Halogenalkanes

Question 1

What are the two types of reactions that can occur with haloalkanes?

Answer 1

Only elimination or substitution reactions can occur

Question 2

Describe and explain nucleophilic substitution

Answer 2

The C atom of the haloalkane is slightly positive, as the halogen it is attached to is more electronegative (and weaker).
This means the negative nucleophile is attracted to the C
The nucleophile bonds to the C, causing the halogen bond to break (weakest bond, can only have 4 bonds)

Question 3

State what is meant by the term nucleophile

Answer 3

An electron pair donater.

Question 4

What is hydrolysis?

Answer 4

Splitting molecules using water

Question 5

How can we test for halogenoalkanes? (brief)

Answer 5

Do a substitution reaction which turns the halogen into a halide ion
Then test for that ion with silver nitrate soln.

Question 6

Describe how you would test for haloalkanes

Answer 6

Warm the haloalkane with NaOH soln. in a mix of ethanol and H2O
Halogen displaced to become an ion
Acidify with dilute nitric acid and then add silver nitrate
Various precipitates may be formed
You can confirm the precipitate by adding ammonia soln.

Question 7

Why do you acidify the halide ion mixture with nitric acid?

Answer 7

To prevent any unreacted OH- ions reacting with the silver and giving a confusing precipitate

Question 8

Adding silver nitrate soln. to a halide ion produces a white precipitate. What ion is it?

Answer 8

Cl-

Question 9

Adding silver nitrate soln. to a halide ion produces a very pale yellow precipitate. What ion is it?

Answer 9

I-

Question 10

Adding silver nitrate soln. to a halide ion produces a very pale cream precipitate. What ion is it?

Answer 10

Br-

Question 11

What are the conditions for haloalkane substitution by OH- ions?

Answer 11

Aqueous and warm

Question 12

What are the conditions for haloalkane substitutions by CN- ions?

Answer 12

Aqueous/alcohol

Warm

Question 13

What is produced by haloalkane substitution with CN- ions?

Answer 13

A nitrile

Question 14

What are the conditions for haloalkane substitution by NH3?

Answer 14

Excess ammonia (NH3)
Sealed tube

Question 15

Give a general reaction for the substitution of a haloalkane by NH3

Answer 15

R-X + 2NH3 –> R-NH2 = NH4+X-

Question 16

In substitution by NH3, NH3 acts in two ways. What are they?

Answer 16

As a nucleophile and as a base

Question 17

What are the reagents and conditions for elimination reactions?

Answer 17

Reagents - NaOH/KOH

Conditions - dissolved in ethanol

Question 18

Describe and explain elimination reaction

Answer 18

The OH- takes one of the Hs - its electrons go to the C it was previously attached to
These two electrons move to form double bond b/wn the 2 C atoms
The approach of those electrons repels the electrons in the C-Br bond right out onto the bromine
The Br is thrown off as a negative ion

Question 19

What does CFC stand for?

Answer 19

Chlorofluorocarbons

Question 20

What are the common uses of CFCs and why?

Answer 20

Low toxicity, low reactivity and low flammability led to common use as refrigerants and aerosol propellants

Question 21

What is the ozone layer?

Answer 21

A thin layer of the atmosphere consisting of O3, that filters out some UV

Question 22

What does the ozone layer do?

Answer 22

Filter out shorter wavelengths of UV light that would be harmful to most life in larger doses

Question 23

Why do HCFCs not affect the ozone layer?

Answer 23

They don’t have the C-Cl bond

Question 24

What is an ozone molecule?

Answer 24

O3

Question 25

CFCs affect the ozone layer. Name the reaction that occurs

Answer 25

Free radical substitution

Question 26

Give an overall balanced formulaic equation for the free radical substitution of ozone

Answer 26

2O3 –> 3O2

Question 27

Why are CFCs such a problem?

Answer 27

Attack ozone
Cl* regenerated so can affect others
CFCs have long lifetime (20-100yrs)

Question 28

Explain why an excess of ammonia is needed in the substitution of NH3 to produce a high yield of amine

Answer 28

NH3 has two roles - firstly as a nucleophile and secondly as a base

Question 29

What is the role of Cl in the decomposition of ozone?

Answer 29

Acts as a catalyst

Question 30

What is the suffix of a nitrile?

Answer 30

-anenitrile

Question 31

CFCs affect the ozone layer. Describe the reaction that occurs

Answer 31

UV light breaks C-Cl bond --> C*F3 + Cl*
Cl radicals attack ozone --> regenerate more Cl*s
Cl* + O3 --> ClO* + O2
ClO* + O3 --> Cl* + 2O2
Cl* + Cl* --> Cl2