Alcohols

Question 1

General formula of alcohols

Answer 1

Cn H2n+1 OH

Question 2

Primary alcohol

Answer 2

The carbon bonded to the OH group is only bonded to one other carbon atom

Question 3

Secondary alcohols

Answer 3

The carbon bonded to the OH group is bonded to two other carbon atoms

Question 4

Tertiary alcohols

Answer 4

The carbon bonded to the OH group is bonded to three other carbon atoms

Question 5

Physical properties of alcohols

Answer 5

The OH group means hydrogen bonding occurs between the molecules so have a higher melting point than alkanes of similar mass. The OH group can bond to water molecules but the non-polar hydrocarbon chain can’t so longer alcohols are harder to dissolve in water.

Question 6

Mechanism for hydration (addition) of ethene

Answer 6

Uses phosphoric acid catalyst. Arrow from double bond to H+ (of catalyst) forming carbocation. Arrow from lone pair on oxygen (of water) to positive carbon so water joins on. Arrow from O-H bond to positive oxygen. Forms alcohol with lone pair on oxygen and H+ separate.

Question 7

Conditions for fermentation

Answer 7

Carbohydrates broken down by yeast to ethanol and CO2. Anaerobic conditions to prevent oxidation of ethanol to vinegar. 35C for fastest rate without enzymes being ineffective.

Question 8

Compare crude oil to fermentation for producing ethanol

Answer 8

Crude oil is faster, continuous (not batch), makes essentially pure ethanol (not aqueous solution of 15% ethanol)

Question 9

What is an elimination reaction?

Answer 9

A reaction in which a small molecule leaves the parent molecule

Question 10

Mechanism for dehydration (elimination)

Answer 10

Can use excess hot concentrated sulfuric acid or pass vapour over heated aluminium oxide. Arrow from lone pair on oxygen to H+ (of acid) so H joins to O making O+. Arrow from C-O bond to O+ so water molecule leaves and forms carbocation. Arrow from one further C-H bond to C-C bond. Water molecule and H+ leaves. Left with alkene. To be dehydrated the alcohol needs a hydrogen atom on a carbon next to the OH group.

Question 11

What can primary alcohols be oxidised to?

Answer 11

Aldehydes. Further oxidation to carboxylic acid if enough oxygen.
In equation, the oxidising agent is written as [O]

Question 12

What can secondary alcohols be oxidised to?

Answer 12

Ketones. No further oxidation.

Question 13

What can tertiary alcohols be oxidised to?

Answer 13

Nothing. Can only be burned.

Question 14

Testing for tertiary alcohols

Answer 14

Add potassium dichromate, acidified with sulfuric acid, to alcohol and warm. If it goes orange to green it means the chromium was reduced and the alcohol was primary or secondary. If it stays orange the alcohol was tertiary.

Question 15

How to oxidise primary alcohol to aldehyde

Answer 15

Add dilute acid and less potassium dichromate than needed for full oxidation to carboxylic acid. Heat mixture to boiling point of aldehyde in distillation apparatus until no more aldehyde is distilled (being careful not to heat above the boiling point of the rest of the mixture).

Question 16

How to oxidise primary alcohol to carboxylic acid

Answer 16

Add concentrated sulfuric acid and more than enough potassium dichromate for complete oxidation. Heat mixture in reflux apparatus for prolonged period of time and distill off ethanoic acid.

Question 17

Apparatus for distillation

Answer 17

Mixture in flask connected to thermometer above it. Condenser joined at an angle (pointing downwards) with water jacket around it (water in below water out). Bulb of thermometer is at junction with condenser. Condenser leads to cooled conical flask. (Cooled to reduce evaporation of product).

Question 18

Apparatus for reflux

Answer 18

Mixture in flask with condenser connected directly above it with water jacket around it (water in below water out). Idea is that vapour condenses and drips back into flask to continue reacting.

Question 19

Describe the Tollens’ reagent test

Answer 19

Tollens’ reagent is a gentle oxidising agent. It is a solution of silver nitrate in aqueous ammonia so contains Ag+ ions. When warmed with an aldehyde, it oxidises the aldehyde so the silver ions are reduced to metallic silver meaning a deposit of this is formed on the inside of the test tube which is known as a silver mirror. The reagent doesn’t oxidise ketones so a silver mirror doesn’t form with them.

Question 20

Describe the Fehling’s solution test

Answer 20

Fehling’s reagent is a gentle oxidising agent. It contains blue Cu2+ ions. When warmed with an aldehyde, it oxidises the aldehyde and the copper ions are reduced to Cu+ ions meaning the solution gradually changes from blue to brick red as the precipitate of Cu(I) oxide forms. The reagent does not oxidise ketones so there is no colour change with them.

Question 21

General equation for oxidation of primary alcohol to aldehyde

Answer 21

ROH + [O] -> RCHO + H2O

Question 22

General equation for oxidation of primary alcohol to carboxylic acid

Answer 22

ROH + 2[O] -> RCOOH + H2O

Question 23

Purpose of anti-bumping granules

Answer 23

Only allows small bubbles to form (stops large bubbles forming) to prevent vigorous boiling.

Question 24

Equations for carbon neutrality of biofuels

Answer 24

Photosynthesis of growing plant takes in 6CO2. Fermentation releases 2CO2. Combustion of ethanol releases 4CO2.

Question 25

Define carbon neutral

Answer 25

A process or series of processes in which as much CO2 is absorbed from the air as is given out.