1. Reactions with alkenes

2. Addition Reactions Hydrogenation Halogenation Hydration
Addition of hydrogen halides
Oxidation to diols

3. Halogenation Alkenes and halogens Produce dihalogenoalkanes
E.g. ethene + bromine  1,2-dibromoethane
E.g. ethane + chlorine  1,2-dichloroethane

4. Hydration Adding water
Adding H and OH to the two atoms in a C=C double bond
Heat the alkene with steam
Pass the mixture over a catalyst of phosphoric acid
Ethene + water  ethanol

5. Addition of hydrogen halides
Adding HBr or HCl usually
Form a halogenoalkane
Ethene + hydrogen bromide  bromoethane
Why do you think this doesn’t show the test for saturation?

6. Oxidation to diols Both addition and oxidation
A diol is a compound containing two OH (alcohol groups)
Oxidising agent – acidified potassium manganate (V)
Potassium manganate (V) provides an oxygen atom
Water in the solution provides the other oxygen atom and two hydrogen atoms
CH2=CH2 + [O] + H2O  CH2OH-CH2OH
Purple to colourless (like the bromine water saturation test)

7. Electrophilic Addition Reactions
Reactions of alkenes are typically electrophilic addition reactions
Electrophile
Addition
4 e- in the double bond make it a centre of high electron density
Electrophiles are attracted to it
Can form a bond by using 2 of the 4 e- in the C=C (the 2 that are in the π bond)
The electrophile may be a positively charged ion OR have a positively charged area

8. Mechanism Main Steps The electrophile is attracted to the double bond
Electrophiles are positively charged and accept a pair of e- from the double bond.
A positive ion (a CARBOCATION) is formed
A negatively charged ion forms a bond with the carbocation

9. 1. Reaction with Hydrogen Halides HBr and Ethene+ Br C H H Br + -
:Br-
Bromoethane
The H-Br bond is polar
 H+ electrophile by breaking H-Br bond
Electrons from the weak π bond of C=C are attracted to the +H atom
The H-Br bond breaks – e- move to more electronegative Br
This produces an intermediate carbocation (C+) and a :Br- ion
Lone pair of electrons on the :Br- form a new bond with the C+

10. 2. Reaction of Alkenes with Halogens: Br2 with Ethene+ Br C H Br + -
:Br-
1,2-Bromoethane
Exactly parallel to addition of HBr except Br2 dipole is induced
This is the test for unsaturation!!

11. Notes about Alkenes and Halogens
React very rapidly with chlorine gas or with solutions of bromine and iodine in an organic solvent
It gives you dihaloalkanes
The halogen atoms add across the double bond
Halogen molecules act as electrophiles

12. Why do you think the reaction happens very quickly??
HINT: Instantaneous dipole

13. At any instance, a halogen molecule is likely to have an instantaneous dipole e.g. Brδ+ – Br δ-.
The δ+ end of this dipole is attracted to the electron-rich double bond in the alkene; the bromine molecule has become an electrophile.

14. 3. With Concentrated Sulfuric Acid
At room temperature, alkenes react with concentrated sulphuric acid whose structure is:
The electrophile is the partially positively charged H atom in the sulfuric acid molecule
This allows the production of alkylhydrogensulphates
It is an exothermic reaction

15. Ethyl hydrogen sulphate Ethyl hydrogen sulphate
3. Conc H2SO4 to Ethene C H C H +
HO3SO C H H O
SO3H + -
:O-
SO3H
Ethyl hydrogen sulphate
HO3SO C H HO C H
+ H2O
+ H2SO4
Ethyl hydrogen sulphate
Ethanol

16. Reaction with water
Water also adds on across the double bond in alkenes.
The reaction is used industrially to make alcohols and is carried out with steam, at a suitable temperature and pressure, using an acid catalyst such as phosphoric acid (see this in Equilibria Topic).

