3.3.5 Alcohols

Question 1

what is the functional group of an alcohol?

Answer 1

R-OH

Question 2

what is the functional group of a diol?

Answer 2

OH-R-OH

Question 3

what is the general formula of an alcohol?

Answer 3

CnH2n+1OH

Question 4

what type of intermolecular forces do alcohols have? why?

Answer 4

hydrogen bonding - H directly bonded to O, difference in electronegativity

Question 5

how do alcohols’ mp and bp compare to other hydrocarbons with the same chain lengths? why?

Answer 5

higher, because they have hydrogen bonding —> stronger than VdW

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 to water
insoluble when long chain - non polarity of C-H bond takes precedence.

Question 7

what makes an alcohol primary?

Answer 7

C bonded to OH is only bonded to one other carbon atom

Question 8

what makes an alcohol secondary?

Answer 8

C bonded to OH is bonded to 2 other C atoms

Question 9

what makes an alcohol tertiary?

Answer 9

C bonded to OH is bonded to three other C atoms

Question 10

how can ethanol be made from crude oil?

Answer 10

hydration of ethene via electrophilic addition

Question 11

what are the advantages and disadvantages of production of ethanol from crude oil?

Answer 11

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

Question 12

how can ethanol be made by fermentation?

Answer 12

plant carbohydrates broken down and fermented by enzymes in yeast —> ethanol

Question 13

what conditions are needed for the fermentation to form ethanol?

Answer 13

enzymes in yeast act as a catalyst
35 degrees
anaerobic conditions

Question 14

write an equation for the fermentation of plant carbohydrates to form ethanol.

Answer 14

C6H12O6(aq) —> 2C2H5OH + 2CO2

Question 15

what are the advantages disadvantages of fermentation of plant carbohydrates to form ethanol?

Answer 15

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 16

define carbon neutral

Answer 16

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

Question 17

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

Answer 17

other “carbon costs” associated with it eg. transport

Question 18

write an equation for the combustion of ethanol.

Answer 18

C2H6O + 3O2 —> 2CO2 + 3H2O

Question 19

what is an elimination reaction?

Answer 19

removal of a smaller molecule from a larger one

Question 20

what leaves the parent molecule when it comes to alcohols?

Answer 20

OH and H (to form water)

Question 21

what physical conditions are needed for the elimination reaction from alcohols to alkenes?

Answer 21

excess hot conc sulphuric acid or pass hot vapour over hot aluminium oxide.

Question 22

what forms if you partially oxidise a primary alcohol?

Answer 22

an aldehyde

Question 23

what conditions are needed to partially oxidise a primary alcohol?

Answer 23

dilute sulphuric acid, potassium dichromate, distill product as it’s produced, gentle heating

Question 24

write an equation for the partial oxidation of ethanol.

Answer 24

C2H6O + [O] —> C2H4O + H2O

Question 25

what forms if you fully oxidise a primary alcohol?

Answer 25

a carboxylic acid

Question 26

what conditions are needed to fully oxidise a primary alcohol?

Answer 26

conc. sulphuric acid, potassium dichromate, reflux for about 20 minutes, strong heating

Question 27

write an equation for the full oxidation of ethanol.

Answer 27

C2H6O + 2[O] —> CH3COOH + H2O

Question 28

what forms if you oxidise a secondary alcohol?

Answer 28

a ketone

Question 29

why can’t a secondary alcohol be oxidised further than a ketone? why can’t a tertiary alcohol be oxidised?

Answer 29

a carbon- carbon bond would have to break

Question 30

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

Answer 30

conc sulphuric acid, potassium dichromate, reflux for around 20 mins, strong heating

Question 31

write an equation for the oxidation of propan-2-ol.

Answer 31

C3H8O + [O] —> CH3COCH3 + H2O

Question 32

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

Answer 32

aldehydes

Question 33

what is in Tollens’ reagent? how does this react with the substance it is testing for?

Answer 33

silver nitrate in NH3(aq) - oxidises aldehydes but not ketones
complex silver (I) reduced to Ag(s)

Question 34

how do you carry out the Tollens’ test?

Answer 34

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

Question 35

what is the result of the Tollens’ test for aldehydes and ketones?

Answer 35

aldehydes - silver mirror (solid Ag)
ketones - no visible change

Question 36

What does Fehling’s sol. give a positive test for?

Answer 36

aldehydes

Question 37

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

Answer 37

Blue copper (II) complex ions - gentle oxidising agent
Reduced to Cu2+ ions (brick red)

Question 38

what conditions are needed to use Fehling’s sol?

Answer 38

heat

Question 39

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

Answer 39

aldehydes - brick red ppt.
ketones - no visible change