1. Alkanes, Alkenes and Halogenoalkanes

2. The only reactions of importance at normal temperatures are:
Reactions of Alkanes
Alkanes are relatively unreactive.
The only reactions of importance at normal temperatures are:
with oxygen (combustion) the reaction is highly exothermic
and so alkanes are widely used as fuels.
with the halogens chlorine and bromine

3. The Alkanes and halogens
with bromine (Br2)
with chlorine (Cl2)
reaction is relatively slow
reaction is faster
reaction still requires light
occurs explosively
reaction requires sunlight
Both produce hydrogen halides
The reaction mechanism involves a FREE RADICAL CHAIN REACTION.
FREE RADICAL = atom or molecule with an unpaired electron

4. H3C -H The Alkanes and halogens Reaction mechanism – alkane + bromine.
R – H will represent the alkane
R is an alkyl group
e.g. in methane R will be methyl – CH3
H3C -H

5. Free radical chain reaction - Revision
steps
Initiation
Propagation x 2
Termination

6. Br2  Br + Br Free radical chain reaction Initiation
The first stage in the reaction.
Light (hn) supplies the energy to break the Br – Br bond
RESULT
Two bromine atoms each with an unpaired electron.
Br2  Br Br
Homolytic fission

7. R-H + Br  R + HBr Free radical chain reaction Another FREE RADICAL
Propagation 1
The second stage in the reaction.
Br are very reactive.
There is a strong tendency for unpaired electrons to pair up.
RESULT
Bromine atom removes H atom from an alkane molecule .
R-H + Br  R + HBr
Another FREE RADICAL

8. R + Br-Br  R-Br + Br Free radical chain reaction Propagation 2
R reacts with a bromine molecule.
R + Br-Br  R-Br + Br
Br can then attack another alkane molecule
This propagates the chain reaction.
The two propagation steps continue in turn until one reactant is used up.

9. The chain is ended when any two free radicals meet up and combine.
Free radical chain reaction
Termination
The chain is ended when any two free radicals meet up and combine.
R + R  R-R
Br + Br  Br-Br
R Br  R-Br

10. Free radical chain reaction
Initiation
Propagation
Termination
Light is needed to start this reaction but once it has started it continues in the dark.
PRODUCTS
Bromoalkane
Hydrogen bromide

11. Free radical chain reaction
Initiation
Propagation
Termination
It is possible for multiple substitutions to take place.
To prevent this (ensure monosubstitution) a large excess of alkane is used.
Boiling point of the bromoalkane
is higher than that of the alkane this means the excess alkane can easily be removed by distillation.
The reaction is not specific
- any hydrogen atom in any alkane can be substituted. The more complex the alkane the more complex the mixture of products.

12. Saturated hydrocarbon (Single bonds)
Free radical chain reaction
Initiation
Propagation
Termination
Useful memory aid – the ‘S’ s
Saturated hydrocarbon (Single bonds)
Slow, Substitution reaction, takes place in Sunlight

13. Mostly ADDITION REACTIONS across double bond  saturated products
Reactions of alkenes
Mostly ADDITION REACTIONS
across double bond  saturated products
Hydrogen (H2)
Hydrogen halides (e.g. HBr)
Halogens (e.g. Br2)

14. Reactions of alkenes ADDITION of Hydrogen (H2)
slightly exothermic
so slow at room temperature  unobservable
Carried out at high temperature with a catalyst
Catalysts = nickel, platinum, palladium
Heterogeneous catalysts – adsorb Hydrogen onto the surface

15. Cl2 reacts more vigorously and I2 reacts less vigorously.
Reactions of alkenes
ADDITION of Halogens (e.g. Cl2, Br2 )
Br2
Rapid decolourisation even in the absence of light
Used to identify the presence of C = C
Cl2 and I2
Also add across C = C.
Which of these additions is more vigorous than that of Br2?
Cl2 reacts more vigorously and I2 reacts less vigorously.

