
-Cl, -Br, -I
generally direct substitution to the ortho and para positions
where substitutions must
take place. With some
exceptions, deactivating substituents direct to the meta position. Deactivating substituents
which orient ortho
and para- positions are the halogens (-F, -Cl, -Br, -I) and -CH2Cl,
When disubstituted benzene molecules undergo electrophilic substitution reactions,
a new substituent is directed depends on the orientation of
the existing substituents and their individual effects; whether the groups have cooperative or antagonistic directing effects.
Ortho position is the most reactive towards electrophile
due to the highest electron density ortho positions.
But this increased reactivity is countervailed by steric hindrance between substituent and
A nucleophilic substitution is a substitution reaction which the nucleophile
displaces a good leaving
group, such as a halide on an aromatic ring. This
mechanism is called SNAr
( the two-step addition-elimination mechanism), where electron withdrawing substituents activate
the ring towards nucleophilic attack. Addition-elimination reactions usually
occur at sp2 or sp
hybridized carbon atoms, in contrast to SN1 and SN2
Chloro and bromobenzene reacts with the very
strong base sodium amide (NaNH2) to give good yields of aniline.
Other nucleophilic aromatic substitution mechanisms
include benzyne mechanism and free radical
(SRN1) mechanism. Common
reactions of substituent groups on benzene ring include:
- Conversion of halogens
into other various substituents
- Modifying activating substituents
- Oxidative degradation of
- Reduction of
nitro or carbonyl substituents
- Reversibility of the aromatic sulfonation reaction
is used as a solvent and as an intermediate for organic
synthesis especially for dyes.