Identification of Organic Molecules

Question 1

What is the first way of determining whether an organic substance is an alkene?

Answer 1

Add bromine water to the substance and it will turn from brown to colourless

Question 2

What is the second way of determining whether an organic substance is an alkene?

Answer 2

Add acidified potassium manganate (purple) and shake which will decolourise the manganate as a diol is formed

Question 3

How do you test for a carbonyl group?

Answer 3

Add small amount of 2,4-dinitrophenylhydrazine and a yellow/orange precipitate will slowly form

Question 4

What is Brady’s reagent?

Answer 4

2,4-dinitrophenylhydrazine dissolved in methanol and conc sulfuric acid

Question 5

What is the main difference between aldehydes and ketones?

Answer 5

Aldehydes are easily oxidised, whilst ketones aren’t

Question 6

How do you make Tollens’ reagent?

Answer 6

Dissolve silver nitrate in aqueous ammonia

Question 7

What happens when an aldehyde is heated in a test tube with Tollens’ reagent?

Answer 7

A silver mirror is formed

Question 8

What is the ionic equation of the formation of a silver mirror?

Answer 8

Ag(NH3)2 + +e- —-> Ag +2NH3

Question 9

What is Fehling’s solution?

Answer 9

A blue solution of complexed copper (2) ions dissolved in NaOH

Question 10

What happens when Fehling’s solution is heated with an aldehyde?

Answer 10

The blue copper(2) ions are reduced to a brick-red precipitate of copper(1) oxide. Cu2+ +e- –> Cu+

Question 11

What is Benedict’s solution?

Answer 11

The same as Fehling’s solution, however the copper (2) ions are dissolved in sodium carbonate instead of sodium hydroxide

Question 12

What happens when Benedict’s solution is heated with an aldehyde?

Answer 12

The blue copper (2) ions are reduced to a brick-red precipitate of copper(1) oxide

Question 13

What happens when you heat an aldehyde with acidified dichromate ions?

Answer 13

A carboxylic acid is formed

Question 14

When an aldehyde is oxidised by acidified dichromate ions, what is the colour change of the dichromate ions?

Answer 14

Orange to green

Question 15

If something contains a methyl carbonyl group, what must it be?

Answer 15

Either ethanal or a ketone with at least one methyl group

Question 16

What happens when carbonyls which contain a methyl carbonyl group are heated with iodine in the presence of an alkali?

Answer 16

If there is a methyl carbonyl group a yellow precipitate of triidomethane will form and an antiseptic smell

Question 17

How do you test for primary or secondary alcohols?

Answer 17

Warm with potassium dichromate and add dilute sulfuric acid and the substance will change from orange to green

Question 18

How do you test for a carboxylic acid?

Answer 18

Add some sodium carbonate solution and carbon dioxide will be formed

Question 19

How do you test for an acyl chloride?

Answer 19

Add substance to water and hydrogen chloride will be formed

Question 20

What happens when phenol is added to sodium carbonate solution?

Answer 20

It dissolves without the evolution of carbon dioxide

Question 21

What happens when you add phenol to water, shake and then add a few drops of iron chloride solution?

Answer 21

A purple colour is seen

Question 22

What happens when you add phenol/phenylamine to brown bromine water and shake?

Answer 22

Bromine colour goes and white precipitate is formed

Question 23

What is the colour of a dichromate ion?

Answer 23

Orange