16. H Br2 C Br C H Reactions of alkenes
ADDITION of Halogens (e.g. Cl2, Br2 )
Only one product C H
Br2 C H Br

17. H H C Br C H H Reactions of alkenes δ+ δ-
ADDITION of Br2 - likely mechanism – step 1
Bromine attacks electron rich C=C bond
C=C bond electrons repel the Br2 electrons – temporary polarisation H H C δ+ δ- Br C
electrophilic H H

18. electron deficient Br atom is attacked by the pair of π bond electrons
Reactions of alkenes
ADDITION of Br2 - likely mechanism
Br-Br bond breaks heterolytically – both bond electrons go with the leaving Br- H H C δ+ δ- Br Br C
electron deficient Br atom is attacked by the pair of π bond electrons H H

19. + H Br C Br Reactions of alkenes ADDITION of Br2 - likely mechanism
Its formation is the rate determining step
the BROMONIUM ION
Stabilised by delocalisation
Evidence suggests the intermediate is a positive cyclic ion

20. + Br H H C Br C Br Reactions of alkenes 1,2 - dibromoethane
ADDITION of Br2 - likely mechanism – step 2
Rapid nucleophilic attack by the bromide ion on one of the carbon atoms of the bromonium ion Br H C C H Br + Br
1,2 - dibromoethane

21. + Reactions of alkenes H Br H C C H Br H Br C C Cl
ADDITION of Br2 in the presence of sodium chloride some 1- bromo chloroethane is also produced H Br C H C
Br2
Br2 with Cl- present H Br C H Br C Cl +

22. Addition of Cl2 to C = C occurs in a similar way.
Reactions of alkenes
ADDITION of Br2 in the presence of sodium chloride some 1- bromo chloroethane is also produced C H Br + Cl
Looks as though
Cl- can compete with Br- as a nucleophile and intercepts the cyclic ionic intermediate.
Addition of Cl2 to C = C occurs in a similar way.

23. CH3 – CH = CH2 Reactions of alkenes H H HI C C I
ADDITION of Hydrogen Halides (HCl, HBr, HI) H C H C I HI
Carried out in gas state or in non-polar solvents
What happens if an unsymmetrical alkene is used e.g. propene?
CH3 – CH = CH2

24. CH3 – CH = CH2 HBr CH3 - CH – CH3 CH3 – CH2 - CH2 - Br Br
Reactions of alkenes
CH3 – CH = CH2
HBr
CH3 - CH – CH3 Br
CH3 – CH2 - CH2 - Br
Minor product
Major product

25. RULE for addition of HX across C = C
Reactions of alkenes
RULE for addition of HX across C = C
Markovnikov’s Rule
When HX is added across C = C the hydrogen atom attaches to the carbon atom that is already bonded to the greater number of hydrogen atoms.
This is thought to involve the attack on
the electron rich C = C by an electrophile.
H – X bond is permanently polarised.

26. Mechanism for addition of HX
Reactions of alkenes
Mechanism for addition of HX C H + C H X δ+ δ- X-
Uses π electrons C
Rate determining step – produces X- and carbonium ion
Carbonium ion is attacked by X- - a nucleophile

27. Mechanism for addition of HX
Reactions of alkenes
Mechanism for addition of HX C H X C H + X-

28. Addition of HBr to propene 2 possible carbocation intermediates
Reactions of alkenes
Addition of HBr to propene
2 possible carbocation intermediates 1
- C - C -- C H +
Electron donating effect 2 H C +

29. Addition of HBr to propene
Reactions of alkenes
Addition of HBr to propene H C + 2 1
- C - C -- C H +
Alkyl groups donate electrons to adjacent atoms
Alkyl groups can help stabilise the positive charge in these carbocations.
Which carbocation is most stable?
Why?
Two alkyl groups are present to help stabilise the positive charge
This carbocation is formed in preference

30. Addition of HBr to propene
Reactions of alkenes
Addition of HBr to propene
Most likely product H C +
Preferred carbocation H C H C Br C H
2-bromopropane

31. Reactions of alkenes HI + H2O  H3O+(aq) + I-(aq)
Acid catalysed addition of water
HI + ethene in an aqueous solution
Strong acid
HI + H2O  H3O+(aq) + I-(aq)
dissociates
Hydronium or oxonium ion
Attacks double bond!
electrophile or nucleophile
electrophile or nucleophile

32. Mechanism for addition of WATER
Reactions of alkenes
Mechanism for addition of WATER C H + O H C H O
- H+ C H O + C H O

33. Reactions of alkenes C H SO3H O C H S O H H2O H H C H H2SO4 H C O H H
Another Mechanism for the addition of WATER
Cold conc. Sulphuric acid or phosphoric acid C H
SO3H O C H
Ethyl hydrogen sulphate S O H
H2O H
Hydrolysed by warming with water H C H
H2SO4 H C O H H

34. Reactions of alkenes Addition of water across a double bond
follows Markovnikov’s rule.
Good method for synthesising specific alcohols.

