Chemistry Class 11 NCERT Solutions: Chapter 13 Hydrocarbons Part 7

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Q: 10. Why is benzene extra ordinarily stable though it contains three double bonds.

Answer:

Benzene is a hybrid of resonating structures given as:

All six-carbon atoms in benzene are hybridized. The two hybrid orbitals of each carbon atom overlap with the hybrid orbitals of adjacent carbon atoms to form six sigma bonds in the hexagonal plane. The remaining hybrid orbital on each carbon atom overlaps with the s-orbital of hydrogen to form six sigma bonds. The remaining unhybridized p-orbital of carbon atoms has the possibility of forming three bonds by the lateral overlap of

The six ’s are delocalized and can move freely about the six carbon nuclei. Even after the presence of three double bonds, these delocalized electrons stabilize benzene.

Q: 11. What are the necessary conditions for any system to be aromatic?

Answer

A compound is said to be aromatic if it satisfies the following three conditions:

(I) It should have a planar structure.

(II) The electrons of the compound are completely delocalized in the ring.

(III) The total number of -electrons present in the ring should be equal to where … etc. This is known as Huckel’s rule.

Q: 12. Explain why the following systems are not aromatic?

(I)

(II)

(III)

Answer

(I)

For the given compound, the number of π-electrons is 6.

By Huckel’s rule,

For a compound to be aromatic, the value of n must be an integer . Since the value of is an integer, the given compound is aromatic in nature.

(II)

For the given compound, the number of π-electrons is 4.

By Huckel’s rule,

For a compound to be aromatic, the value of n must be an integer which is not true for the given compound. Hence, it is not aromatic in nature.

(III)

For the given compound, the number of π-electrons is 8.

By Huckel’s rule,

For a compound to be aromatic, the value of n must be an integer . Since the value of is not an integer, the given compound is not aromatic in nature.