Organic Analysis: R.P.6 - Tests for Functional Groups

Question 1

What oxidizing agent is used to oxidize primary and secondary alcohols?

Answer 1

Potassium Dichromate (VI)

Question 2

What happens as primary and secondary alcohols are oxidized?

Answer 2

Potassium Dichromate (VI) is reduced. There is a color change from orange to green

Question 3

What is formed when a primary alcohol is oxidized?

Answer 3

Aldehyde

Question 4

What is formed when a secondary alcohol is oxodized?

Answer 4

Ketone

Question 5

What is formed when a tertiary alcohol is oxidized?

Answer 5

No reaction

Question 6

Describe the experiment for testing alcohols.

Answer 6

1. Add 10 drops of the alcohol to 2cm3 of acidified potassium dichromate solution in a test tube
2. Warm the mixture in a hot water bath

Primary and secondary alcohols - color change from orange to green as an aldehyde or ketone forms
Tertiary alcohols - No color change, no reaction

Question 7

Describe the solutions used to further differentiate between the primary alcohol and the secondary alcohol.

Answer 7

Fehling’s Solution - Blue solution of copper (II) ions dissolved in NaOH

Benedict’s Solution - Blue solution of copper (II) ions dissolved in Na2CO3

Question 8

Describe the Fehling’s and Benedict’s test for aldehydes and ketones.

Answer 8

1. Add 2cm3 of Fehling’s or Benedict’s solution to a test tube
2. Add 5 drops of the aldehyde or ketone to the test tube
3. Put the test tube in a hot water bath to warm it

Ketone - No color change, no reaction
Aldehyde - Color change from blue to brick red Cu2O precipitate

Question 9

How are carboxylic acids formed?

Answer 9

Oxidizing aldehydes or primary alcohols

Question 10

Describe the experiment for testing carboxylic acids.

Answer 10

1. Add 2cm3 of the solution you want to test to a test tube
2. Add 1 small spatula of solid Na2CO3 (or 2cm3 of Na2CO3)
3. If the solution begins to fizz, bubble the gas produced through some limewater in a 2nd test tube

If the solution tested contains a carboxylic acid, CO2 gas will be produced. When CO2 is bubbled through limewater, the limewater turns cloudy

Question 11

What does testing for Alkenes involve?

Answer 11

Testing for unsaturation (Double bonds)

Question 12

Describe the experiment for testing alkenes.

Answer 12

1. Add 2cm3 of the solution that you want to test to a test tube
2. Add 2cm3 of bromine water to the test tube
3. Shake the test tube

If an alkane is present, the bromine water will turn from orange to colorless

Question 13

What does Tollens’ Reagent test for?

Answer 13

Aldehydes and Ketones

Question 14

Describe the Tollens’ Reagent test

Answer 14

1. Put 2cm3 of 0.10 mol/dm3 of AgNO3 in a test tube
2. Add a few drops of dilute NaOH solution. A light brown precipitate should form
3. Add drops of dilute ammonia solution until the brown precipitate dissolves completely

Aldehyde - The silver ions in Tollens’ reagent are reduced to silver metal which will coat the inside of the test tube to form a silver mirror

Ketone - No reaction, No silver mirror