15.1 Chemistry of Haloalkanes

Question 1

What is a haloalkane?

Answer 1

An alkane

with at least 1 hydrogen substituted for a halogen

Question 2

What is a primary haloalkane?

Answer 2

Haloalkane

where halogen is bonded to a carbon

which is bonded to 1 other carbon atoms.

Question 3

What is a secondary haloalkane?

Answer 3

Haloalkane

where halogen is bonded to a carbon

which is bonded to 2 other carbon atoms.

Question 4

What is a tertiary haloalkane?

Answer 4

Haloalkane

where halogen is bonded to a carbon

which is bonded to 3 other carbons.

Question 5

Haloalkanes undergo hydrolysis reactions. Explain how reactivity changes with different haloalkanes:

Answer 5

Lower carbon-halogen bond enthalpy = faster rate of hydrolysis

- MORE SHELLS in halogen atom
- larger ATOMIC RADIUS  
- weaker ELECTROSTATIC ATTRACTION between shared pair of electrons + nuclei of bonded atoms
- less ENERGY required to break bond
- FASTER rate of hydrolysis

Question 6

Why is there a difference in reactivity of haloalkanes?

Answer 6

BOND ENTHALPY in carbon-halogen bond changes

depending on halogen used.

Question 7

If a haloalkane has a halogen which has a LARGE ATOMIC RADIUS, how reactive would the haloalkane be?

Answer 7

more reactive,

weaker bond enthalpy

Question 8

If a haloalkane has a halogen which has a SMALL ATOMIC RADIUS, how reactive would the haloalkane be?

Answer 8

less reactive,

weaker bond enthalpy

Question 9

Why are fluoroalkanes very

unreactive?

Answer 9

fluorine has only 2 shells,

small atomic radius&raquo_space; stronger electrostatic attraction between shared pair of electrons + nuclei of bonded atoms&raquo_space; bond enthalpy is too high

Question 10

What is a nucleophile?

Answer 10

a species

ATTRACTED to an ELECTRON-DEFICIENT CENTRE

where it DONATES an ELECTRON PAIR

forming a NEW COVALENT BOND

Question 11

What are some different types of nucleophiles?

Answer 11

:OH-

H2O:

:NH3

Question 12

Outline the mechanism for nucleophilic substitution:

Answer 12

1. NUCLEOPHILE ATTRACTED to 𝛅+ carbon
   
   • approaches from opposite 𝛅- halogen&raquo_space; minimise repulsion
2. LONE PAIR on nucleophile DONATED to 𝛅+ carbon
3. New COVALENT BOND formed between Carbon + nucleophile.
4. Carbon-Halogen bond breaks by HETEROLYTIC fission

Products = halide + molecule

Question 13

What is nucleophilic substitution?

Answer 13

Substitution reaction

where nucleophile is ATTRACTED to ELECTRON DEFICIENT centre

where it DONATES an electron PAIR

+ REPLACES a group of atoms

Question 14

How do you measure the rate of hydrolysis of haloalkanes?

Answer 14

1. Dissolve haloalkane in ETHANOL.
2. Put in water bath @ 60°C. Put Silver Nitrate test tube in water bath.
3. Start stopwatch + add AgNO3 to haloalkane + record time taken for precipitate to form + cross underneath to disappear.
   
   • AgX(s)

Question 15

Do bromoalkanes react faster than iodoalkanes?

Answer 15

no

bromoalkanes have a smaller ATOMIC RADIUS

(stronger electrostatic attraction) so stronger bond ENTHALPY

so slower RATE of reaction.

Question 16

Do Iodoalkanes react faster than bromoalkanes?

Answer 16

yes

Iodialkanes have a bigger ATOMIC RADIUS

(weaker electrostatic attraction) so weaker bond ENTHALPY

so faster RATE of reaction

Question 17

What factor(s) determine the rate of hydrolysis of haloalkanes?

Answer 17

atomic radius / bond enthalpy