Oxidation of alcohols Flashcards
Oxidising by burning in oxygen
Forms carbon dioxide and water as products
What oxidising agent can be used to oxidise alcohols instead of burning?
Acidified potassium dichromate ions
What are the rules of oxidising alcohols?
You do not break C-C bonds when oxidising but the C-H bonds
How many times is primary alcohols oxidised?
Twice (has 2 C-H bonds because the other bonds is C-OH and C-C)
How many times is secondary alcohols oxidised?
Once (has only 1 C-H bond, the other bonds are 2 C-C bonds and C-OH)
How many times are tertiary alcohols oxidised?
Never: because they have no C-H bonds.
Oxidation products of a PRIMARY ALCOHOL
Once to aldehyde
Then change experimental condition to a carboxylic acid
Oxidation products of a SECONDARY ALCOHOL
A ketone
and water
What is a aldehyde?
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
How to name aldehydes?
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
What is a ketone?
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
How to name ketones?
Branching + longest chain-X-one
Where X= the carbon the C=O is at
For example, pentan-2-one
Carboxylic acid
Molecule with functional group C =O which is also bonded to -OH
How to form an aldehyde in first oxidation of an alcohol
In distillation set up
Where aldehyde is condensed following addition of potassium dichromate (acidified) and can be separated by distillation into a new flask
Why must the aldehyde be distilled as it’s formed following oxidation of alcohol?
Because otherwise it will continue to oxidise into a carboxylic acid
How can we ensure we are distilling the ALDEHYDE and not unreacted alcohol?
Use a thermometer to ensure it is boiling + condensing the formed aldehyde because aldehyde will have a lower bp than the alcohol
Why does aldehyde have lower boiling point than an alcohol?
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
How can we oxidise an alcohol straight to carboxylic acid and skip turning it to an aldehyde?
Using a reflux system
Reflux of alcohol?
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
How can we differentiate an aldehyde and ketone?
Based on ability to be oxidised by oxidising agents
Aldehyde vs ketone oxidising ability
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
Testing the oxidation of ketones vs aldehydes using fehling’s solution
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
Testing the oxidation of ketones vs aldehydes using Benedict’s solution
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
potassium dichromate ions colour change once oxidises the alcohol
orangeto green
because the chromium in dichromate Cr2O7 -2 are reduced to form green chromate ions Cr 3+
Method for carrying out PARTIAL oxidation of an alcohol to an aldehyde
In a peat shaped, dissolve 2g of potassium dichromate in 10cm3 of concentrated sulfuric acid and add anti bumping granules
Use a pipette to transfer 3cm3 of ethanol to flask drop wise and shake
Set up distillation apparatus and allow to GENTLY HEAT by monitoring temp to be near aldehyde’s bp then condense and collect product
Where is the thermometer in a distillation apparatus?
Between T junction of pear shaped flask and condenser
Why use anti bumping granules?
To provide a surface for smaller bubbles to form rather than larger bubbles forming
Detailed method for FULL oxidation of an alcohol to a carboxylic acid
Pour 5cm3 of ethanol into a pear shaped flask with anti bumping granules
Place into reflux apparatus with vertical condenser
Add drop wise using a pipette , 2cm3 of acidified potassium dichromate into the flask down the condenser followed by concentrated sulfuric acid using the same method
Heat released by reaction will boil mixture and allow reflux of alcohol to carboxylic acid, but use Bunsen burner to maintain heat
Rearrange for distillation to collect carboxylic acid /separate from mixture
