Alkenes preparing, reactions, physical properties

General formula of alkenes - C_nH_2n [n=2,3,4....]

Ex: if n=3;
Respective hydrocarbon : C₃H_2*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.

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

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.

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)

Preparing alkene from alcohols

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

Preparing alkenes from alcohols

Alcohols are heated with anhydrous alumina( Al₂O₃ )

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

Alkene and halogen reaction

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

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

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.

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.

unsymmetrical alkene and halogen acids

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.

More examples of addition of halogens 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.

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.

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 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 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.

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.

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).

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.

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

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

Question 1

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

Answer

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.

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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

Answer

When we consider about molecular formula C₅H₁₀ , we can identify this as an alkene.Alkene’s general formula of molecular formula C_nH_2n. 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.

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Question 3

Addition of HBr into alkenes

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.

Inductive effect

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.

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