Preparation of Aldehydes and Ketones
From alcohols, alkenes, alkynes, acid chlorides and nitriles. (Chemistry › Aldehydes, Ketones and Carboxylic Acids, NEET UG syllabus.)
What is Preparation of Aldehydes and Ketones?
An organic compound containing a formyl group (–CHO), consisting of a carbonyl center (a carbon double-bonded to oxygen) bonded to hydrogen and an R-group.
Key formula / rule: Oxidation of Primary Alcohol to Aldehyde
Key points
- Identify appropriate reagents and conditions for converting primary and secondary alcohols to aldehydes and ketones.
- Explain the mechanism and products of ozonolysis of alkenes.
- Describe the hydration of alkynes and predict the products formed.
- Outline the Rosenmund reduction for synthesizing aldehydes from acid chlorides.
Common exam trap
Using strong oxidizing agents (e.g., KMnO4, CrO3/H2SO4) for primary alcohols, leading to carboxylic acids instead of aldehydes.
Definitions
- Term
Aldehyde
- Meaning
An organic compound containing a formyl group (–CHO), consisting of a carbonyl center (a carbon double-bonded to oxygen) bonded to hydrogen and an R-group.
- Term
Ketone
- Meaning
An organic compound containing a carbonyl group (C=O) bonded to two alkyl or aryl groups.
- Term
Oxidation (Organic Chemistry)
- Meaning
In organic reactions, typically involves an increase in the number of bonds to oxygen or a decrease in the number of bonds to hydrogen.
- Term
Reduction (Organic Chemistry)
- Meaning
In organic reactions, typically involves a decrease in the number of bonds to oxygen or an increase in the number of bonds to hydrogen.
- Term
Tautomerization
- Meaning
A chemical phenomenon where two isomers (tautomers) exist in dynamic equilibrium, interconverting by the migration of a proton and a change in the location of a double bond (e.g., keto-enol tautomerism).
Learning objectives
Identify appropriate reagents and conditions for converting primary and secondary alcohols to aldehydes and ketones.
Explain the mechanism and products of ozonolysis of alkenes.
Describe the hydration of alkynes and predict the products formed.
Outline the Rosenmund reduction for synthesizing aldehydes from acid chlorides.
Detail the Stephen reaction and DIBAL-H reduction for converting nitriles to aldehydes.
Explain how ketones can be prepared from nitriles and acid chlorides using Grignard reagents or dialkylcadmium.
Formulae
- Name
Oxidation of Primary Alcohol to Aldehyde
- Note
PCC is a mild oxidizing agent.
- Expression
R-CH2OH + [O] --(PCC)-→ R-CHO + H2O
- Name
Oxidation of Secondary Alcohol to Ketone
- Note
Stronger oxidizing agents can be used.
- Expression
R2CHOH + [O] --(CrO3/H2SO4)-→ R2C=O + H2O
- Name
Ozonolysis of Alkene
- Note
Cleaves the double bond.
- Expression
R-CH=CH-R' + O3 --(1. O3; 2. Zn/H2O)-→ R-CHO + R'-CHO
- Name
Hydration of Terminal Alkyne
- Note
Follows Markovnikov's rule, forms enol which tautomerizes.
- Expression
R-C≡CH + H2O --(HgSO4/H2SO4)-→ R-CO-CH3
- Name
Rosenmund Reduction
- Note
Poisoned catalyst prevents over-reduction.
- Expression
R-COCl + H2 --(Pd-BaSO4)-→ R-CHO + HCl
- Name
Stephen Reduction
- Note
Nitrile reduced to imine, then hydrolyzed to aldehyde.
- Expression
R-CN + SnCl2 + HCl --(1. SnCl2/HCl; 2. H3O+)-→ R-CHO
- Name
Nitrile to Ketone (via Grignard)
- Note
Forms an imine intermediate.
- Expression
R-CN + R'-MgX --(1. R'MgX; 2. H3O+)-→ R-CO-R'
Prerequisites
Basic knowledge of functional groups (alcohols, alkenes, alkynes, nitriles, acid chlorides, aldehydes, ketones).
Understanding of oxidation and reduction reactions in organic chemistry.
Knowledge of reaction mechanisms (e.g., electrophilic addition, nucleophilic addition).
Familiarity with common reagents like Grignard reagents, PCC, DIBAL-H.
Common mistakes
Using strong oxidizing agents (e.g., KMnO4, CrO3/H2SO4) for primary alcohols, leading to carboxylic acids instead of aldehydes.
Incorrect workup in ozonolysis (oxidative workup yields carboxylic acids/ketones).
Forgetting the tautomerization step in alkyne hydration.
Not using a poisoned catalyst in Rosenmund reduction, leading to over-reduction to alcohol.
Confusing Stephen reaction with other nitrile reductions.
Keywords
Aldehyde
Ketone
Oxidation
Reduction
PCC
Ozonolysis
Hydration
Rosenmund reduction
Stephen reaction
DIBAL-H
Grignard reagent
Acid chloride
Nitrile
Alcohol
Alkene
Alkyne
Practice preview
Which of the following reagents can be used to convert cyclohexanol into cyclohexanone?…
medium
Which of the following statements regarding the preparation of aldehydes and ketones is INCORRECT?…
hard
Propyne is treated with aqueous H2SO4 containing HgSO4 to form compound 'X'. Compound 'X' is then reduced with NaBH4 to form compound 'Y'. Finally, 'Y' is oxidized with PCC to form compound 'Z'. What is compound 'Z'?…
hard