CH.18 Carbonyl Compounds

Question 1

How can you prepare aldehydes?
Give conditions as well
give general equation

Answer 1

Oxidation of primary alcohol
acidifies potassium dichromate (VI) and acidified dilute sulfuric acid
heat gently
distil of aldehyde
colour change-
orange to green

Question 2

How can you prepare ketones?
Give conditions and give the general equation

Answer 2

Oxidation of secondary alcohol
acidifies potassium dichromate (VI) and acidified dilute sulfuric acid
heat gently
colour change-
orange to green

Question 3

reduction of aldehydes and ketones

Answer 3

NaBH4 OR LiAlH4 as a reducing agent
LiAlH4 should be in dry ether

Question 4

nucleophilic addition

Answer 4

the mechanism of the reaction in which a nucleophile attacks the carbon atom in a carbonyl group and addition across the C=O bond occurs

Question 5

hydrolysis of the nitrile group
give equation

Answer 5

refluxing with dilute hydrochloric acid -COOH
can reflux with alkali but -COO-Na+ formed

Question 6

draw the nucleophilic addition mechanism

Answer 6

pg.355-356

Question 7

test for carbonyl group

Answer 7

2,4-DNPH
if aldehyde or ketone is present a deep-orange precipitate is formed
ppt can be purified by recrystallization and its m.p can be measured to identify the identity of the precipitate compound
condensation reaction occurs
carboxylic acids do not form precipitates

Question 8

test between ketones and aldehydes

Answer 8

tollens’ reagent
aqueous solution of silver nitrate in excess ammonia solution ( ammonical silver nitrate solution)
Ag+ ions act as mild oxidising agents, when warmed they will oxidise aldehyde to form a carboxylate ion. In alkaline conditions any carboxylic acid is immediately neutralised to -COO-
in redox reaction, Ag+ ions get reduced to silver atoms that form a ‘mirror’ on the inside of the tube
no change with a ketone remains a colourless solution

Fehling’s solution
alkaline solution containing copper (II) ions
Cu2+ acts as an oxidising agent, so Aldheyde gets oxidised to carboxylate ion and the Cu2+ ions get reduced to Cu+ ion
colour change- blue to red/ orange ppt
ketones do not get oxidised

Question 9

Reaction to form tri-iodomethane

Answer 9

forms a yellow precipitate with methyl ketone or secondary alcohol with a methyl group (yellow ppt gives evidence for CH3CO- group)
reagent- an alkaline (NaOH) solution of iodine which is warmed with the substance being tested
reaction steps:
1. carbonyl compound is halogenated so hydrogen atoms in CH3 are replaced by iodine
2. intermediate is hydrolysed to form a yellow precipitate of tri-iodomethane

secondary alcohols will get oxidised by iodine in an alkaline solution to give a ketone with CH3CO-
ethanol will also give a positive result because it gets oxidised to ethanol which has the CH3CO- group