Aldehydes and Ketones
What’s The Alpha Carbon In Carbonyl Compounds?
Last updated: February 4th, 2021 |
Alpha Carbon? Beta Carbon? Alpha, Beta Unsaturated Carbonyl?
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.
Table Of Contents
- The “Alpha Carbon” Is The Carbon Adjacent To The Carbonyl.
- “1,2” and “1,4” Additions To “Alpha, Beta” Unsaturated Carbonyls
- Quiz Yourself!
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?
- In organic chemistry, it’s common to use Greek letters to denote this. So the carbon adjacent to a carbonyl is called an “α (alpha) carbon”, two carbons away is called a “β carbon”, and so on.
- This nomenclature can be used to depict different kinds of substituted carbonyl groups. For example a ketone with an OH on the beta carbon would be called a “β-hydroxy ketone”. If it was one carbon further down it would be a “γ hydroxy ketone”.
- If we have a double bond between the α carbon and the β carbon it’s common to call it “α,β-unsaturated”. So we can have α,β unsaturated ketones, aldehydes, esters, and so on.
- It can keep going beyond gamma, of course, but it’s rare to see it progress beyond ε (epsilon).
- Another thing: aldehydes, esters, carboxylic acids, and so on, can only have one “alpha” carbon each, wheras ketones can have two. Sometimes you’ll see one set of Greek symbols marked with ‘ (prime) symbols to distinguish them. The location of the prime is completely arbitrary.
- For esters, the OR group is not denoted “alpha”. It’s usually just called the “alkoxy” group.
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.)