Why no numeral is needed as a prefix in the naming of butanal and butanone?

1 Answer
Dec 9, 2014

No numeral is needed in the naming of butanal and butanone because, in each case, only one number is possible.

The structural formula of butanal is

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We don't need to number the aldehyde group, because it cannot be anywhere else except C-1.

If the C=O were on the left hand end, that would become C-1.

If the C=O group were in the middle, as in CH₃CH₂COCH₃, the compound would not be an aldehyde.

The structural formula for butanone is

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The C=O carbon must receive the lowest possible number (C-2).

We could write the formula as

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But the C=O group is still at C-2.

And there is no butan-1-one. That would be butanal.

So we don't need a number for butanone, either.