Aldehydes and Ketones

Question 1

Aldehydes oxidised

Answer 1

readily, to carboxylic acids

Reagent: potassium dichromate (VI) solution and dilute sulfuric acid.

Conditions: heat under reflux

Observation: the orange dichromate ion (Cr2O72-) reduces to the green Cr 3+ ion

Question 2

Fehling’s

Answer 2

Reagent: Fehling’s solution containing blue Cu 2+ ions.

Conditions: heat gently

Reaction: aldehydes only are oxidised by Fehling’s Solution into a carboxylic acid. The copper (II) ions are reduced to copper(I) oxide . .
Observation: Aldehydes :Blue Cu 2+ ions in solution change to a red precipitate of Cu2O. Ketones do not react.

CH3CHO + 2Cu2+ + 2H2O->CH3COOH + Cu2O + 4H+

Question 3

Tollen’s

Answer 3

Reagent: Tollens’ reagent formed by mixing aqueous ammonia and silver nitrate. The active substance is the complex ion of [Ag(NH3)2]+ .

Conditions: heat gently
Reaction: aldehydes only are oxidised by Tollens’ reagent into a carboxylic acid. The silver(I) ions are reduced to silver atoms

Observation: with aldehydes, a silver mirror forms coating the inside of the test tube. Ketones result in no change.

CH3CHO + 2Ag+ + H2O -> CH3COOH + 2Ag + 2H+

Question 4

Aldehydes

Answer 4

If the C=O is on the end of the chain with an H attached it is an aldehyde.

The name will end in –al

Question 5

Ketones

Answer 5

If the C=O is in the middle of the chain it is a ketone

The name will end in -one

Question 6

Intermolecular forces

Answer 6

Pure carbonyls cannot hydrogen bond to themselves, but are attracted instead by permanent dipole forces

Question 7

Solubility

Answer 7

The smaller carbonyls are soluble in water because they can form hydrogen bonds with water.

Question 8

Reactions of Carbonyls

Answer 8

The C=O bond is polarised because O is more electronegative than
carbon. The positive carbon atom attracts nucleophiles.

In comparison to the C=C bond in alkenes, the C=O is stronger and does not undergo addition reactions easily.

Question 9

Reduction

Answer 9

Aldehydes-> primary alcohol
Ketones-> secondary alcohol
NaBH4

Nucleophilic addition

Room temp and pressure

Question 10

Nucleophilic Addition Mechanism

Answer 10

:H- to C

C=O to O

then
O:- to H+

Question 11

Catalytic Hydrogenation

Answer 11

Reduction
Reagent: hydrogen and nickel catalyst
Conditions: high pressure

Question 12

Nucleophilic addition HCN

Answer 12

carbonyl -> hydroxynitrile

Reagent: sodium cyanide (NaCN) and dilute sulfuric acid.
Conditions: Room temperature and pressure

nucleophile: CN-

Question 13

KCN advantage

Answer 13

higher concentration of the CN- ion as these compounds will completely ionise. HCN is a weak acid an will only partially ionise

Question 14

Enantiomers

Answer 14

Nucleophilic addition of HCN to aldehydes and ketones (unsymmetrical) when the trigonal planar carbonyl is approached from both sides by the HCN attacking species: results in the formation of a racemate.

Question 15

dangers of KCN

Answer 15

irritant
dangerous if ingested or inhaled
react with moisture to produce hydrogen cyanide, toxic gas