Module 6 - Carbonyls

Question 1

What is the test and results for aldehydes

Answer 1

Tollens reagent

Silver mirror/ precipitate produced

Question 2

What is the test and result for ketones

Answer 2

2,4-DNP
Yellow/orange/red precipitate

Question 3

What is the test and observations for carboxylic acids

Answer 3

Sodium carbonate
Fizzing/effervescence/ bubbling

Question 4

Suggest the chemical test to distinguish a ketone from two esters

Answer 4

Reagent = tollens reagent
Observation = silver mirror

Question 5

Suggest the test to identify esters

Answer 5

H+/CrOO7^2- (acidified potassium dichromate
Orange to green (with CH3OH)

Question 6

What is the carbonyl group

Answer 6

C=O

Question 7

What type of intermolecular forces do molecules with the carbonyl group have?
Why?

Answer 7

Permanent dipole-dipole due to the polar C=O bind
(O is delta -ive)

Question 8

How soluble are carbonyls in water?
What influences solubility?

Answer 8

Soluble - form hydrogen bonds between water molecules and oxygen of C=O bind

As chain length increases, solubility decreases

Question 9

What is the name of the product when HCN is added to a carbonyl group

Answer 9

Hydroxynitriles

Question 10

What is Fehling’s solution?
What colour is it?

Answer 10

Copper complex ions

Blue

Question 11

What happens when an aldehyde is added to Fehling’s solution

Answer 11

Reduced to Cu+ ions —> colour change to red brick ppt

Question 12

What happens when a ketone is added to Fehling’s solution

Answer 12

No visible change —> stays blue

Question 13

How do you test for a carbonyl compound

Answer 13

Use Brady’s reagent - 2,4-DNP

If a carbonyl is present - an orange ppt is formed

Question 14

What is tollen’s reagent

Answer 14

Silver complex in colourless solution

Question 15

What happens when an aldehyde is added to tollens reagent

Answer 15

Silver mirror forms as Ag+ reduced to Ag (s)

Question 16

What is a reducing agent for aldehydes sand ketones?
What ions does this release?

Answer 16

NaBH4 (sodium tetrahydridooborate (III))
Releases an H- ion

Question 17

Why do aldehydes react with tollens reagent and not ketones

Answer 17

Because aldehydes are oxidised to form carboxylic acids and ketones cannot be further oxidised

Question 18

Describe how you would make tollens reagent and carry out the test for aldehydes in the lab

Explain what happens to both the tollens reagent and the aldehyde

Answer 18

Silver nitrate + ammonia (warm/heat)
Silver mirror forms
Explanation :
Silver ions reduced Ag+ +e- = Ag
Aldehyde oxidised to carboxylic acids