35. Reactions of alkenes alkanes halogenoalkanes alkene impurities alkanes
H2SO4 method of adding water
is used to purify substances
contaminated with alkenes.
alkanes
halogenoalkanes
alkene impurities
Alkanes and halogenoalkanes are insoluble in conc. sulphuric acid
bubbling through or
shaking with sulphuric acid
alkanes
halogenoalkanes
alcohol
Alcohol dissolves in the acid.
Alkane or halogenoalkane is then obtained by distillation

36. Halogenoalkanes Rare in the natural world Synthesised in the lab
Widely used in medicine, manufacture of plastics and agriculture
1847 CHCl3 (chloroform) – general anaesthetic first used by James Simpson (Bathgate)
Problem – overuse of pesticide and implicated in damage to ozone layer and

37. Naming Halogenoalkanes
Halogenoalkanes (also haloalkanes and alkylhalides)
Halogen atom  prefixes CHLORO- BROMO-, FLUORO-, IODO-
More than one halogen atom of a type  prefixes mono-, di-, tri-, tetra-
Basic name  longest unbranched chain
Position of halogen  number in front of prefix

38. Naming HalogenoalkanesC H Cl
numbering
2,3-dichloropentane

39. Naming HalogenoalkanesC H Br
numbering
3-bromobut-1-ene

40. Classification of Halogenoalkanes
monohalogenoalkanes
primary 1o
secondary 2o
tertiary 3o C X R H C X H R C X R

41. Naming HalogenoalkanesC H Br Cl
numbering
2,3-dibromo-1-chloro-2-methylbutane

42. They are immiscible in water.
Halogenoalkanes
Physical Properties
The carbon-halogen bond is polar but this doesn’t have a great influence on physical properties.
They are immiscible in water.
The boiling point depends on the size of the molecule and the halogen. Compounds with Cl or Br have higher boiling points than those with F.

43. Reactions of Halogenoalkanes Reactions depend on 2 factors
The halogen present  strength of C – X bond
R – I > R- Br > R – Cl > R - F
Weakest Strongest
bond bond
Reactive Very unreactive
The position of C – X bond in the molecule

44. Reactions of Halogenoalkanes 1) Nucleophilic substitution
2 main reactions
1) Nucleophilic substitution
2) Elimination

45. Nucleophilic substitution Reactions
Polarised bond
Carbon atom is susceptible to
nucleophilic attack δ+ δ- C X
higher
electronegativity

46. C X δ+ δ- Nucleophilic substitution Reactions Polarised bond higher
Carbon atom is susceptible to
nucleophilic attack
If bond breaks heterolytically an X- ion is formed.
Cl-, Br-, I- are all good leaving groups because they are stable. δ+ δ- C X
higher
electronegativity

47. Nucleophilic substitution Reactions Nucleophilic substitution
Halogens facilitate (make possible) the heterolytic cleavage (breaking) of the bond
Nucleophilic substitution C Y C X δ+ δ-
+ X- Y-

48. C2H5Br(l) + OH-(aq)  C2H5OH(aq) + Br-(aq)
Nucleophilic substitution – 2 mechanisms
1) Hydrolysis of bromoethane (PRIMARY halogenoalkane)
Using aqueous alkali
C2H5Br(l) + OH-(aq)  C2H5OH(aq) + Br-(aq)
This reaction is
first order with respect to HYDROXIDE IONS
and
first order with respect to BROMOETHANE
Rate = k[C2H5Br][OH-]
RDS involves one mole of each

49. C Br OH- C Y Br - δ+ δ- transition state
Nucleophilic substitution Reactions C Br δ+ δ-
OH-
This type of reaction is known as an SN2 reaction C Y Br -
transition state

