4.2 Alcohols, Haloalkanes & Analysis

Question 1

What is a primary alcohol?

Answer 1

The OH functional group is attached to a carbon atom with no more than one alkyl group

Question 2

What is a secondary alcohol?

Answer 2

The OH functional group is attached to a carbon atom with two alkyl groups

Question 3

What is a tertiary alcohol?

Answer 3

The OH functional group is attached to a carbon atom attached to three alkyl groups

Question 4

Why does the boiling point for alcohols increase as the chain gets longer?

Answer 4

There are more surface area contacts so more London forces

More energy is required to overcome these forces

Question 5

Why are the boiling points for alcohols higher than corresponding alkanes?

Answer 5

There are hydrogen bonds between the alcohol molecules - strongest type of intermolecular force of attraction

More energy is required to overcome this force so higher boiling point

Question 6

Why are alcohols less volatile than corresponding alkanes?

Answer 6

A substance that is volatile evaporates easily at RTP

Volatility increases as boiling point decreases

Hydrogen bonds make alcohols less volatile than corresponding alkanes

Question 7

Why are alcohols soluble?

Answer 7

Water molecules are polar and so is the OH functional group in alcohols

The alcohol can form hydrogen bonds with water

As alkyl chain length increases, the solubility of the alcohol decreases because the aliphatic chain cannot form hydrogen bonds

Question 8

What is reflux?

Answer 8

The constant boiling and condensing of a reaction mixture to ensure that the reaction goes to completion without losing reactants or products as vapour to be air

Question 9

What is the general formula for alcohols?

Answer 9

CnH2n+1OH

Question 10

Why must the aldehyde be distilled immediately after being made by oxidation of a primary alcohol?

Answer 10

To prevent any further reaction like making a carboxylic acid

Question 11

What are the conditions for oxidising primary alcohols?

Answer 11

Gentle heating = aldehyde and water (colour change from orange to green) [O] DISTIL

Stronger heating under reflux with excess potassium dichromate = carboxylic acid and water (colour change from orange to green) 2[O]

Question 13

What is dehydration?

Answer 13

The chemical reaction in which water is lost from an organic compound - a type of elimination reaction

When water is heated with a strong acid (concentrated sulfuric acid/phosphoric acid - catalyst) water is eliminated to make an alkene from an alcohol

OH group and H atom are lost to form water and pi bond forms between two C atoms

Question 14

What is halide substitution?

Answer 14

When halide ions react with alcohols and substitutes the alcohol group to form a haloalkane

ROH + HX –> RX + H2O

Question 15

What are the conditions for oxidising a tertiary alcohol?

Answer 15

No reaction

A tertiary alcohol is very resistant to oxidation and are not oxidised by common oxidising agents like potassium dichromate

Question 16

What are the conditions for oxidising a secondary alcohol?

Answer 16

Heating under reflux = ketone and water (colour change from orange to green) [O] HEAT

Question 17

What is nucleophilic substitution?

Answer 17

Atoms or group of atoms is exchanged for a nucleophile

Curly arrow from nucleophile to slightly positive carbon atom donating its lone pairs

Carbon-halogen bond breaks by heterolytic fission as two electrons move to halogen forming X-

Question 18

What are nucleophiles?

Answer 18

Species that are electron pair donors

Question 19

How can you measure the rate of hydrolysis (a nucleophilic substitution reaction)?

Answer 19

By measuring how long it takes for a coloured precipitate to form (halogen ion and silver nitrate)

IV - halogen present in haloalkane

DV - time taken for precipitate to form

CV - length of alkyl chain, moles of haloalkane used, concentrations of reactants, volume of reactants, temperature of water bath

Question 19

What are the trends in the rates of hydrolysis?

Answer 19

Bond enthalpy is the most important factor in the hydrolysis of primary haloalkanes

Out of Cl, Br and I, C-Cl has the highest polarity however it has the longest rate of hydrolysis as it has a higher bond enthalpy

The least polar has the highest rate of hydrolysis because the bond is weaker and easier to break

Question 19

What happens when a haloalkane is heated under reflux by an aqueous solution containing hydroxide ions?

