Aldehydes and Ketones

Question 1

What is the carbonyl group?

Answer 1

It consists of a carbon-oxygen double bond

Question 2

Whar group is present in both Aldehydes and Ketones?

Answer 2

The Carbonyl group

Question 3

What is the general formula of an aldehyde?

Answer 3

RCHO

C atom is bonded to atleast 1 H atom

Question 4

What is the general formula of a Ketone?

Answer 4

RCOR

C atom is bonded to 2 other groups

Question 5

How do you name Aldehydes?

Answer 5

• They are named using the suffix -al
• Carbon of the aldehyde group is counted as part of the C chain
• Aldehyde group only occurs at end of the chain so no numbering needed

Question 6

How do you name Ketones?

Answer 6

• They are named using the suffix -one
• Carbon atom of group is counted part of the C chain
• No ketone with less then 3 C atoms is possible

Question 7

Name the following compound:

Answer 7

Methanal

Question 8

Name the following compound:

Answer 8

Propanal

Question 9

Name the following compound:

Answer 9

Ethanal

Question 10

Name the following compound:

Answer 10

Propanone

Question 11

Name the following compound:

Answer 11

Pentan-2-one

Question 12

What is the physical property of carbonyl compunds in terms of polarity?

Answer 12

• The carbonyl group is strongly polar so permanent dipole-dipole forces occur between molecules.
• This means BPt’s are higher than those of alkanes with simialr Mr but not as high as alcohols where hydrogen bonding can occur

Question 13

What is the solubilty of carbonyl compounds?

Answer 13

Smaller short chain carbonyl compounds are soluble in water but as the size increases solubility decreases due to the greater significance of a large hydrophobic chain

Question 14

What is the reactivity of Carbonyl compunds?

Answer 14

• The C=O bond is strong but is reactive due to its polar nature. Nucleophiles will attack the positive carbon and because of the double bond addition reactions are possible too.
• So Nucleophilic addition reactions tend to take place

Question 15

How are aldehydes made?

Answer 15

• Gently heat the primary excess alcohol with some oxidisng agent(potassium dichromate)through distillation and sulfuric acid.
• The aldehyde boiling at a lower temperature than the alcohol is distilled off immediately
• Equation:

Primary alcohol + [O] → Aldehyde + water

Question 16

How are ketones made?

Answer 16

• A secondary alcohol is refluxed using an oxidising agent (potassium dichromate) and dilute sulfuric acid
• Ketones can be oxidised eaily because even with refluxing for longer nothing else will be produced

Question 17

Describe a general Nucleophilic addition reaction:

Answer 17

1. 1 arrow from nucleophile to delta+ Carbon
2. 1 arrow from double bond to to delta- Oxygen
3. 1 arrow from negative oxygen with lone pair to H+

Question 18

Describe the Nucleophilic addition of HCN ( Hydrogen Cyanide)

Answer 18

2 Stages:

1. Nucleophilic attack by :CN- leaving intermediate :O-
2. Reaction of intermediate with H+ from solvent or diute acid leaving OH attached

• The end product = 2-hydroxynitrile
• When CN is added to an aldheyde or assymetric ketone a racemic mixture of 2 enantiomers/optical isomers produced because CN may attck from above or below as C=O group is planar.

Question 19

Why are Nucleophilic addition reactions useful in organic synthesis?

Answer 19

• The C chain is lengthened by the forming of a new C-C bond
• The hydroxynitrile formed is useful becasue the OH and CN groups can be readily converted into other functional groups

Question 20

What is the Fehlings test?

Answer 20

• Aldehyde - when warmed with Fehlings solution a brick red precipitate of copper(I) oxide produced as copper(II) oxides aldehyde to carboxylic acid so is reduced itself. It goes from blue to green then brick red precipitate forms.
• Ketone - no reaction . C-C bond needs breaking and stronger agent required which would result in short chain molecule of water and CO2 instead.

Question 21

What is The silver mirror test?

Answer 21

• Tollens reagent contains [Ag(NH3)2]+ from mixing aqueous ammmonia and silver nitrate
• Aldehyde - is oxidised to carboxylic acid by Tollens and Ag+ is reduced to metallic silver which is shown as silver mirror on the inside of test tube

[Ag(NH3)2] + e- → Ag(s) + 2NH3 (Silver Reduced)

• Ketone - no reaction

Question 22

What 2 tests can distinguish between aldehydes and ketones?

Answer 22

• Fehlings test and Sliver mirror test
• Both are weak oxidising agents that can oxidise aldehydes and not ketones

Question 23

How is the reduction of aldehydes and ketones achieved?

Answer 23

• It isn’t easy and requires a powerful reducing agent like sodium tetrahydridoborate(III) / NaBH4 in aqueous solution which generates :H- / hydride ion as a nucleophile
• The carbonyl compounds are reduced to alcohols
• The nucleophile is repelled by high electron density in C=C bonds but is attracted to C=O bond because of delta+ C

Question 24

What is the mechansim for the reactions reducing aldehydes and ketones?

Answer 24

• These are Nucleophilic addition reactions

Question 25

What is the difference between primary and secondary alcohols?

Answer 25

• primary alcohol - OH is attached to carbon with 1 alkyl group
• secondary alcohol - OH is attached to a carbon with 2 alkyl groups