Aldehydes, Ketones and Carboxylic Acids - Alpha hydrogen reactivity in aldehydes
Review the key concepts, formulae, and examples before starting your quiz.
πConcepts
The -hydrogen atoms in aldehydes and ketones are acidic in nature. This acidity is primarily due to the strong electron-withdrawing inductive effect ( effect) of the carbonyl group () and the resonance stabilization of the resulting conjugate base, known as the enolate ion.
Aldol Condensation: Aldehydes and ketones possessing at least one -hydrogen undergo a reaction in the presence of dilute alkali (such as or ) to form -hydroxy aldehydes (aldols) or -hydroxy ketones (ketols).
Dehydration of Aldols: The -hydroxy carbonyl compounds (aldols) readily lose a molecule of water upon heating to form -unsaturated carbonyl compounds. The conjugation of the double bond with the carbonyl group provides extra stability.
Cross Aldol Condensation: When aldol condensation is carried out between two different aldehydes and/or ketones, it is called cross aldol condensation. If both reactants contain -hydrogens, a mixture of four products is typically obtained.
Mechanism of Aldol Condensation: It involves three main steps: (1) Deprotonation of the -carbon by a base to form an enolate ion, (2) Nucleophilic attack by the enolate ion on the carbonyl carbon of another molecule, and (3) Protonation of the alkoxide ion to form the aldol.
πFormulae
π‘Examples
Problem 1:
Predict the product formed when Propanal () reacts with dilute followed by heating.
Solution:
The product is 2-methylpent-2-enal ().
Explanation:
First, the base abstracts an -hydrogen from propanal to form the enolate ion . This enolate acts as a nucleophile and attacks the carbonyl carbon of another propanal molecule to form 3-hydroxy-2-methylpentanal (the aldol). Upon heating, this aldol undergoes dehydration (loss of ) to form the -unsaturated aldehyde, 2-methylpent-2-enal.
Problem 2:
Why does Formaldehyde () not undergo Aldol condensation?
Solution:
Formaldehyde does not undergo Aldol condensation because it lacks -hydrogen atoms.
Explanation:
Aldol condensation requires the formation of an enolate ion, which is generated by the removal of a hydrogen atom from the carbon atom adjacent to the carbonyl group (the -carbon). Since Formaldehyde () has only one carbon (the carbonyl carbon) and no -carbon, it cannot form an enolate ion.
Problem 3:
Write the chemical equation for the Cross Aldol condensation between Benzaldehyde () and Ethanal ().
Solution:
Explanation:
Benzaldehyde has no -hydrogen, while ethanal has three. Ethanal forms the enolate ion, which then attacks the more reactive carbonyl group of benzaldehyde. The resulting -hydroxy aldehyde dehydrates to form Cinnamaldehyde (3-phenylprop-2-enal).