4.2 - Alcohols, Haloalkanes and Analysis

Question 1

What is the general formula for alcohols?

Answer 1

CnH2n+1OH.

Question 2

Describe the polarity of alcohols.

Answer 2

Alcohols contain an OH group. The oxygen is much more electronegative than the hydrogen. This results in dipoles.

Question 3

Describe and explain the solubility of alcohols in water.

Answer 3

Alcohol group forms hydrogen bonds with water and the compound dissolves. Hydrogen chain is non-polar and cannot form bonds with the water. Hydrogen chain disrupts the bonding between other water molecules. The longer the alcohol, the less soluble it is.

Question 4

Describe and explain the volatility of alcohols.

Answer 4

Alcohols can form hydrogen bonds with each other. Gives them a much lower volatility than comparable alkanes. The longer the chain, the less volatile they are due to increased van der Waals’.
Note - volatility is how easily a substance turns into a gas. The less volatile a substance is, the higher it’s boiling point.

Question 5

How are alcohols classified into primary, secondary or tertiary?

Answer 5

Primary - the carbon with the -OH group is bonded to 1 other carbon atom.
Secondary - the carbon with the -OH group is bonded to 2 other carbon atoms.
Tertiary - the carbon with the -OH group is bonded to 3 other carbon atoms.

Question 6

Describe the combustion of alcohols.

Answer 6

Complete: CH3CH2OH + 3O2 –> 2CO2 + 3H2O.
Incomplete: CH3CH2OH + 2O2 –> 2CO + 3H2O.

Question 7

Describe the oxidation of a primary alcohol.

Answer 7

1. Gentle heating with acidified dichromate (VI). Acidified dichromate e.g. potassium dichromate and sulphuric acid (K2Cr2O7/H2SO4 or Cr2O7 2-/H+ with spectator ions removed). An aldehyde is produced.
2. Stronger heating with acidified dichromate (VI) under reflux with Cr2O7 2-/H+. The aldehyde is converted to a carboxylic acid.

Question 8

Describe the oxidation of a primary alcohol under reflux.

Answer 8

Stronger heating with acidified dichromate (VI) under reflux with Cr2O7 2-/H+. The product may not be pure as there may still be some aldehyde left.

Question 9

Describe the oxidation of a secondary alcohol.

Answer 9

Reflux with acidified dichromate (VI). Produces a ketone.

Question 10

What will happen if you try to oxidise a tertiary alcohol.

Answer 10

It will not oxidise. tertiary alcohols are resistant to oxidation.

Question 11

What would you observe during the oxidation of primary and secondary alcohols?

Answer 11

The initial colour will change. The final colour will be green. This is because the dichromate (VI) used is an orange colour. This is then reduced to Cr3+ (aq) which is a green colour.

Question 12

Describe the formation of an alkene from an alcohol.

Answer 12

This is called an elimination reaction (or dehydration). Requires an acid catalyst like sulphuric acid and heat.

Question 13

Describe the substitution of alcohols with halide ions.

Answer 13

Requires acidic conditions (H2SO4). Reagent = sodium bromide or similar.

Question 14

What is the general formula for halogenoalkanes?

Answer 14

CnH2n+1X (where X is a halogen).

Question 15

How are halogenoalkanes classified?

Answer 15

Primary, secondary and tertiary.

Question 16

What type of reaction is a hydrolysis of a halogenalkane?

Answer 16

Substitution reaction.

Question 17

What is a nucleophile?

Answer 17

An electron pair doner.

Question 18

Describe using a chemical equation, the hydrolysis of a halogenoalkane with hot aqueous alkali.

Answer 18

Heat and reflux needed. E.g. using NaOH (Na+ removed as spectator ion). CH3CH2Br + OH- –> CH3CH2OH + Br-.

Question 19

Describe the rates of hydrolysis and explain them.

Answer 19

Hydrolysis becomes easier as you go down the group. Bond enthalpies decrease significantly as you go down the group. This means the bonds are broken more easily as you go down the group. The weaker the bond, the faster it will hydrolyse. C-F will not undergo nucleophilic substitution as the bond is too strong.

Question 20

State why the ozone layer is important to life on Earth.

Answer 20

Ozone absorbs harmful UV radiation.

Question 21

Describe how UV radiation acts of CFCs to produce halogen radicals.

Answer 21

Takes place in the upper atmosphere. CFCs stand for chloroflurocarbons.
C2F2Cl2 –> C2F2Cl• + Cl•.

