3.3 Halogenoalkanes

Question 1

What is a halogenoalkane?

Answer 1

Compounds which one or more hydrogen atoms in an alkane have been replaced by halogen atoms

Question 2

What are the halogen prefixes?

Answer 2

Fluoro
Chloro
Bromo
Iodo

Question 3

What are some nomenclature rules for halogenoalkanes?

Answer 3

The different halogens are listed alphabetically

Question 4

What are some uses for halogenoalkanes?

Answer 4

Refrigerator
Solvents
Anaesthetics
Aerosol propellants 
Insecticides

Question 5

What is a primary, secondary and tertiary structure?

Answer 5

Primary- only has 1 other carbon attached to the carbon with the halogen attached

Secondary- has 2 carbons attached to the carbon with the halogen attached

Tertiary - has 3 carbons attached to the carbon with the halogen attached

Question 6

What is the ozone layer?

Answer 6

O3 forms naturally in the atmosphere

It protects us from harmful UV radiation

Question 7

What affects the ozone layer?

Answer 7

CFC’s - Chlorofluorocarbons

They form holes in the layer

Question 8

What are used instead of CFC’s?

Answer 8

HFC’s - Hydrofluorocarbons

Question 9

What happens when CFC’s reach the ozone layer?

Answer 9

Chlorine free radicals are formed when CFC’s are exposed to UV radiation

Question 10

What reaction shows what happens to the ozone layer? Type of reaction?

Answer 10

O3 + Cl• -> ClO• + O2
ClO• + O3 -> 2O2 + Cl•

Chain reaction

Question 11

Name the types of reactions that can take place with halogenoalkanes?

Answer 11

Free radical substitution
Nucleophilic substitution
Elimination

Question 12

What is free radical substitution?

Answer 12

A chain reaction

Where one thing replaces another with 3 stages

Question 13

What are the three stages of free radical substitution?

Answer 13

Initiation
Propagation
Termination

Question 14

What is the initiation mechanism?

Answer 14

Cl2 ——> 2Cl•

With UV light over the arrow

Question 15

What takes place in initiation?

Answer 15

Homolytic bond fission

When the bond breaks each atom takes an electron from the bond now called free radicals

Question 16

What is meant by propagation step?

Answer 16

Where free radicals are used up and then regenerated

Question 17

What is the mechanism for propagation in free radical substitution? (Use CH4 and Cl)

Answer 17

CH4 + Cl• -> •CH3 + HCl

•CH3 + Cl2 -> CH3Cl + Cl•

Question 18

What is the mechanism for termination in free radical substitution? (Use CH4 and Cl)

Answer 18

Cl• + Cl• -> Cl2
•CH3 + Cl• -> CH3Cl
•CH3 + •CH3 -> C2H6

Question 19

In free radical substitution how can you make sure you get more of one particular product?

Answer 19

Use an excess of the reactant that will help produce more of the product

Therefore collisions are more likely between the one in excess and the other free radical

Question 20

What are the strongest to weakest bonds of carbon-halogens?

Answer 20

Strongest 
C-F
C-Cl
C-Br
C-I
Weakest

Question 21

What is a nucleophile? What do they do?

Answer 21

Attracts positive charge

Attack a halogenoalkane and form a bond with it
As the carbon will have a delta + charge therefore the nucleophile will be attracted to it

Question 22

Examples of a nucleophile?

Answer 22

:OH- (hydroxide ion)
:NH3 (ammonia)
:CN- (cyanide ion)

Question 23

How do you know it’s likely nucleophilic substitution?

Answer 23

A polar molecule - nucleophilic

Saturated - substitution

Question 24

How does carbon-halogen bonds affect reactions?

Answer 24

The weaker the carbon-halogen bond the more reactive it will be

Question 25

In nucleophilic substitution how does a nucleophile react with a halogenoalkane? (Where do the arrows go)

Answer 25

From the lone pair of the nucleophile to the carbon From the middle of the carbon halogen bond to the halogen

Question 26

In nucleophilic substitution what does the arrow from the middle of the carbon to the halogen show?

Answer 26

Shows the electrons that were in the bond but now broken they go to the carbon

Question 27

What are the products in nucleophilic substitution?

Answer 27

The nucleophile attached to the alkane And A free radical halogen

Question 28

What are the arrows of nucleophilic substitution if the reaction requires two steps? (Ammonia)

Answer 28

The arrow goes from the middle of the hydrogen/nitrogen bond on to the nitrogen The arrow goes from the ammonia free radical to the hydrogen

Question 29

What are the final products in nucleophilic substitution of a halogenoalkane and ammonia?

Answer 29

The alkane with NH2 attached Ammonium ion The halogen free radical

Question 30

What is produced in the nucleophilic substitution of a halogenoalkane and ammonia in the first step?

Answer 30

An unstable intermediate that has to lose something in order to become a stable molecule An alkane with NH3 attached

Question 31

What are the conditions for nucleophilic substitution?

Answer 31

In water or aqueous solution Using ethanol to enable the reactants to mix together Low temperature

Question 32

What are nucleophilic reactions useful for making?

Answer 32

Alcohols Nitriles Amines Used for drugs in medicine

Question 33

What is used to heat any organic chemicals?

Answer 33

Reflux apparatus

Question 34

What is used in reflux apparatus?

Answer 34

A condenser To prevent the liquid from evaporating therefore you have product to collect

Question 35

What is an elimination reaction?

Answer 35

A reaction between a halogenoalkane and an OH- ion Something is lost and nothing is gained

Question 36

Where do the arrows go in elimination reactions?

Answer 36

From the :OH- to the hydrogen From the middle of the c-h bond you drew the other arrow to - to the middle of the c-c bond From the middle of the carbon-halogen bond to the halogen

Question 37

What is produced in elimination reactions?

Answer 37

An alkene Water A halogen free radical

Question 38

What conditions do elimination reactions need?

Answer 38

Ethanolic conditions and high temperature