Topic 17.2 Carbonyl compounds

Question 1

Carbonyl group

Answer 1

The carbonyl group is a carbon atom joined by a double bond to an oxygen atom
-C=O
Carbonyl compounds can be classed as either aldehydes or ketones

Question 2

Aldehydes

Answer 2

-Carbonyl compounds is where a hydrogen atom is bonded to the carbonyl group
-General formula: RCHO
- =O at the end of carbon chain
(Aledehydes are oxidised further to produce carboxylic acids …..

Question 3

Ketone

Answer 3

Carbonyl compounds where there are only one hydrocarbon groups joined to the carbonyl group.

Question 4

Bonding in carbonyl compounds

Answer 4

-Contain -C=O group
-Polar bond
-Electron density is greater near the δ- O atom

Question 5

Physical properties in carbonyl compounds

Answer 5

-Distinctive smells
-Boiling temperature are between those of alkanes (low) and alcohol (high)
-Boiling points increase with chain length
-No hydrogen bonding but permanent dipole dipole attraction

Question 6

Physical properties in ketones and aldehydes

Answer 6

-Smaller aldehydes and ketones are soluble in water - they can form hydrogen bonds with water
-Solubility decreases as chain length increases - hydrocarbon part of the molecules become more significant

Question 7

Primary alcohol

Answer 7

Oxidised:
Primary alcohol —> aldehyde —>
carboxylic acid

Question 8

Secondary alcohol

Answer 8

Oxidised:
Secondary alcohol —> ketone —> X

Question 9

Tertiary alcohol

Answer 9

Tertiary alcohol —> X

Question 10

Reagent observations: Felling’s reagent

Answer 10

Aldehyde: brick red, precipitate solid
Ketone: stays blue, no visible change

Question 11

Reagent observations: Tollen’s reagent

Answer 11

Aldehyde: silver mirror, black precipitate
Ketone: stays colourless, no visible change

Question 12

Reagent observations: sodium dichromate

Answer 12

Aldehydes: decolourises
Ketones: stays purple, no visible change

Question 13

Reagent observations: K2Cr2O7

Answer 13

Aldehyde: colour change, orange to green
Ketone: stays orange, no visible change

Question 14

Reaction with proponal/anone: Sodium dicromate

Answer 14

Propanal: green
Propanone: orange (no change)

Question 15

Reaction with proponal/anone: Felling’s solution

Answer 15

Propanal: light blue
Propanone: dark blue

Question 16

Reaction with proponal/anone: Tolling’s reagent

Answer 16

Propanal: white precipitate
Propanone: colourless

Question 17

Reduction reactions (eg. 2° alcohol –> 1° alcohol)

Answer 17

Aldehydes and ketones can be reduced to alcohols using lithium tetrahydridoaluminate (LiAlH4) dissolved in dry ether.
-Reducing agent represented by [H]

Question 18

Oxidation reactions

Answer 18

-Aldehydes can be oxidised to form carboxylic acids
-Ketones are not easily oxidised (assume they aren’t)
-Oxidising agents shown as [O]

Question 19

Reactions with iodine

Answer 19

Triidomethane reaction
-Carbonyl compound added to an alkaline solution of iodine
-Mixture warmed then cooled
-Yellow precipitate forms - this is a positive result
-Test for CH3CO group (found in ethanal and all methyl ketones)

Question 20

Reaction with HCN

Answer 20

-Aldehydes and ketones react with HCN in an alkaline solution of KCN
-These are addition reactions where a hydrogen atom joins to the oxygen in the carbonyl group and the cyanide group attaches to the carbon of the carbonyl group

Question 21

Naming products

Answer 21

OH group- hydroxy
CN group- nitrile
The products are known as hydroxynitriles

Question 22

Nucleophilic addition mechanism (Step 1)

Answer 22

Step 1: nucleophilic attack by a cyanide ion on the δ+ carbon atom of the carbonyl group.

Question 23

Nucleophilic addition mechanism (Step 2)

Answer 23

Step 2: the intermediate reacts with a hydrogen cyanide molecule.

Question 24

Optical activity

Answer 24

-As C=O is planar there is an equal chance of the CN- ion attacking from each side of the plane
-Even though the product contains a chiral centre, the product is not optically active as there are equal amounts of each enantiomer - a racemic mixture.

Question 25

Reaction with 2,4-dinitrophenylhydrazine (Brady’s reagent)

Answer 25

A bright orange precipitate indicates the presence if a carbonyl compound.

Question 26

Howto find the melting temperature of derivatives

Answer 26

-The compounds formed, known as derivatives, can be used to identify individual compounds
-The derivatives are purified and dried and their melting point measured

Question 27

Reagent observations: 2,4 DNPH

Answer 27

Aldehyde: orange precipitate
Recyrstalise and find melting point to find which compound it is.

Question 28

Recrystalisation

Answer 28

1) Dissolve min hot solvent
2) Filter
3) Cool & crystalise
4) Filter with Buchner funnel
5) Wash with minimum cold solvent
6) Dry in warm oven

Question 29

Iodoform test (triodomethane)

Answer 29

CH3CO in a molecule reacts with Iodine/OH- to form a yellow precipitate.

Question 30

Ester hydrolysis

Answer 30

Acidic conditions: ……..?