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.....)
Ex: if n=3;
Respective hydrocarbon : C3H2*3 = C3H6
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.
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.
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.
Chain and position isomers are demonstrated by alkenes as structural isomers.
There are two geometrical isomerism in 2-butene.
Geometrical isomerism structures for are figured below.
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.
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.
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.
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.
Alkynes are reduced to alkenes using Linda catalyst ( palladium- charcoal catalyst partially deactivated with poison like sulfur compounds or quinoline ) and hydrogen(H2)
Alcohols are heated with sulphuric or phosphoric acid at about 443K.
Alcohols are heated with anhydrous alumina( Al2O3 )
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
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
Halogens( specially Cl2 and Br2 ) reacts with alkenes and produces dihalogen derivatives
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.
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.
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.
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.
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.
One part of the halogen acid attaches to one carbon atom( in the double bond ) and other part to other carbon atom.
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.
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.
More examples of addition of halogens to alkenes
This reaction occurs in two steps.
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.
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.
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.
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.
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.
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.
Finally, a proton is eliminated from the ion and give the alcohol as the product.
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.
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).
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.
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.
A lot of alkenes are used to prepare polymers. These polymers are very useful in our domestic purposes.
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.
Hydrogen peroxide add to alkenes to form glycols.
Propane-1,2-diol include two -OH groups. Diols can be prepared from alkenes from reaction of alkenes and OH-/ KMnO4. Also it can be prepared from halo carbons. But two halogen atoms should exist and be on two carbon atoms. Study the figure how alkene is prepared from halo alkane and some reactions of alkenes.
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
When we consider about molecular formula C5H10 , we can identify this as an alkene.Alkene’s general formula of molecular formula CnH2n. Therefore, when n=5, molecular formula is C5H10 According to the question we know, A reacts with dilute H2SO4 and Br2/CCl4. Alkenes react with H2SO4 and HBr. So, we can prove A is an alkene.
What are the major and minor products when following alkene react with HBr
Addition of HBr to alkenes is decided by Markovnikov's rule. But in this example we see there are no H atoms which have bonded to C atoms which exist in double bond. So we have to find first, Where H atom (in HBr molecule) will be bonded? Therefore we consider about mechanism of addition of HBr to alkenes.
there may be two intermediate compounds (carbocation). If one intermediate compound is more stable than other intermediate compound, the stable intermediate compounds forms mostly.
Alkyl groups such as -CH3, -CH2CH3 can release electrons and when C chain gets longer inductive effect(releasing electrons) is increased. Therefore inductive effects of different alkyl groups are changed as below.
So we can understand, plus charge of intermediate compound i is low. So it is more stable than other carbocation . Therefore major product is given according to the most stable intermediate compound.