Alcohols Oxidation Flashcards
What’s oxidation defined as?
the loss of electrons
What’s the definition of oxidation in organic chemistry?
Whenever a carbon forms a bond with a more electronegative element
e.g Oxygen
C-O -> polar due to Oxygen being more electronegative
when a C-O forms, a molecule is oxidised by this definition
When a molecule goes from a single bond to a double bond what do we say?
the molecule has been oxidised as the number of C-O bond has increased by 1
e.g
What about this molecule? why is it an oxidation molecule ?
Number of C-O bond have increased from 0 to 4
Why isn’t this molecule an oxidation molecule?
as the number of C-O bonds have decreased from 3 to 1
Which of these molecules are oxidation molecules?
A and B as they’ve formed bonds with oxygen
As A went from 1 C-O bond to 2 C-O bonds
B went from 0 C-O bonds to 2 C-O bonds
So in organic chemistry we say a molecule has been oxidised whenever
a molecule forms a bond with a more electronegative molecule
it can be C-O, C-F, C-N etc
What can we use to oxidise alcohols?
acidified potassium dichromate
(K2C2O7)
Ethanol (alcohol) (Orange) + Cr2O72- + H+ -> ethanal (aldehyde) (Green) + Cr (III) ions + water
ethanol oxidised to ethanal half equation
Primary alcohol is oxidised to?
Aldehydes (H-C^=O-C)
What’s an aldehyde oxides to?
Carboxylic acid (COOH)
Secondary alcohol is acidoses to?
ketone
We can oxidise a species using
an oxidising agent
What’s the oxidising agent for alcohols
acidified Potassium dichromate
K2Cr2O7
so meaning we add a dilute acid to our potassium dichromate the dilute acid being sulfuric acid (H2SO4)
When e.g an alcohol like ethanol is acidified what happens to our reagent?
our potassium dichromate and sulfuric acid dissociate into their ions
but we only care about our Cr2o72- and H+ ions as they’re the ones that react
So if we oxidise Ethanol what happens?
ethanol and cr2O72- + H+ -> ethanal + Cr (III) ions and water
What happened to our reaction what’s occurred
reduced reaction occured from Cr2O72- to Cr3+
from +6 to +3
If there’s a colour change
oxidisation occured
as alcohol was orange and turned into an aldehyde to green
Which of these molecules would turn green if we add acidified potassium dichromate?
B is primary alcohol
C is Secondary alcohol
D Secondary alcohol
so these will turn orange to green
How to write the oxidation equation from Ethanol to ethanal to ethanoic acid?
1) write the oxidation equation from ethanol to ethanal
CH3CHOH + [O] -> CH3CHO + H2O
2) Write the oxidation equation from ethanal to ethanoic acid
CH3CHO + [O] -> CH3COOH + H2O
(Cancel the repeated molecules & add the [O])
3) what’s the oxidation from ethanol to ethanoic acid?
CH3CH2OH + 2[O] -> CH3COOH + H2O
What about diols?
1) molecule is bonded to 2 carbons so it’s a secondary molecule C-OH is oxidised to a ketone
divide it into 2 OH see if they’re secondary or not and follow the steps
& then combine equations which will be
CH3CH(OH)CH2OH + 3[O] —>CH3COCOOH + 2H2O
Compared to alcohols, ketones have a
lower boiling point as alcohols have van der waals, dipole dipole and hydrogen bonding
whilst ketones have only van der waals and dipole dipole
What does it mean when it says a reaction is heated under reflux?
means substance that turns into a vapour returns to the reaction mixture as a liquid due to the condenser
Why do we need an excess amount of reagent?
To ensure that our reagent potassium dichromate is oxidised as much as possible in the solution
what’s the method to produce ketones?
Chemists heat the reaction mixture (K2Cr2O7/H+ + excess oxidising agent )
under reflux. During reflux, the mixture is heated any vapour produced condenses back into a liquid due to the condenser which is placed vertically above the mixture flask and drops back down into the reaction mixture
How do you seperate ketones from reaction mixture
Using distillation.
where the mixture is heated, any vapour produced condensed into a liquid and pours into the new flask to seperate the mixture.
the Condenser is places horizontally between the new flash and the reaction mixture. A thermometer is placed in the mixture flask to record the temperature change as ketones are volatile.
What’s the difference between reflux and distillation ?
reflux don’t have a thermometer or a second flask to seperate the products
Reflux occurs to produce primary alcohols and ionise them with our reagent excess potassium dichromate which then gets ionised to aldehydes and carboxylic acids if the aldehyde stay for longer to get ionised
distillation is where secondary alcohols and get mixed with our reagent excess potassium dichromate & het ketones then are and get separated to get ketones alone
What’s the chemical test for alcoholls if it’s primary secondary or tertiary
1) to identify if the alcohol is tertiary we mix it with acidified potassium dichromate. the colour of the tertiary will remain orange as it can’t be oxidised whilst the 1st n 2nd will turn green
2) to identify which is 1st and 2nd alcohol we add Tollens reagent
when added to an aldehyde a silver mirror will form whereas the ketone will remain its colour
so silver mirror = primary
no colour change = secondary
1) primary
2) tertiary
( as it remained orange when acidified potassium dichromate got added - tertiary)
what is benedict’s / fehlings when added to a solution to identify primary and secondary alcohols
when either benedict or fehlings solution are added to an aldehyde, solution turns red
when it’s an ketone there’s no colour change
so red = primary alcohol
no change= secondary alcohol
Summary, using acidified potassium dichromate
primary alcohol are oxidised to…(1)
which can be further oxidised to… (2)
(3)…… alcohols are oxidised to …..(4)
1) aldehyde
2) carboxylic acids
3) secondary
4) ketones
