Effect of Substituents: MDCAT Chemistry notes

Why substituents matter

A group already on the benzene ring changes two things:

1. Reactivity: whether the ring reacts faster or slower than benzene.
2. Orientation: where the next electrophile goes (ortho/para or meta).

Electrophiles look for electrons. A group that releases electrons into the ring makes the ring a better nucleophile, so it is activating. A group that withdraws electrons is deactivating.

Classification of groups

Type	Examples	Effect on ring	Directs to
Activating (electron-donating)	$-\mathrm{NH_2}$, $-\mathrm{NHR}$, $-\mathrm{NR_2}$, $-\mathrm{OH}$, $-\mathrm{OCH_3}$, $-\mathrm{CH_3}$ (alkyl)	More reactive than benzene	Ortho and para
Weakly deactivating	Halogens: $-\mathrm{F}$, $-\mathrm{Cl}$, $-\mathrm{Br}$, $-\mathrm{I}$	Less reactive than benzene	Ortho and para
Deactivating (electron-withdrawing)	$-\mathrm{NO_2}$, $-\mathrm{NR_3^+}$, $-\mathrm{CN}$, $-\mathrm{CHO}$, $-\mathrm{COR}$, $-\mathrm{COOH}$, $-\mathrm{SO_3H}$	Less reactive than benzene	Meta

How donation and withdrawal work

• Lone-pair donors (−NH₂, −OH) feed their lone pair into the ring by resonance. This raises the electron density, especially at the ortho and para positions.
• Alkyl groups (−CH₃) push electrons into the ring by the inductive effect (and hyperconjugation). So toluene is more reactive than benzene, and the methyl group makes the ring a better nucleophile.
• Meta directors pull electrons out of the ring. The loss is greatest at the ortho and para positions. The ring as a whole becomes less reactive, and meta is simply the least-depleted position, so the electrophile goes there.
• Groups with a positive charge or a multiple bond to an electronegative atom on the atom attached to the ring are withdrawing, e.g. −NO₂, −CN, −CHO, −NR₃⁺.
• Halogens are a special case. They withdraw electrons inductively, so they deactivate the ring, but their lone pairs still direct to ortho and para.

Reactivity order: worked comparison

Take nitration or sulphonation of four compounds. Reactivity falls as the substituent becomes more electron-withdrawing:

$$\text{toluene} > \text{benzene} > \text{chlorobenzene} > \text{nitrobenzene}$$

So toluene (methylbenzene) is the most readily sulphonated. Methylbenzene is also more reactive than benzaldehyde, benzoic acid and nitrobenzene, which all carry meta-directing, deactivating groups.

Predicting products

• Nitration of toluene gives o- and p-nitrotoluene.
• Nitration of nitrobenzene is slower and gives m-dinitrobenzene.
• Bromination of phenol is very fast, because −OH is strongly activating.

Common MDCAT traps

• $-\mathrm{NR_3^+}$ has no free lone pair and carries a positive charge, so it withdraws electrons. Only −NH₂ donates.
• A meta director does both: it lowers the o/p electron density and makes the ring less reactive.
• −CHO is electron-withdrawing. −NH₂ and −N(CH₃)₂ are donating.
• Halogens deactivate the ring but are still o/p directors.
• −CH₃ activates the ring towards electrophiles. It does not make the ring an electrophile.

Quick revision

• Electron donors activate the ring and direct ortho/para.
• Electron withdrawers deactivate the ring and direct meta.
• −NH₂ is the classic electron-releasing group.
• −NO₂ and −CN are classic withdrawing groups.
• Toluene is sulphonated fastest among the common examples.