3.3- mechanisms of halogenoalkanes

Question 1

what are free radicals (2)

Answer 1

-a reaction intermediate (made during a reaction then are destroyed)
-highly reactive species with one or more unpaired electrons

Question 2

how do free radicals form (2)

Answer 2

-through homolytic fission of a covalent bond
-the bond is broken through a source of high energy eg. UV light/ high temps

Question 3

what is the initiation step in free radical substitution

Answer 3

-creates 2 free radicals
X₂ ->2X⋅

Question 4

what is the dot next to an element/compound in free radical susbtitution

Answer 4

a single, unpaired electron

Question 5

what are the propagation steps in free radical substitution (2)
show the 2 propagation steps from a chlorine radical and methane

Answer 5

-when a free radical has formed, the unpaired electron is transferred to other molecules to create a series of new products
-halogenoalkanes are produced in the second equation/part
1. CH4 + Cl⋅ -> HCl + ⋅CH3
(starting molecule- CH4- reacts with one free chlorine radical to form methyl free radical)
2. ⋅CH3 + Cl₂ -> CH3Cl + Cl⋅
(methyl free radical reacts with diatomic halogen- Cl2- to produce a halogenoalkane and a free radical)

Question 6

what is the termination step in free radical substitution
show 2 possible termination steps

Answer 6

-2 free radicals combine and the reaction ends
-goes from 2 free radicals to no free radicals
1. ⋅CH3 + ⋅CH3 -> C2H6
2. ⋅CH3 + Cl⋅ -> CH3Cl

Question 7

what is the overall equation for the reaction between methane and chlorine

Answer 7

CH4 + Cl₂ -> CH3Cl +HCl

Question 8

what is ozone and why is it bad (3)

Answer 8

protects us from UV radiation (can cause skin cancer)
-when they rise to the atmosphere, the initiation step occurs
-UV light causes the C-Cl bond to break

Question 9

what are chloroflurorocarbons (2)

Answer 9

-hydrocarbons where all H atoms are substituted by Cl and F
-very unreactive due to C-F and C-Cl bonds

Question 10

2 equations to show how ozone is formed

Answer 10

O2 –> 2O⋅
⋅O(g) + O2(g) –> 03(g)

Question 11

why is it beneficial that ozone reduces the amount of UV reaching earth

Answer 11

reduces risk of skin cancer as UV damages DNA

Question 12

what is a nucleophile (2)

Answer 12

an electron pair donor- they have a lone pair
-they attack electron-defficient species

Question 13

what are the 3 nucleophiles

Answer 13

hydroxide ions, cyanide ions, ammonia

Question 14

reagent for hydroxide ion in nucleophilic substitution

Answer 14

aqueous sodium/potassium hydroxide
(NaOH or KOH)

Question 15

conditions for hydroxide ion in nucleophilic substitution(2)

Answer 15

heated under reflux
aqueous reagent

Question 16

reagent for cyanide ion in nucelophilic substitution

Answer 16

potassium/sodium cyanide dissolved in ethanol
(NaCN or KCN)

Question 17

conditions for cyanide ion in nucleophilic subsitution

Answer 17

heated under reflux

Question 18

reagent for ammonia in nucleophilic substitution

Answer 18

excess concentrated solution of ammonia in ethanol (ethanolic ammonia solution)

Question 19

conditions for ammonia in nucleophilic substitution

Answer 19

heated under pressure

Question 20

what is the new functional group formed in hydroxide ions nucleophilic substitution

Answer 20

alcohol

Question 21

what is the new functional group formed in cyanide ions nucleophilic substitution

Answer 21

nitrile

Question 22

what is the new functional group formed in ammnia nucleophilic substitution

Answer 22

amine

Question 23

what happens in nucleophilic substitution (3 stages)

Answer 23

1. nucleophile donates a pair of electrons to the partially positively charged carbon in the halogenoalkane
2. the C-H bond breaks heterolitically, with both electrons going to the halide ion
3. a new bond forms between the carbon atom and the nucleophile- the halogen has been replaced by another functional group

Question 24

why must the reaction cyanide ions in nucleophilic substitution have to take place under reflux conditions and in ethanol, not water

Answer 24

Water in an aqueous solution would act as a nucleophile and undergo nucleophilic substitution with the halogenoalkane, forming an alcohol instead.

Question 25

how does ammonia nucleophilic substitution happen (3 stages )

Answer 25

1. lone pair on NH3 attacks the partially charged carbon so that the C-halogen bond is replaced by a C-N bond 2.NH3 displaces the halogen and gains a positive charge 3.the leaving halide ion/another ammonia molecule in solution picks up a H+ from the NH3 by heterolytic fission and leaves an amine group

Question 26

why must the reaction ammonia in nucleophilic substitution have to take place in ethanol and in excess ammonia (2)

Answer 26

-in aqueous solution, water would act as a nucleophile and an alcohol would form instead -amines are strong nucleophiles so in the absence of excess ammonia, they would undergo nucleophilic substitution with the halogenoalkane instead

Question 27

how are ammonium salts formed as a by product in ammonia nucleophilic substitution (2)

Answer 27

-the extra H+ is picked up by another NH3 molecule -ammonium ion (NH4+) forms, which along with the halide ions in solution can form ammonium salts

Question 28

why does the carbon–halogen bond enthalpy influences the rate of reaction (2)

Answer 28

The carbon-halogen bond has to be broken during the reaction. The harder the bond is to break (the higher the bond enthalpy) the slower the haloalkanes react.

Question 29

what is a base

Answer 29

a proton acceptor

Question 30

what happens if ethanol is used in a reaction between halogenoalkanes and OH- ions (4)

Answer 30

elimination reaction -alkene is produced - -OH ion acts as a base -condition: reflux

Question 31

elimination reaction 4 stages

Answer 31

1. :OH- bonds to H atom 2. electrons move to form double bond 3. the newly formed double bond repulses the electrons in the carbon-halogen bond onto the halogen 4. halogen is eliminated as a negative halide ion

Question 32

what always has to happen in elimination reactions

Answer 32

:OH- always attaacks the H atom bonded to the C which is ADJACENT to the C-halogen bond

Question 33

why were CFCs not restricted until research found they were harmful (3)

Answer 33

lack of evidence that ozone was being depleted lack of alternatives to CFCs commercial interest to continue using CFCs

Question 34

how do CFCs contribute to global warming

Answer 34

absorbs infrared radiation as the molecule has polar bonds