15.1 - Chemistry of Haloalkanes

Question 1

How should haloalkanes be classed and named?

Answer 1

When naming haloalkanes with more than 1 halogen, list them alphabetically in the name.

Primary: Halogen bonded to a carbon bonded to 1 R group
Secondary: Halogen bonded to a carbon bonded to 2 R groups
Tertiary: Halogen bonded to a carbon bonded to 3 R groups

Question 2

What are the properties of the C-X bond, where the X is a halogen?

Answer 2

• Halogens more electronegative than carbon
• Electron pair is closer to the halogen
• Making the C-X bond polar

Question 3

What does the partially positive carbon in the C-X do?

Answer 3

Attract a species called a nucleophile, that has a lone pair of electrons.

Question 4

Define nucleophile.

Answer 4

Atom/group of atoms that are attracted to an electron deficient carbon atom - where a pair of electrons are donated to form a carbon bond.

E.g. hydroxide ions, water molecules, ammonia molecules

Question 5

Briefly define nucleophilic substitution.

Answer 5

• Where the nucleophile is attracted to the haloalkane, replacing the halogen bonded to the carbon.
• Primary haloalkanes undergo nucleophilic substitutions with many nuclephiles

Question 6

What is formed during the hydrolysis of haloalkanes?

Answer 6

• Halogen replaced by -OH group,
• Either water or (aq) hydroxide reacts under hydrolysis
• Forming an alcohol

Question 7

Describe the steps of haloalkane hydrolysis.

Answer 7

1. Hydroxide ion attacks the C (with partial positive charge) from the opposite side of the molecule to the halogen to avoid repulsion from the partially negatively charged halogen.
2. Lone pair on hydroxide attracted to C and is donated
3. New bond between oxygen of hydroxide and C
4. C-X breaks by heterolytic fission
5. Halide ion and alcohol formed.

Question 8

What are the reaction conditions for hydrolysis of haloalkanes?

Answer 8

• Heat under reflux for good yield

- Slow reaction at room temperature.

Question 9

What does the rate of hydrolysis depend on?

Answer 9

The strength of the C-X bond.

Question 10

What is the strongest and weakest C-X bond?

Answer 10

C-F STRONGEST
C-Cl
C-Br
C-I WEAKEST

Iodoalkanes react the fastest down to chloroalkanes due to the lack of bond strength

• Fluoroalkanes are quite unreactive due to the amount of energy required to break the bond.

Question 11

Describe the practical procedure needed to measure the rate of hydrolysis of primary haloalkanes.

Answer 11

1. Add 1cm3 of each haloalkane in separate test tubes
2. Add 1cm3 of ethanol into each test tube (haloalkanes are insoluble in water, this helps the water and haloalkanes mix)
3. Stand the test tubes in 60 degrees C water bath
4. Add 1cm3 of AgNO3. - The X- ions will react with the Ag+ ions to form AgX silver halide precipitate. Start the stop clock
5. Time how long each test tube takes to form precipitate.