3.3.3 - halogenoalkanes

Question 1

what does CFCs stand for?

Answer 1

chlorofluorocarbons

Question 2

what are CFCs?

Answer 2

halogenoalkane molecules where all hydrogen atoms have been replaced by chlorine and fluorine atoms

Question 3

what is the ozone layer made from?

Answer 3

(O)3

Question 4

what does the ozone layer do?

Answer 4

absorbs lots of UV radiation from the sun

Question 5

dangers of UV radiation

Answer 5

• sunburn
• skin cancer

Question 6

how is the ozone layer formed?

Answer 6

oxygen molecule is broken down into 2 free radicals by UV radiation

Question 7

first step of ozone layer formation

Answer 7

O2 (+UV) = O. +O.

Question 8

second step of ozone layer formation

Answer 8

O2 + O. = O3

Question 9

CFCs and the ozone layer

Answer 9

CFCs contribute to damaging the ozone layer

Question 10

what is the first step of how CFCs damage the ozone layer?

Answer 10

chlorine radicals are formed in the upper atmosphere when
C-Cl bonds in CFCs are broken down by UV radiation
e.g.
CCl3F (+UV) =.CCl2F +Cl.

Question 11

what is the second step of how CFCs damage the ozone layer

Answer 11

Cl. + O = O2 + ClO.
ClO. + O3 = 2(O2) + Cl.

Question 12

what safer alternatives to CFCs have been made?

Answer 12

• HCFCs (hydrochlorofluorocarbons)
• HFCs (hydrofluorocarbons)

Question 13

what did CFCs mainly used to be used in?

Answer 13

• propellers in aerosols
• fire extinguishers
• coolant gas in fridges
• added to foam plastics to make insulation/packaging materials

Question 14

properties of CFCs

Answer 14

• pretty unreactive
• non flammable
• non toxic

Question 15

what do most aerosols use now instead of CFCs as a propellant

Answer 15

-pump spray system
- nitrogen as a propellant

Question 16

what is now used as a coolant gas in fridges instead of CFCs

Answer 16

ammonia or hydrocarbons

Question 17

what is now used to make foamed polymers instead of CFCs?

Answer 17

carbon dioxide is used to make foamed polymers

Question 18

what are halogenoalkanes

Answer 18

an alkane with at least one halogen atom in place of a hydrogen atom

Question 19

polarity of carbon-halogen bonds

Answer 19

most carbon-halogen bonds are polar

Question 20

what is the reasoning for the polarity of carbon-halogen bonds

Answer 20

halogens are usually much more electronegative than carbon

Question 21

what charges would the carbon and halogen have in a carbon-halogen bond

Answer 21

carbon has a slight positive charge and the halogen has a slight negative charge

Question 22

what is the reason for the charges in a carbon-halogen bond

Answer 22

halogen atom is more electronegative than carbon atom so withdraws electron density from the carbon atom

Question 23

what is a nucleophile

Answer 23

nucleophile is an electron pair donor, donates an electron pair to somewhere without enough electrons

Question 24

what are some examples of nucleophiles that will react with halogenoalkanes

Answer 24

-cyanide ion
-ammonia
-hydroxide ion

Question 25

how would you illustrate a cyanide ion?

Answer 25

-
: CN

Question 26

how would you illustrate an ammonia ion?

Answer 26

: NH3

Question 27

how would you illustrate a hydroxide ion?

Answer 27

-
: OH

Question 28

what happens in a nucleophilic substitution reaction?

Answer 28

nucleophile attacks a polar molecule, kicks out a functional group and takes its place

Question 29

what is the general formula for the nucleophilic substitution of a halogenoalkane

Answer 29

CH3,CH2,X + Nu-
= CH3,CH2,Nu + X-
where X represents a halogen
and Nu represents a nucleophile

Question 30

what is produced in the nucleophilic substitution reaction of halogenoalkanes with hydroxides

Answer 30

halogenoalkanes react with hydroxides to produce alcohols

Question 31

what are the conditions used to make an alcohol from a halogenoalkane by using nucleophilic substitution

Answer 31

warm aqueous sodium or potassium hydroxide
(water=solvent)

Question 32

what group do nitriles have

Answer 32

nitriles have CN groups, C atom and N atom are held together with a triple bond

Question 33

how do you form nitriles

Answer 33

if you warm a halogenoalkane with ethanolic potassium cyanide, you get a nitrile
(happens under reflux, ethanol used as solvent)

Question 34

what are amines

Answer 34

-an amine has the structure R3N
-R groups can be hydrogen or another group
- nitrogen always has a lone pair, shown as a pair of dots

Question 35

how are amines formed in nucleophilic substitution reactions

Answer 35

if you warm a halogenoalkane with excess ethanolic ammonia (ethanol as solvent) in a sealed tube the ammonia swaps place with the halogen to form an amine

Question 36

bond enthalpy from highest to lowest of carbon-halogen bonds

Answer 36

highest bond enthalpy
C-F
C-Cl
C-Br
C-I
lowest bond enthalpy

Question 37

what carbon-halogen bonds are weakest

Answer 37

C-I as it has the lowest bond enthalpy

Question 38

if you have a molecule with more than one halogen, how do you know which halogen gets replaced first?

Answer 38

the halogen with the lowest bond enthalpy will get replaced first

Question 39

what is an elimination reaction?

Answer 39

in an elimination reaction, a small group of atoms breaks away from a larger molecule, this small group isn’t replaced by anything else

Question 40

what are the conditions for an elimination reaction to occur with a halogenoalkane and hydroxide ions

Answer 40

• hydroxide ions are dissolved in ethanol (ethanol used as solvent)
• reaction under reflux

Question 41

what is formed in elimination reactions of halogenoalkanes with hydroxide

Answer 41

alkenes

Question 42

in a nucleophilic substitution reaction with a halogenoalkane and hydroxide, what is the role of the OH-

Answer 42

under aqueous conditions, OH- acts as a nucleophile, donates electron pair to slightly positive carbon atom

Question 43

in an elimination reaction with a halogenoalkane and hydroxide, what is the role of the OH-

Answer 43

OH- acts as a base - removes a hydrogen atom from the halogenoalkane