Properties of Alkene

• Physical Properties of Alkenes

All are colourless & have no characteristic odour. Ethene has pleasant smell.

Lower members (C₂ to C₄) are gases, middle one (C₃ to C₁₇) are liquids, higher are solids.

The boiling points, melting points, and specific gravities show a regular increase with increase in molecular weight, however less volatile than corresponding alkanes.

A cis isomer has high boiling and melting point than trans isomer because of more polar nature.

Like alkanes, these too are soluble in non polar solvents.

Alkenes are weak polar. The polarity of cis isomer is more than trans which are either non polar or less polar. (e.g. trans butene-2 is non polar; trans pentene-2 is weak polar).

• Chemical Properties of Alkenes

Alkenes are highly reactive due to unsaturation. The double bond consist of a strong σ and a weaker Π bond. The Π electrons are loosely held and are easily polarized on the need of an attacking reagent as a result of electromeric effect. This gives rise to the formation of

C⁺ & C^- centres at the unsaturation point provide sites for electrophilic additions 

The reactivity order for alkenes has been given as:

CH₂==CH₂ > R-CH==CH₂ > RCH==CHR > Cis R₂C == CHR >  trans-R₂C == CR₂

The reactivity order of alkenes has been dealt in terms of heat of hydrogenation of alkene. More is heat of hydrogenation (ΔH = -ve), more is reactivity of alkene. The reactivity of alkene is however also related to

1. Steric effect

2. Hyper conjugation

3. Heat of combustion

• Conversion to Alkanes

?

The relative rates of hydrogenation

H₂C=CH₂  >  RCH=CH₂  >  R₂C=CH₂,  RCH=CHR  >  R₂C=CHR  >  R₂C=CR₂

Indicate that the rate is decreased by steric hindrance.

• Electrophilic Addition Reactions

Table shows the results of electrophilic addition of polar reagents to ethylene.

1.  Addition of Hydrogen Halides to Alkenes ?

Hydrogen halides (HCl, HBr and HI) add to the double bond of alkenes : >C=C< + HX → >CH-CX<

Mechanisms for addition of hydrogen halide to an alkene involves the following two steps :

The addition of HBr to some alkenes give a mixture of the expected alkyl bromide and an isomer formed by rearrangement.

With this understanding of the mechanism for the ionic addition of hydrogen halides to alkenes, a statement can be made :

In the ionic addition of an unsymmetrical reagent to a double bond, the positive portion of the reagent attaches itself to a carbon atom of the double bond so as to yield the more stable carbocation as intermediate.  Because this is the step that occurs first, it is the step that determines the overall orientation of the reaction.

When HI is added to 1-butene the reaction leads to the formation 2-iodobutane, that contains a stereocenter.

The product, therefore, can exist as a pair of enantiomers.  The carbocation that is formed in this first step of the addition is trigonal planar and is achiral.  When the iodide ion reacts with this flat carbocation, reaction is equally likely at either face.  The reaction leading to the two enantiomers occur at the same rate, and the enantiomers, therefore, are produced in equal amounts as a racemic form.

Markovnikov’s Rule:   Arrording to thies rule negative part of the addendum (adding molecule) gets attached to that carbon atom which possesses lesser number of hydrogen atoms.

So according to Markonikov’s rule, during addition of hydrogen halide to alkene, the hydrogen does to that double bonded carbon atom which contains greater number of H atoms already attached to it while the  halide ion goes to the one which contians leseer number of H atoms alredy attached to it.

i.e.  CH₃-CH=CH₂+ HX → CH₃-CH(Br)-CH₃

Mechanism of Markonikov’s Addition:

Refer following Video for Markonikov’s Addition

Effect of Peroxide( Markonikov’s Addition): 

In presence of peroxides addition of HX to alkenes takes place contarary to Markonikov’s Rule.This happens only with HBr and not HCl and HI

Mechanism of Aniti-Markonikov’s Addition: Follows free radical mechanisms.

Peroxide effect proceeds via free radical chain mechanism.

2.  Addition of Bromine And Chlorine To Alkenes

Alkenes react rapidly with bromine at room temperature and in absence of light.  If bromine is added to an alkene, the red-brown colour of the bromine disappears almost instantly as long as the alkene is present in excess.  The reaction is one of addition .

• Oxidation of Alkenes

1. Combustion : The combustion of alkenes is also exothermic with high calorific values and thus used for welding purposes in oxy-ethylene welding.

