15.1 The chemistry of the haloalkanes

Question 1

What are haloalkanes

Answer 1

Compounds containing the elements carbon, hydrogen and at least one halogen

Question 2

When naming haloalkanes…

Answer 2

…a prefix is added to the name of the longest chain to indicate the identity
Eg. Fluoro- Chloro- Bromo- Iodo-

Question 3

Haloalkanes have a carbon-halogen bond in their structure. Halogen atoms are more _______ than carbon atoms. The electron pair in the carbon-halogen bond is therefore _______ to the halogen atom than the carbon atom. The carbon-halogen bond is ______.

Answer 3

Electronegative
Closer
Polar

Question 4

Carbon atoms in haloalkanes

Answer 4

Have a slightly positive charge and can attract species containing a lone pair of electrons

Question 5

What are nucleophiles

Answer 5

Species that donate a lone pair of electrons

An atom or group of atoms that is attracted to an electron deficient carbon atom

Question 6

Examples of nucleophiles

Answer 6

Hydroxide ions, :OH-
Water molecules, H2O:
Ammonia molecules, :NH3

Question 7

What happens when a haloalkane reacts with a nucleophile

Answer 7

The nucleophile replaces the halogen in a substitution reaction
New compound is produced containing different functional group
Reaction mechanism is nucleophilic substitution

Question 8

What is substitution

Answer 8

A reaction in which one atom or group of atoms is replaced by another atom or group of atoms

Question 9

What is hydrolysis

Answer 9

A chemical reaction involving water or an aqueous solution of a hydroxide that causes the breaking of a bond in a molecule

Question 10

Steps in nucleophilic substitution

Hydrolysis of haloalkane

Answer 10

1. Nucleophile, OH- approaches carbon attached to halogen on opposite side of molecule
2. Direction of attack by OH- minimises repulsion between nucleophile and δ- halogen
3. Lone pair of electrons on OH- ion attracted and donated to δ+ carbon atom
4. New bond formed between oxygen atom of OH - ion and carbon atom
5. Carbon-halogen bond breaks by heterolytic fission
6. New organic product is alcohol, halide ion formed

Question 11

How can haloalkanes be converted to alcohols

Answer 11

By using aqueous sodium hydroxide

Very slow reaction at room temp. so mixture heated under reflux

Question 12

What do curly arrows show

Answer 12

The movement of electron pairs

Start from either lone pair or bonding pair of electrons and point to atom to which it moves

Question 13

What happens to the carbon-halogen bond in hydrolysis

Answer 13

It is broken and the -OH group replaces the halogen in the haloalkane

Question 14

What does the rate of hydrolysis depend on

Answer 14

The strength of the carbon-halogen bond in the haloalkane

Question 15

Which bond is the strongest
Which is the weakest
C-F, C-Cl, C-Br, C-I

Answer 15

C-F is the strongest
C-I is the weakest
Less energy is needed to break C-I bond

Question 16

Which reacts faster, iodoalkanes or bromoalkanes

Answer 16

Iodoalkanes

Question 17

Which reacts faster, bromoalkanes or chloroalkanes

Answer 17

Bromoalkanes

Question 18

Fluoroalkanes are unreactive because…

Answer 18

…a large quantity of energy is required to break the C-F bond

Question 19

How can we measure the rate of hydrolysis of primary haloalkanes

Answer 19

Carry out rewaction in presence of aqueous silver nitrate
Halide ions produced which react with Ag ions to form precipitate of silver halide
Nucleophile in reaction is water
Haloalkanes are insoluble in water and reaction carried out in presence of ethanol solvent

Question 20

Why do we use ethanol when measuring the rate of hydrolysis of primary haloalkanes

Answer 20

It allows water and the haloalkane to mix and produce a single solution rather than two layers

Question 21

Required practical detailed method

Hydrolysis of haloalkanes

Answer 21

1. Set up 3 test tubes of haloalkanes with 1cm^3 ethanol
2. Stand tubes in water bath at 60 degrees
3. Place test tube containing 0.1moldm^-3 silver nitrate in water bath, allow all to reach constant temp.
4. Add 1cm^3 silver nitrate quickly to each test tube
5. Start stop clock
6. Observe for 5 mins and record time taken for precipitate to form

Question 22

Hydrolysis of haloalkanes - The compound with the slowest rate of reaction is the one that has the ______ carbon-halogen bond

Answer 22

Strongest

Question 23

The rate of hydrolysis increases as the strength of the carbon-halogen bond…

Answer 23

…Decreases.

Question 24

Which is hydrolysed the fastest, a primary haloalkane, secondary or tertiary

Answer 24

Primary is slowest, tertiary is fastest

Primary reacts by one-step mechanism while tertiary is two-step

Question 25

Two step mechanism of a tertiary haloalkane

Answer 25

1. Carbon-halogen bond breaks by heterolytic fission forming tertiary carbocation and halide ion
2. Hydroxide ion attacks carbocation to form organic product