Alcohols

Question 1

what are alcohols?

Answer 1

organic compounds which are characterised by the presence of a hydroxyl (-OH) functional group.

Question 2

how do you name an alcohol?

Answer 2

by using the number of carbon atoms in the longest chain and end with the suffix ‘-ol’.

Question 3

what is the general formula of alcohols?

Answer 3

CnH2n+1OH

Question 4

what are primary alcohols?

Answer 4

primary alcohols are alcohols in which the carbon atom bonded to the -OH group is attached to one other carbon atom (or alkyl group), they are the least stable.

Question 5

what can you say about the flammability of primary alcohols?

Answer 5

primary alcohols are more flammable as the flammability of the alcohols is decreased with an increase in the size and the mass of the molecule.

Question 6

what can you say about the solubility of primary alcohols?

Answer 6

primary alcohols are completely soluble in the water and if two of them are mixed in any proportion, it produces a single solution.

Question 7

what may affect the solubility of alcohols?

Answer 7

an increase in the length of hydrocarbon chain, the solubility is decreased.

Question 8

what are secondary alcohols?

Answer 8

secondary alcohols are alcohols in which the carbon atom bonded to the -OH group is attached to two other carbon atoms (or alkyl groups).

Question 9

what are tertiary alcohols?

Answer 9

tertiary alcohols are alcohols in which the carbon atom bonded to the -OH group is attached to three other carbon atoms (or alkyl groups)

Question 10

which alcohols can be mildly oxidised with acidified K2Cr2O7

Answer 10

primary and secondary alcohols.

Question 11

what do primary alcohols get oxidised into?

Answer 11

primary alcohols get mildly oxidised to aldehydes.

Question 12

what does the further oxidation of aldehydes produces?

Answer 12

carboxylic acid.

Question 13

what do secondary alcohols oxidise into?

Answer 13

secondary alcohols get mildly oxidised to ketones.

Question 14

do tertiary alcohols oxidise, and why?

Answer 14

no, tertiary alcohols do not undergo oxidation under typical conditions because they lack a hydrogen atom attached to the carbon atom bonded to the hydroxyl (-OH) group.

Question 15

explain the colour change when primary and secondary alcohols oxidation?

Answer 15

only the oxidation of primary and secondary alcohols will change the colour of K2Cr2O7 solution as the orange Cr2O72- ions are reduced to green Cr3+ ions.

Question 16

what does their high boiling point affect?

Answer 16

alcohols are less volatile than alkanes due to their high boiling points.

Question 17

what bonding is involved in alcohols?

Answer 17

due to their hydroxyl group, they can form london dispersion forces which are weak intermolecular forces which do not require as much energy to break and hydrogen bonds which are strong intermolecular forces that require large amounts of energy to break.

Question 18

why do alcohols have hydrogen bonding?

Answer 18

alcohols have hydrogen bonding because they contain hydrogen atoms bonded to highly electronegative oxygen atoms, which allows them to form strong hydrogen bonds with other electronegative atoms.

Question 19

in terms of water, how does hydrogen bonding occur for alcohols?

Answer 19

water molecules are polar, with the oxygen atom being more electronegative than the hydrogen atoms. this polarity leads to partial positive (δ+) charges on the hydrogen atoms and a partial negative (δ-) charge on the oxygen atom.
the partially positively charged hydrogen atoms in water molecules can form hydrogen bonds with the lone pairs of electrons on the oxygen atom of alcohol molecules.
similarly, the partially negatively charged oxygen atom in water molecules can form hydrogen bonds with the hydrogen atoms of alcohol molecules.

Question 20

explain the trend in boiling points of alcohols in comparison to alkanes?

Answer 20

due to the presence of hydrogen bonding, the boiling points of alcohols is slightly higher than alkanes with the same number of carbon atoms, but only slightly. this is due to there being more london dispersion forces present than hydrogen bonds.

Question 21

explain the solubility of alcohols?

Answer 21

highly soluble, due to hydrogen bonds being formed with water. but as length of carbon chain increases, the solubility decreases as non-polar carbon chain cannot form hydrogen bonds, it is only the -OH (hydroxyl) functional group.

Question 22

what methods may be used to form alcohols?

Answer 22

electrophilic addition
oxidation
nucleophilic substitution
reduction of aldehydes/ketones
reduction of carboxylic acids
hydrolysis of esters.

Question 23

outline the reagent, conditions and product of electrophilic addition.

Answer 23

reagent- alkene steam
conditions- heat + H3PO4 catalyst
product- alcohol

Question 24

outline the reagent, conditions and product of oxidation.

Answer 24

reagent- alkenes + cold dilute, KMnO4
conditions- shaking reagents
product- diol

Question 25

outline the reagent, conditions and product of nucleophilic substitution.

Answer 25

reagent- haloalkanes + aqueous NaOH
conditions- heat
product- alcohol

Question 26

outline the reagent, conditions and product of reduction of aldehydes/ketones.

Answer 26

reagent- aldehydes or ketones
conditions- NaBH4 or LiAlH4
product- primary/secondary alcohols

Question 27

outline the reagent, conditions and product of reduction on carboyxlic acids.

Answer 27

reagent- carboxylic acid
conditions- NaBH4 or LiAlH4 or H2 using a nickel catalyst and heat
product- primary alcohol

Question 28

outline the reagent, conditions and product of hydrolysis.

