Question

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

Short answer

Answer

[16]-annulene - non-aromatic

[20]-annulene - non-aromatic

[22]-annulene - aromatic

Step-by-step solution

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.

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.