Chapter 14 Alcohols

Question 1

What is the functional group of alcohols?

Answer 1

-OH group
Hydroxyl group

Question 2

Explain the shape and bond angle around an -OH group

Answer 2

Non-linear, 104.5
Two bonding pairs and two lone pairs
This is because electron pairs repel, bond pairs repel equally and lone pairs repel more.

Question 3

What are the three levels of classification for alcohols?

Answer 3

Primary,
Secondary
Tertiary

Question 4

How are alcohols classified?

Answer 4

Dependent on the number of carbon atoms or alkyl groups that are bonded directly to the OH group.

Question 5

Explain the trend in the boiling point of alcohols in comparison to alkanes

Answer 5

Alcohols have a higher boiling point than the corresponding alkanes, this is due to the presence of the OH functional group. Therefore more energy is required to overcome the Hydrogen bonds and induced dipole: dipole interactions between alcohols, than the
induced dipole: dipole interactions between alkane molecules.
However, as the Carbon chain increases, the number of electrons in the molecules also increases. Therefore, the Induced dipole: dipole interactions get stronger and decrease the gap in boiling points between alcohols and alkanes. As a result, the Induced dipole interactions become more significant while the influence of the Hydrogen bonds becomes less significant.

Question 6

Explain the solubility of alcohols

Answer 6

Small alcohols infinitely dissolve in water
Larger alcohols become less soluble in water due to the strengthening of the induced dipole: dipole interactions between the alcohol molecules as there are more electrons. Therefore, more energy is required to overcome the induced dipole- dipole interactions.

Question 7

How do alcohols dissolve in water?

Answer 7

For an alcohol to dissolve in water, the energy released when Hydrogen bonds form between the alcohol and water has to overcome the intermolecular forces (the hydrogen bond and induced dipoles) between the alcohol molecules.

Question 8

Explain the volatility of alcohols in comparison to alkanes

Answer 8

Alcohols have a lower volatility than corresponding alkanes
This is because, as a liquid, hydrogen bonds and London forces hold the alcohol molecules together, therefore these bonds require a lot more energy to break in order to change the liquid alcohol into a gas than just overcoming the weaker London forces of alkanes.

Question 9

How do alcohols undergo complete combustion?

Answer 9

Alcohols can undergo complete combustion to produce carbon dioxide and water- this is an exothermic reaction so energy is released in the form of heat.

Question 10

When do alcohols undergo incomplete combustion?

Answer 10

Alcohols can undergo incomplete combustion the longer the carbon chain.

Question 11

How do alcohols undergo incomplete combustion?

Answer 11

Alcohols undergo incomplete combustion to produce carbon monoxide and water

Question 12

What classification levels of alcohols can be oxidised?

Answer 12

Primary and secondary alcohols can be oxidised by an oxidising agent, while tertiary alcohols cannot easily be oxidised.

Question 13

What is the oxidising agent used to oxidised primary and secondary alcohols?

Answer 13

Potassium dichromate (VI) K2Cr2O7 acidified with dilute sulfuric acid

Question 14

During the oxidation of alcohols what is the colour change?

Answer 14

Orange to green

Question 15

Explain the preparation of aldehydes from alcohols

Answer 15

Aldehydes can be prepared as a result of the partial oxidation of primary alcohols.
The primary alcohols are heated with the oxidised agent
Water is also produced
This is prepared through the use of distillation as the aldehyde distills out of the reaction mixture, preventing further oxidation.

Question 16

Explain the preparation of carboxylic acids from alcohols

Answer 16

Carboxylic acids can be prepared as a result of the complete oxidation of primary alcohols
The primary alcohols are heated with the oxidation agent
Water is also produced
This is prepared through the use of reflux- the aldehyde remains in the mixture so it is oxidised.

Question 17

Explain the preparation of ketones from alcohols

Answer 17

Keaton’s can be prepared as a result of the complete oxidation of secondary alcohols.
The secondary alcohols are heated with the oxidising agent.
Water is also produced
This is prepared through the use of reflux to ensure all the alcohol is reacted however distillation can also be used.

Question 18

Explain the elimination reaction of alcohols

Answer 18

This is known as the dehydration reaction where a water molecule is removed from the starting material.
To do this an alcohol is heated under reflux in the presence of a concentrated phosphoric acid catalyst or concentrated Sulfuric acid to produce an alkene and water.

Question 19

Explain the substitution reaction of alcohols

Answer 19

Alcohols react with hydrogen halides to form haloalkanes
The halogen atom is substituted with the OH group of the alcohol
To do this the alcohol is then heated under reflux with Sulfuric acid and a sodium halide (eg NaCl or NaBr)

Question 20

During the oxidation of alcohols what happens to the dichromate (VI) ions?

Answer 20

The dichromate (VI) ions CrO3^2- are reduced to Chromium (III) ions

Question 21

How do Branched isomers affect the boiling point of alcohols?

Answer 21

Branched chain isomers have the same number of electrons, however they have a lower boiling point than straight chain alcohols , this is because branching reduces the surface contact between the molecules. Therefore, the London forces are weaker and don’t require a lot of energy to be overcome.

Question 22

When are alcohols soluble in polar and non polar solvents?

Answer 22

The longer the carbon chain of an alcohol, the lower solubility in polar solvents and higher solubility in non-polar solvents. This is because as the chain length increases there are more electrons so the London forces are stronger and require more energy to be overcome so the hydrogen bonds become less significant

Question 23

Are alcohols more soluble in polar or non polar solvents?

Answer 23

Alcohols would be more soluble in polar solvents like water due to their ability to form strong hydrogen bonds with the water molecules as a result of their -OH functional group.
While alcohols and non polar solvents would tend to interact through weaker London forces