What are Alpha Carbons and Alpha Protons? 

The carbon directly attached to the functional group in an organic molecule is referred to as the alpha carbon and the hydrogen attached to an alpha carbon are termed as the alpha hydrogens or alpha protons. These alpha carbon atoms and alpha hydrogen atoms are of importance because they undergo certain characteristic reactions in organic chemistry. 

Alpha Carbons 

Any organic compound with a functional group is attached to a carbon which is called the alpha carbon, the carbon next to alpha carbon is called beta carbon, and the carbon attached to beta carbon is called gamma carbon and so on. 

Examples: 

• In this molecule (5-pentanol), the functional group is at the terminal position and the carbon attached to the functional group is the alpha carbon as indicated. (Even the beta, gamma and delta carbons have been indicated). 

• In a molecule like 3-pentanone where the functional group is a carbonyl group (a ketone) and is not present terminally but within the carbon chain, we can see two alpha carbons as there are two carbons on either side of the functional group which are directly attached to it.  

• In case of presence of multiple functional groups within the same molecule, there can be alpha carbons to that specified functional group. 

Different Compounds with Alpha Carbons and Their Significance 

The branch of chemistry that deals with the study of alpha carbons is known as alpha carbon chemistry and it particularly deals with reactivity and reactions of the alpha carbon. 

Alpha carbons are found in amino acids. The carbon that connects the amino and the carboxylic acid group is the alpha carbon and hence, these amino acids are called alpha amino acids. During protein formation, several such alpha amino acids polymerize and they do so by forming a carbon backbone consisting of primarily alpha carbons only. 

We know that a carbonyl compound (aldehydes/ketones) undergo tautomerization to give enol-enolate ions. Basically in tautomerization of carbonyl compounds, the alpha hydrogen present on the alpha carbon atom migrates to the oxygen of the carbonyl group which leads the migration of the pi-bond to the alpha carbon. This is specifically referred to as the keto-enol tautomerization. Keto form is when the alpha hydrogen is bonded to the alpha carbon and the enol form when the alpha hydrogen is bonded to the carbonyl oxygen atom (along with the migration of the pi-bond). Although both these forms exist in a kind of equilibrium, the keto form is most stable. However, this is of significance to us because a number of reactions are possible when the compound is in the enol form. 

During the conversion of the carbonyl compound from keto to enol / enolate form, the formation of an intermediate wherein the alpha carbon acts as a nucleophile takes place. This nucleophile in the presence of haloalkanes gets alkylated. 

Note:  enol isomers are relatively extremely acidic in comparison with the keto isomers. This point brings us to the topic of alpha-hydrogen atoms. 

Alpha Hydrogen Atoms 

Alpha hydrogen is the one found attached to the alpha carbon in a molecule. These alpha hydrogen atoms in a carbonyl compound have a characteristic property that it is acidic in nature. 

 In aldehydes/ketones, due to the presence of an electron withdrawing group (the carbonyl group), an inductive effect is observed in addition to which resonance structures can be written for the two isomers  (keto-enol forms) which leads to stabilization. Owing to this, the alpha hydrogen atom(s) becomes especially acidic and is prone to removal. 

Although various reactions are studied, the major ones are concerning carbonyl compounds because aldehydes and ketones possess alpha hydrogens which undergo a set of reactions specific to them. One such reaction with utmost significance is the aldol condensation reaction. This reaction is a type of electrophilic substitution reaction at the alpha hydrogen of the enolate ion and these are base catalysed reactions.In an aldol condensation reaction, the beta hydrogen of one carbonyl compound attacks the oxygen of the carbonyl group of the second carbonyl compound in the presence of a base to give a beta-hydroxy ketone. 

Alkyl halides in which the alpha carbon atom is a chiral centre, decides the fate of the reaction the compound would undergo. We know that, as discussed above, the carbon atom attached to the halogen is known as alpha carbon. This alpha carbon in an alkyl halide has a partial positive charge around it and will hence be expected to undergo nucleophilic substitution reaction at that carbon. 

Key takeaways    

• Why do alpha carbons and alpha hydrogens undergo reactions specific to them? 
•  Why is the alpha hydrogen in carbonyl compounds acidic? 
• What are the different compounds we observe alpha carbons and alpha hydrogens in? 
•  Some relevant reactions of significance.  

Practice Problem 

Q1) How many alpha hydrogens are seen in a benzene ring? 

Solution-  

In a benzene ring, we know that there are 3 pi-bonds but however, these 6 pi- electrons are delocalized as a result of which all the 6 carbons of the ring experience conjugation and have a partial double bond character. 

This implies that all 6 carbons are alpha carbons. Therefore, there are 6 alpha hydrogen atoms. 

Context and Applications 

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for   

• Bachelors and Masters Chemistry, Biochemistry and Organic chemistry   
• Masters in Biotechnology