Q.64. - Organic Chemistry

Question

Question: Explain why A is aromatic but B is not aromatic.

Step-by-Step Solution

Answer

Answer

The lone pair on nitrogen is available for resonance in A while it is not available for resonance in B. therefore, A is aromatic while B is not aromatic.

Step 1: Aromatic compounds

Cyclic compounds with alternating single and double bonds which satisfy the Huckel rule for aromaticity are called aromatic compounds.

Aromatic compounds are highly stable and undergo substitution reactions rather than addition reactions.

The bond length in these compounds will be between the bond length of the C=C double bond and the C-C single bond.

Step 2: Conditions for aromaticity

For a compound to be aromatic, it should have an uninterrupted cyclic cloudof pi electrons above and below the plane of the ring.

For the pi cloud to be uninterrupted, all the atoms in the ring must have a p-orbital.

The ring should be planar and the ring must contain (4n+2) pi electrons.

Step 3: Identifying aromatic and non-aromatic compounds

Compound A-

The number of pi electrons in the ring will be 10 because the lone pair in nitrogen can take part in resonance. Therefore, there is a continuous electron cloud making compound A aromatic.

Compound B-

When the hydrogen on compound A is replaced with the ethoxy carbonyl group, the electron-withdrawing carbonyl group attracts the electron from the nitrogen. This disrupts the continuous electron cloud. The number of pi electrons in the ring will not be (4n+2). Therefore, compound B is not aromatic.