What are Aldehydes and Ketones?
Aldehydes and ketones are carbonyl compounds containing a C=O group. Aldehydes have the carbonyl at a terminal carbon (R-CHO), while ketones have it between two carbons (R₂C=O). They are easily synthesized from alcohols and undergo nucleophilic addition and condensation reactions.
Aldehydes and ketones contain the carbonyl (C=O) functional group. Aldehydes are R-CHO (terminal), ketones are R₂C=O (internal). Both react with nucleophiles, undergo aldol condensation, and form stable derivatives.
- •R-CHO (terminal)
- •From primary alcohols
- •More reactive
- •Forms semicarbazones
- •R₂C=O (internal)
- •From secondary alcohols
- •Less reactive
- •Forms acetals, oximes
Step-by-step worked examples
What aldehyde is produced from the oxidation of ethanol (CH₃CH₂OH)?
Ethanol is a primary alcohol Oxidation converts R-CH₂OH → R-CHO Product: acetaldehyde CH₃CHO The carbonyl group is at the terminal carbon
Acetone (CH₃COCH₃) is formed from which alcohol?
Acetone is a ketone: R₂C=O Ketones come from secondary alcohols The precursor is 2-propanol (CH₃)₂CHOH Carbonyl is between two methyl groups
Benzaldehyde (C₆H₅CHO) undergoes nucleophilic addition with HCN. What is the product?
HCN adds to the C=O: nucleophile attacks C Intermediately forms C-H and C-N bonds Product: C₆H₅CH(OH)CN (cyanohydrin) Cyanohydrins are stable α-hydroxy nitriles
Flashcards
Quick quiz
Q1.Which is an aldehyde?
Q2.From oxidation of 2-propanol, the product is…
Q3.Aldehydes are more reactive than ketones because…
Q4.A typical test for aldehydes is…
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Common mistakes
Ketones have a terminal carbonyl. — Correct: Only aldehydes are terminal (R-CHO); ketones are internal (R₂C=O).
Aldehydes and ketones oxidize at the same rate. — Correct: Aldehydes are more reactive to nucleophilic addition due to less steric hindrance.
Both can form stable hemiacetals. — Correct: Hemiacetals form from both, but acetals (full protection) come from aldehydes more readily.
Ketones easily oxidize to carboxylic acids. — Correct: Ketones resist oxidation; only aldehydes easily oxidize to carboxylic acids.
FAQ
What is the difference between aldehydes and ketones?
Position of carbonyl: aldehydes are R-CHO (terminal), ketones are R₂C=O (internal).
How are aldehydes synthesized?
Primarily by oxidation of primary alcohols with K₂Cr₂O₇, Jones reagent, or PCC.
What is nucleophilic addition in carbonyl reactions?
A nucleophile attacks the electron-deficient carbonyl carbon, forming a new C-Nu bond.
Why are aldehydes more reactive than ketones?
Lower steric hindrance around the carbonyl carbon and the presence of a terminal hydrogen.