50. (CH3)3CBr(l) + H2O(l)  (CH3)3COH(aq) + HBr(aq)
Nucleophilic substitution – 2 mechanisms
2) Hydrolysis of 2-bromo-2 methylpropane
(TERTIARY halogenoalkane)
Using water
(CH3)3CBr(l) + H2O(l)  (CH3)3COH(aq) + HBr(aq)
This reaction is
first order with respect to HALOGENOALKANE
but
Zero order with respect to water
Rate = k[(CH3)3CBr]
RDS only involves the halogenoalkane

51. - + CH3 H3C CH3 H3C C Br C Br Rate = k[(CH3)3CBr] slow
Nucleophilic substitution – 2 mechanisms
2) Hydrolysis of 2-bromo-2 methylpropane
(TERTIARY halogenoalkane) using water
Rate = k[(CH3)3CBr]
RDS only involves the halogenoalkane C Br δ+ δ-
CH3
H3C C Br -
CH3
H3C +
slow
heterolytic cleavage
planar carbocation

52. + CH3 H3C CH3 H3C C C OH + H+ + H2O fast slow
Nucleophilic substitution – 2 mechanisms
2) Hydrolysis of 2-bromo-2 methylpropane
(TERTIARY halogenoalkane) using water C OH
CH3
H3C C
CH3
H3C +
+ H2O
fast
slow
+ H+
This type of reaction is known as an SN1 reaction

53. Order of stability: 1o < 2o < 3o
Nucleophilic substitution
Mechanism depends (partly) on the type of halogenoalkane
Primary and secondary halogenoalkanes:
hydrolysed by SN2 mechanism
Tertiary halogenoalkanes: hydrolysed by SN1 mechanism.
(elimination can take over)
This is because stability of the carbocation intermediate is important
Order of stability:
1o < 2o < 3o

54. Nucleophilic substitution
Alkyl groups push electrons towards neighbouring carbon atoms.
In tertiary carbocations,
3 alkyl groups help stabilise the positive charge.
A primary carbocation,
with only one alkyl group will be much less stable.

55. Uses of substitution of halogenoalkanes
Reactants: water or aqueous alkali.
Products : specific alcohols which can then
be converted to aldehydes or ketones.
Aldehydes can then be oxidised to alkanoic acids.
R - X  R - OH
e.g. CH3CH2CH2CH2Cl  CH3CH2CH2CH2OH

56. Uses of substitution of halogenoalkanes
Reactant: ammonia
(weakly nucleophilic due to lone pair of electrons on the N atom
and the polar nature of N-H bond)
Products : amines
R - X  R – NH2
e.g. CH3CH2I  CH3CH2NH2

57. Uses of substitution of halogenoalkanes
Reactant: ammonia
Products : amines I- N H C
CH3 + N H C H
CH3 I
- H+ N H C
CH3

58. Uses of substitution of halogenoalkanes
sodium + dry alcohol  hydrogen + sodium alkoxide
Na(s) C2H5OH(l)  H2(g) C2H5O- Na+
The alkoxide ion is a powerful base
and a powerful nucleophile
Reactant: sodium alkoxide
Products : ethers
sodium ethoxide + bromomethane  methoxyethane + sodium bromide
C2H5O- Na BrCH  C2H5OCH Na + Br -
This reaction is favoured at lower temperature

59. Uses of substitution of halogenoalkanes Reactant: cyanide ion (CN-)
[potassium cyanide in ethanol/heated under reflux]
Products : nitriles
CH3CH2CH2I CN-  CH3CH2CH2CN I-
1-iodo propane butanenitrile
Main advantage:
The carbon chain has been increased by one carbon
Nitriles can be converted into the corresponding acid by acid hydrolysis or into an amine by reduction using lithium aluminium hydride.(LiAlH4)

60. Uses of substitution of halogenoalkanes
CH3CH2CH2 - I 
CN-
CH3CH2CH2 - C N
[nitrile]
Acid hydrolysis
LiAlH4
CH3CH2CH2 C O OH
CH3CH2CH2 CH2 - NH2
[amine]

61. Products of substitution
alcohols
ethers
amines
nitriles
R-X
reagents
NH3
OH-
RO-
in alcohol
CN-
in alcohol

62. Elimination Reactions
Reactant: strong base [eliminates hydrogen halide molecule]
Product : alkene
Good nucleophiles are often bases –
elimination and substitution can take place at the same time
The reaction conditions determine which process dominates.
In water [polar solvent] substitution predominates.
In ethanol it’s elimination.

