15.1 Chemistry Of Haloalkanes

Question 1

What are haloalkanes?

Answer 1

Compounds containing Carbon, Hydrogen, and at least one Halogen

Question 2

Prefix for Fluorine

Answer 2

Fluoro

Question 3

Prefix for Chlorine

Answer 3

Chloro

Question 4

Prefix for Bromine

Answer 4

Bromo

Question 5

Prefix for Iodine

Answer 5

Iodo

Question 6

What type of bond is the Carbon-Halogen bond?

Answer 6

Polar
Halogen is more electronegative than the Carbon
Halogen will have slightly negative charge
Carbon will have slightly positive charge

Question 7

What are species that donate a lone pair of electrons called?

Answer 7

Nucleophiles

Question 8

What is a nucleophile?

Answer 8

An atom or group of atoms that is attracted to an electron deficient Carbon atom, where it donates a pair of electrons to form a new covalent bond.

Question 9

What types of substances are nucleophiles?

Answer 9

Anions (negatively charged ions)

Molecules with lone pairs

Question 10

What happens when a haloalkane reacts with a nucleophile?

Answer 10

The nucleophile replaces the halogen in a substitution reaction

Question 11

What is a substitution reaction?

Answer 11

A reaction where one atom or group of atoms is replaces by another atom or group of atoms

Question 12

What is hydrolysis?

Answer 12

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

Question 13

What happens in the hydrolysis of a haloalkane?

Answer 13

The halogen atom is replaced by an OH group

Question 14

Steps for a nucleophilic substitution reaction

Answer 14

The nucleophile, OH-, approaches the Carbon atom which is attached to the halogen from the opposite side to the halogen atom
The direction of attack by the OH- ion minimises repulsion from the halogen atom which has a slight negative charge
Lone pair of electrons on the OH- ion is attracted and donated to the Carbon atom
New bond is formed between the oxygen atom of the hydroxide ion and the Carbon atom
The Carbon-halogen bond breaks by heterolytic bond fission
New product - alcohol is formed, as well as a halide ion

Question 15

How does a nucleophilic substitution reaction take place?

Answer 15

Using aqueous Sodium Hydroxide

Reaction is slow at room temperature, so mixture is heated under reflux to obtain a good yield of product

Question 16

What does the rate of hydrolysis depend on?

Answer 16

Strength of the Carbon-halogen bond

Question 17

Which Carbon-halogen bond is the strongest?

Answer 17

C—F

Question 18

Which Haloalkane reacts the fastest?

Answer 18

Iodoalkanes, as bond requires less energy to break

Question 19

Which haloalkane is unreactive?

Answer 19

Fluoroalkanes as a large amount of energy is required to break the C-F bond

Question 20

How can the rate of hydrolysis of haloalkanes be measured?

Answer 20

Carry out the reaction in the presence of aqueous silver nitrate
As the halide ions are produced during the reaction, they form a precipitate of the silver halide

Question 21

Are haloalkanes soluble in water?

Answer 21

No

Question 22

When haloalkanes react with water, what is used?

Answer 22

An ethanol solvent - allows water and haloalkane to mix and produce a single solution

Question 23

Practical steps for haloalkanes

Answer 23

3 haloalkanes used 1-chlorobutane, 1-bromobutane, 1-fluorobutane

1) Add 1cm3 of ethanol and two drops of each haloalkane into 3 test tubes
2) Place the test tubes in a water bath of 60oC
3) Place a tube of silver nitrate in the water bath as well, so all substances reach a constant temperature
4) Add 1cm3 of silver nitrate to each test tube, start stop watch immediately
5) Observe test tubes for 5 minutes and record time take for precipitate to form

Question 24

Observations from hydrolysis of haloalkanes

Answer 24

1-chlorobutane - white precipitate forms very slowly
1-bromobutane - cream precipitate forms at medium speed
1-iodobutane - yellow precipitate forms rapidly

Question 25

Relationship between rate of hydrolysis and strength of carbon-halogen bond

Answer 25

Rate of hydrolysis increases as strength of carbon-halogen bond decreases

Question 26

Which would hydrolyse faster - primary, secondary, tertiary haloalkane?

Answer 26

Tertiary haloalkane

Question 27

What happens during hydrolysis of a tertiary haloalkane?

Answer 27

There is a 2 step mechanism
Carbon-halogen bond of tertiary haloalkane breaks by heterolytic fission, forming tertiary carbocation and a halide ion
Hydroxide ion attacks the carbocation to form the organic product

Question 28

Why is the hydrolysis of a tertiary haloalkane faster?

Answer 28

The tertiary carbocation is more stable than a primary carbocation