Aldehydes and ketones

Question 1

Aldehyde test- tollens reagent I

Answer 1

• add solution of silver nitrate containing Ag+ ions to solution of excess ammonia to form a complex

Complex is added to aldehyde and is warmed gently - it is oxidised to the corresponding. Carboxylic acid

Ag+ ions are reduced to Ag - REDUCATION RXN

SILVER MIRROR RESULT SEEN
Doesn’t work for ketones

Question 2

Aldehyde test- fehlings

Answer 2

It is a complex containing copper (II) sulfate containing Cu2+ ions, which are BLUE

Complex is added to the aldehyde which is oxidised to a carboxylic acid

Copper sulfate is reduced to copper oxide containing cu+ ions which are red

REDOX RXN- COLOIR CHANGE BLUE TO RED
doesn’t work for Ketones

Question 3

Triiodomethane / iodoform

Answer 3

Add unknown compound to a solution of iodine and sodium hydroxide

A yellow ppt and antiseptic smell due to. Triiodomethane forming

A positive result is only seen if CH3CO is present

Ethanal is only aldehyde to give positive result
Ketones can have positive results

Question 4

Name tests for to distinguish between aldehydes and ketones

And why?

Answer 4

Fehlings reagent

Tollens reagent

As they both don’t work for ketones only aldehydes

Question 5

Give the observation of an aldehyde with tollens reagent ?

Answer 5

A silver mirror

Question 6

What is the observation of an aldehyde with fehlings?

Answer 6

Blue to red colour change

Question 7

What type of reaction is tollens test?

Answer 7

Redox

Question 8

What type of reaction is fehlings test?

Answer 8

Redox reaction

Question 9

What is 2-4-DNP also known as?

What is it a test for?

Answer 9

Brady’s reagent

Both aldehydes and ketones

Question 10

2-4-DNP test to identify aldehyde and ketones

Answer 10

Add Unknown to 2,4-DNP In methanol or dilute acid

Orange crystals formed

Filter and wash cold solvent

Dry and take the MP and compare it with literature value

Question 11

Describe 2-4-DNP

Answer 11

Aldehydes and ketones react to produce orange crystals (hydrazine deravatives)

It is a NUCLEOPHILIC addition + elimination RXN
Also called condensation as a molecule of water is eliminated

MPS of crystals are high due to more IMF (VDW+H bonds)- these MPS can be used to identify the original carbonyl compound by comparing it with literature value

If product is pure then MP is sharp
If product is impure then MP is over a range

Question 12

How can the original carbonyl compound be identified in 2,4-DNP?

Answer 12

The MPS of the products can be used to identify original carbonyl compound by comparing it with the literature value

Question 13

What type of reaction is aldehyde and ketone test 2,4-DNP?

Answer 13

NUCLEOPHILIC addition and elimination

Question 14

What is a test for both aldehydes and ketones

Answer 14

2,4-Dinitrophenylhydrazine

Question 15

What is formed in:

Tollens test

Fehlings test

Triiodomethane / iodoform

2,4-DNP

Answer 15

Tollens: silver mirror

Fehlings: COLOIR change blue to red

Iodoform: yellow ppt if positive result occurs

2,4-DNP: orange crystals

Question 16

Give a test to distinguish between aldehyde and ketone

Answer 16

Fehlings reagent- add both to this complex which contains copper sulfate (Cu2+ ions which are blue), when aldehyde Is gently heated is oxidises to carboxylic acid- and there’s a blue to red colour change with aldehyde as the cooper sulfate reduced to Cu+ ions which are red. Ketones won’t react with fehlings reagent

Question 17

What is a positive result when aldehyde or ketone reacts with a reagent

(Name reagent also)

Answer 17

Reagent is sodium hydroxide (aq) and iodine

Positive result is CH3CO

Question 18

What produces orange crystals?

What type of reaction is this?

Answer 18

Aldehydes and ketones do with 2,4-DNP

This is a NUCLEOPHILIC addition / elimination reaction

Question 19

What is produced in the addition of hydrogen cyanide and aldehyde or ketones

Answer 19

2-hydroxynitrile

Question 20

Step by step of hydrogen cyanide adding to an aldehyde or ketone

Answer 20

HCN Ionises or disassociates into a H+ ion and cyanide ion - Heterolytic fission

The C on cyanide ion acts as a NUCLEOPHILE attacking the C on the carbonyl- NUCLEOPHILIC addition

A negative charge is produced in the oxygen and electron density increases - the oxygen becomes an electron pair donor attracting the H+ from the solvent eg water or the original HCN

A molecule of HCN has added across the C=O producing a hydroxy nitrile

Question 21

Type of reaction occurring in 2,4-DNP?

Answer 21

NUCLEOPHILIC addition and elimination

Question 22

What is the only aldehydes to produce a positive result with Triiodomethane/iodoform (NaOH(aq)/I2) ?

Answer 22

Ethanal

Question 23

What is the positive result group for an aldehyde or ketone with iodine and sodium hydroxide (aq)?

Answer 23

CH3CO

Question 24

Why is is better to use one step processes in synthesis ?

Answer 24

Better yield (in each stage there is waste products)

Question 25

Why are 2 step processes carried out despite them being not as good as 1 step?

Answer 25

Because it’s cheaper