Answer 28

reagent- esters
conditions- dilute acid or alkali heat
product- alcohol

Question 29

what does dehydration of alcohols produce?

Answer 29

an alkene.

Question 30

in dehydration what is eliminated and why?

Answer 30

water is eliminated from ethanol as ethanol is heated with a concentrated sulfuric acid catalyst.

Question 31

what is the chemical equation for this dehydration reaction?

Answer 31

C2H5OH → CH2=CH2 + H2O

Question 32

describe the mechanism of a dehydration reaction.

Answer 32

1- a lone pair of electrons from the oxygen bonds to an H+ from the acid. the alcohol is protonated, giving the oxygen a positive charge.
2- the positively charged oxygen pulls electrons away from the carbon. H2O molecule leaves, creating an unstable carbocation intermediate.
3- the carbocation intermediate loses an H+ and an alkene is formed.

Question 33

what may occur within dehydration of longer, unsymmetrical alcohols?

Answer 33

may result in more than one product because the double bond can go on either side of the carbon that had the hydroxyl (-OH) functional group attached to it. (this is because isomerism is shown, specifically position isomerism)

Question 34

how can you purify (remove unwanted by-products or leftover reactants from the reaction mixture) the product of a reaction?

Answer 34

distillation is a technique which uses the fact that different chemicals have different boiling points, thus this can be used to separate them.

Question 35

explain distillation in terms of purification. (potential 6 marker, so parrot this!!!!)

Answer 35

(example of cyclohexene from cyclohexanol used)
1- add conc. H2SO4 amd H2PO4 to a round bottom flask comtaining cyclohexanol. mix the solution by swirling the flask and add carborundum boiling chips (these make the mixture boil more calmly, also called anti-bumping granules).
2- connect the flask to the rest of the distillation apparatus, including a thermometer, condenser and cooled collection flask.
3- gently heat the mixture in the flask to around 82°C(boiling point of cyclohexene, for whatever product the question asks the boiling point will be listed) using a waterbath.
4- chemicals within the specific boiling point will evaporate and warm gas will rise out the flask into the condenser.
5- condenser, which has cold water running through the outside, will cause the gas to convert gas back into liquid.
6- liquid product will then be collected at cooled flask.

Question 36

how can you purify this product collected at the cooled flask even further?

Answer 36

1- transfer the product mixture to a separating funnel and add water to dissolve the water soluble impurities and create an aqueous solution.
2- allow mixture to layer. drain off the aqueous layer at the bottom leaving behind the impure cyclohexene.
3- drain off this into a round-bottomed flask.
4- add anhydous CaCl2 and stopper to the flask. let mixture dry for 20 mins with occasional swirling.
5- cyclohexene may still have small amounts of impurities so distil it again.

Question 37

what does hydrating alkenes with the presence of an acid catalyst produce?

Answer 37

alcohols.

Question 38

what is the general equation for the reaction?

Answer 38

CnH2n + H2O ⇌ CnH2n+1OH

Question 39

list the conditions,drawbacks and positives for steam hydration ?

Answer 39

strong acid catalyst (conc. H3PO4)
high temperatures of 300°C
high pressures of 60 atm
uses a lot of energy
expensive
fast+continuous- higher purity of product.

Question 40

outline the mechanism of hydration of alkenes.

Answer 40

1- a pair of electrons from the double bond bonds to an H+ from the acid. forms a carbocation.
2- a lone pair of electrons form a water molecule bonds to the carbocation.
3- the water loses a H+, as the oxygen is not positively charged, forming an alcohol.

Question 41

list the conditions,drawbacks and positives for fermentation of glucose?

Answer 41

requires no O2
low temperatures of 30-40°C
requires yeast
renewable and low set up cost.
batch process
labour intensive
needs distillation to purify
ethics using food

Question 42

what is the equation for fermentation of glucose?

Answer 42

C6H12O6 → 2C2H5OH + 2CO2

Question 43

what is a biofuel?

Answer 43

made from a biological material that recently died. e.g. sugars from sugar cane can be fermented to form ethanol which can be added to petrol. ethanol formed in this way can be called bioethanol

Question 44

list the advantages of biofuel use.

Answer 44

renewable energy sources- wont run out, thus more sustainable.
carbon neutral- no net release of carbon dioxide into the atmosphere.

Question 45

list the disadvantages of biofuel use.

Answer 45

impact on food supply: leads to an increase in food prices.
environmental problems: deforestation to grow crops for biofuels + release of CO2
fertilisers are often added to soils in order to increases biofuel crop production. these may release nitrous oxides, green house gas.

Question 46

how do you test for aldehydes and ketones?

Answer 46

Fehling’s solution and Benedict’s solution are both deep blue Cu2+ complexes, which reduce to a brick-red Cu2O precipitate wjen warmed with an aldehyde but stay blue with a ketone.
Tollens’ reagent is colourless- it’s reduced to silver when warmed with an aldehyde (silver mirror)

Question 47

how do you test for carboxylic acid?

Answer 47

1- add 2cm^3 of the solution you are testing into a test tube.
2- add 1 small spatula pf solid sodium carbonate.
3- if the solution begins to fizz and bubble put the gas that is produces through some limewater in a second test tube.
4- CO2 should be produced in presence of a carboxylic acid so limewater should turn cloudy.