Alkenes

These are hydrocarbons that contain aa double bond

The general formula is C_nH_2n n ≥ 2

Examples are

Ethene               CH₂=CH₂

Propene           CH₃CH=CH₂

But-1-ene         CH₃CH₂CH=CH₂

But-2-ene         CH₃CH=CHCH₂

For branched alkenes we number the carbon atoms from the side nearest the double bond

For example

Physical properties

They like those of alkane

• they are insoluble in water
• they are soluble in organic solvents
• they are solvents
• they range from gases to liquids to waxy solids

Chemical properties

Combustion of alkenes

They burn in air to produce carbon dioxide, water, and heat.

C₂H₄ (g) + 3O₂ (g)    →     2CO₂(g) + 2H₂O   + heat 

Because they produce heat they are used as fuel.

Reduction of alkenes

They are reduced by hydrogen in presence of catalysts; nickel or platinum  to alkenes

Example

Hydration reaction of alkenes

The reactiontion of alkenes with KMnO₄ solution

Dilute alkaline KMnO4 solution convert alkenes into compounds called GLYCOLS (dihydroxyl alcohols)

Electrophilic addition reactions

Alkenes react with electron-deficient groups called electrophiles to form saturated compounds.

Definitions

Electrophilic addition reaction is a reaction where an electron-deficient group adds to unsaturated compounds (alkenes or alkynes)  to form saturated compounds.

(a) The reaction of alkenes with halogens

Halogens add across a double bond to form dihalogeno compounds. The reaction is carried out in an inert solvent (like tet

Example 1

Propene reacts with bromine in the presence of inert solvents (tetrrachloromethane) to form 1,2-dibromopropane

The mechanism for the reaction of bromine with alkenes

Example 2

2-methylpropene plus chlorine in presence of tetrachloromethane give 1,2-dibromo-2-methylpropane

The mechanism for the reaction 2-methylpropene with chlorine

Example 3

Phenylethene plus bromine in presence of tetrachloromethane give 1,2-dibromophenylethane

Mechanism

Example 4

2-methylpropene plus chlorine in presence of tetrachloromethane gives 1, 2-dichloro-2-methylpropane

(b) The reaction of alkenes with HX (X = Cl, Br, I)

The reaction is carried out by bubbling HX through alkenes.

Example 5

Propene plus hydrochloric acid gives 2-chloropropane

Example 6

Phenylethene plus hydrogen bromide give 1-bromo-1-phenylethane

Trial 1

Complete and write a mechanism

Solution to the exercise

(i) 2-phenylpropene plus HCl gives 2-chloro-2-phenylpropane

(ii) 2-methylbut-2-ene plus HBr gives 2-bromo-2-methylbutane

(c) The reaction of alkenes with halogens in the presence of water.

The reaction is carried out by bubbling alkenes through aqueous halogens.

Trial  2

Complete the following and write mechanism (do not look at the answer before)

Trial 3

Complete and write a mechanism

(d) The reaction of an alkene with water.

The reaction is catalyzed by an acid sulphuric acid or phosphoric acid. Alcohols are produced

Example

Ethene reacts with acidified water to produce ethanol.

Example 8

Propene reacts with dilute sulphuric acid to form propan-2-ol

Trial 4

Complete the following equations and suggest mechanisms

NB. The charges must be accounted for at every stage

(e) Addition of hydrogen bromide in presence of a peroxide

In presence of a peroxide, alkenes react with HBr by free radial mechanisms; but in this case, hydrogen atom goes a carbon atom that carries the least number of hydrogen atoms of those that form a double bond otherwise the bromine atom goes to a carbon with the highest number of hydrogen atoms.

Examples 9

Mechanism

Trial 5

Complete and write a mechanism (it advisable you attempt these questions and mark yourself)

Solutions to the exercise

(f) Ozonolysis (reaction of ozone with alkenes)

Ozone reacts with alkenes to form unstable ozonide that is reduced by zinc in the presence of hydrochloric acid to form carbonyl compounds.

Example 10

From the products of ozonolysis, it is possible to tell the position of the double bond in the compound that was ozonolyzed because ozonolysis breaks a compound at the double bond.

