Alkenes By Dr. Bbosa Science (A-level Organic chemistry)
These are hydrocarbons that contain aa double bond
The general formula is CnH2n 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
- 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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