3.3.5- Alcohols (PAPER 2) Flashcards
What are alcohols?
Homologous series containing hydrocarbon chain joined to OH functional group.
Named using suffix -ol but if more important functional groups present (such as carboxylic acid), the prefix hydroxy- is used.
eg 2-hydroxypropanoic acid
What are the physical properties of alcohols?
O bonded to end C, H bonded to O
Soluble in water as O directly bonded to H so H bonding occurs.
Higher boiling point than same sized alkanes due to presence of H bonding.
Boiling point increases with chain length as VDW’s forces increase.
Become insoluble past certain chain length as C chain interrupts H bonding.
What are the 2 industrial methods of ethanol production?
Fermentation of glucose or hydration of alkene in presence of acid catalyst.
What are the conditions for fermentation of glucose?
Low temp around 35C
Low pressure
Yeast enzyme/ lack of oxygen
What is a biofuel?
A fuel made from plant materials- eg ethanol made from sugars obtained from crops like sugar cane.
What is the equation for complete combustion of ethanol?
C2H5OH + 3O2 —-> 2CO2 + 3H2O
What is the equation for fermentation of glucose?
C6H12O6 —-> 2C2H5OH + 2CO2
What are the strengths of fermentation of glucose to produce ethanol?
Sugars come from renewable resources.
Low temperature requirements so inexpensive- so enzymes do not denature.
Carbon-neutral process: photosynthesis of plants containing sugar removes CO2 from air, fermentation produces CO2.
What are the weaknesses of fermentation of glucose to produce ethanol?
Ethanol formed is in an aqueous solution so heat energy needed for fractional distillation of the solution- expensive.
Slow ROR- low temperature
Renewable crops require large SA to grow.
What are the conditions for hydration of ethene?
High temp of 300C
High pressure of 60-70kPa
Presence of phosphoric acid catalyst.
Write equations to support the statement that ethanol produced by fermentation is a carbon neutral fuel.
Removal of CO2 by photosynthesis
6CO2 + 6H2O➜ C6H12O6 + 6O2
Production of CO2 by fermentation and combustion
C6H12O6 ➜ 2C2H5OH + 2CO2
2CH3CH2OH + 6O2➜ 4 CO2 + 6 H2O
What is the equation for hydration of ethene?
CH2=CH2 + H2O ⇌ (H3PO4) CH3CH2OH
What are the strengths of hydration of ethene to produce ethanol?
Ethanol formed is pure so no separation required after.
High rate of reaction.
Process is continuous so ethene and steam added automatically.
What are the weaknesses of hydration of ethene to produce ethanol?
Ethene obtained from cracking of crude oil which requires a large amount of energy.
Crude oil is non-renewable so cannot be restored once used up.
Requires high temp and pressure- both costly.
Draw the formation of ethanol from ethene and H3PO4= HYDRATION
Curly arrow from double bond to H+ in H3PO4.
H joins to C, 2nd C is carbocation.
Curly arrow from :O which is joined to 2H.
O binds to carbocation with 2H attached, has + charge.
1H+ removed, oxygen no charge.
Draw the formation of ethene from ethanol and acid catalyst= DEHYDRATION/ ACID CATALYSED ELIMINATION.
O on ethanol has 2 lone pairs, curly arrow from one lone pair to H+
H attached to O, curly arrow from C-O bond to O and H on adjacent C atom to C-C bond.
Double bond forms, water molecule removed.
What is the difference between primary, secondary and tertiary alcohols?
Primary= OH group attached to C with one other C attached.
Secondary= OH attached to C with 2 other C attached.
Tertiary= OH group attached to C with 3 other C attached.
What is oxidising agent for alcohols?
Acidified potassium dichromate
How are primary alcohols oxidised?
To aldehydes which can be further oxidised to carboxylic acids.
Write equations for oxidation of ethanol to ethanal and ethanal to ethanoic acid.
CH3CH2OH + (O) —–> CH3CHO + H2O
CH3CHO + (O) —–> CH3COOCH
What is the colour change when going from alcohol to aldehyde to carboxylic acid?
orange- dark green.
Method used to obtain an aldehyde from a primary alcohol?
Distillation apparatus
Lower conc of acidified potassium dichromate or excess alcohol
Reagent= acidified potassium dichromate
aldehyde boils at lower temp than alcohol so can be distilled off.
Method used to obtain a carboxylic acid from a primary alcohol?
Reflux apparatus
More concentrated acidified potassium dichromate/ in excess.
Reagent= acidified potassium dichromate.
How are secondary alcohols oxidised?
To ketones
Write an equation for the oxidation of propan-2-ol to form propanone.
C₃H₇OH + [O] → C₃H₆O + H₂O
Method used to obtain a ketone from a secondary alcohol?
Reflux apparatus
More concentrated acidified potassium dichromate/ in excess.
Ketone separated from reaction mixture by distillation.
Are tertiary alcohols oxidised?
No, acidified potassium dichromate remains orange.
What are the 2 ways of distinguishing between aldehydes and ketones?
1) Tollen’s reagent
ammonia mixed with silver nitrate solution.
ketone shows no visible change but aldehyde forms a silver mirror as silver ions are reduced- aldehyde is also oxidised to a carboxylic acid.
2) Fehling’s solution
oxidises the aldehyde to give brick red precipitate of Cu (I) oxide.
ketone shows no visible change.
THESE DO NOT OXIDISE ALCOHOLS.
