Which one of the following statements is not correct for benzene ?
- A.Each carbon atom forms sigma bonds with two other carbon atoms with a bond angle of 120°.
- B.Delocalized electrons create symmetrical 'cloud' of electrons above and below the plane.
- C.The length of C-C bonds is intermediate between single and double bonds.
- D.It has six isomers.
Correct answer
Explanation
The correct answer is D, It has six isomers. This is the statement that is not correct; the other three describe the benzene molecule correctly, and UPSC's official key also gives D.
Benzene, C6H6, is a flat ring of six carbon atoms. Each carbon is sp2 hybridised and forms sigma bonds with two neighbouring carbons and one hydrogen, at angles of 120°. The six remaining p electrons are delocalised in rings of charge above and below the plane of the ring. Because of this, all six C-C bonds are equal, about 139 pm long, between a C-C single bond (154 pm) and a C=C double bond (134 pm). Benzene is not described as having six isomers; the idea is simply false.
Option A is wrong because the 120° sigma framework is correct. Option B is wrong because the delocalised electron cloud is correct. Option C is wrong because equal, intermediate bond lengths are correct.
Exam tip: benzene means planar ring, sp2 carbons, 120° angles and six equal C-C bonds of about 139 pm.
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