Oxidation of alcohols

Question 1

Oxidising by burning in oxygen

Answer 1

Forms carbon dioxide and water as products

Question 2

What oxidising agent can be used to oxidise alcohols instead of burning?

Answer 2

Acidified potassium dichromate ions

Question 3

What are the rules of oxidising alcohols?

Answer 3

You do not break C-C bonds when oxidising but the C-H bonds

Question 4

How many times is primary alcohols oxidised?

Answer 4

Twice (has 2 C-H bonds because the other bonds is C-OH and C-C)

Question 5

How many times is secondary alcohols oxidised?

Answer 5

Once (has only 1 C-H bond, the other bonds are 2 C-C bonds and C-OH)

Question 6

How many times are tertiary alcohols oxidised?

Answer 6

Never: because they have no C-H bonds.

Question 7

Oxidation products of a PRIMARY ALCOHOL

Answer 7

Once to aldehyde
Then change experimental condition to a carboxylic acid

Question 8

Oxidation products of a SECONDARY ALCOHOL

Answer 8

A ketone
and water

Question 9

What is a aldehyde?

Answer 9

A carbonyl compound (this means it’s functional group is C=O)
However the carbon the =O is attached to is only bonded to one other alkyl group aka C=O is at the end of the molecule with no branching

Question 10

How to name aldehydes?

Answer 10

Branching + Longest chain chain (eg butan, propan) + Al
No need for numbers to show which carbon the C=O is because it will always be at the end
Examples: pentanal, butanal

Question 11

What is a ketone?

Answer 11

Also a carbonyl compound (functional group is C=O) but the C=O is bonded to 2 alkyl groups, meaning it can be in the middle of the chain

Question 12

How to name ketones?

Answer 12

Branching + longest chain-X-one
Where X= the carbon the C=O is at
For example, pentan-2-one

Question 13

Carboxylic acid

Answer 13

Molecule with functional group C =O which is also bonded to -OH

Question 14

How to form an aldehyde in first oxidation of an alcohol

Answer 14

In distillation set up
Where aldehyde is condensed following addition of potassium dichromate (acidified) and can be separated by distillation into a new flask

Question 15

Why must the aldehyde be distilled as it’s formed following oxidation of alcohol?

Answer 15

Because otherwise it will continue to oxidise into a carboxylic acid

Question 16

How can we ensure we are distilling the ALDEHYDE and not unreacted alcohol?

Answer 16

Use a thermometer to ensure it is boiling + condensing the formed aldehyde because aldehyde will have a lower bp than the alcohol

Question 17

Why does aldehyde have lower boiling point than an alcohol?

Answer 17

Because alcohol can form hydrogen bonds as well permanent dipole-dipole forces and London forces which aldehyde cannot form hydrogen bonds thus less energy is needed to boil aldehyde

Question 18

How can we oxidise an alcohol straight to carboxylic acid and skip turning it to an aldehyde?

Answer 18

Using a reflux system

Question 19

Reflux of alcohol?

Answer 19

This is vigorous oxidation where the acidified potassium dichromate begins oxidising alcohol and the heat below pear shaped flask boils aldehyde which is then condensed by reflux condenser and falls back due to vertical cylinder
To be oxidised again into a carboxylic acid

Question 20

How can we differentiate an aldehyde and ketone?

Answer 20

Based on ability to be oxidised by oxidising agents

Question 21

Aldehyde vs ketone oxidising ability

Answer 21

Aldehyde can be oxidised again to form a carboxylic acid because carbon with C=O has C-H bond to be broken and oxidised
But ketones cannot be oxidised twice

Question 22

Testing the oxidation of ketones vs aldehydes using fehling’s solution

Answer 22

Using the Fehling’s solution which has Cu2+ ions (blue) which will oxidise aldehydes to form Cu 1+ ions (red)
NO COLOUR CHANGE FOR KETONES SO REMAINS BLUE

Question 23

Testing the oxidation of ketones vs aldehydes using Benedict’s solution

Answer 23

Using Benedict’s solution blue solution) which has Cu2+ ions which will oxidise aldehydes to form brick red precipitate of copper oxide(ions reduced) but NO COLOUR CHANGE OBSERVEDWITH KETONES

Question 24

potassium dichromate ions colour change once oxidises the alcohol

Answer 24

orangeto green
because the chromium in dichromate Cr2O7 -2 are reduced to form green chromate ions Cr 3+