A2 AQA CHEM4 3.4.5

3.4.5 Compounds containing the carbonyl group

Aldehydes and Ketones

·         Aldehydes –COH

·         Ketones –CO-

·         1^o alcoholsàaldehydesàcarboxylic acids

·         2^o alcoholsàketones

o   Ketones resist further oxidation

·         Tollen’s reagent and heat

o   Aldehydes form a silver mirror but ketones do not react

o   This is because the aldehyde is oxidised to a carboxylic acid here

·         Fehling’s’ solution and heat

o   Aldehydes form a brick red ppt whereas ketones do not react

·         Adding K₂Cr₂O₇/H⁺ causes a colour change from orange (+6) to green (+3) as 1^o alcohols and 2^o alcohols are oxidised to form aldehydes and ketones

·         Turning aldehydes/ketones back into alcohols is a reduction process

o   Reversal of oxidation

o   Uses NaBH₄ as the reducing agent [H] as it can reduce carbonyl compounds

o   LiAlH₄ could also be used

o   The [H] form H^- ions which can act as nucleophiles and attach the carbon in the carbonyl group to form alcohols

§  The C=O bond is very polar since O is a lot more electronegative than C

o   CH₃CH₂COH + 2[H] àCH₃CH₂CH₂OH

o   CH₃COCH₃ + 2[H] à CH₃CHOHCH₃

·         Aldehydes and ketones react with HCN/KCN to produce hydroxynitriles

o   CH₃COH + HCN à CH₃CH(OH)CN

§  2-hydroxypropanenitrile

o   CN^- has a lp and acts as a nucleophile in nucleophilic addition

·         HCN is a colourless gas which is toxic as it stops cells from respiring

·         Hydroxynitriles can have a chiral carbon and therefore show optical activity

Carboxylic acids and esters

·         Carboxylic acids are weak acidsà high pK_a

·         They react with carbonates to produce CO₂ (effervescence), H…