CD.9

Question 1

What is the formula of a cyanide ion?

Answer 1

C≡N-

Question 2

What is the functional group known as when a cyanide group is present in organic molecules?

Answer 2

Nitrile.

Question 3

What is the advantage of introducing a cyanide/nitrile group into a moleucle during synthesis?

Answer 3

It provides a way of lengthening the carbon chain in a molecule.

Question 4

What are two things that can be done to a nitrile group once it is in a molecule?

Answer 4

• Hydrolysed to a carboxylic acid group.

- Reduced to an amine group.

Question 5

What are two types of organic compounds that contain a carbonyl group, C=O?

Answer 5

Aldehyde and ketone.

Question 6

What is the key difference betweent the structures of an aldehyde and a ketone?

Answer 6

Aldehydes contain a hydrogen atom attached to the carbonyl group, whereas a ketone doesn’t.

Question 7

Can both aldehydes and ketones by oxidised?

Answer 7

Only aldehydes can be oxidised because it has that hydrogen atom on the carbonyl group.

Question 8

What would you observe if a sample of aldehyde was heated under reflux with acidified potassium dichromate?

Answer 8

The solution turning from orange to green.

Question 9

What would you observe if a sample of ketone was heated under reflux with acidfied potassium dichromate?

Answer 9

No change.

Question 10

What does the acidified dichromate act as in the test for aldehydes?

Answer 10

An oxidising agent.

Question 11

What other compounds can turn acidified dichromate from orange to green when heated under reflux?

Answer 11

Primary and secondary alcohols.

Question 12

What are two tests for aldehydes?

Answer 12

Fehling’s test and Tollen’s solution.

Question 13

What are aldehydes oxidised to in Fehling’s test?

Answer 13

Carboxylic acids.

Question 14

What are three step to carrying out a Fehling’s test?

Answer 14

1. Prepare Fehling’s solution by mixing together Fehling’s A and B.
2. Add the solution containing presumablly aldehyde to the Fehling’s solution.
3. Warm the solution in a water bath.

Question 15

What would you observe if the solution contained an aldehyde?

Answer 15

The colour of the solution would change from transparent blue to cloudy brick red.

Question 16

What would you observe if the solution contained a ketone?

Answer 16

No change, the solution remains blue.

Question 17

What does Fehling’s A and B each contain?

Answer 17

Fehling’s A contains Cu2+ ions, which will act as a weak oxidising agent.
Fehling’s B is alkaline solution containing bidentate ligand.

Question 18

What is the chemical change associated with the colour change in Fehling’s solution?

Answer 18

The blue Cu2+ ions are reduced to Cu+ ions with form a brick red precipitate of Cu2O.

Question 19

What are aldehydes oxidised to by Tollen’s reagent?

Answer 19

Carboxylate ions.

Question 20

How do you make a Tollen’s reagent?

Answer 20

By dissolving silver(I) oxide, Ag2O in ammonia solution to form a complex ion of [Ag(NH3)2]+.

Question 21

How do you test an aldehyde using Tollen’s reagent?

Answer 21

Add the solution containing the aldehyde to the Tollen’s reagent and warm it using a water bath.

Question 22

What would you observe if the solution contained an aldehyde?

Answer 22

A silver mirror forming on the inside of the test tube (silver-mirror effect).

Question 23

What would you observe if the solution contained a ketone?

Answer 23

No change.

Question 24

What is the chemical change associated with the silver mirror effect in Tollen’s reagent?

Answer 24

Silver(I) ions are reduced to elemental silver, which appears as a silvery layer on the inside of the test tube.

Question 25

What compound will an aldehyde be reduced to?

Answer 25

Primary alcohol.

Question 26

What compound will a ketone be reduced to?

Answer 26

Secondary alcohol.

Question 27

What is required to reduce aldehydes and ketones back to alcohols?

Answer 27

A very powerful reducing agent such as sodium tetrahydridoborate, NaBH4, a metal hydride.

Question 28

What type of reaction is it when aldehyde or ketone react with acidified cyanide?

Answer 28

Nucleophilic addition.

Question 29

What type of compound is formed when aldehyde or ketone react with acidified cyanide?

Answer 29

Cyanohydrin, which contains a nitrile group and a hydroxyl group on the same carbon.

Question 30

In practice, what is used instead of hydrogen cyanide, as it is too hazardous?

Answer 30

Acidified potassium cyanide for exmaple.

Question 31

How many steps is mechanism of the nucleophilic addition between aldehyde/ketone and acidified cyanide?

Answer 31

2

Question 32

What happens in the first step of the mechanism?

Answer 32

CN-, cyanide ion acts as a nucleophile and attacks the ẟ+ carbon in the carbonyl group. A carbon carbon bond is formed and a pair of electrons in the C=O moves onto the ẟ- oxygen, which now becomes negative. The bond between the carbon and the oxygen is now a single bond.

Question 33

What happens in the second step of the mechanism?

Answer 33

The negatively charge oxygen picks up a H+ ion from the solution (acidic conditions or from HCN) and a hydroxyl group is formed.