Chapter 1.4, Problem 1.15P

Interpretation Introduction

Interpretation:

The aromatic compound from the given options has to be identified.

Concept Introduction:

Criteria for aromaticity:

• The compound must be cyclic and planar.  That means there must be a ring comprised of continuously overlapping $p$ orbitals.  Each of the carbon atom or heteroatom that is a part of the ring must be $s{p}^2$ hybridized.
• The ring must contain an odd number of pairs of $\pi$ electrons.  An odd number of pairs of $\pi$ electrons are to say that the number of $\pi$ electrons must be among the following series of numbers: $2,6,10,14$ etc.  These numbers are called Huckel numbers.  The formula is $4n+2$ where, $n=0,1,\mathrm{2...}$
• If a compound is cyclic, planar and has $4n\pi$ electrons where, $n=0,1,\mathrm{2...}$ that means it doesn’t obey Huckel’s rule, then it will fall under the category of anti aromatic.
• The compound lacking of continuous overlapping of $p$ orbitals falls under the category of nonaromatic.

Calculation of number of $\pi$ electrons:

One double bond is equivalent to two electrons and one negative charge is equivalent to two electrons.  These electrons are called $\pi$ electrons.

Total number of $\pi$ electrons $=$ Number of double bonds $\times 2+$ Number of lone pairs $\times 2$

If an atom has two lone pairs, then only one lone pair is included in counting to check whether it is satisfying Huckel number.

Calculation of hybridization:

Number of hybrid orbitals $=$ Number of sigma bonds $+$ Number of lone pairs

If hybrid orbitals are two, then it belongs to $sp$ hybridization.  If hybrid orbitals are three, then it belongs to $s{p}^2$ hybridization.  If hybrid orbitals are four, then it belongs to $s{p}^3$ hybridization.