3.3.8 Aldehydes and ketones

Question 1

Carbonyl group

Answer 1

H- C=Oat the end

benzene with a c attacjed and a double bond o of the c and and a h
Benzenecarbaldehyde

Question 2

Ketone functional group

Answer 2

C=O in the middle

Question 3

Physical properties of carbonyl compounds

Answer 3

Polar - pd - pd forces
Boiling point higher than alkanes of similar length (not as high as alcohols)
Soluble when short - as H bonds form
Methanal is gas at room temp (other short chains are liquids)
Benzenecarbaldehyde smells of almonds - (used in soaps)

Question 4

Nucleophilic addition reactions

Answer 4

1. nucleophile atacks the c diuble bind to o
   attaches the nu to the c and 0 single bonded with a neg
   charge and a lp

the lp on the oxygen attcks the h+ makes an oh group

High difference in electronegativity between the c and the o
Nucleophiles attach the C
Unsaturated as = - addition reactions used

Question 5

Oxidation of aldehydes and ketones

Answer 5

Aldehydes - oxidise to carboxylic acids
Example of oxidising agent - acidified potassium dichromate
Ketones cannot be oxidised - as the c-c must t be broken
Stronger oxidising agents break hydrocarbon chain of ketones into shorter molecule anD C02 and H20

Question 6

Reduction of aldehydes and ketones

Answer 6

Reducing agent - tetrahydridoborate(III), NaBH4 in aq
Generates a H- nucleophile (hydride ion)
H- repealed by the high electron density of the c=c bond

Question 7

Reduction mechanisms -

Answer 7

Reducing an aldehyde into a primary alcohol
H- acts as a nucleophile (nucleophilic addition reactions)

Reducing a ketone into a secondary alcohol
H- acts as a nucleophile (nucleophilic addition reactions)

Question 8

Distinguishing aldehydes from ketones

Answer 8

Fehling’s test
Fehlings made of - fehling’s a (containing Cu 2+ ions ) and fehling’s b (contains aldehyde and a complexing agent )
Pos result = brick red ppt (copper (i) oxide (as act as an oxidising agent)
Blue - brick red ppt
Tollens test
complex ion [Ag(NH3)2]+
Formed when aq NH3 is added to aq silver nitrate
Aldehyde warmed with tollens = metallic silver formed
Ag ions reduced to Ag