A2 AQA CHEM4 3.4.5

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  • Created on: 23-04-14 12:49

3.4.5 Compounds containing the carbonyl group

Aldehydes and Ketones

·         Aldehydes –COH

·         Ketones –CO-

·         1o alcoholsàaldehydesàcarboxylic acids

·         2o 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 1o alcohols and 2o 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 pKa

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

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