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.