3.3.5 Alcohols Flashcards
What is the functional group of the alcohol homologous series?
OH
Hydroxyl group
What is the difference between primary, secondary and tertiary alcohols?
Primary- one alkyl group attached to the carbon that is attached to the OH
Secondary- two alkyl group attached to the carbon that is attached to the OH
Tertiary- three alkyl group attached to the carbon that is attached to the OH
What is the trend in boiling points of alcohols?
The boiling point increases as the size of alcohols increases.
The size of the van-der-waals forces between molecules increases
What is the difference between boiling points of alcohols and alkanes and why?
The boiling points of alcohols is higher than alkanes of similar molecular mass.
The presence of oxygen results in alcohols being polar molecules.
There is also hydrogen bonding which is stronger than van der waals forces.
Why are alcohols soluble in water?
They hydrogen bond due to presence of oxygen.
Why are larger alcohols less soluble?
There becomes greater proportion of the molecule which isn’t polar.
Why is hydrogen bonding weaker in tertiary alcohols?
Steric hindrance means other alkyl groups get in the way of hydrogen bonding.
The OH in primary alcohol allows other molecules to come closer to increases the strength of hydro-green bonding.
Why is hydrogen bonding weaker in tertiary alcohols?
Steric hindrance means other alkyl groups get in the way of hydrogen bonding.
The OH in primary alcohol allows other molecules to come closer to increases the strength of hydro-green bonding.
What is the bond angle of alcohols and why?
105*
The oxygen in the OH group has 2 bonding pairs and 2 lone pairs
Lone pairs repel more strongly.
What is dehydration of alcohols used for?
To produce alkenes from alcohols
Advantages of dehydration of alcohols compared to producing alkenes using crude oil
It is sustainable if the alcohols was made via fermentation of glucose from plants
Normally alkenes are made from crude oil which is non-renewable so this is more sustainable when making polymers and plastics
What is dehydration?
Elimination of a water molecule
What are the catalysts for dehydration of alcohols?
Aluminium oxide OR sulphuric/phosphoric acid catalyst
What mechanism illustrates the dehydration of alcohols?
Elimination
Why does dehydration of secondary or tertiary alcohols produce multiple products?
Because the C=C double bond can form either side of the C that had the OH group
Why is distillation needed after dehydration of alcohols?
As we get impure product so used to separate the useful alkene
How does distillation separate different alkenes?
By boiling point
What is hydration of alkenes used for?
To produce alcohols
What are the conditions for hydration of alkenes?
High pressure of 60 atm
High temperature of 300*c
Steam
Acid catalyst (concentrated phosphoric)
What are the advantages of producing alcohols via direct hydration?
Continuous process that makes pure ethanol
Fast rate of reaction
What is fermentation?
An alternative way to produce alcohols using glucose from plants
What are the conditions for fermentation and why?
Anaerobic conditions (no oxygen)
Yeast (enzyme to convert glucose to ethanol and CO2)
Temperature of 30-40*c - optimum temperature for yeast to operate / not denature
How is pure ethanol obtained after fermentation?
Fractional distillation
Give reasons why/why not fermentation is carbon neutral?
-number of moles of carbon used is the same as the number of moles of carbon produced when burnt so no net carbon change
-however fuels are burnt to transport biofuels and machinery to harvest crops so increased carbon emissions.
What is a biofuel?
Fuel made from dead biological matter eg. Ethanol made by fermentation of glucose
Advantages of using biofuels?
Renewable
More sustainable than crude oil
Classed as carbon neutral
Disadvantages of using biofuels?
Expensive to convert petroleum engines to take fuels with a higher concentration of ethanol
Land used to make fuel that could have been used to grow food so could cause food shortages in countries that grow sugarcane (ethical issues)
Advantages and disadvantages of using fermentation over hydration to produce alcohols?
Renewable so sustainable
Classed as carbon neutral
Slow rate of reaction
Batch process
What can alcohols be oxidised into?
Aldehydes
Ketones
Carboxylic acids
What is the oxidising agent used to oxidise alcohols?
Acidified potassium dichromate
K2Cr2O7
What is the colour change when potassium dichromate oxidises alcohols?
Orange - green (as dichromate ions are reduced to form chromium ions)
What can primary alcohols be oxidised into?
Aldehydes and then carboxylic acids
How are primary alcohols oxidised?
Oxygen reacts with 2 hydrogen atoms to form water and an aldehyde
Oxygen then reacts with a hydrogen to form OH and therefore a carboxylic acid.
What are secondary alcohols oxidised into?
Ketones
How are secondary alcohols oxidised/
Oxygen reacts with one hydrogen and the hydrogen in the OH group to form water and a ketone.
Can tertiary alcohols be oxidised? Why?
Not using potassium dichromate, but can be by burning but it produces no useful product.
Because there are no hydrogens attached to the carbon attached to the OH for the oxygen to react with.
What are anti-bumping granules?
Prevent bubbling during distillation and reflux
Why is distillation used when oxidising primary alcohols?
To remove aldehyde from the oxidising agent so that it does not further oxidise to form carboxylic acids.
Aldehydes have a lower boiling point than alcohols so evaporate out to form the distillate.
Why is reflux used when oxidising primary alcohols?
Once aldehydes form, they evaporate, condense and fall back into the flask so they are further oxidised to form carboxylic acids.
What are 2 reagents used to distinguish between aldehydes and ketones?
Fehling’s solution and tollen’s reagent
Why can Fehling’s and tollen’s distinguish between aldehydes and ketones?
They are both oxidising agents that oxidise aldehydes but not ketones.
How does Fehling’s solution distinguish between an aldehyde and a ketone?
It is blue as it contains copper Cu2+ ions
Add it to solution and warm
Aldehydes are oxidised, so copper ions are reduced to Cu+ copper (1) ions in copper oxide (brick red)
The colour changes from blue to brick red if aldehydes are present
No colour changes if ketones are present
How does Tollen’s reagent distinguish between an aldehyde and a ketone?
It contains an active complex of silver ions [Ag(NH3)2]+
Add to solution and warm
Silver coats the test tube to produce a silver mirror in aldehydes
No silver precipitate will form in ketones
