2.3 The Versatility of the Condensed Formula

If there is a double bond, we can represent it with a double line, as in leaf alcohol, CH₃CH₂CH=CHCH₂CH₂OH. (More about leaf alcohol a little later.) A triple line means a triple bond, as in acetylene: HC≡CH. We are free to write HC or CH, H₃C or CH₃, and so on, as we wish; it is understood that the heavy atom, not any of the hydrogens, is what bonds to the next heavy atom in the formula.

If there is a branch in the carbon chain, we can use parentheses, e.g. CH₃CH(CH₃)CH₂CO₂H, isovaleric acid, responsible for sweaty feet smells. The first and second CH₃, called methyl groups, are each single-bonded to the CH. We could also write it as (CH₃)₂CHCH₂CO₂H. But we have to be careful; (CH₃)_n means n methyl groups bonded to one shared atom, but (CH₂)_n means a straight chain of n saturated carbon atoms.

The CO₂H is shorthand for C(=O)OH, meaning one oxygen is double-bonded to the carbon while the other is single-bonded and has an oxygen atom. In older literature you will sometimes see this written COOH. Parentheses are also useful for compounds like trimethylamine, a fishy odor compound, whose condensed formula can be written N(CH₃)₃, representing a nitrogen atom with three methyl groups single-bonded to it.

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