3.3 Haloalkanes

Question 1

What fission occurs during free radical substitution?

Answer 1

Homolytic fission

Question 2

What happens during initiation? (FRS)

Answer 2

A halogen is subjected to UV light, and the single bond breaks, forming two radical halogen atoms

Question 3

What happens during propagation (FRS)?

Answer 3

An alkane reacts with a radical halogen, forming a radical alkane and an acid (eg hydrochloric acid)

Question 4

What happens during the second stage of propagation (FRS)?

Answer 4

The radical alkane reacts with a diatomic halogen, forming a haloalkane and a radical halogen. This creates a chain reaction as the radical halogen can react with more alkanes

Question 5

What happens during termination (FRS)?

Answer 5

There are many methods, eg a radical alkane reacts with a radical halogen to form a normal haloalkane

Question 6

What will occur if the halogen is in excess during Free radical substitution?

Answer 6

There will be a higher proportion of dihalo, trihalo, tetrahalo, etc.

Question 7

What will occur if the alkane is in excess during free radical subsitution?

Answer 7

The product will mostly be a haloalkane

Question 8

Describe how CFCs are destroying the ozone layer ?

Answer 8

1) Chlorine free radicals are formed in the upper atmosphere with UV light
2) The free radicals are catalysts that react with ozone to break it down into diatomic oxygen molecules
3) the overall reaction is 2O3 -> 3O2

Question 9

Show the complete reaction for the destruction of ozone layer

Answer 9

Cl° + O3 -> O2 + ClO°
ClO° + O3 -> 2O2 + Cl°
Overall 2O3 -> 3O2

Question 10

What allows for mechanisms in haloalkanes?

Answer 10

The polar carbon-halogen bond, with the delta+ charge on carbon

Question 11

Describe a nucleophilic substitution reaction

Answer 11

1) an arrow is drawn form a nucleophile electrons(eg :-OH) to the delta+ carbon
2) an arrow is drawn from the carbon-halogen bond to the halogen
3) The halogen leaves, taking both electrons, and a new bond is formed between the carbon and nucleophile

Question 12

Give examples of nucleophiles that can react with haloalkanes

Answer 12

-OH, -CN, NH3

Question 13

What is formed when a haloalkane reacts with hydroxide in an aqueous, warm environment?

Answer 13

Alcohol is formed through nucleophilic substitution

Question 14

What are the conditions needed to form an alcohol using nucleophillic substitution?

Answer 14

• Warm

- aqueous solution of hydroxides, dissolved in water

Question 15

What is formed when a haloalkane reacts with cyanide in a warm aqueous environment?

Answer 15

A nitrile is formed through nucleophilic substitution

Question 16

What are the conditions needed to form a nitrile through nucleophilic substitution?

Answer 16

• Warm

- aqueous solution (including cyanide) dissolved in ethanol

Question 17

Describe a nucleophilic substitution reaction between a halogen and excess ammonia

Answer 17

1) An arrow goes from the lone pair of electrons on ammonia to to delta+ carbon connected to a halogen, and an arrow is drawn from this bond to the halogen. This forms a CH3CH2N+H3
2) an arrow is drawn from the lone pair of another ammonia to a hydrogen on the NH3 of the product of above. An arrow is drawn from the NH bond to the nitrogen, and a CH3CH2NH2 is formed (an amine). The amine group still has a lone pair of electrons.

Question 18

Describe the trend in the rate of reactions of nucleophilic substitution reactions with haloalkanes down group 7, and explain why.

Answer 18

Increase in rate as you move down as the bond enthalpy decreases as you move down (C-Halogen bond) meaning less energy is required to break this bond.

Question 19

What are the conditions needed to form an amine through nucleophilic substitution?

Answer 19

• warm

- excess ammonia dissolved in ethanol

Question 20

Describe the conditions needed to form an alkene through elimination from a haloalkane

Answer 20

• warm
• hydroxide dissolved in ethanol (instead of water)
• Reflux

Question 21

Describe an elimination reaction with a haloalkane to form an alkene

Answer 21

1) An arrow is drawn from the lone pair of electrons on -OH (which acts as a base) to a hydrogen bonded to a carbon neighbouring a C-Halogen bond.
2) An arrow is drawn from this C-H bond to the C-C bond with a C-Halogen bond (as it has a spare electron). An arrow is drawn from from the C-Halogen bond to the halogen, and the halogen leaves.
3) An alkene is formed.

Question 22

Describe an electrophilic addition reaction between an alkene and a hydro-halogen (eg HCl)

Answer 22

1) The hydrogen is delta+. This causes an arrow from the C=C bond to go to this hydrogen as it is attracted.
2) An arrow is drawn from the H-Cl bond to the Cl
3) A CH3C+H2 is formed, with the positive carbon being the most stable carbocation (carbon with most carbons surrounding it).
4) The negatively charged Chlorine is attracted to this C+ and bonds, forming a haloalkane.

Question 23

Describe the electrophilic reaction between an alkene and sulfuric acid

Answer 23

1) An arrow is drawn from the C=C bond to the delta+ H on H2SO4, and an arrow is drawn from the OH bond to the O
2) The HSO4- molecule is attracted to the C+ and a bond is formed, forming an alkyl hydrogen sulfate
3) Then in warm water, an alcohol is formed through hydrolysis, and the sulfuric acid is regenerated.

Question 24

Describe the conditions needed for electrophilic addition reactions with H2SO4

Answer 24

• Cold concentrated sulfuric acid

- Then warm water.

Question 25

What is a nucleophile?

Answer 25

An electron pair donator