3.3 Organic Chemistry: .3 Halogenoalkanes

Question 1

what are halogenoalkanes? what is the difference between them and alkanes and what do they contain?

Answer 1

-alkanes that contain a halogen usually: fluorine chlorine bromine iodine -they are more reactive than alkanes -they contain a polar bond

Question 2

for each halogen state what prefix is used Fluorine Chlorine Bromine Iodine

Answer 2

Fluorine= fluoro

Chlorine= Chloro

Bromine= bromo

Iodine= iodo

Question 3

are halogenoalkanes soluble in water?

Answer 3

no because the carbon-halogen bond is not polar enough

Question 4

how does the boiling point of halogenalkanes change with chain lenght and halogen group (from fluorine to iodine)

Answer 4

• the boiling point increases with increased chain lenght
• the boiling increases going down the halogen group

this is due to the fact that there are increased van der Waals forces because the larger the molecules, the greater the number of electrons (and therefore the larger the van der Waals forces)

Question 5

what is the trend reactivity in the halogenalkanes (going down the halogen group from fluorine to iodine)

Answer 5

• it is the carbon halogen bond that determines the reactivity of halogenalkanes
• as you go down the group 7 halgoens, the atoms increase in size
• this means that nuclues of the halogen becomes more shielded so the nucleus has less of an attraction for the shared electron pair in the covalent bond
• this means the carbon-halgoen bond becomes weaker and the rate of nucleophillic subsitituion increases

Question 6

what is a nucleophile?

Answer 6

an electron pair donor that bonds with psotively or partially positively charged carbon atoms

Question 7

what happens in nucleophillic substitution?

Answer 7

in nucleophillic subsitution the nucleophile replces a halogen in halogenoalkane

Question 8

show the process of nucleophillic substitution were bromoethen reacts with Pottasium hydroxide

state what molecule is made at the end

Answer 8

ethanol is made

Question 9

show the process of nucleophillic substitution were bromoethen reacts with Pottasium cyanide

state what molecule is made at the end and give an equation for the reaction

why is this mechanism important?

Answer 9

propanenitrile is made

CH3CH2Br + KCN → CH3CH2CN + KBr

this mechanism is important because the carbon chain increase by one

Question 10

what type of product is formed when you react halogenoalkanes with ammonia?

Answer 10

amines

Question 11

show the process of nucleophillic substitution were bromoethen reacts with ammonia

state what molecule is made at the end and give an equation for the reaction

Answer 11

ethylamine

CH3CH2Br + 2NH3 → CH3CH2NH2 + NH4Br

Question 12

what conditions are needed for an alcohol to be produced in the reaction of a halogenoalkane with sodium hydroxide

what does the OH- act as here?

what is the name of the mechanism that takes place when this happens?

Answer 12

cold and aqueous

the OH- ion acts as a nucleiophile

the mechanism that takes place is called nucleophillic substitution

Question 13

what are the conditons needed for an alkene to be produced from the reaction between a halogenoalkane and sodium hydroxide?

what does the OH- act as?

what is the name of the mechanism that takes place in this reaction?

Answer 13

• hot and alcoholic
• the OH- ion acts as a base
• the mechanism that takes place is elimination

Question 14

what is a base?

Answer 14

A “base” (or, “Brønsted base”) is just the name we give to a nucleophile when it’s forming a bond to a proton (H+)

Question 15

show the elimination reaction where 2-bromopropane reacts with Pottasium hydroxide

name the product made and write and eqaution for this reaction

Answer 15

propene is made

CH₃CHBrCH₃ + KOH → CH₂CHCH₃ + H₂O + KBr

Question 16

explain how CFCs create a hole in the ozone layer and give an eqaution for each step.

give an overall eqaution at the end

Answer 16

1.) chlorine free radicals (Cl•) are formed in the upper atmosphere when C-Cl bonds in CFCs are broken down by ultraviolet radiation.

CCl₃F → CCl₂F• + Cl•

2.) These free radicals are catalysts. They react with ozone to form and intermediate (ClO•) and oxygen molecule

Cl• + O₃ → O₂ + Cl•

ClO• + O₃ → 2O₂ + Cl•

Overall Equation: 2O₃ → 3O₂

the chlorine free radical is regenerated and can go straight on to attack another ozone moleule. so it only takes one chlorine free radical to destroy loads of ozone molecules.

Question 17

what are the alterntives to CFCs?

Answer 17

hydrofluorocarbons

Question 18

what is the overall eqaution for the depletion of the ozone

what is the intiation step, 1st propogation and 2nd propogation step?

Answer 18

2O3 –> 3O2