CH₂=CH₂ + 3O₂ → 2CO₂ + 2H₂O; ΔH = -ve

C_nH_2n + (3n/2)O₂ → nCO₂ + nH₂O; ΔH = -ve

2. Oxidation by Baeyers' reagent or hydroxylation : A test for unsaturation

Alkenes on passing through dilute alkaline, 1% cold KMnO₄ (i.e. Baeyers reagent) decolorize the pink colour of KMnO₄ and forms dihydroxy compounds (e.g. glycols)

3. Oxidation by alkaline KMnO₄ : Oxidation of alkenes by hot alkaline KMnO₄gives two acid salts showing fission of C=C bond

RCH=CHR'  RCOOK + R'COOK

                   ^{alk. KMnO}₄

4. Oxidation by acidic KMnO₄ or K₂Cr₂O₇ :

Oxidation of alkenes by acidic KMnO₄ or K₂Cr₂O₇ gives carboxylic acids. If HCOOH is acid, it is further oxidized to CO₂ & H₂O.

CH₃-CH = CH₂  CH₃COOH + HCOOH

RCH = CHR'   RCOOH + R'COOH

Same products are obtained if oxidation is made by per iodic acid or lead tetra acetate.

The nature of acid formed decides the position of unsaturation in molecule.

Note : Alkenes on oxidation by osmium tetraxide gives an intermediate product which on refluxing with NaHSO₃ (alc.) gives glycols.

• Ozonolysis of Alkenes

Ozonolysis of alkenes : A test for unsaturation in molecule.

A more widely used method for locating the double bond of an alkene involves the use of ozone (O₃).  Ozone reacts vigorously with alkenes to form unstable compounds called initial ozonides, which rearranges spontaneously to form compounds known as ozonides.  Ozonides, themselves are unstable and reduced directly with Zn and water.  The reduction produces carbonyl compounds that can be isolated and identified.

(a)On passing ozone through a solution of alkenes in inert solvent i.e. CHCI₃ or CCI₄ or ether, addition of ozone takes place round double bond of alkene to form ozonides.  

(b)The mono ozonides are highly explosive in nature and are generally decomposed during hydrolysis or by reduction with hydrogen in presence of catalyst to give two molecules of carbonyl compounds.  

(c)The complete process of ozonide formation (step a) and then their decomposition to give carbonyl compounds (step b) is known as ozonolysis.  

(d)The ozonolysis thus involves the replacement of an olefinic bond C=C by two carbonyl groups C =O. The total number of carbon atoms in two carbonyl compound is equal to total number of carbon atoms in alkene.  

(e)The ozonolysis is used to detect the position and nature of unsaturation in a molecule. For this purpose first ozonides are formed. The solution is evaporated to get the ozonides as viscous oil which are then either hydrolysed directly with water using Zn dust as reducing agent or reduced by H₂ is presence of Pd or Pt. The Zn dust used during hydrolysis checks the formation of H₂O₂which can otherwise oxidize the products (carbonyl compounds) to respective acids. Identification of aldehydes & ketones formed during ozonolysis suggests the nature & position of unsaturation in molecule.  

(f)A symmetrical alkene give rise to two molecules of same carbonyl compound.  

Examples:

An alkene of the type RCH=CHR’ gives two aldehydes RCHO & R’CHO  

An alkene of the type R₂C = CHR’ gives R₂C = O + R’CHO  

An alkene of the type R₂C=CR₂’ gives ketones only R₂C=O&R₂’C=O 

Reduction of ozonide can also be made by Zn/Acid, H2-Raney Ni or triphenyl phosphine to carbonyl compounds.    

Reduction of ozonide by LiAIH₄ or sodium borohydride gives corresponding alcohols. 

• Isomerization  of Alkenes

Alkenes on heating to 500 to 700^oC or on heating in presence of catalyst {AICI₃ or AI₂(SO₄)₃} undergo isomerization. The isomerisation involves migration of olefinic bond or alkyl group. 

 CH₃—CH₂—CH = CH₂  CH₃—CH=CH—CH₃ or (CH₃)₂C=CH₂  

              ^{butene-1                            Δ                         butene-2                         isobutene}  

• Allylic Substitution of Alkenes

The alkyl group of the alkene (except C₂H₄) undergoes substitution at high temperature in presence of CI₂ or Br₂.  

It is free radical substitution  

The substitution occurs at a–carbon to the double bond. 

