4.2 Haloalkanes

Question 1

Haloalkanes

Answer 1

• alkanes with at least one halogen atom in place of a hydrogen atom

Question 2

Why is the carbon-halogen bond in haloalkanes polar?

Answer 2

Because halogens are generally much more electronegative than carbon.

Question 3

Nucleophiles

Answer 3

• -> electron pair donors
• -> a nucleophile could be a negative ion or an atom with a lone pair of electrons
• -> donate an electron pair to somewhere without enough electrons
• -> water is a nucleophile
• -> OH-, CN- and NH3 are all nucleophiles

Question 4

What type of reaction is the hydrolysis of haloalkanes to make alcohols?

Answer 4

A nucleophilic substitution reaction

Question 5

How are haloalkanes hydrolysed to alcohols?

Answer 5

1) OH- is a nucleophile so it provides a pair of electrons for the C in the carbon-halogen bond
2) The carbon-halogen bond breaks heterolytically - both electrons from the bond are taken by Br-
3) Br- is separated from the molecule as OH- bonds to the carbon

Question 6

What is the general equation for the reaction between a haloalkane and water?

Answer 6

R-X + H2O –> R-OH + H+ + X-

Where:
R = alkane
X = halogen

Question 7

The speed at which different haloalkanes are hydrolysed depends on what?

Answer 7

Bond enthalpy

Question 8

Why do haloalkanes with weaker carbon-halogen bonds react faster?

Answer 8

Because weaker carbon-halogen bonds break more easily

Question 9

Which haloalkanes hydrolyse the fastest?

Answer 9

Iodoalkanes

Question 10

Which haloalkanes hydrolyse the slowest?

Answer 10

Fluoroalkanes because they have the strongest bonds

Question 11

How to compare the reactivity of chloroalkanes, bromoalkanes and iodoalkanes

Answer 11

1) Set up three test tubes, each containing a different haloalkane, ethanol (as a solvent) and silver nitrate solution.

Iodoalkanes - quickly form a yellow precipitate
Bromoalkanes - react less quickly to form a cream precipitate
Chloroalkanes - react more slowly to form a white precipitate