17. Reaction with Asymmetrical Alkenes
With propene:
e.g. CH2=CHCH3(g) +H2SO4(l) CH3CH(OSO2OH)CH3(l)
propylhydrogensulphate
The reaction with alkenes other than ethene
proceeds via the more stable carbocation.
The main use of this reaction is that the product produces an alcohol on heating with water.

18. Reminder : Electrophilic Addition of HBr to Ethene+ Br C H H Br + -
:Br-
Bromoethane
Symmetry of ethene  SINGLE addition product
Similarly for any SYMMETRICAL alkene
e.g.1 But-2-ene + HBr  2-Bromobutane ONLY
e.g.2 Hex-3-ene + HBr  3-Bromohexane ONLY

19. C + H Br H CH3 H C Br CH3 C + H Br H CH3 Br C H CH3
Addition to Unsymmetrical Alkenes C
+ H Br H
CH3
eg Propene + HBr H C Br
CH3 C
+ H Br H
CH3 Br C H
CH3
Is 1-bromopropane or 2-bromopropane formed?
BOTH, but ----
NB The major product depends on the TYPE OF INTERMEDIATE CARBOCATION corresponding to that product

20. + at junction of 2 C chains
Three types of carbocation
PRIMARY
SECONDARY
TERTIARY C H + C H + C +
+ at end of C chain
+ at middle of C chain
+ at junction of 2 C chains
Increasing stability
because MORE (1  2  3) alkyl groups “push” e- towards C+, thus stabilising it
Unsymmetrical alkenes form the product related to the MORE STABLE carbocation

21. The Positive Inductive Effect
Alkyl groups have a tendency to release electrons.
This is known as a positive inductive effect.
Sometimes we can see this effect by an arrow along the bond to show the direction of release:

22. Electrophilic Addition of HBr to Propene C
CH3 H + H C
CH3 Br
δ δ- H Br C
CH3 H
:Br-
Secondary carbocation or
2-Bromopropane or or C
CH3 H + Br C
CH3 H
δ δ+ H Br
 (2-bromopropane) is the predominant product
:Br-
Primary carbocation
1-Bromopropane
because reaction proceeds via MORE STABLE SECONDARY CARBOCATION

23. Task
Decide which is the major product in the following reactions. You must draw out each mechanism:
2-methylpropene + hydrogen bromide
Pent-2-ene + water
2-methyl-but-2-ene + hydrochloric acid

24. (CH3)2CBr-CH3 eg 1. 2-methylpropene + HBr ? H+ (CH3)2C=CH2 + H-Br
(CH3)2CH-CH2 +
(CH3)2C-CH3 + or
Primary
+ Br-
Tertiary
- more stable
(CH3)2CBr-CH3
NOT (CH3)2CH-CH2-Br

25. CH3CH2CH2-CH(OH)CH3 and CH3CH2CH(OH)-CH2CH3 eg 2. Pent-2-ene + H2O ?
CH3CH2CH=CHCH3 + H-OH
CH3CH2CH2-CHCH3 +
CH3CH2CH-CH2CH3 + or
Secondary
+ (OH-)
Secondary
- EQUALLY stable
CH3CH2CH2-CH(OH)CH3
and
CH3CH2CH(OH)-CH2CH3

26. (CH3)2CCl-CH2CH3 eg 3. 2-methyl-but-2-ene + HCl H+
(CH3)2C=CHCH3 + H-Cl
(CH3)2CH-CHCH3 +
(CH3)2C-CH2CH3 + or
Secondary
+ Cl-
Tertiary
- more stable
(CH3)2CCl-CH2CH3
NOT (CH3)2CH-CHBrCH3

27. Summary Questions
Write the equation for the complete combustion of propene.
What is the typical reaction of alkenes?
What are the two possible products of the reaction between propene and hydrogen bromide? Which is the main product? Explain why this product is the more likely.
What is the product of the reaction between ethane and hydrogen chloride?
Which test tests for a carbon-carbon double bond?
17 mark questions and
 June 2012 Unit 2 Question 7 (QS )
June 2010 Unit 2 Question 6a (QS )

28. Apply your Knowledge!
Past paper Questions
17 Marks: 17 Minutes