Example 11

Write the structure of an alkene which on ozonolysis gave

(i) Propanal (CH₃CH₂CHO) and methanol (HCHO)

By removing the oxygen atoms from each compound create a double bond and hence the compound.

Trial 5

An alkene X has molecular formula C₄H₈

(a) Write an name all structural formula of compound X.

(b) On ozonolysis X gave propanone (CH₃COCH₃) and methanal (HCHO). Identify X.

Solution

(b) By removing oxygen atoms from the products of ozonolysis, the position of a double bond is created.

Polymerization

This is the joining of very many small molecules called monomers to form big molecules called polymers

In the presence of a catalyst, (acid) alkenes polymerize to form polyalkenes.

For example

(i) ethane plus a catalyst you get polyethene

(ii) Propene polymerize to form polypropene

(iii) Vinyl chloride polymerize to form polyvinylchloride (PVC)

(iv) Styrene polymerizes to form polystyrene.

All polyalkenes can be used as insulators, packing materials, and pipes.

Alkenes undergo a type of polymerization called addition polymerization in that when the monomers of alkenes add to form a polymer, there is no loss of any atom or particle from them.

Note that when given a polymer without a double bond the monomer is a substituted two carbon compound with a single double bond.

Trial 11

Write the structure and the names of monomers for the following polymers

Solutions

Uses of polyalkenes

• packing materials
• water pipes
• insulators

Alkenes undergo a type of polymerization called addition polymerization because monomers add to form a polymer without loss of an atom.

Polyalkenes are also called thermoplastics because they soften on heating and can be remolded.

Polymerization of dienes

Alkenes with conjugated double bond undergo polymerization to form polyalkenes with double bonds

Example

Preparation of alkenes

1. From alcohols

Alkenes are produced by dehydration of alcohols by hot sulphuric or phosphoric acid. Alcohols are organic compounds that contain hydroxyl group (-OH). The mechanism depends on the class of alcohol.

Classification of alcohols,

They are three classes of alcohols according to the number of alkyl groups attached to a carbon atom that carry -OH group.

(a) Primary alcohols have one alkyl group on the carbon atom that carry OH group

(b) Secondary alcohols have two alkyl groups attached to a carbon atom that carry OH group

(c) Tertiary alcohols

Tertiary alcohols have three alkyl groups attached to the carbon atoms that carry OH group

Examples

Mechanism

It depends on the class of alcohol.

(a) Primary alcohol: undergo a mechanism called elimination bimolecular(E2). It is an elimination reaction because the water molecule is removed from the alcohol. It bimolecular because the slow step involves two species.

(b) Secondary alcohols undergo the same mechanism as primary alcohol or that of a tertiary alcohol

(c) Tertiary alcohol undergoes a mechanism called E1 or elimination unimolecular because a water molecule is eliminated and the slowest step involves one molecule only.

Mechanism

Trial 17

Complete and write a mechanism

Preparation of alkenes from alkyl halide (R-X where R is an alkyl group and X is Cl, Br or I)

The mechanism depends on the class of alkyl halide

Classification of alkyl halide

They are three classes of alkyl halide according to the number of alkyl groups attached to a carbon atom that carry a halide

(a) Primary alkyl halides have one alkyl group on the carbon atom that carries a halide.

Examples

(b) Secondary alkylhalide have two alkyl groups attached to a carbon atom that carry halogen atom

Examples

(c) Tertiary alkyl halides

Tertiary alkyl halide have three alkyl groups attached to the carbon atoms that carry OH group

Examples

  Mechanism

It depends on the class of alcohol.

(a) Primary alkyl halide: undergo a mechanism called elimination bimolecular(E2). It is an elimination reaction because HX is removed from the alkyl halide. It bimolecular because the slow step involves two species.

Example 17

Complete and write a mechanism

Solution

(b) Secondary alkyl halides undergo the same mechanism as primary alcohol or that of a tertiary alcohol

(c) Tertiary alkyl halides undergo a mechanism called E1 or elimination uni-molecular because a water molecule is eliminated and the slowest step involves one molecule only.

Example

Trial 8

Complete and write a mechanism

Solution

Mechanism

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