Q.10. - Organic Chemistry

Question

Question: Would [16]-, [20]- or [22]-annulene be aromatic if each ring is planar?

Step-by-Step Solution

Answer

Answer

[16]-annulene - non-aromatic

[20]-annulene - non-aromatic

[22]-annulene - aromatic

Step 1: Criteria for aromaticity

According to Huckel’s rule, for an organic molecule to be aromatic, the molecule should be:

1. Cyclic
2. Planar
3. Conjugated
4. Follow 4n+2 = pi electrons (n - 1,2,3…)

 

If the molecule follows 4n = pi electrons, the molecule is anti-aromatic.

 

Molecules which do not obey any or all of the criteria are non-aromatic.

Step 2: Aromatic or non-aromatic annulene Considering that the rings are planar

1. [16]-annulene

          [16]-annulene 

Cyclic

Conjugated

8 pi bonds = 16 pi electrons

 $$\begin{gathered} 4n + 2= 16 \\ 4n = 14 \\ n = 3·5 \end{gathered}$$

n value is fractional

Hence, [16]-annulene is not aromatic.

2. [20]-annulene

       [20]-annulene 

Cyclic

Conjugated

10 pi bonds = 20 pi electrons

$$\begin{gathered} 4n+2 = 20 \\ 4n = 18 \\ n = 4·5 \end{gathered}$$

 n value is fractional

Hence, [20]-annulene is not aromatic.

 

 3. [22]-annulene

           [22]-annulene 

Cyclic

Conjugated

11 pi bonds = 22 pi electrons

 $$\begin{gathered} 4n+2 = 22 \\ 4n = 20 \\ n = 5 \end{gathered}$$

 n value is not fractional

Hence, [22]-annulene is aromatic.