Answer 19

An alcohol and halogen anion is made

Rate of reaction is faster than using water

Question 20

What are CFCs?

Answer 20

Chlorofluorocarbons - class of organic compounds that contain chlorine and fluorine atoms

CFCs are gas at room temperature and after use they disperse into the atmosphere

As gases diffuse up layers of the atmosphere, they eventually become exposed to UV radiation in the stratosphere

C-Cl bonds in C2F2Cl2 undergo homolytic fission due to the UV light - C2F2Cl2 –> C2F2Cl’ + Cl’

Question 21

What do CFCs do to the ozone layer?

Answer 21

The ozone layer is in the stratosphere which is a layer of O3 molecules - protects us from harmful UV radiation and stabilises temperature

The chlorine radicals from CFCs can catalyse the decomposition of ozone to make diatomic oxygen

Cl’ + O3 –> ClO’ + O2
ClO’ + O3 –> Cl’ + O2
(Overall - 2O3 –> 3O2)

Also, nitrogen monoxides can decompose ozone (from lightning, combustion engines)
NO’ + O3 –> NO2’ + O2
NO2’ + O –> NO’ + O2
(Overall - O3 + O –> 2O2)

Question 22

What quickfit apparatus is needed for distillation?

Answer 22

Round bottomed flask - pour organic mixture with a few anti bumping granules to ensure smooth boiling

Heating mantle/Bunsen burner/Water bath

Thermometer holder

Thermometer

Condenser - connect to water supply

Connector

Collecting flask

Question 23

What quickfit apparatus is needed for reflux?

Answer 23

Pear shaped flask - pour the reaction mixture with anti bumping granules to ensure smooth boiling

Heating mantle

Condenser

Question 24

How do you use a separating funnel to remove an organic layer from an aqueous layer?

Answer 24

1. put separating funnel in a clamp stand and remove stopper, ensuring tap is closed at the bottom
2. pour in the mixture so it is half full, wash out reaction vessel and add this, then putting the stopper back
3. take the funnel out of the ring and invert it, open tap to equalise pressure then close tap - repeat until you no longer hear a whistle
4. replace funnel in iron ring and give mixture time to separate, remove the stopper, put a beaker under spout and open tap
5. collect lower water layer in a beaker, turn the tap off then use a clean beaker to collect organic upper layer and then shake liquid with a drying agent

Question 25

What is the purpose of drying the organic liquid with a an anhydrous salt?

Answer 25

These absorb water slowly to become hydrated so can be used to dry other chemicals

If drying agent clumps tiger add more
When drying agent remains free moving, the organic product is dry
Use gravity filtration to collect the filtrate which is the dry organic product

Question 26

What is redistillation?

Answer 26

The purification of a liquid by performing multiple distillations

Question 27

What is the test used to identify an unsaturated hydrocarbon?

Answer 27

Add a few drops of bromine water to the sample and shake

Positive test - bromine water decolourises

Question 28

What is the test used to identify a haloalkane?

Answer 28

Add silver nitrate, ethanol and water

White precipitate - chloro-
Cream precipitate - bromo-
Yellow precipitate - iodo-

Question 29

What is the test used to identify a carbonyl?

Answer 29

Acidified potassium dichromate; Ketones - no change/Aldehyde - turns green

Fehling’s solution; Ketones - no change/Aldehydes - dark red precipitate

Tollens’ reagent; Ketones - no silver mirror/Aldehydes - silver mirror

Question 30

What is the test used to identify a carboxylic acid?

Answer 30

Universal indicator or pH probe - pH of a weak acid

Reactive metal - effervescence (hydrogen)

Metal carbonate - effervescence (carbon dioxide)

Question 31

What is the test used to identify an alcohol?

Answer 31

Warm with an equal volume of carboxylic acid and a few drops of sulfuric acid

Positive test - sweet smell of an ester after a short time