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Question: Which compound is optically active? (A) \( Butan - 1 - amine\) (B) \( Butan - 2 - amine\) (C...

Which compound is optically active?
(A) Butan−1−amine Butan - 1 - amine
(B) Butan−2−amine Butan - 2 - amine
(C) 2−methylpropan−1−amine 2 - methylpropan - 1 - amine
(D) 2−methylpropan−2−amine 2 - methylpropan - 2 - amine

Explanation

Solution

The chemical compounds consisting of at least one chiral carbon is optically active. Chiral carbon is the carbon attached to the four different groups. When all the carbon atoms in the compound are attached to two or more same groups, then that compound can be known as optically inactive.

Complete answer:
The presence of chiral carbon gives optical activity to the molecule.
Option A consists of Butan−1−amine Butan - 1 - amine , in which the carbon chain consists of four carbon atoms. The amine group is at 1st{1^{st}} position. The structure of Butan−1−amine Butan - 1 - amine will be CH3CH2CH2CH2NH2C{H_3}C{H_2}C{H_2}C{H_2}N{H_2} . None of the carbon is chiral. So, it is not optically active.
Option B consists of Butan−2−amine Butan - 2 - amine . The structure of Butan−2−amine Butan - 2 - amine will be CH3CH2CH(NH2)CH3C{H_3}C{H_2}CH\left( {N{H_2}} \right)C{H_3} . The carbon to which an amine group is attached is chiral carbon. So, it is optically active.
Option C consists of 2−methylpropan−1−amine 2 - methylpropan - 1 - amine . The structure of 2−methylpropan−1−amine 2 - methylpropan - 1 - amine will be CH3CH(CH3)CH2NH2C{H_3}CH\left( {C{H_3}} \right)C{H_2}N{H_2} . None of the carbon is chiral carbon. Thus, it is optically inactive.
Option D consists of 2−methylpropan−2−amine 2 - methylpropan - 2 - amine . The structure of 2−methylpropan−2−amine 2 - methylpropan - 2 - amine will be CH3C(CH3)(NH2)CH3C{H_3}C\left( {C{H_3}} \right)\left( {N{H_2}} \right)C{H_3} . None of the carbon is chiral carbon. Thus, it is optically inactive.
Thus, in the given compounds, CH3CH2CH2CH2NH2C{H_3}C{H_2}C{H_2}C{H_2}N{H_2} is an optically active compound.
Option A is the correct one.

Note:
When any chemical compound is optically active, it can be classified into two forms namely d-form and l-form. These two forms rotate the plane-polarized light in different directions. d-form rotates the plane polarised light towards the right side which can be known as Dextro and the l-form rotates the plane polarised towards the left side which can be known as leavo.