Alpha-carbon is the carbon atom located adjacent to a functional group in an organic chemical compound. On the other hand, beta-carbon is the carbon atom next to the carbon attached to the functional group. Similarly, gamma carbon is the carbon atom of an organic compound, which is bonded, with the beta carbon atom.
Alpha vs. Beta Carbon
| Alpha carbon | Beta carbon |
| A carbon atom bonded to the functional group | A carbon atom bonded to the alpha carbon |
| Linked hydrogen atoms are called alpha hydrogen | Linked hydrogen atoms are called Beta hydrogen |
| Highly acidic alpha proton | Beta proton is less acidic than an alpha proton |
| Alpha proton participates in several reactions | Beta proton is rarely involved in reactions |
| Alpha proton attracts electrophiles | Slight attraction to electrophiles |
Related Content
- Alpha(α) and Beta(β) Glucose: Comparison, Structures, Explanation
- Mutarotation: The α,β Sugars Interconversion
- Anomeric Carbon: The alpha-beta Anomerization
Examples of α & β Carbons
Aldehydes, esters, carboxylic acids, and other functional groups, can only have one “alpha” carbon each, however, ketones can have two.
If a compound contains more than one functional group, the terms α-carbon, β-carbon, etc. are meaningful only if the functional group is specified.
If there is a double bond between the alpha and beta carbons, it’s common to call it “α,β-unsaturated”. Therefore, we can have α,β unsaturated ketones, aldehydes, esters, etc.
Chemistry of Alpha Carbon
When a hydrogen (proton) atom is attached to alpha carbon, it is an alpha proton (H+). Not all alpha carbons will have alpha protons. For example, benzaldehyde (C6H5CHO).
In carbonyl compounds (ketones, aldehydes, and carboxylic acid derivatives), alpha carbon can serve as a nucleophile, and the alpha protons are fairly acidic, with a pKa around 19. Hence, removing the alpha proton from alpha carbon will result in a carbanion, but the carbon atoms do not typically like to have a negative charge, because of low electronegativity, therefore carbanion is a very reactive species.
The carbonyl group makes the alpha hydrogen acidic via a resonance stabilization mechanism. This results in several different reactions including:
- Acetoacetic ester synthesis
- Aldol Condensation Reaction
- Claisen Condensation reaction
- Synthesis of Enols and Enolates
- Enamine Reactions
- Malonic Ester Synthesis
- Alpha Halogenation reaction
Chemistry of Beta Carbon
When a hydrogen (proton) atom is attached to beta carbon, it is defined as a beta proton (H+). They are typically less acidic than alpha hydrogen, making them less reactive, and thus are rarely involved in chemical reactions.
To explain this difference, consider the protons in a carbonyl compound. The solid electron-withdrawing effect of the carbonyl group makes the release of alpha hydrogen easier; the abstraction of hydrogen causes a negative charge on the alpha carbon. This negative charge is stabilized by resonance. The stability of the conjugate base (anion form) reflects the stronger acidity of the alpha protons.
Key Differences
Concepts Berg
What is the primary difference between alpha and beta carbon?
Alpha carbon refers to the first carbon atom attached to a functional group, whereas, beta carbon refers to the second carbon atom attached to the functional group.
What is the difference between alpha and beta-amino acids?
Alpha-amino acids have carboxylic acid groups and amine groups on the adjacent carbon atoms, whereas in beta-amino acids the amine group is attached to the secondary (beta) carbon atom from the carboxylic acid group.
How do alpha and beta glucose differ?
The difference between alpha glucose and beta glucose is just in the orientation of the hydroxyl group. In alpha glucose, the –OH group attached to the first carbon atom is located on the same side as the –CH2OH group whereas the –OH group attached to the first carbon atom of in beta glucose is located on the opposite side from the –CH2OH group.
