Alcohols 1
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What is the general formula of an alcohol
CnH2n+1OH or ROH
Describe and explain the molecular ‘shape’ of alcohols
• The oxygen atom has two bonding pairs and two lone pairs • The C-O-H angle is about 105 degrees because the 109.5 angle of the tetrahedron is ‘squeezed down’ by the presence of lone pairs. • These two lone pairs will repel each other more than the pairs of electrons in a covalent bond.
What is a primary alcohol
An alcohol where the carbon that contains the OH functional group only has one R group attached to it. Therefore, the carbon has an OH group, the R group and two hydrogen atoms attached.
What is a secondary alcohol
A secondary alcohol is an alcohol where the carbon containing the OH functional group has two R groups attached to it. The carbon therefore has the OH group, two R groups and one hydrogen attached.
What are tertiary alcohols
Tertiary alcohols have three R groups attached to the carbon atom that the OH functional group is attached to. This carbon therefore has no hydrogen atoms attached to it.
Why do alcohols have higher boiling points than alkanes with a similiar relative molecular mass
The -OH group means that hydrogen bonding occurs between the molecules.
Explain why short chain alcohols are soluble in water but long chain alcohols are insoluble in water
The -OH group in alcohols can form hydrogen bonds with water molecules but the non-polar hydrocarbon chain cannot. This means that the alcohols with short hydrocarbon chains are soluble because the hydrogen bonding predominates. In long chain alcohols the non-polar hydrocarbon chain dominates and this makes these alcohols insoluble in water.
Describe the solubility of alcohols in water
Short chain alcohols are soluble in water but long chain alcohols are not.
What are the two methods of producing ethanol
Fermentation Hydration of ethene
What is the equation for the production of ethanol by fermentation from glucose
C6H12O6 —> 2C2H5OH + 2CO2
What is the equation for the production of ethanol by the hydration of ethene (reaction of ethene with steam)
C2H4 + H2O —> C2H5OH
What are the conditions needed for the production of ethanol via fermentation of glucose
Temperature= 35 degrees Celsius Yeast present
What are the conditions needed for the production of ethanol via hydration of ethene
• Temperature= High temp of 300 degrees Celsius • Pressure= 65 atmospheres • Catalyst= Phosphoric acid H3PO4
Compare the raw materials needed for the production of ethanol via fermentation vs hydration of ethene
The raw material needed for fermentation is sugar (glucose) which is renewable whereas the raw material for hydration of ethene is oil which is non-renewable.
Compare the type of process in the production of ethanol via fermentation vs hydration of ethene
Fermentation is a batch process and is ‘carbon neutral’ whereas the hydration of ethene is a continuous process.
Compare the reaction rate/workers needed in the production of ethanol via fermentation vs hydration of ethene
The reaction rate of fermentation is slow and many workers are needed whereas the reaction rate of the hydration of ethene is fast and few workers are needed.
Compare the product made when obtaining ethanol via fermentation vs hydration of ethene
Fermentation produced impure ethanol (15%) whereas the hydration of ethene produces pure ethanol.
Why is air kept out of the fermentation vessels when producing ethanol
To prevent the ethanol oxidising to ethanolic acid.
Why does fermentation produce an impure ethanol solution of 15%
Because once the solution contains 15% ethanol, the enzymes in yeast can no longer function and fermentation stops. The ethanol can be distilled from the solution via fractional distillation.
What is the equation for the complete combustion of ethanol
C2H5OH + 3O2 —> 2CO2 +3H2O
Why are the elimination reaction in alcohols always dehydrations
Because a molecule of water is always lost.
By which two methods can alcohols be dehydrated
• By being passed over hot aluminium oxide • dehydrated using excess of a hot concentrated acid catalyst (sulphuric acid or phosphoric acid)
What happens when alcohols are passed over hot aluminium oxide
A molecule of water is lost. There is no mechanism- just drawn out formulas.
Describe what happens when alcohols are dehydrated using a concentrated acid catalyst
• the lone pair of electrons on the oxygen atom are attracted to the positive hydrogen atom from the acid catalyst. • The lone pair forms a covalent bond to the hydrogen ion. • An intermediate molecule is formed with a positive oxygen atom. • The pair of electrons in the covalent bond between the carbon and oxygen atom move onto the oxygen atom which causes a molecule of water to be released. • At the same time, a pair of electrons between a carbon and hydrogen now move between the two carbon atoms. The hydrogen is released as a hydrogen ion and a double bond is formed between the two carbon atoms.