(a) (i)Enantiomers are chiral molecules that are mirror images of one another. These molecules are non-superimposable.So, chirality is the property of a molecule to have non-superimposable mirror image. This means that the molecules cannot be placed on top of one another so that they become superimposed. Chiral molecules with one or more stereocenters can be enantiomers. They do not have a plane of symmetry.
Examples of enantiomers.
(ii) We know,enantiomers are non-superimposable, chiral molecules that are mirror images to one another. A racemic mixture is nothing but a mixture of equal volume(50:50) of two enantiomers.
Each enantiomer can rotate the plane-polarized light in an equal but opposite direction (as they are the mirror images) and are optically inactive. The mixture is said to have been resolved if the enantiomers are separated.
Racemic mixture (50:50 mixture of enantiomers)
(b) Chlorobenzene is extremely less reactive towards a nucleophilic substitution reaction. This is due to:-
(i) Resonance effect- Due to resonance, the electron pair on chlorine atom is in conjugation with the electrons of the benzene ring which results in the following resonating structures
Resonance structures of chlorobenzene
Due to delocalization of electrons, a partial double bond character develops in the C-Cl bond, which makes it difficult for the nucleophile to cleave the C-Cl bond.
(ii) Due to repulsion from the increased electron density on the benzene ring, the nucleophile finds it difficult to attack on the molecule.
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