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Weird Nomenclature In Carbonyl Chemistry

Making it through the chemistry of carbonyl derivatives (ketones, aldehydes, carboxylic acids, esters, and more) there are at least two “weird” nomenclature issues that repeatedly come up to baffle students: Greek letters, and “1,2-” or “1,4-” addition reactions.  In this post I’ll try to address them both.

 1. Greek letters

The functional group C=O is called a “carbonyl”. The carbon itself is called the “carbonyl carbon”, and the oxygen is called “the carbonyl oxygen”. But what do you call a carbon adjacent to the carbonyl carbon… or 3 carbons away?

1,2 and 1,4 additions

Another item of confusion are the terms “1,2-addition” and “1,4-addition”. This originates in the discussion of hydrogen halides to alkenes and dienes. Usually, addition to straight alkenes is just called “addition”. But when two or more alkenes are next to each other, at least two different products become possible. The numbers “1,2-” and “1,4-” are used to distinguish the two products from each other.

Using HBr as an example, in the first case, we’re forming C-H on carbon 1 and C-Br on carbon 2. Hence, “1,2-addition”.

In the second reaction, we form C-H on carbon 1 and C-Br on carbon 4. Hence, “1,4-addition”.

The same analogy holds for additions to carbonyls and to alpha-beta unsaturated carbonyls, but it’s a little bit confusing because the “1” in this case refers to oxygen: not an atom we’re used to numbering.

But if you can get over that little bit of weirdness, the analogy to “1,2-addition” and “1,4-addition” with alkenes is dead on. As an example, the 1,2-addition of water to an α,β unsaturated ketone gives a C-OH bond on position 2 and an O-H bond on position 1; the 1,4-addition of water to the same ketone forms C-OH on position 4 and O-H on position 1.

(It doesn’t just stop at 1,4-additions by the way. If you have a triene (3 adjacent alkenes), it’s possible to have “1,6-additions”. Rare, but it can happen.)


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