alcohols Flashcards
why will wine end up tasting sour if left opened without cork?
This is because the alcohol (ethanol) has been oxidised to a carboxylic acid (ethanoic acid).
how can we distinguish between primary, secondary and tertiary alcohols?
by looking at the reaction of alcohols with oxidising agents such as acidified potassium dichromate (VI)
what is oxidation?
oxidation is regarded as the gain of 1
oxygen atom (or the loss of 2 hydrogen atoms).
what is an oxidising agent?
The chemical that supplies the oxygen (or removes the hydrogen)
what symbol shows 1 atom of oxygen? or shows the oxidant
In balanced symbol equations [O] is 1 atom of oxygen.
what is a primary alcohol oxidised to form?
to form an aldehyde.
the aldehyde can then be further oxidised into a…?
By adding..?
carboxcylic acid
Excess potassium dichromate
give the full displayed formula reaction when ethanol is warmed with excess acidified potassium dichromate.
ethanol + [O] ——> ethanal and water
CH3CH2OH + [O] → CH3CHO + H2O
then
ethanal + [O] ———-> ethanoic acid
CH3CHO + [O] → CH3COOH
what is the overall reaction for the above reaction?
CH3CH2OH + 2 [O] → CH3COOH + H2O
ethanol + 2 [O] —–> carboxylic acid + water
what are the reaction conditions for the oxidation of primary alcohols?
K2Cr2O7 + (H2SO4) dilute and warm sulphuric acid
what colour change is observed when primary alcohols are oxidised using acidified potassium dichromate (VI)?
The orange solution of potassium dichromate (VI) changes to
a green solution containing chromium (III) ions.
how can we control the oxidation of primary alcohols?
By altering the reaction conditions
How do we control the oxidation of a primary alcohol to give us an aldehyde?
using distillation apparatus
How do we control the oxidation of a primary alcohol to give us a carboxylic acid?
Alcohol is oxidised under reflux
if a primary alcohol is oxidised using distillation apparatus…?
why does this work?
then any aldehyde formed will distil off immediately from the mixture.
This works because the aldehyde boiling point is lower than the corresponding alcohol and carboxylic acid.
if a primary alcohol is oxidised under reflux, then…?
why does this work?
the aldehyde cannot escape and is oxidised to form a carboxylic acid.
When reactants are heated under reflux, any volatile components will rise up the condenser, get condensed and
then return to the reaction mixture.
what are secondary alcohols oxidised to?
ketones
give the full displayed formula reaction when butan-2-ol is warmed with acidified potassium dichromate.
makes butanone
why can aldehydes be further oxidised but not ketones?
Aldehydes can be oxidised further because there is still a hydrogen atom attached to the carbon with the C=O bond.
Ketones do not have this hydrogen atom.
what are the reaction conditions for the oxidation of secondary alcohols
K2Cr2O7 + dilute H2SO4 + warm
When secondary alcohols are oxidised using
acidified potassium dichromate (VI) what colour change is observed?
The orange solution of potassium dichromate (VI) changes to a green solution containing chromium (III) ions.
what observation is made from tertiary alcohols and oxidising agents?
Tertiary alcohols are not oxidised by acidified
dichromate (VI) ions.
Therefore, no colour change is observed – the
solution stays orange.
why do tertiary alcohols have no reaction?
In contrast to 1° and 2° alcohols, 3° alcohols do not have a hydrogen atom bonded to the carbon carrying the ―OH group.
Such atoms are removed as part of the oxidation process – thus 3° alcohols are not oxidised under normal conditions.
Aldehydes and ketones
What does the carbonyl, aldehyde and ketone group look like?
- C=O
Which two homologous series have the same general formula?
Aldehydes and ketones
CnH2nO
What is the difference between the aldehydes and ketones?
The aldehydes have a hydrogen atom attached to the carbonyl carbon atom
Where do aldehydes have their carbonyl group n the carbon chain?
At the end
Where do ketones have their carbonyl group in the carbon chain?
The middle
Higher ketones have …? isomers
Positional isomer
Why do aldehydes and ketones show functional group isomerism?
Give an example.
They’re different homologous series but have the same functional groups
E.g C3H6O can be propanal or propanone
How can aldehydes and ketones be distinguished?
By using mild oxidising agents
E.g Tollen’s reagent or Fehling’s solution
what is the active substance in tollens reagent?
