Alkenes preparing, reactions, physical properties

When, there are double bonds between carbon atoms, those compounds are introduced as alkene or olefin.Due to existing of double bonds, alkene are considered as unsaturated hydrocarbons. Every alkene contain a single or several double bond(s) between two carbon atoms. The common formula of alkene is CnH2n.(n=2,3,4.....)

Examples and IUPAC names of alkenes

examples of alkene ethene and propene


General formula of alkenes - CnH2n [n=2,3,4....]

Ex: if n=3;
Respective hydrocarbon : C3H2*3 = C3H6

What is the simplest alkene

Simplest alkene contains only two carbon atoms. According to the general formula of alkene, number of hydrogen atoms are four in the simplest alkene. Therefore ethene( C2H4 ) is the simplest alkene.

Physical states of alkenes at room temperature

  • First 3 of alkenes are gases.
  • Alkenes containing carbon atoms 5 to 18 are liquids.
  • More than 18 carbon atoms in alkenes are solids.

Melting and Boiling points of alkenes

Alkenes have higher melting and boiling points than the corresponding alkanes
alkenes have a pi ( Π ) bond in it's double bond. Pi ( Π ) bonds are more polarizable than sigma ( σ ) bonds. Therefore intermolecular forces are more stronger than alkanes.
Boiling and melting points are increased with molecular mass.

Melting and Boiling points of alkenes

Isomerism of alkenes

Geometrical isomerism is the important isomerism case we study in alkenes due to double bond of alkenes. Other isomerism cases such as optical , chain and position isomerism cases also may be exist.

Structural isomers of alkenes

Chain and position isomers are demonstrated by alkenes as structural isomers.

Structural isomers of butene

Structural isomers of butene

Geometrical isomerism of 2-butene

There are two geometrical isomerism in 2-butene.

butene geometrical isomerism

Geometrical isomerism of C5H10

Geometrical isomerism structures for are figured below.

Geo isomerism of alkenes

Study the figure. You can see the number 2 carbon in the molecule A has two different groups as -CH3 and -H. If we interchange that two groups we get a new compound. Physical and chemical properties of A and B are different. A structure is cis isomerism and B is trans isomerism.

Hybridization of alkenes

Hybridization of ethene

Ethene only has two carbon atoms which are bonded by a double bond. Each carbon atom has three hydrogen atoms. When consider one carbon atom, it has three σ bonds and one Π bond. Therefore hybridization of that carbon ato is sp3.

Hybridization of alkenes

Hybridization of propene

Hybridization of propene

Sources of alkenes

Many industrial important alkenes are taken from breaking petroleum. Petroleum is heated upto very high temperature. Then molecules of petroleum are gone under thermolysis. These alkenes are converted into other important compounds.

Preparing Alkenes

Alkenes are prepared by several ways under different conditions.
  1. Heating alkyl halides with alcoholic potash(KOH).
  2. Preparation of alkene from alkyne.
  3. Preparation of alkene from alcohols.

Preparing alkene by Alkyl halides with alcoholic potash(KOH).

A H-X molecule is removed from alkyl halide to give alkene.Heated KOH is acted as an alkaline and remove a H atom which is linked to a C atom which is adjoin to the Br linked C.

alkene prepare-alkyl halide and potash KOH

Preparing alkene from alkyne

Alkynes are reduced to alkenes using Linda catalyst ( palladium- charcoal catalyst partially deactivated with poison like sulfur compounds or quinoline ) and hydrogen(H2)

alkene preparing from alkynes

Preparing alkene from alcohols

Alcohols are heated with sulphuric or phosphoric acid at about 443K.

alkynes preparing by alcohol and conc sulfuric

Preparing alkenes from alcohols

Alcohols are heated with anhydrous alumina( Al2O3 )

alkenes preparing by alcohol and anhydrois alumina

Reactions of alkenes

Alkenes show different kinds of reactions, those reaction kinds are defined according to the mechanisms of reactions. Reason for alkene's characteristic reactions is the double bond. A strong σ and a weak Π bond include in double bond. Two electrons of Π bond are attached very lightly. Therefore these Π electrons are more movable than σ electrons. Due to this reason, Π electrons are easily polarized than σ electrons by reagents

