How does one tell if a compound is optically active or inactive by looking at it? For examples: 1-4-dichloro-2-methylpentane and 1,2-dichloro-2-methylpentane

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Answer 1 · 2016-03-08T20:58:03

You look for a chiral carbon — a carbon atom that has four different groups attached.

1,4-Dichloro-2-methylpentane

The structure of 1,4-dichloro-2-methylpentane is

1,4-dichloro

You see that $"C2"$ and $"C4"$ are chiral carbons.

The 4 groups attached to $"C2"$ are $"H, CH"_3", CH"_2"Cl"$ , and $"CH"_2"CHClCH"_3$ .

The 4 groups attached to $"C4"$ are $"H, Cl, CH"_3$ , and $"CH"_2"CH"("CH"_3)"CH"_2"Cl"$ .

The compound has two chiral carbons, so it will be optically active.

1,2-Dichloro-2-methylpentane

The structure of 1,2-dichloro-2-methylpentane is

1,2-dichloro

Here, $"C2"$ is a chiral carbon.

The 4 groups attached to $"C2"$ are $"CH"_3", Cl, CH"_2"Cl"$ , and $"CH"_2"CH"_2"CH"_3$ .

The compound has a chiral carbon, so it will be optically active.