[Ag(NH3)2]+. Silver ions in the +1 oxidation state.
how is the active substance made?
The active substance is made by mixing silver nitrate and dilute aqueous ammonia.
what happens when an aldehyde is warmed with Tollens reagent?
When Tollen’s reagent is warmed with an aldehyde the silver ions are reduced (by the aldehyde) to silver metal atoms.
The aldehyde is oxidised to a carboxylic acid.
A silver mirror is produced on the walls of the test tube.
what doesn’t react with Tollens reagent?
so colour remains..?
ketones
colourless
what is Fehling’s solution?
blue solution containing Cu2+ ions.
what happens when an aldehyde is warmed with Fehling’s solution?
When an aldehyde is warmed with Fehling’s solution the Cu2+ ions are reduced to Cu+ ions (Cu2O, copper (I) oxide).
The Cu+ ions in copper (I) oxide are brick red and copper (I) oxide is insoluble.
A red precipitate is produced.
The aldehyde is oxidised to a carboxylic acid.
what doesn’t react with Fehling’s solution?
so colour remains…?
ketones
blue
Fermentation of ethanol
What is fermentation?
In the fermentation process an aqueous solution of plant sugar e.g glucose is converted into ethanol and carbon dioxide by living yeast cells
Write the equation for fermentation?
C6H12O6 —(yeast, 35degrees) -> 2C2H5OH + 2CO2
What does the yeast contain?
The heat contains enzymes which catalyse the fermentation reaction
What are the reaction conditions?
The reaction is carried out in
- approximately 35 degrees C in aqueous solution
- anaerobic conditions ( no oxygen)
What would the presence of oxygen cause?
Cause ethanol to oxidise into ethanoic acid
Why can’t fermentation take place under a high or low temperatures?
A hight temperatures - Yeast cannot survive as the enzymes are denatured
At low temperatures - enzymes in yeast are not activated and the reaction is very slow
What is a batch process?
The reactant are mixed together, once the reaction is over the products are removed and the process is started all over again
How is the ethanol separated from fermented products?
Fractional distillation
What is fermentation used to make?
Alcoholic drinks, bread and biofuel
Compare the advantages and disadvantages of making ethanol by direct hydration of ethene compared to fermentation
Direct hydration - continuous process- cheap on manpower, requires expensive equipment and high energy, reaction if fast, pure ethanol and high yield, finite resource used
Fermentation - batch process- expensive on manpower, cheap equipment low energy, slow reaction, ethanol is impure and low yield, renewable resource used
What type of fuel is ethanol?
Ethanol made by fermentation is a biofuel
What is biofuel?
Biofuels are made from plants or biomass (material derived from plants)
Are biofuels non-renewable or renewable?
Renewable because plants can be grown again quickly
Compare biofuels with fossil fuels
Fossil fuels take millions of years to form and are finite whereas biofuels are quickly grown and renewable
Ethanol as a biofuel can be considered…?
Carbon neutral
What is a carbon neutral activity?
A carbon neutral activity is one that has no net annual carbon (greenhouse gases) emissions to the atmosphere
Why is ethanol biofuel considered carbon neutral?
- amount of carbon dioxide made during ethanol combustion + amount of carbon dioxide released by ethanol formation (by fermentation) = amount of carbon dioxide removed from the atmosphere during photosynthesis
Why is fermentation considered by some people not carbon neutral?
- processing steps require energy
- production of this energy may involve the release of additional carbon dioxide
- the land area used to grow crops for fermentation may lead to deforestation
- trees act as natural stores of co2 and cutting them will release
- also less land available for growing crops for food (developing countries)
Dehydration of alcohols
Alcohols can also undergo…? Reactions
Elimination reactions
Why is this reaction referred to as a dehydration reaction?
Water is eliminated in this reaction
Draw a word equation to show the dehydration of an alcohol to produce an..?
Alcohol —-> (heat/acid catalyst) –> Akene + water
Outline the dehydration of ethanol by elimination
Alkanes made by elimination can be used as..?
Starting material for addition polymerisation reactions
Why is this alternative route to making addition polymers better than the usual route
The usual route starts or with crude oil
Fermentation of plant sugars —> crude oil (fractional distillation) —> alkanes (cracking) —-> alkenes (addition polymerisation) —> polymers
Draw using arrows the alternative route
Alcohols —(dehydration)—> Alkenes —(addition polymerisation) —> polymers
FINISH
