Step 2: Enolate ion 1 preferentially adds to the non-enolizable aldehyde, which has the sterically less hindered and, therefore, more accessible carbonyl carbon. When performing both reactions together always consider the aldol product first then convert to the enone. The double bond always forms in conjugation with the carbonyl. Aldol condensation of aldehydes and ketones, mechanism Aldol condensation is occurred only in carbonyl compounds which have a alpha hydrogen. Step 3: Alkoxide ion 2 is protonated by water. 1) Form enolate. These alpha hydrogen is acidic, therefore they are removed easily with bases (alkalis) and give carbanion. Carbonyl compounds are aldehydes and ketones. Note! An aldol condensation is a condensation reaction in organic chemistry in which an enol or an enolate ion reacts with a carbonyl compound to form a β-hydroxyaldehyde or β-hydroxyketone (an aldol reaction), followed by dehydration to give a conjugated enone. Aldol condensations between different carbonyl reactants are called crossed or mixed reactions, and under certain conditions such crossed aldol condensations can be effective. Step-1:In reverse order, The hydroxide ion deprotonates the aldehyde.Step-2:Here Enolate ion 1 adds to the unreacted aldehyde.Step-3:Alkoxide ion 2 is protonated by water.Step-4:A small amount of aldol is converted into enolate ion (4) by hydroxide ion.Step-5:Here Enolate Ion(4) loses a hydroxide ion.Step 1 to step 3 illustrates the aldol reaction. Aldol Condensation Mechanism.
2) Form enone. An aldol reaction between two different carbonyl compounds is called a crossed aldol or mixed aldol reaction. An aldol condensation is a condensation reaction in organic chemistry in which an enol or an enolate ion reacts with a carbonyl compound to form a β-hydroxyaldehyde or β-hydroxyketone, followed by dehydration to give a conjugated enone. crossed aldol condensation: eg: Mechanism: Step 1: The hydroxide ion deprotonates the enolizable aldehyde reversibly. For example, if we mix acetaldehyde and propanal in presence of sodium hydroxide, a crossed aldol reaction is expected to occur with four different products.
A reactant without a hydrogens cannot self-condense because it cannot form an enolate. Aldol condensations are important in organic synthesis, because they provide a good way to form carbon–carbon bonds.
Crossed Aldol Condensations Using Weak Bases: Crossed aldol reactions are possible with weak bases such as hydroxide or an alkoxide when one carbonyl reactant does not have an a hydrogen. Example 3: Mixed Aldol Reaction The success of these mixed aldol reactions is due to two factors.