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Concepts of Aldehydes, Ketones, and Carboxylic Acids - Chemical Reactions of Aldehydes and Ketones - Reactions Due to α-hydrogen

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Estimated time: 6 minutes
CBSE: Class 12

Key Points: Chemical Reactions of Aldehydes and Ketones - Reactions Due to α-hydrogen

Aldol condensation:

\[\ce{CH3 - \overset{\displaystyle O}{\overset{||}{C}} - H + H - CH2 - \overset{\displaystyle O}{\overset{||}{C}} - H} \xrightarrow{\text{dil. NaOH}} \ce{H3C - \underset{\underset{\displaystyle OH}{|}}{CH} - \underset{\underset{\displaystyle H}{|}}{CH} - \overset{\displaystyle O}{\overset{||}{C}} - H} \\ \xrightarrow{\text{dil. }\ce{H2SO4}\Delta, \ce{-H2O}} \ce{CH3 - CH = CH - \overset{\displaystyle O}{\overset{||}{C}} - H}\]

Mechanism:

Cross aldol condensation:

\[\begin{array}{ccccccccccccc} & \ce{O} & & \ce{O} & & & & \ce{OH} & & \ce{O} & & & \\ & || & & || & & & & | & & || & & & \\ \ce{C6H5 -} & \ce{C} & \ce{+ HCH2 -} & \ce{C} & \ce{- H} & \overset{\text{dil. NaOH}}{\rightleftharpoons} & \ce{C6H5 -} & \ce{C} & \ce{- CH2 -} & \ce{C} & \ce{- H} & \xrightarrow[\Delta]{\ce{H2O}} & \ce{C6H5CH=CHCHO} \\ & | & & & & & & | & & & & & \text{Cinnamaldehyde} \\ & \ce{H} & & & & & & \ce{H} & & & & & \end{array}\]

Cannizzaro reaction: It is a self-oxidation reduction reaction.

\[\begin{array}{r@{\;}c@{\;}l} \ce{H} & & \\ & \backslash & \\ & & \ce{C=O} \\ & / & \\ \ce{H} & & \end{array} + \begin{array}{r@{\;}c@{\;}l} \ce{H} & & \\ & \backslash & \\ & & \ce{C=O} \\ & / & \\ \ce{H} & & \end{array} + \text{conc. KOH} \xrightarrow{\Delta} \ce{H - \underset{\underset{\displaystyle H}{|}}{\overset{\overset{\displaystyle H}{|}}{C}} - OH} + \ce{H - \overset{\displaystyle O}{\overset{||}{C}} - OK}\]

Haloform Reaction:

The reaction can be used to transform acetyl groups into carboxyl groups or to produce chloroform or iodoform. This reaction has been used in qualitative analysis to indicate the presence of a methyl ketone in which excess iodine is used to halogenate the compound. The product iodoform precipitates as a yellow-coloured substance and has a characteristic odour.

\[\underset{\substack{\text{Acetone} \\ \text{Iodoform Test}}}{\ce{H3C - \overset{\displaystyle O}{\overset{||}{C}} - CH3}} \xrightarrow[4\ce{NaOH}]{3\ce{Cl2}} \underset{\text{Sodium acetate}}{\ce{H3C - \overset{\displaystyle O}{\overset{||}{C}} - O^\ominus Na^\oplus}} + \underset{\text{Chloroform}}{\ce{CHCl3}}\]

Shaalaa.com | Aldehydes Ketones part 18 (Alpha hydrogen acidity, aldol condensation)

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Related QuestionsVIEW ALL [44]

Which of the following compounds would undergo aldol condensation, which the Cannizzaro reaction and which neither? Write the structures of the expected products of aldol condensation and Cannizzaro reaction.

  1. Methanal
  2. 2-Methylpentanal
  3. Benzaldehyde
  4. Benzophenone
  5. Cyclohexanone
  6. 1-Phenylpropanone
  7. Phenylacetaldehyde
  8. Butan-1-ol
  9. 2, 2-Dimethylbutanal

Compound ‘A’ was prepared by oxidation of compound ‘B’ with alkaline \[\ce{KMnO4}\]. Compound ‘A’ on reduction with lithium aluminium hydride gets converted back to compound ‘B’. When compound ‘A’ is heated with compound B in the presence of \[\ce{H2SO4}\] it produces fruity smell of compound C to which family the compounds ‘A’, ‘B’ and ‘C’ belong to?

A compound 'A' of molecular formula C2H3OCl undergoes a series of reactions as shown below. Write the structures of A, B, C and D in the following reactions :

Which of the following compounds will undergo self-condensation in the presence of dilute NaOH solution?

Why is there a large difference in the boiling points of butanal and butan-1-ol?

Write the products formed when CH3CHO reacts with the following reagents: CH3CHO in the presence of dilute NaOH

Explain Aldol condensation of ethanal.

Assertion (A): The final product in Aldol condensation is always α, β-unsaturated carbonyl compound.

Reason (R): α, β-unsaturated carbonyl compounds are stabilised due to conjugation.

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