 CH₃—CH=CH₂ + CI₂  CH₃CICH=CH₂ + HCI  

CH₃CH₂CH=CH₂ + CI₂  CH₃CHCICH = CH₂ + HCI 

Note :At normal temperature halogens show addition reactions with alkenes.  

• Wohl-Ziegler Bromination. (Allylic Bromination with NBS)  

• Oxy-Mercuration-Demercuration  

?Alkenes react with mercuric acetate in the presence of water to give hydroxymercurial compounds which on reduction yield alcohols.

The first stage, oxymercuration, involves addition of -OH and HgOAc to the C-C double bond.  Then, in demercuration, HgOAc is replaced by H.  The reaction sequence amounts to hydration of the alkene, but is much more widely applicable than direct hydration. Oxymercuration-demercuration gives alcohols corresponding to Markovnikov addition of water to the carbon-carbon double bond.  For example :

• Heats of Hydrogenation of Alkenes

Hydrogenation provides a way to measure the relative stabilities of certain alkenes. The reaction of an alkene with hydrogen is an exothermic reaction; the enthalpy change involved is called the heat of hydrogenation. Most alkenes have heats of hydrogenation near - 30 kcal mol^-1.

For example,

In each reaction the product is the same. A different amount of heat is evolved in each and this difference is related to different relative stabilities of the individual butenes. 1-Butene evolves the greatest amount of heat when hydrogenated, and trans-2 butene evolves the least. Hence, 1-butene must have the greatest potential energy and be the least stable isomers. Trans - 2 butene must have the lowest P.E and be the most stable isomer.

• Hydroboration-Oxidation

With the reagent diborane, (BH₃)₂, alkenes undergo hydroboration to yield alkylboranes, R₃B, which on oxidation gives alcohols.  For example:

The hydroboration oxidation process gives products corresponding to anti-Markovnikov addition of water to the C-C double bond.  This reaction is free from rearrangement.

Examples :

• Dienes or Diolefins or Alkadienes

Hydrocarbons having two double bonds ae known as alkadienes. Alkadienes are classified into three categories on the basis of location of two double bonds.

Cumulative dienes: Two double bonds are on adjacent carbon atoms e.g.

CH₂ = C = CH₂ allene or 1, 2-propadiene

Conjugated dienes: Molecules having alternate single & double bonds e.g.

CH₂ = CH-CH = CH₂ buta-1, 3-diene

Isolated diene : Molecules having two double bonds separated by more than one single bond.

CH₂=CH-CH₂-CH=CH₂                     1, 4-pentadiene

CH=CH-CH₂-CH₂-CH=CH₂                1, 5-hexadiene

Among the three types of dienes, conjugated alkadienes have some characteristic nature and undergo addition reactions in a peculiar manner. The simplest conjugated alkadiene is buta-1, 3-diene. It has following note worthy features.

All carbon atoms in CH₂=CH-CH=CH₂ (buta-1, 3-diene) are sp² hybridized

The delocalization of Π electrons results in resonance in molecule to show extra stable nature than corresponding non conjugated alkadienes (Resonance energy of buta-1, 3-diene is 3 kcal mol^-1)

CH₂=CH-CH=CH₂ ↔ CH₂⁺ -CH=CH-CH₂ ↔ CH₂-CH=CH-CH₂

The addition of H₂, Br₂ or HBr ........ etc on conjugated alkadienes takes place in two ways either 1, 2 or 1, 4-addition.

Non ionizing solvent favours 1, 2-addition whereas ionizing solvent favours 1, 4-addition. However in each case mixture of both type of addition products are formed, the one predominating on the other as the case may be.

It undergoes polymerization in presence of peroxides to give polybutadiene (Buna Rubber).

 nCH₂=CH-CH=CH₂     (CH₂-CH=CH-CH₂)_n

                                                             ^{Buna rubber}

• Uses of Alkenes

1. In plastic formation i.e., polyethene, polypropene etc. and synthetic rubber formation.  

2. In oxy-ethylene welding. 

3. As food preservatives (C₂H₄) and ripening of fruits.  

4. As general anesthetic (C₂H₄ with 10% O₂)  

5. In preparation of mustard gas – An oily liquid having high vaporizing tendency. Its vapours have high penetrating power and penetrate even thick boots and causes painful blisters on skin as well as inside the body, causing death ultimately. It was used I world war.

You might be interested in referring IIT JEE Organic Chemistry Syllabus, Books and Revision Notes on Organic Chemistry.