Aldehydes & Ketones
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Carbonyl group functional group
C = O
Aldehyde functional group
-CHO
How are aldehydes formed?
Oxidation under distillation of primary alcohols using acidified potassium dichromate
How are carboxylic acids formed?
Oxidation under reflux of primary alcohols using acidified potassium dichromate
Ketone functional group
-C=O
How are ketones produced?
Oxidation of secondary alcohols with acidified potassium dichromate
Can oxidation of alcohols be reversed?
Yes
What is the reducing agent for alcohols?
NaBH4
What is the mechanism for reducing alcohols?
Nucleophilic addition
Conditions for nucleophilic addition
NaBH4 dissolved in water with methanol (acidic conditions) for an extra H+ ions
Process of nucleophilic addition reactions
• CN⁻ attacks the partially positive carbon atom and donates a pair of electrons. Both electrons from the oxygen double bond transfer to the oxygen. • H⁺ ions add to the oxygen to form the hydroxyl group. Acidified KCN means that there is a source of H⁺ ions. • If you start with an unsymmetrical ketone or any aldehyde except methanal, a mixture of enantiomers will be produced, due to the planar bond (nucleophile can attack from above or below).
General formula of an aldehyde with a nitrile group
RCHO + KCN + H+ -> RCH(OH)CN + K+
General formula of a ketone with a nitrile group
RCOR’ + KCN + H+ -> RCR’(OH)CN + K+
Hazards of using KCN
Irritant. Dangerous if ingested or inhaled. React with moisture to produce hydrogen cyanide, a highly toxic gas.
This question is about ketones. Solution X reacts with liquid ketones to form a crystalline solid. This reaction can be used to identify a ketone if the crystalline solid is separated, purified by recrystallisation, and the melting point determined. Describe how the crystalline solid is separated and purified. [5]
[1] Filter / decant [2] Dissolve in minimum vol [3] of hot solvent [4] Cool / leave (to crystallise) AND filter (under reduced pressure) [5] Wash with cold solvent/water, and dry (with method)
Propanone (CH3COCH3) reacts with the weak acid HCN to form a hydroxynitrile. This hydroxynitrile is usually made by reaction of propanone with KCN followed by dilute acid, instead of with HCN State the hazard associated with the use of KCN. Suggest a reason, other than safety, why KCN is used instead of HCN. [2]
[1] Toxic / poisonous [2] HCN to weak for a reaction
Aqueous NaBH4 reduces aldehydes but does not reduce alkenes. Show the first step of the mechanism of the reaction between NaBH4 and 2-methylbutanal. You should include two curly arrows. Explain why NaBH4 reduces 2-methylbutanal but has no reaction with 2-methylbut-1-ene. [5]
[1] Structure of 2-methylbutanal [2] 2 curly arrows. 1 from lone pair on H+ to the C attached to C=O. 1 from middle of double bond to O [3] nucleophile is attracted to δ+ C [4] electron rich C=C [5] nucleophile is repelled by C=C