63. Elimination v substitution
CH3CH2CH2CH2 - Br
1-bromobutane
KOH
in aqueous solution
KOH
in ethanol solution
CH3CH2CH2CH2 - OH
CH3CH2CH CH2
butan-1-ol
but-1-ene
There are two possible mechanisms for elimination reaction

64. - Elimination H Br C R’’ R’ δ+ δ- HO H C R’’ R’ - H 2 O Br-
One step process:
removal of H+ ion
and
loss of halide ion
occur simultaneously
CH3CH2CH2CH2 - Br
1-bromobutane
KOH
in ethanol solution H Br C
R’’ R’ δ+ δ-
CH3CH2CH =CH2
but-1-ene HO - H C
R’’ R’
- H 2 O
This mechanism is known as an E2 reaction
[Elimination , TWO particles involved in RDS]
Br-

65. E1 reaction - Elimination R’ R’’ H C Br R δ+ δ- R’ R’’ H C R + + Br -
CH3CH2CH2CH2 - Br
1-bromobutane
+ Br -
slow
KOH
in ethanol solution
CH3CH2CH =CH2 R’
R’’ H C R +
but-1-ene
This mechanism is known as an
E1 reaction
[Elimination , ONE particle involved in RDS]
Two step process:
1) C-X bond breaks heterolytically to form a carbocation.
2) Hydrogen ion is removed from adjacent carbon atom forming the C=C. R’
R’’ H C R
fast
+ H2O HO -

66. Try these questions
Name and draw the products for the following reactions and indicate the product that will be formed in the higher yield.
The reaction of propene with hydrogen bromide
The reaction of 2-methylbut-2-ene with hydrogen chloride
The catalytic hydration of 2-methylbut-1-ene
The addition of hydrogen bromide to 3-methylbut-1-ene
The answers are shown on the following slides

67. Answers a) The reaction of propene with hydrogen bromide
Names of products
structures
Higher yield
Higher yield H C
propene H C Br H C Br
2-bromopropane
1-bromopropane

68. Answers b) The reaction of 2-methylbut-2-ene with hydrogen chloride
Names of products
structures
Higher yield
2-methylbut-2-ene H C
CH3 H
Higher yield H C
CH3 Cl H C
CH3 Cl
2-chloro-2-methylbutane
3-chloro-2-methylbutane

69. Answers c) The catalytic hydration of 2-methyl-but-1-ene
Names of products
structures
Higher yield
2-methylbut-1-ene H C
CH3 H
Higher yield H C OH
CH3 H C
CH3 OH
2-methylbutan-1-ol
2-methylbutan-2-ol

70. Answers d) The addition of hydrogen bromide to 3-methylbut-1-ene
structures
Names of products
Higher yield
3-methylbut-1-ene H C
CH3
Higher yield H C
CH3 Br H C
CH3 Br
4-bromo-2-methylbutane
3-bromo-2-methylbutane

71. More questions
Draw structures for the possible products when 2-chloro-2-methylbutane is reacted with:
aqueous potassium hydroxide
a solution of potassium hydroxide in ethanol
a solution of sodium ethoxide in ethanol?
Structure of 2-chloro-2-methylbutane H C Cl
CH3
Answer to a)

72. Answer a)
Possible product when 2-chloro-2-methylbutane is reacted with:
aqueous potassium hydroxide
Substitution
Nucleophilic
Alcohol produced H C Cl
CH3 H C OH
CH3
2-chloro-2-methylbutane
2-methylbutan-2-ol
Answer to b)

73. Elimination predominant
Answer b)
Possible product when 2-chloro-2-methylbutane is reacted with:
b) a solution of potassium hydroxide in ethanol
Elimination predominant
Alkene produced H C Cl
CH3 H C
CH3
2-chloro-2-methylbutane
2-methylbut-1-ene H C
CH3
Answer c)
2-methylbut-2-ene

74. Answer c)
Possible product when 2-chloro-2-methylbutane is reacted with:
b) a solution of sodium ethoxide in ethanol H C Cl
CH3
Substitution
Ether produced H C O
CH3
C2H5
2-chloro-2-methylbutane