Q16-31E - Organic Chemistry (Mcmurry)

Question

The N,N,N-trimethylammonium group $-{\mathrm{N}}^{+}{\left({\mathrm{CH}}_3\right)}_3$ , is one of the few groups that is a meta-directing deactivator yet has no electron-withdrawing resonance effect. Explain

Step-by-Step Solution

Answer

N,N,N trimethyl ammonium ion is a meta-directing deactivator because its p orbital does not overlap with pi orbital and ${\left({\mathrm{CH}}_3\right)}_3{\mathrm{N}}^{+}$ group is inductively deactivating due to positive charge.

Step 1: N,N,N-trimethylammonium group

It is a group without an electron-withdrawing effect due to the absence of p electron. As a result, no delocalization occurs with the pi electrons because it is deactivating in nature due to the presence of a positive charge.

Step 2: N,N,N-trimethylammonium group ortho attack

When the electrophile attack occurs at the ortho position to the N,N,N- trimethylammonium group, destabilization of the resonance benzene occurs due to the presence of the two electropositive charge next to each other. No ortho attack occurs shown below.

Meta-Attack

Step 3: N,N,N-trimethylammonium group para attack

When the electrophile attack occurs at the para position to the N,N,N trimethylammonium group, destabilization of the resonance benzene occurs due to the presence of two electropositive charge next to each other. No ortho attack occurs as shown below.

Para-attack

Step 4: N,N,N-trimethylammonium group para attack

When the electrophile attack occurs at the meta position to the N,N,N -trimethylammonium group, destabilization of the resonance benzene occurs due to the presence of two electropositive charge at a para position to each other. Meta attack occurs as shown below.

Ortho-attack