Just a short explanation on:
- reactivity of chloroarenes vs. chloroalkanes
- reactivity of chloroarenes vs. arenes
Contrasting the reactivity between cholorobenzene & chloroalkanes
Consider the structure of chlorobenzene:
The σ bond between C & Cl comes from the overlap between 1 C’s sp2 orbital & 1 of the Cl’s p-orbitals. Usually, it would be considered as a regular SINGLE BOND.
HOWEVER, the presence of a delocalised electron ring causes chlorobenzene to be different.
There is actually a slight overlap between the delocalised ring & 1 of the Cl’s OTHER p orbitals (the orbital in the same orientation as the ring – above & below the carbon ring).
Due to this overlap, the C-Cl bond has a SLIGHT DOUBLE BOND character.
Thus, the bond is STRONGER & SHORTER than a C-Cl bond in an aliphatic compound, & the group is LESS REACTIVE.
Thus, it is less reactive to NUCLEOPHILIC SUBSTITUTION unlike aliphatic chloroalkanes.
REMINDER: this only applies if the Cl is DIRECTLY bonded to a C in the benzene ring. If it’s bonded to a C on a side chain, there is no difference.
Contrasting the reactivity between cholorobenzene & benzene
Generally, halobenzenes react SLOWER than benzene.
There are 2 effects to consider:
- Inductive effect: Cl draws electrons in due to the difference in electronegativity between C & Cl
- Resonance effect: Cl donates electrons out due to the overlap between the p-orbital & the delocalised ring
With experimental data, it turns out that the Inductive Effect is STRONGER than the resonance effect, so there is a LOWER electron density in the π ring.
Thus, the benzene ring is LESS REACTIVE.
HOWEVER, the Cl directs any incoming electrophile to the 2/4 positions on the benzene ring, just like an electron-donating group would.