3.3.5- Alcohols (PAPER 2)

Question 1

What are alcohols?

Answer 1

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

Question 2

What are the physical properties of alcohols?

Answer 2

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.

Question 3

What are the 2 industrial methods of ethanol production?

Answer 3

Fermentation of glucose or hydration of alkene in presence of acid catalyst.

Question 4

What are the conditions for fermentation of glucose?

Answer 4

Low temp around 35C
Low pressure
Yeast enzyme/ lack of oxygen

Question 5

What is a biofuel?

Answer 5

A fuel made from plant materials- eg ethanol made from sugars obtained from crops like sugar cane.

Question 6

What is the equation for complete combustion of ethanol?

Answer 6

C2H5OH + 3O2 —-> 2CO2 + 3H2O

Question 7

What is the equation for fermentation of glucose?

Answer 7

C6H12O6 —-> 2C2H5OH + 2CO2

Question 8

What are the strengths of fermentation of glucose to produce ethanol?

Answer 8

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.

Question 9

What are the weaknesses of fermentation of glucose to produce ethanol?

Answer 9

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.

Question 10

What are the conditions for hydration of ethene?

Answer 10

High temp of 300C
High pressure of 60-70kPa
Presence of phosphoric acid catalyst.

Question 11

Write equations to support the statement that ethanol produced by fermentation is a carbon neutral fuel.

Answer 11

Removal of CO2 by photosynthesis
6CO2 + 6H2O➜ C6H12O6 + 6O2

Production of CO2 by fermentation and combustion
C6H12O6 ➜ 2CH3CH2OH + 2CO2
2CH3CH2OH + 6O2➜ 4 CO2 + 6 H2O

Question 12

What is the equation for hydration of ethene?

Answer 12

CH2=CH2 + H2O ⇌ (H3PO4) CH3CH2OH

Question 13

What are the strengths of hydration of ethene to produce ethanol?

Answer 13

Ethanol formed is pure so no separation required after.

High rate of reaction.

Process is continuous so ethene and steam added automatically.

Question 14

What are the weaknesses of hydration of ethene to produce ethanol?

Answer 14

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.

Question 15

Draw the formation of ethanol from ethene and H3PO4= HYDRATION

Answer 15

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.

Question 16

Draw the formation of ethene from ethanol and acid catalyst= DEHYDRATION/ ACID CATALYSED ELIMINATION.

Answer 16

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.

Question 17

What is the difference between primary, secondary and tertiary alcohols?

Answer 17

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.

Question 18

What is oxidising agent for alcohols?

Answer 18

Acidified potassium dichromate

Question 19

How are primary alcohols oxidised?

Answer 19

To aldehydes which can be further oxidised to carboxylic acids.

Question 20

Write equations for oxidation of ethanol to ethanal and ethanal to ethanoic acid.

Answer 20

CH3CH2OH + (O) —–> CH3CHO + H2O

CH3CHO + (O) —–> CH3COOCH

Question 21

What is the colour change when going from alcohol to aldehyde to carboxylic acid?

Answer 21

orange- dark green.

Question 22

Method used to obtain an aldehyde from a primary alcohol?

Answer 22

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.

Question 23

Method used to obtain a carboxylic acid from a primary alcohol?

Answer 23

Reflux apparatus

More concentrated acidified potassium dichromate/ in excess.

Reagent= acidified potassium dichromate.

Question 24

How are secondary alcohols oxidised?

Answer 24

To ketones

Question 25

Write an equation for the oxidation of propan-2-ol to form propanone.

Answer 25

C₃H₇OH + [O] → C₃H₆O + H₂O

Question 26

Method used to obtain a ketone from a secondary alcohol?

Answer 26

Reflux apparatus

More concentrated acidified potassium dichromate/ in excess.

Ketone separated from reaction mixture by distillation.

Question 27

Are tertiary alcohols oxidised?

Answer 27

No, acidified potassium dichromate remains orange.

Question 28

What are the 2 ways of distinguishing between aldehydes and ketones?

Answer 28

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.