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Hydrocarbons

Short Answer Type

201.

You are given samples of ethane, ethene and ethyne in three different containers. How will you distinguish them?

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Long Answer Type

202.

What are aromatic hydrocarbons or arenes?

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203.

Explain isomerism in arenes.

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204.

Discuss the structure of benzene with special reference to Kekule structure.

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205. Discuss the molecular orbital structure of benzene (Delocalisation of $straight pi$ -electrons).

The orbital structure of benzene: All the carbon atoms in benzene are sp² hybridised. The three sp² hybrid orbitals are lying in one plane and oriented at an angle of 120°. The fourth unhybridized p-orbital having two lobes is lying perpendicular to the plane of the hybrid orbital. Two out of the three sp² hybrid orbitals of each carbon atom overlap axially with sp² hybrid orbitals of the neighbouring carbon atoms on either side to form carbon-carbon σ bonds. The third hybrid orbital of each carbon atom overlaps axially with the half filled 1s- orbital of the hydrogen atom to form carbon-hydrogen sigma bonds. Thus there is six sigma C-C bonds and six sigma C-H bonds. There is still one unhybridized 2p_z orbital on each carbon atom. This orbital consists of two lobes, one lying above and the other below the plane of the ring.

The unhybridized 2p_z orbital on each carbon atom can overlap sidewise with the 2p_zorbital of the two adjacent carbon atoms in two different ways as shown below giving rise to two sets of

-bonds. Since 2p_z orbital on any carbon atom can overlap sideways with the 2p_z orbital on adjacent carbon atom on either side equally well, a continuous

-molecular 3 orbitals will result which embraces all the six p-electrons as shown:

The net result is that there are two continuous rings-like electron clouds, one above and the other below the plane of atoms as shown. This delocalisation of π-electrons imparts unique stability to the benzene molecule.

In benzene molecule all

and

are of 120° each and each C-C bond length is 139 pm.

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Short Answer Type

206.

What do you mean by delocalisation of $straight pi$ -electrons?

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207.

Benzene ring has three double bonds in it but is still quite stable. Explain.

Or

Why is benzene extra-ordinary stable though it contains three double bonds?

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