Reaction Due To Alpha Hydrogen: Definition, Reaction, Sample Questions

Alpha hydrogen is a hydrogen atom attached to alpha carbon in an organic molecule. The aldehydes and ketones presence in alpha hydrogen is essential for aldol condensation. The alpha hydrogen atoms are acidic in nature due to the presence of the electron-withdrawing carbonyl group and resonance stabilization of the conjugate base

Key Terms: Alpha Hydrogen, Alpha Carbon, Functional Groups, Hydrocarbons, Ketones, Aldehydes, Aldol Condensation , Organic Chemistry, Organic Compound

Also Read: Chemistry Formula

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What Is Alpha Hydrogen? 

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Alpha in organic chemistry refers to the first position. The alpha hydrogen is specified below:

• The hydrogen atom attached to an alpha carbon is known as alpha hydrogen. 
• The closest hydrogen atom to the functional group is also called alpha hydrogen. Both definitions explain the concept of alpha hydrogen.
• In hydrocarbons, the first carbon which is situated closest to the functional group or at the first position is termed alpha carbon. 
• The position of carbons in a hydrocarbon can be denoted using the terms alpha, beta, and gamma.
• Here alpha refers to the first position, beta refers to the second position and gamma refers to the third position with respect to the functional group.
• Hence, the atom that is at the first position of any group or closest to the functional group is known as the alpha atom

The Above Diagram Of Hydrocarbons Shows Alpha Hydrogens With Respect To Alpha Carbons. Therefore, Alpha Hydrogen Refers To The Position Or Placement Of Hydrogen In A Hydrocarbon. 

Also read: Organic Compound

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Acidic Property Of Alpha Hydrogen

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Alpha hydrogen is acidic and due to this property, alpha hydrogen in ketones and aldehydes undergo different types of reactions. This is because of the high electron-withdrawing nature of the carbonyl group and resonance stabilization of the conjugate base. The figure below is an example of the acidity of alpha hydrogen.

Acidity Of Alpha Hydrogen

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Reaction Due To Alpha Hydrogen

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Alpha hydrogen can react with a neighboring molecule to form a covalent bond. This process is called hydrogen bonding, and it is the reason for many of the unique properties of water. 

Like, water molecules can absorb large amounts of heat by forming a network and this property makes water a good solvent for many ionic and polar molecules. Alpha hydrogens in ketones and aldehydes are acidic in nature and due to this they undergo many reactions and we will discuss each of them.

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Aldol Condensation Reaction

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In an aldol condensation reaction, an enolate ion (which are derivatives of ketones and aldehydes) reacts with a carbonyl compound in the presence of acid or base in order to form a beta-hydroxy aldehyde or beta-hydroxy ketone, followed by dehydration to produce a conjugate enone. 

Aldol Condensation reaction has some applications

1. Preparation of (E)-6-(2,2,3-trimethyl-cyclopentane-3-enyl)-hex-4-en-3-one.
2. Enzymatic synthesis of fatty acids.
3. Synthesis of high polymers of poly(glutaraldehyde).

Also Read: List of Important Named Reactions in Organic Chemistry

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Cross Aldol Condensation Reaction

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Cross Aldol Condensation reaction is an aldol condensation between two different carbonyl hydrocarbons. If both the carbonyl hydrocarbons have alpha hydrogen atoms, then four different products can be obtained. The reaction below represents a cross aldol condensation reaction.

Cross Aldol Condensation Reaction

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Cannizzaro Reaction

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The cannizzaro reaction is a process in which two molecules of aldehyde or ketone (not containing alpha hydrogen) combine with a base (hydroxide) to produce alcohol and a carboxylic acid. Some examples of Cannizzaro reactions are vanillin, benzaldehyde, and formaldehyde. 

Cannizzaro Reaction

Also Check: Difference Between in Chemistry

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Electrophilic Substitution Reaction

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In the electrophilic substitution reaction, electrophiles (atoms or molecules that seek an atom or molecule containing an electron pair available for bonding) displace a functional group. The displaced functional group is typically a hydrogen atom.

Electrophilic Substitution Reaction

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Things To Remember

1. The first carbon which is situated closest to the functional group or at the first position is termed alpha carbon.
2. The hydrogen atom attached to an alpha carbon is known as alpha hydrogen.
3. Alpha hydrogen is acidic in nature
4. Alpha hydrogen can react with a neighboring molecule to form a covalent bond.