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what is rotation about double bond and in what kind of organic compound is it possible?

what is rotation about double bond and in what kind of organic compound is it possible?

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1 Answers

Saurabh Kumar
askIITians Faculty 2411 Points
6 years ago
A double bond consists of a sigma bond and a pi bond .....pi bond is formed by sideways overlapping of unhybridized p-orbitals of two carbon atoms above and below the plane of carbon atoms..if now one of the carbon atoms of double bond is rotated with respect to the other ,the p-orbitals will no longer overlap and pi bond should break....but the breaking of pi bond recquires 251 kj/mole of energy which is not provided by collision of molecules at room temperature...consequently the rotation about a carbon-carbon double bond is not free but is strongly hindered or restricted....
while in carbon-carbon single bond only 12.55 kj/mole of energy is recqired (this data is for ethane molecule) .....at room temperature the collission of molecules supply sufficient kinetic energy to overcome this energy barrier....

The carbon-carbon double bond is formed between two sp2 hybridized carbons, and consists of two occupied molecular orbitals, a sigma orbital and a pi orbital. Rotation of the end groups of a double bond relative to each other destroys the p-orbital overlap that creates the pi orbital or bond. Because the pi bond has a bond energy of roughly 60 kcal/mole, this resistance to rotation stabilizes the planar configuration of this functional group. As a result, certain disubstituted alkenes may exist as a pair of configurational stereoisomers, often designated cis and trans. The essential requirement for this stereoisomerism is that each carbon of the double bond must have two different substituent groups (one may be hydrogen). This is illustrated by the following general formulas. In the first example, the left-hand double bond carbon has two identical substituents (A) so stereoisomerism about the double bond is not possible (reversing substituents on the right-hand carbon gives the same configuration). In the next two examples, each double bond carbon atom has two different substituent groups and stereoisomerism exists, regardless of whether the two substituents on one carbon are the same as those on the other.

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