6: Organic Analysis

Question 1

What is potassium dichromate used for?

Answer 1

An oxidising agent

Question 1

What colour is potassium dichromate?

Answer 1

Orange

Question 2

What conditions should you use when oxidising a primary alcohol?

Answer 2

Distillation

Question 3

What conditions should you use when oxidising a secondary alcohol?

Answer 3

Acidified potassium dichromate with sulfuric acid

Question 4

Describe the colour change when acidified potassium dichromate gets reduced.

Answer 4

Orange to green

Question 5

What colour are dichromate ions?

Answer 5

Orange

Question 5

Give an ionic equation for the reduction of potassium dichromate and production of Cr3+ ions.

Answer 5

Cr2O7 2- + 6e- + 14H+ -> 2Cr 3+ + 7H2O

Question 6

What colour are chromium 3+ ions?

Answer 6

Green

Question 7

What can primary alcohols be reduced to?

Answer 7

Aldehydes.
With further oxidation, carboxylic acids are produced

Question 8

What can secondary alcohols be oxidised into?

Answer 8

Ketones

Question 9

Explain the test for the presence of primary, secondary and tertiary alcohols.

Answer 9

Add 10 drops of alcohol to 2cm3 of potassium dichromate
Warm in a hot water bath
If primary/secondary alcohols are present, the orange solution changes colour to green. If tertiary alcohols are present then there is no colour change.

Question 10

When heating alcohols, why do we use a water bath instead of a bunsen burner?

Answer 10

Alcohol is flammable

Question 11

If we see a positive result for primary and secondary alcohols, what can we do to see whether the alcohol is primary? (2 methods)

Answer 11

Oxidise under reflux
Test for carboxylic acid
If the test is positive then it is a primary alcohol

Oxidise under distillation conditions
Test for aldehyde
If the test is positive then it is a primary alcohol

Question 12

What conditions are needed to convert an aldehyde to carboxylic acid?

Answer 12

Oxidation under reflux

Question 13

What conditions are needed to convert a primary alcohol to aldehyde?

Answer 13

Oxidation under distillation conditions

Question 14

Explain the test for aldehydes using Fehling’s solution.

Answer 14

Add 2cm of Fehlings
Add 5 drops of sample
If aldehydes are present then there will be a colour change from blue to brick red

Question 15

Give the ionic equation for the reduction of Cu2+ ions to Cu2O

Answer 15

2Cu2+ + 2OH- + 2e- -> Cu2O + H2O

Question 16

State the complex ion present in Tollens’

Answer 16

[Ag(NH3)2]+

Question 17

What happens to silver metal ions in Tollens’ when added to an aldehyde?

Answer 17

They are reduced

Question 18

What will you observe if you add Tollens’ reagent to an aldehyde?

Answer 18

Silver mirror formed

Question 19

Why must aldehydes and ketones always be heated in a water bath?

Answer 19

They are flammable

Question 19

Describe the test for carboxylic acid.

Answer 19

1. Add 2cm3 of test sample
2. Add a spatula of sodium carbonate
3. If bubbles are produced, bubble through limewater
4. If the gas is CO2 then the limewater turns cloudy.
5. This means that a carboxylic acid is present.

Question 20

Describe the test for alkenes.

Answer 20

Add 2cm3 of sample solution
Add 2cm3 of orange bromine water
Shake
Solution changes from orange to colourless if alkenes are present

Question 21

What does the molecular ion peak mean on a mass spectra?

Answer 21

The Mr of a compound.

Question 22

What is high resolution mass spectrometry used for?

Answer 22

Identifying compounds with the same Mr when rounded to the nearest whole number

Question 23

Explain how infrared spectroscopy works.

Answer 23

Beam of infrared radiation passed through a chemical and is absorbed by covalent bonds.

Question 24

What is the fingerprint region?

Answer 24

The area on the spectrum that is unique to a specific compound.

Question 25

How can we use infrared spectra to discover impurities in molecules?

Answer 25

There may be an extra fingerprint region