  • Reducing reactions
  • Oxidizing reactions
  • Electrophilic additive reactions

Electrophilic additive reactions of alkenes

The area of double bond has high electron density. Therefore alkenes allure to react with electrophiles(positively charged or neutral species). Weak Π bond is breaked easily and added the reagent through the double bond. This kind of reactions are defined as electrophilic additive reactions

Alkene and halogen reaction

Halogens( specially Cl2 and Br2 ) reacts with alkenes and produces dihalogen derivatives

alkene and halogen reaction

Stability of carbocation in intermediate states of reactions

Carbocations holds positive charges. Carbocations are formed as intermediate compounds in the reactions. If positive charge density is low, that carbocation show some stability. That positive charge density depends on number of other alkyl groups which are around positively charged C atom and state of those alkyl groups. Alkyl groups (CH3CH2- , CH3- ) can release electrons.

  • Therefore alkyl groups can reduce positive charge of carbocation.
  • Also, when number of alkyl group increases, positive charge of carbocation decreases.

This carbocation of stability of carbocation is useful when we learn some reactions of alkenes. Some examples are addition of halogen acids to alkenes, hydration of alkenes.

Alkenes and halogens reaction mechanism

Addition of halogens to alkenes is an electrophilic addition reaction. Normally, halogen molecules(Cl2, Br2) are not electron deficiency compounds. But due to higher electrons density of double bond of alkenes, halogen molecule gets polarized. The most closed atom of halogen molecule to the double bond is positively charged and other halogen atom gets negatively charged. The positively charged halogen atom(electron deficiency atom) react with double bond and make bonds with both carbon atoms of double bond. Other halogen molecule forms halide ion.

Ethene and bromine

Cyclic brominium cation

A cyclic brominium ion is given as the intermediate product. Next, one carbon atom in the brominium ion is attacked by bromide ion( Br -). Then a bond between Br - and that carbon atom forms and bromine-carbon bond in the ring is breaked. A dibromide forms as the product.

Cyclic brominium cation give dibriomidee

Reaction is started through Br δ+ . Br δ+ is an electrophilic. And this is an addition reaction. Therefore this reaction is an Electrophilic Addition Reaction.

Alkene and halogen acids (HCl, HBr , HI)

symmetrical alkene and halogen acids

One part of the halogen acid attaches to one carbon atom( in the double bond ) and other part to other carbon atom.

symmetric alkene and halogen acid reaction example

unsymmetrical alkene and halogen acids

unsymmetric alkene and halogen acid reaction example

Addition of halogen acids to unsymmetrical alkenes ( Markovnikov rule)

negative part of the addendum ( here Cl, Br,I ) joins with carbon atom which carries the smaller number of hydrogen atoms and positive part (H) goes to the carbon atom which has more hydrogen atoms.

Anti Markovnikov rule

When HBr ( HF , HCl and HI does not show this effect ) is added to an unsymmetrical alkene in the presence of organic peroxides, the reaction takes place opposite to the markovnikov rule.
That means, Br joins with carbon atom which carries the higher number of hydrogen atoms and H goes to other carbon atom.

anti markovnikov rule example

More examples of addition of halogens to alkenes

Examples of halogen acids addition to alkenes

Alkene and halogen acids reaction mechanism

This reaction occurs in two steps.

