Aldehydes and Ketones

By James Ashenhurst

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

  1. The “Alpha Carbon” Is The Carbon Adjacent To The Carbonyl.
  2.  “1,2” and “1,4” Additions To “Alpha, Beta” Unsaturated Carbonyls
  3. Quiz Yourself!

1. The “Alpha Carbon” Is The Carbon Adjacent To The Carbonyl

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.

what is the alpha carbon in organic chemistry - it is the carbon adjacent to a carbonyl

2. “1,2” and “1,4” Additions To “Alpha, Beta” Unsaturated Carbonyls

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.

types of addition reactions to alkenes 1 2 addition and 1 4 addition nomenclature

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.

types of addition reactions to carbonyls - 1 2 addition and 1 4 addition

(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.)

Quiz Yourself!

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Comment section

9 thoughts on “What’s The Alpha Carbon In Carbonyl Compounds?

  1. Doesn’t change the weirdness of numbering an oxygen, but at least its clearly explained. Good work James, thanks.

    Some other odd terms (which I think I’ve seen you explain elsewhere) include ‘geminal’ and ‘vicinal’ which refer to the same and adjacent carbon respectively. Geminal means twin and refers to two atoms attached to the same carbon. Vicinal means neighbor and refers to two atoms attached to adjacent carbons. These terms are usually used in reference to alkene addition/elimination.

    And to think, this is easy compared to what the alchemists had to deal with. I should be able to list Organic Chemistry as a foreign language on my resume

    1. heteroatoms are numbered all the time in organic chemistry and are often given higher priorities than their adjacent carbon atoms. think of the carbon-oxygen double bond as similar to a carbon-carbon bond but with higher priority

  2. First of all, I should appreciate your “magnanimity” in sharing your knowledge. I have a question about Alpha, Beta, and gamma hydrogen. I have seen a definition for these nomenclature in a paper available in below link:
    As you can see, the author define Alpha, Beta, and gamma hydrogen as:
    H alpha: Aliphatic hydrogen on C alpha to aromatic rings.
    H beta: Aliphatic hydrogen on C beta and the CH2, CH beyond the C beta to aromatic rings.
    H gamma: Aliphatic hydrogen on C gamma and the CH3 beyond the C gamma to aromatic rings.
    What do the CH2, CH beyond … and CH3 beyond …mean?
    Please consider an aliphatic chain with 10 or more carbons attached to an aromatic ring. According to these definitions, gamma hydrogen atoms are those bond with penultimate carbon as well as those belong to the CH3 at the end of chain.
    Accordingly, the gamma carbon is not the third carbon next to aromatic ring, but the penultimate one. Moreover, based on the definition presented above, we have only two and five alpha and gamma hydrogen atoms,respectively whereas there may be several beta hydrogen atoms.
    The main question is that whether my inference about those definitions are true?
    Thank you very much.

    1. I am not familiar with that journal and I cannot gain access. Looks like they are doing an NMR study and want to group the hydrogens according to where they show up in the spectra. I would not take what you read in this paper as any official statement of proper nomenclature.

  3. Thanks for this website.
    I was unknow of the term addition but now organic is my best friend. Thanks for making organic my friend. Credit goes to u.

  4. James, are you sure the second last example with the 2-Oxo-hex-3-ene is an 1,4 addition? If this was the case, the product IMHO should be 2-Hydroxyhexane-4-thiol. I’d vote for 1,2 therefore.

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