Question 22

Describe how radicals catalyse the breakdown of ozone.

Answer 22

Overall equation: O3 + O –> 2O2. The radicals are not consumed in the reaction.
Radicals can come from CFCs giving chlorine radicals or nitrogen oxides coming from thunderstorms/aircrafts giving nitrogen monoxide radicals.
General equation: R + O3 –> RO + O2. RO + O –> R + O2. R represents a chlorine or nitrogen monoxide radical.
For chlorine radicals: Cl• + O3 –> ClO• + O2. ClO• + O –> Cl• + O2.
For nitrogen monoxide radicals: NO• + O3 –> •NO2 + O2. •NO2 + O –> NO• + O2.

Question 23

What is distillation?

Answer 23

Separates liquids with different boiling points. The mixture is gently heated. Substances evaporate out the mixture in order of increasing boiling point. A thermometer tells you the temperature of the substance evaporating.

Question 24

What is reflux?

Answer 24

Continual evaporation and condensation of the reacting mixture. Without reflux, some organic reactants would evaporate or catch fire before they have time to react. The Leibig condenser is fitted vertically to allow substances that have evaporated to condense and return to the reaction mixture.

Question 25

What is the purpose of redistillation?

Answer 25

Redistillation allows you to purify a product that contains volatile impurities with different boiling points. The impure product is heated in the same manner as a normal distillation. When the temperature reaches the boiling point of the product you want, place a new collection flask at the open end of the condenser. When the thermometer shows a change from this temperature, place a different flask at the end of the condenser so the impurities are not collected with the product.

Question 26

What is the use of a separating funnel?

Answer 26

The mixture at the end of an organic reaction will often contain an aqueous layer and an organic layer. The organic layer is normally less dense and floats on top. Most of the impurities will have dissolved in the aqueous layer. The separating funnel has a stopper at the bottom. This allows you to run off the aqueous layer, leaving the product for collection.

Question 27

What is the purpose of drying with an anhydrous salt?

Answer 27

The organic product left in the separating funnel will contain trace amounts of water. It will need to be dried. An anhydrous salt is added which acts as a drying agent - it binds to the water and becomes hydrated. If the drying agent added looks lumpy, you need to add some more. All the water has been removed when swirling the mixture makes it look like a snow globe. The mixture can then be filtered to remove the drying agent.

Question 28

How does infrared spectroscopy work?

Answer 28

Absorption of infrared radiation causes covalent bonds to vibrate. Different bonds absorb different wavelengths. This produces a spectrum that can be analysed.

Question 29

How do molecules contribute to global warming?

Answer 29

Infrared radiation is absorbed by bond vibration within the molecules. CO2: C=O bond vibration. H2O: O-H bond vibration. CH4: C-H bond vibration.

Question 30

Describe how the link to global warming has resulted in changes in energy uses.

Answer 30

Governments are now producing policies for energy that are based on renewable energy supplies. Renewable energy supplies do not produce the gases responsible for global warming.

Question 31

What does the peak at 3000cm-1 on an IR spectrum represent?

Answer 31

C-H bonds.

Question 32

Which area of the IR spectrum should you not use when identifying functional groups?

Answer 32

0-1500cm-1.

Question 33

How can you distinguish between a carboxylic acid and an aldehyde/ketone on an IR spectrum?

Answer 33

A carboxylic acid should have an O-H absorption and a C=O absorption. A ketone/aldehyde should only have a C=O absorption.

Question 34

What is the language that should be used when describing an IR spectrum?

Answer 34

E.g. a very broad absorption between 2500-3300 indicates an O-H bond in a carboxylic acid.

Question 35

Describe 2 uses of IR spectroscopy.

Answer 35

Modern breathalysers measure ethanol in the breath by analysis using IR spectroscopy. Gases that cause air pollution can be monitored by IR spectroscopy.

Question 36

What can mass spectroscopy be used for?

Answer 36

An indication of the mass of the molecule. Every molecule will produce a different mass spectroscopy readout which can be referenced against a database to identify the molecule.

Question 37

How can you identify fragments from a mass spectrum?

Answer 37

A reading at 15 suggests CH3+. A reading at 14 suggests CH2+. If you know what functional groups are present from an IR spectra, add them to a CH3 and check the mass of the fragment. If this does not fit, add a CH2, then the functional group again and check. If the functional group is at the chain end, check this first and then add to it.

Question 38

Represent the formation of a fragment using an equation.

Answer 38

C4H10 --> C3H7 + CH3+.