CHEM C15: Halogenoarenes

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.


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