alcohols

Question 1

what is the general formula for the alcohol homologous group?

Answer 1

C_nH_2n + 1OH

Question 2

explain the difference between primary, tertiary and secondary alcohols

Answer 2

depends on which carbon atom the OH group is bonded to

Question 3

explain the naming with more than one OH group

Answer 3

2 - diol

3 - triol etc

Question 4

does a carboxylic acid take precedence over alcohols in naming

Answer 4

yes use the prefix hydroxy

Question 5

what is the equation for fermentation

Answer 5

C₆H₁₂O₆ → 2C₂H₅OH + CO₂

Question 6

what are the conditions for fermentation

Answer 6

yeast (biological catalyst)

anaerobic (without oxygen)

30-40 degrees

aqueous

when the solution reaches about 15% ethanol the yeast dies, fractional distillation to increase the concentration

Question 7

what is the equation for the steam hydration of ethene

Answer 7

CH₂=CH₂ (g) + H₂O (g) ⇔ C₂H₅OH

Question 8

what are the conditions for steam hydration of ethene?

Answer 8

solid phosphoric acid catalyst (H₃PO₄)

300 degrees and 60 atm

at the moment most industrial ethanol is produced by steam hydration of ethene

yield is low - 5%, however as it is a reversible reaction so can reuse unreacted ethene gas making the yield 95%

Question 9

compare the two methods of producing alcohols

Answer 9

fermentation

rate: slow
quality: impure + low yield

raw material: sugars (renewable)

process: batch process, expensive on manpower but cheap equipment

hydration of ethene:

rate: fast
quality: pure + high yield

raw material: ethene, crude oil and is finite

process: continuous process, not much manpower but expensive equipment

Question 10

show the mechanism for making ethanol from ethene

Answer 10

Question 11

explain how ethanol can be used as a biofuel

Answer 11

ethanol burns to give carbon dioxide and water and can be used as a fuel in its own right or in mixtures with petrol (gasoline) to produce ‘gasohol’ containing 10-20% ethanol

the equation for the combustion of ethanol is

C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O

Question 12

why is using ethanol as a biofuel a good idea?

Answer 12

as it can be produced by fermentation it can be useful for countries without an oil industry

eg. `brazil, sugars from sugar canes are fermented to produce alcohol which is added to petrol to produce biofuel (fuel produced from a renewable biological resource)

Question 13

how is ethanol thought to be a carbon-neutral fuel

Answer 13

carbo neutral = an activity that has no net annual carbon emissions to the atmosphere

photosynthesis : 6CO₂ + 6H₂O → 6O₂ + C₆H₁₂O₆

fermentation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
combustion: C₂H₅OH + 6O₂ → 6H₂O + 4CO₂

there is no net gain of carbon dioxide

however may not be carbon neutral due to:

transport, harvesting crops and fertilisers

Question 14

explain what primary, secondary and tertiary alcohols can be oxidised into

Answer 14

primary → aldehyde → carboxylic acid

secondary → ketone

tertiary → X

Question 15

what is a carbonyl compound

Answer 15

anything containing a C=O

the general formula of C_nH_2nO

includes aldehydes and ketones

Question 16

what is an aldehyde

Answer 16

have the suffix ‘-al’ and the function group is always on C1

Question 17

what is a ketone

Answer 17

the suffix -one and function group position must be stated for ketones with 4 or more carbons

Question 18

what is a carboxylic acid

Answer 18

have a function group COOh at the end of their carbon chain

the suffix - oic acid

Question 19

explain the oxidation of primary alcohols to aldehydes

Answer 19

first oxidised to an aldehyde then further to a carboxylic acid

oxidising agent : [O] = H⁺/K₂Cr₂O₇ ( acidified potassium dichromate VI)

conditions: distillation, aldehyde boils off as BP is lower than that of alcohols to prevent further oxidation

C₂H₅OH + [O] (orange) → CH₃CHO + H₂O (ethanal green)

Question 20

explain the oxidation of primary alcohols to carboxylic acids

Answer 20

to produce carboxylic they must be vigorously oxidised

C₂H₅OH + 2[O] (XS) →CH₃COOH + H₂O

conditions: reflux, reflux allows the temp to be increased without losing volatile solvents + reactants or products, air can escape at the top or would be a closed system and would explode

Question 21

explain the oxidation of secondary alcohols

Answer 21

refluxing secondary alcohol with acidified dichromate(VI) will produce a ketone

ketones can’t be oxidised easily so even prolonged refluxing won’t produce anything more

Question 22

explain the oxidation of tertiary alcohols

Answer 22

don’t react with acidified potassium dichromate (IV) at all and the solution stays orange

the only way to oxidise is burning

Question 23

explain the testing for aldehydes and ketones

Answer 23

they can be distinguished using oxidising agents as aldehydes are oxidised easily but ketones aren’t

can be distinguished using Tollen’s reagent or Fehling’s solution - two mild oxidising agents

a positive test indicates the presence of an aldehyde

Tollen’s reagent: [Ag(NH₃)₂]⁴ ammoniacal silver nitrate

-with an aldehyde, you get the silver mirror effect, the aldehyde is oxidised to a carbox and the colourless silver ions are reduced to silver and coats the test tube

Fehling’s solution

Blue solution containing Cu²⁺

aldehyde = brick red precip and decolourise the solution, Cu²⁺ reduced as aldehyde is oxidised

Question 24

explain dehydration of ethanol

Answer 24

ethene can be made by eliminating water from ethanol by a dehydration reaction

CH₃CH₂OH → C₂H₄ + H₂O

conditions: reflux + conc. sulfuric acid catalyst or 600 k and Al₂O₃ catalyst

the reaction happens in two stages:

1) CH₃CH₂OH + H₂SO₄ → CH₃CH₂OSO₂OH + H₂O
2) CH₃CH₂OSO₂OH → CH₂=CH₂ + H₂SO₄

The sulfuric acid is regenerated and chemically unchanged at the end of the reaction so is a catalyst

Question 25

explain the processes of producing alkenes from renewable resources

Answer 25

plants → glucose → ethanol → ethene

(extract) (fermentate) (eliminate)

Question 26

show the mechanism of dehydration of ethanol

Answer 26

Question 27

what must be remembered with dehydration of longer chain or branched alcohols

Answer 27

may produce a mixture of alkenes including ones with Z and E isomers