Chemistry is an essential branch of science. It is that branch of science that deals with chemicals and their properties. When we talk about chemistry, we also deal with terms such as oxygen, hydrogen, carbon dioxide, sulphur, etc. Here, in this article, we shall talk about hydrogen. You must have come across the word hydrogen many times during your science lectures. You also know that hydrogen is a fast-reacting chemical. We will talk about the different reactions that take place due to alpha –hydrogen. Alpha hydrogen is hydrogen that is attached to a carbon.
Alpha hydrogen is a hydrogen attached to the carbon that is adjacent to a carbonyl carbon. This type of hydrogen is noted for its acidic nature. It can easily be abstracted by a base forming a reactive carbanion. Let us know what a carbonyl group is. A carbonyl group is a functional group in organic chemistry where oxygen bonds twice to a carbon atom, C=O. Some examples of carbonyl compounds are carboxylic acids, anhydrides, amides, esters, and acyl halides.
You know that alpha is denoted by the symbol α. So, alpha hydrogen is α hydrogen. The α-hydrogen atom is one that binds to a carbon alongside some functional group, e.g. ketone or an aldehyde group. Without this bonding, no hydrogen can be isolated by the process of Enolization. The hydrogen atoms in the alpha carbon atoms are known as alpha hydrogen. You will find almost seven α- hydrogen atoms that are present in this compound.
The term alpha is not a very clear term. The meaning of the word alpha changes in the atom or group whose alpha or emerging position is under discussion. The atom or group that is located closest, at a first position, is called an alpha group or alpha atom. It is essential to note that H or hydrogen atoms that bond with the alpha atom is called alpha Hydrogen atoms. They stabilise alkenes and alkynes, and carbocations by a process known as hyperconjugation. Here, in this process, the alpha Hydrogen atoms must be in conjugation (combines) with the carbocation. Fig.1 below
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Two carbonyl groups increase the acidity. It is due to the acidic nature of aldehydes and ketones that they see several reactions. This strong acidity is due to the robust electron-withdrawing feature of carbonyl groups and the vibration of the joined structure. Transformation of a carbonyl compound to an enol by Enolization at a useful rate requires a basic catalyst or an acidic catalyst and at least one hydrogen or α carbon.
In the presence of a proton source, the product reverts back into the initial ketone or aldehyde or forms a new product, the enol. The equal reaction between the ketone and aldehyde and the enol form is referred to as keto-enol tautomerism. The ketone or aldehyde gets a strong favourite in the reaction.
Aldol Condensation - β-hydroxyl aldehydes and β-hydroxyl ketones result when aldehydes and ketones with one α-hydrogen undergo reaction with a dilute base as a catalyst. The term Aldol is a combination of aldehydes and alcohol. β- Unsaturated carbonyl compounds are the resultants of Aldol condensation.
Cross Aldol Condensation - in this reaction, two different aldehydes and ketones are in use. What you get is a mix of four products if two of them have α –hydrogen.
Cannizzaro Reaction - aldehydes and ketones undergo self-oxidation and reduction when heated with a basic solution. What you get due to the reduction is one molecule of aldehydes and one alcohol molecule.
Electronic Substitution Reactions - aldehydes and ketones undergo electronic substitution reaction. In this process, the deactivating mode is the carbonyl group.
Tautomers are isomers of a compound. You know that Isomers are each of two or more compounds that have the same formula but have a different arrangement of atoms in their molecule and have different properties. However, tautomers are isomers that differ only in the position of protons and electrons. The carbon framework of the compound remains the same. A reaction that involves a simple proton transfer into an intermolecular form is known as a tautomeric reaction. Keto-enol tautomerism is a standard process. It consists of an acid or base as the catalyst.
1. What is Enolization?
Enolization is a molecular process in which a carbonyl compound is converted into an enol. It is a reversible tautomerization process but can be irreversible too. The transformation of carbonyl compounds requires a basic or acid as a catalyst. And the presence of one hydrogen on α -carbon. Enols are isomers of aldehydes or ketones in which one α hydrogen has replaced oxygen of the carbonyl group. The molecule result has both a C=C (-ene) and an –oh (-ol), hence it is as enol. Only those carbonyl compounds which have alpha hydrogens can form enols. Example of an enol is acetone.
2. What are the Uses of Aldol Condensation?
As you already know that this is part of organic chemistry. Aldol condensation is a condensation reaction - an essential part of organic chemistry in which an enolate or enol reacts with a carbonyl compound and the result is a β – hydroxy aldehyde or β –hydroxy ketone (all part of Aldol reaction). This reaction requires the presence of alkali (base) or acid as the catalyst. The Aldol reaction is reversible. The beta-hydroxy carbonyl products can revert to their original aldehyde or ketone reactant positions. Aldol condensation is in use in the manufacture of pharmaceuticals such as unsaturated ketones, plasticizers, and aromatic ketones.