Alpha Carbon

Aug 24, 2020 An alpha (symbol: α) carbon is a carbon atom bonded to a functional group in an organic compound; the carbon atom next to the α carbon is the beta (symbol: β) carbon, and so on (α, β, γ, δ). Eg: A compound containing only one functional group may have more than one α carbon.
An alpha carbon can also be a beta carbon, depending upon the reference point. Related terms: β-carbon, primary carbon, 1 o carbon, secondary.
When there are two ester groups on the same carbon chain (a diester), the enolate formed at the alpha carbon atom of one ester group can react with the carbonyl carbon of the other ester group. The resulting Claisen condesation makes a ring of carbon atoms. If this ring has five or six carbons in it, it forms readily.
Beta carbon in an ester. Beta carbon in an alkyl bromide. A beta carbon can also be an alpha carbon, depending upon the reference point.

Practice Problems
The following problems review many aspects of aldehyde and ketone chemistry. The first three questions concerns their nomenclature. The fourth also requires knowledge of nomenclature, together with simple chemical properties.. The fifth question asks you to match descriptions with corresponding structures. The sixth reviews oxidation-reduction terminology. The seventh examines three common reactions as applied to eight carbonyl compounds. You should only be concerned with the aldehyde and ketone reactants. The eighth question asks you to select those reactions, among a large group, that would yield aldehyde or ketone products. The next question focuses on possible reactions of a simple aldehyde. The tenth illustrates three ways questions about chemical reactions may be formulated. The eleventh and twelfth questions require you to draw the structural formulas for the products of many aldehyde and ketone reactions. Next, two questions about the mechanism of acetal and enamine formation are given. Finally, the last question concerns the aldol condensation.

Practice Problems

The following problems review many aspects of aldehyde and ketone chemistry. The first three questions concerns their nomenclature. The fourth also requires knowledge of nomenclature, together with simple chemical properties.. The fifth question asks you to match descriptions with corresponding structures. The sixth reviews oxidation-reduction terminology. The seventh examines three common reactions as applied to eight carbonyl compounds. You should only be concerned with the aldehyde and ketone reactants. The eighth question asks you to select those reactions, among a large group, that would yield aldehyde or ketone products. The next question focuses on possible reactions of a simple aldehyde. The tenth illustrates three ways questions about chemical reactions may be formulated. The eleventh and twelfth questions require you to draw the structural formulas for the products of many aldehyde and ketone reactions. Next, two questions about the mechanism of acetal and enamine formation are given. Finally, the last question concerns the aldol condensation.

What is an alpha, beta-unsaturated ketone?

A useful carbon-carbon bond-forming reaction known as the Aldol Reaction or the Aldol Condensation is yet another example of electrophilic substitution at the alpha carbon in enols or enolate anions. Three examples of the base-catalyzed aldol reaction are shown in the following diagram, and equivalent acid-catalyzed reactions also occur.

Alpha Carbonyl

1 Answer

It looks something like this:

Alpha Carbon Vs Beta Carbon

The #alpha#-carbon is the one adjacent to (on the right of) the carbonyl carbon. The #beta#-carbon is the very bottom-right carbon. Hence, #mathbf(alpha,beta)#-unsaturated ketones have a double bond across the#mathbf('C'_alpha-'C'_beta)#connection.

Alpha Carbon Amino Acid

You would see these in second-semester organic chemistry courses, and they often react on the #beta#-carbon due to the electron-withdrawing behavior of oxygen in these contexts (the #beta#-carbon becomes electropositive).

An interesting example called the Robinson Annulation is shown below, where an #alpha,beta#-unsaturated ketone reacts with another ketone in base with some heat applied.

If you have heard of it before, then you should know that the steps are as follows:

Formation of an enolate using a base such as sodium hydroxide to steal a proton at the #alpha#-carbon.
Nucleophilic attack of the #alpha,beta#-unsaturated ketone at the #mathbf(beta)#-carbon, which is the common target for a nucleophile that is of the strength of a typical enolate.
The enolate takes a proton from the water that just formed from the hydroxide instead of attacking the carbonyl carbon because it cannot make any ring (a four-membered ring is too unstable).
The formation of another enolate occurs---this time, five carbons away from the leftmost carbonyl carbon, so a ring can be made (thus giving the sixth bond in a six-membered ring).
A ring is made via an intramolecular enolate nucleophilic attack.
This step is to show how the bond is connected. It is not actually realistically stable. :P
The water that just formed from the hydroxide that was used in step 4 donates a proton to the oxide intermediate.
Yet another enolate formation occurs. This time, facilitated by heat.
There is heat applied, so that, in addition to how the atoms are aligned, are such that the hydroxide can be eliminated from the product.

Alpha Carbon In Amino Acids

We have formed our product! :D As you can see, this is a ring formation reaction. It makes not one (like the acetylene synthesis), but TWO #'C'-'C'# bonds! NICE!

SIDENOTE: Steps 1-3 are called the Michael reaction. Steps 4-7 are an intramolecular aldol addition. Steps 8-9 form another#alpha,beta#-unsaturated ketone, from a #beta#-hydroxyketone. Steps 4-9 are collectively called an intramolecular aldol condensation.