3.5 Alcohols

Question 1

What is the functional group of an alcohol?

Answer 1

Hydroxyl group -OH

Question 2

What is the general formula of an alcohol?

Answer 2

CnH2n+1OH

Question 3

How do you name alcohols (one prefix, one suffix)?

Answer 3

Hydroxyl- or -OH

Question 4

What kind of intermolecular forces do alcohols have? Why?

Answer 4

Hydrogen bonding, due to the electronegativity difference in the OH bond

Question 5

How do alcohols’ m.p. And b.p. Compare to other hydrocarbons’ of similar C chain lengths? Why?

Answer 5

Higher, because they have hydrogen bonding → stronger than Van der Waals forces

Question 6

Are alcohols soluble in water? Why does solubility depend on chain length?

Answer 6

Soluble when short chain - OH hydrogen bonds
to hydrogen bond in water
Insoluble when long chain - non-polarity of C-H
bond takes precedence

Question 7

How can ethanol be made from crude oil?

Answer 7

Hydration of ethene via electrophilic addition (phosphoric acid catalyst H₃PO₄)

Question 8

What are the advantages and disadvantages of hydration of ethene via electrophilic addition method?

Answer 8

Advantages - fast, continuous process, ethanol has a high purity
Disadvantages - not renewable as from crude oil

Question 9

How can ethanol be made by fermentation?

Answer 9

Plant carbohydrates broken down and fermented by enzymes in yeast → ethanol

Question 10

What conditions are needed for fermentation to take

place?

Answer 10

Enzymes in yeast as catalyst, 35°C, anaerobic conditions

Question 11

What are the advantages and disadvantages of fermentation?

Answer 11

Advantages - renewable as from plants
Disadvantages - slow, batch process, enzymes stop working at 15% alcohol so solution is not pure, needed to be fractionally distilled

Question 12

In the future, how might most ethene be made? Why is it not made like this at the moment?

Answer 12

Dehydrate ethanol made by fermentation → ethene

Not economical at the moment

Question 13

Define carbon neutral

Answer 13

No net addition of CO2 to the atmosphere - carbon dioxide released when combusted = carbon dioxide absorbed as a plant

Question 14

Explain how using ethanol in petrol engines could be considered to be carbon neutral.

Answer 14

Carbon dioxide released in fermentation and combustion = carbon dioxide absorbed when growing

Question 15

Why would it probably not be entirely carbon neutral to use ethanol?

Answer 15

Other “carbon costs” associated with it e.g. Transport

Question 16

What is a commercial fuel that uses ethanol? What else does it contain and why?

Answer 16

Methylated spirits - methanol (toxic, so it can’t be drunk)

Question 17

What is an elimination reaction?

Answer 17

The removal of a smaller molecule from a larger one

Question 18

Which group leaves the parent molecule in the case of alcohols?

Answer 18

OH and a H (to form water)

Question 19

What physical conditions are needed for the elimination reaction from alcohols to alkenes? (2 alternatives)

Answer 19

Excess hot concentrated sulphuric acid or pass vapour over hot aluminium oxide

Question 20

What forms if you partially oxidise a primary alcohol?

Answer 20

Aldehyde

Question 21

What conditions are needed to partially oxidise a primary alcohol?

Answer 21

Dilute sulphuric acid, potassium dichromate (VI), distill product as it’s produced, gentle heating

Question 22

What forms if you fully oxidise a primary alcohol?

Answer 22

Carboxylic acid

Question 23

What conditions are needed to fully oxidise a primary alcohol?

Answer 23

Concentrated sulphuric acid, potassium dichromate (VI), reflux for about 20 mins, strong heating

Question 24

What forms if you oxidise a secondary alcohol?

Answer 24

Ketone

Question 25

Why can it not be oxidised further and why can’t a tertiary alcohol be oxidised?

Answer 25

A carbon-carbon bond would have to break

Question 26

What conditions are needed for the oxidation of a secondary alcohol?

Answer 26

Concentrated sulphuric acid, potassium dichromate (VI), reflux for about 20 mins, strong heating

Question 27

What does the Tollens’ test give a positive result for?

Answer 27

Aldehydes

Question 28

What is in Tollens’ reagent? How does this react with the substance to be tested?

Answer 28

Silver nitrate in NH3 (aq) - oxidises aldehydes but
not ketones

Complex silver (I) ions reduced to Ag(s)

Question 29

How do you carry out the Tollens’ test?

Answer 29

Add equal volumes of substance being tested and tollen’s reagent to a test tube, leave in water bath for 10mins and observe any changes

Question 30

What is the result of the Tollen’s test for aldehydes and ketones?

Answer 30

Aldehydes - silver mirror forms (solid Ag) Ketones - no visible change

Question 31

What does Fehling’s solution give a positive test result for?

Answer 31

Aldehydes

Question 32

What is in Fehling’s? How does this react with the substance to be tested?

Answer 32

Blue copper (II) complex ions - gentle oxidising agent
Reduced to Cu+ ions (brick red)

Question 33

What conditions do you need to use the Fehling’s

solution?

Answer 33

Heat

Question 34

What result do aldehydes and ketones give in the Fehling’s test?

Answer 34

Aldehydes- brick red ppt

Ketones- no visible change