Alcohols and Haloalkanes

Question 1

Explain why alcohols have a lower volatility and higher boiling point than alkanes with the same chain length

Answer 1

The higher boiling point if alcohol is due to the hydrogen bonds that form between the Hydroxyl groups on different alcohol molecules. These hydrogen bonds require a larger amount of energy to be broken and therefore the boiling point is higher.

Question 2

Describe the intermolecular forces between alcohols

Answer 2

Like all molecules, alcohol molecules form London forces with one another. However the OH groups on alcohols create a polarity in the molecule and allow the delta negative oxygen atoms to become attracted to the delta positive atoms in the OH group, this attraction is stronger than London forces and is called a hydrogen bond.

Question 3

Describe alcohols’ solubility in water

Answer 3

As well as forming hydrogen bonds with one another, alcohol molecules also form hydrogen bonds with water molecules, meaning they are soluble in water. However as the chain length increases, the influence of the OH bond on the molecule decreases so longer chain alcohols are less soluble.

Question 4

What is a primary alcohol

Answer 4

A primary alcohol is one in which the carbon atom that the OH group is attached to only forms bonds with only 1 other carbon and 2 hydrogens.

Question 5

What is a secondary alcohol

Answer 5

In a secondary alcohol the carbon that the OH group is attached to is bonded to 2 other carbon atoms and only one hydrogen.

Question 6

What is a tertiary alcohol

Answer 6

A tertiary alcohol is when the OH group is bonded to a carbon that is only bonded to 3 other carbons atoms, usually in the presence of an alkyl group.

Question 7

What is the equation for the combustion of an alcohol (ethanol)?

Answer 7

CH3CH2OH + 3O2 ———> 2CO2 + 3H2O

Question 8

Describe the effect of chain length of an alcohol on the energy released during combustion

Answer 8

The combustion of alcohols in exothermic and as the chain length increases the energy releases per mole also increases. Energy is released in the form of heat.

Question 9

What two molecules can primary alcohols be oxidised into?

Answer 9

Aldehydes and carboxylic acids

Question 10

Describe the conditions and method required for the oxidation of an alcohol into an aldehyde

Answer 10

A primary alcohol must be gently heated in the presence of acidified potassium dichromate (K2Cr2O7/H2SO4). The aldehyde is distilled out of the reaction mixture to prevent further reactions with the oxidising agent.

Question 11

Describe the conditions and method required for the oxidation of an alcohol into a carboxylic acid

Answer 11

A primary alcohol must be heated under reflux in the presence of K2Cr2O7/H2SO4. Heating under reflux ensures that any aldehyde formed is further oxidised into a carboxylic acid

Question 12

Describe the conditions and method required for the oxidation of an alcohol into a ketone

Answer 12

A secondary alcohol is heated under reflux in the presence of K2Cr2O7/H2SO4 to produce a ketone, further oxidisation is not possible so a ketone is the only product. The K2Cr2O7/H2SO4 changes from orange to green if the reaction is successful

Question 13

Describe the conditions and method required for the oxidation of a tertiary alcohol

Answer 13

Tertiary alcohols do not oxidise

Question 14

Describe the conditions and required to dehydrate an alcohol and state the product of the reaction.

Answer 14

When an alcohol is heated under reflux in the presence of an acid (H2SO4 or H3PO4) an alkene is formed as well as a water molecule.

Question 15

Describe the conditions and method required for the substitution of a haloalkane on an alcohol

Answer 15

For a substitution reaction to occur, an alcohol must be heated under reflux with sulphuric acid and a sodium halide (NaBr). H2SO2 and NaBr react to form a hydrogen halide (HBr) in situ. The HBr formed then reacts with the alcohol to form a haloalkane and a water molecule.

Question 16

Explain why haloalkanes attract nucleophiles

Answer 16

Halogens are more electronegative than carbon meaning the bonding pair is closer to the halogen in the C-Halogen bond. This creates a delta + carbon atom and a delta - halogen. A nucleophile is a species with an attraction to an electron deficient carbon atom.

Question 17

What is a nucleophile?

Answer 17

A nucleophile is an atom or group of atoms attracted to an electron deficient carbon atom where it donates a pair of electrons to from a new covalent bond

Question 18

Describe the reactions and conditions to produce a haloalkane from an alkene

Answer 18

Addition of a hydrogen halide- attraction to the electron dense pi bond which heterolytically splits the H-Halogen bond when H bonds with the alkene creating a carbocation. The negative halogen ion then forms a bond to the carbocation

Question 19

Describe the reactions and conditions to produce a haloalkane from an alkane

Answer 19

Substitution of a halogen which is a non polar molecule so requires UV light to provide the activation energy.

Question 20

Describe the reactions and conditions to produce a haloalkane from an alcohol

Answer 20

Alcohols react with hydrogen halides to from haloalkanes. However the hydrogen halide is produced in situ from a sodium halide and H2SO4.

Question 21

Describe the reactions and conditions to produce an alkene from an alcohol

Answer 21

When an alcohol is heated under reflux in the presence of an acid the product is an alkene and water.

Question 22

Describe the reactions and conditions to produce an alkane from an alkene

Answer 22

The addition of H2 molecules to an alkene in the presence of a nickel catalyst forms the corresponding alkane

Question 23

Describe the reactions and conditions to produce an alcohol from an alkene

Answer 23

To form an alcohol from an alkene you must hydrate the molecule, you do this by adding steam (gaseous water) with an acid (H3PO4) catalyst.

Question 24

Describe the reactions and conditions to produce an alcohol from a haloalkane

Answer 24

In order to produce an alcohol from a halo alkane you must add NaOH to the reaction mixture. This forms an alcohol by nucleophilic substitution of an OH- ion