Reactions with alkenes

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

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

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

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?

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)

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

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

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+

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!!

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

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

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.

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

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

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).

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.

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

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

+ 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

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:

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

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

(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

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

(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

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 (QS12.2.07) June 2010 Unit 2 Question 6a (QS10.2.06)

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