Ethene and hydrogen bromide reaction mechanism

First, H the electron deficiency part of halogen acid, is added to the π bond of alkene. Meanwhile HBr bond breaks and both electrons in the bond, gets displaced towards bromine atom. This reaction slowly occurs.

alkene bromination mechanism

Formed carbocation is very reactive.The nucleophile, Br - can give two electrons to carbon atom in the carbocation. This is the second step in the reaction and fastly occurs.

ethyl bromide formation

Alkene and strong oxidizing agent

When alkenes react with strong oxidizing reagents such as acidic potassium permanganate(H+/KMnO4) , acidic potassium dichromate(H+/ K2Cr2O7) double bond between carbon atoms are breaked. Breaked carbon atoms (in the double bond) will be oxidized. Carboxylic acids or carbon dioxides are formed according to the position of double bond. single bonds between carbon atoms remain unchanged in the carbon chain.

propene and acidic permanganate reaction

alkene and permanganate strong oxidizing agent

Alkene hydration - H2O addition to alkenes

All alkenes react with dilute H2SO4 and give give alcohols. This is a hydration. A H2O molecule is added through double bond and give primary, secondary, titary alcohols. Alkene hydration is also decided by Markovnikov rule.

alkene hydration - alkene and dilute sulfuric

Alkene and sulfuric acid reaction mechanism

First, a hydrogen atom (proton) is added to the alkene to give carbocation as a intermediate product. Two carbocations are formed in this reaction. According to the stability of carbocation, most stable carbocation forms more than other carbocation.

alkene and sulfuric carbocations

Therefore, tertiary carbocation forms more. Next, carbocation is attacked by a water molecule. Water molecule can performed as nucleophile because oxygen atom in the water molecule has two lone pair electrons. Therefore electrons density of oxygen is high. After making a bond between oxygen atom and carbon atom, oxygen atom receives a positive charge. alkene and sulfuric reaction mechanism

Finally, a proton is eliminated from the ion and give the alcohol as the product.

carbocation stability and give alcohols

When consider total reaction, a proton is used in initial step. But in final step a proton is released again. Therefore in this reaction, H2SO4 is used as a catalyst. When there is no acid to provide proton, reaction does not occur. A parts of water molecule(H and OH) is added to double bond. Therefore this reaction is a hydration reaction.

Oxidation of alkenes

Diol preparing - reaction of alkenes and cold dilute alkaline potassium permanganate solution

When alkenes react with dilute alkaline potassium permanganate solution (OH- / KMnO4), two -OH groups are attached to the two carbon atoms of the double bond. Also manganese dioxide(MnO2)a brown colour precipitate forms. As an example ethylene(CH2=CH2) is converted onto ethylene glycole(HO-CH2-CH2-OH).

diol preparing - alkenes and cold dilute alkaline potassium permanganate solution

Natural sources of alkenes

There are alkenes in aromatic oils in plants. Limonene includes in lemon and orange aromatic oils. Polyisoprene is an alkene included in rubber. There is α - pinene in turpentine.

limonene structure molecule repeating unit rubber polymer

Industrial and domestic uses of alkenes

Polyethylene, ethylene dioxide, ethylene oxide, vinyl chloride are produced by ethene. Polymerization of these compounds, very important industrial artificial polymers are produced. Ethanol, acetaldehyde and acetic acid are produced by ethene.
Polypropylene, acrylonitrile, propylene oxide and cumene are prepared by propene.

Producing polymers using alkenes

A lot of alkenes are used to prepare polymers. These polymers are very useful in our domestic purposes.

Alkene Questions, Problems and answers

We target to prepare questions of alkenes,how they are prepared, reactions of alkenes, isomerism, IUPAC names of alkenes. We presents problems as essay questions and MCQ questions.

What are the products when alkenes react with hydrogen peroxide?

Hydrogen peroxide add to alkenes to form glycols.

Ethene and hydrogen peroxide reaction

Ethene alkene and hydrogen peroxide reaction

Question 1

Synthesis propane-1,2-diol from 2-bromopropane

alkene questions and answers

Question 2

Identify isomrisms of alkenes from reactions

A is an aliphatic organic compound. It’s molecular formula is C5H10. When A is treated with dilute H2SO4, it gives B. B don’t show optical isomerism. B gives a cloudy solution instantly with NaOH(aq). A gives C when A react with Br2 / CCl4. C has a carbon atom which shows optical isomerism.
Identify different compounds for A, Write IUPAC names of A

Question 3

Addition of HBr into alkenes

What are the major and minor products when following alkene react with HBr

alkenes with HBr