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Rotation around carbon-carbon single bond of ethane is not completely free. Justify the statement.

Answers (1)

(i)Alkanes have carbon-carbon sigma bond where the distribution of electron of sigma molecular orbit is symmetrical around the internuclear axis of C-C bond, viz., not distributed due to rotation about its axis.

(ii) Hence, C-C single bond is permitted for free rotation which results in different spatial arrangements of atoms in space which can change into one another.

Such spatial arrangements of atoms are called conformations or rotamers or conformers.

(iii) Due to rotation around C-C bonds alkenes have infinite no. of conformations. However, rotation around C-C single bond is not completely free and is hindered by small energy barrier of1-20 kJ mol-1 because of weak repulsive interaction between adjacent bonds. It is called a torsional strain.

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