★ Alkenes as a class resemble alkanes in physical properties, except in types of isomerism and difference in polar nature.

★ The first three members are gases, the next fourteen are liquids and the higher ones are solids.

★ Ethene is a colourless gas with a faint sweet smell.

★ All other alkenes are colourless and odourless, insoluble in water but fairly soluble in nonpolar solvents like benzene, petroleum ether.

★ They show a regular increase in boiling point with increase in size i.e., every – `color{red}(CH_2)` group added increases boiling point by `20–30 K`.

★ Like alkanes, straight chain alkenes have higher boiling point than isomeric branched chain compounds.

★ Alkenes are the rich source of loosely held pi `color{red}((π))` electrons, due to which they show addition reactions in which the electrophiles add on to the carbon-carbon double bond to form the addition products.

★ Some reagents also add by free radical mechanism. There are cases when under special conditions, alkenes also undergo free radical substitution reactions.

★ Oxidation and ozonolysis reactions also occurs in alkenes.

★ `color{green}("𝐀𝐝𝐝𝐢𝐭𝐢𝐨𝐧 𝐨𝐟 𝐝𝐢𝐡𝐲𝐝𝐫𝐨𝐠𝐞𝐧:")` Alkenes add up one molecule of dihydrogen gas in the presence of finely divided nickel, palladium or platinum to form alkanes (already discussed).


★ `color{green}("𝐀𝐝𝐝𝐢𝐭𝐢𝐨𝐧 𝐨𝐟 𝐡𝐚𝐥𝐨𝐠𝐞𝐧𝐬 :")` Halogens like bromine or chlorine add up to alkene to form vicinal dihalides. However, iodine does not show addition reaction under normal conditions. The reddish orange colour of bromine solution in carbon tetrachloride is discharged when bromine adds up to an unsaturation site. This reaction is used as a test for unsaturation.

★ Addition of halogens to alkenes is an example of electrophilic addition reaction involving cyclic halonium ion formation.

(i) `color{red}(underset("Ethene")(CH_2 = CH_2 ) + Br - Br overset( C Cl_4)→ underset("1 , 2 Dibromoethane")(underset(underset(Br)(|))CH_2 - underset(underset(Br)(|))CH_2))` .............(13.38)

(ii) `color{red}(underset("Propene")(CH_2 - CH = CH_2 + Cl - Cl) → underset("1,2-Dichloropropane")(CH_2 - underset(underset(Cl)(|))CH - underset(underset(Cl)(|))CH_2)` .....................(13.39)



★ `color{green}("𝐀𝐝𝐝𝐢𝐭𝐢𝐨𝐧 𝐨𝐟 𝐡𝐲𝐝𝐫𝐨𝐠𝐞𝐧 𝐡𝐚𝐥𝐢𝐝𝐞𝐬:")` Hydrogen halides `color{red}((HCl, HBr,HI))` add up to alkenes to form alkyl halides. The order of reactivity of the hydrogen halides is `color{red}(HI > HBr > HCl)`.

Addition of hydrogen halides is also an example of electrophilic addition reaction that can be illustrated as:


`color{green}("★ 𝐀𝐝𝐝𝐢𝐭𝐢𝐨𝐧 𝐫𝐞𝐚𝐜𝐭𝐢𝐨𝐧 𝐨𝐟 𝐇𝐁𝐫 𝐭𝐨 𝐬𝐲𝐦𝐦𝐞𝐭𝐫𝐢𝐜𝐚𝐥 𝐚𝐥𝐤𝐞𝐧𝐞𝐬:")`

Addition reactions of `color{red}(HBr)` to symmetrical alkenes (similar groups attached to double bond) take place by electrophilic addition mechanism.

`color{red}(CH_2 = CH_2 + H - Br → CH_3 - CH_2- Br)` .............(13.40)

`color{red}(CH_3 - CH = CH - CH_3 + HBr → CH_3 - CH_2 - underset(underset(Br)(|))CHCH_3)` ................(13.41)


`color{green}("★ 𝐀𝐝𝐝𝐢𝐭𝐢𝐨𝐧 𝐫𝐞𝐚𝐜𝐭𝐢𝐨𝐧 𝐨𝐟 𝐇𝐁𝐫 𝐭𝐨 𝐮𝐧𝐬𝐲𝐦𝐦𝐞𝐭𝐫𝐢𝐜𝐚𝐥 𝐚𝐥𝐤𝐞𝐧𝐞𝐬 (𝐌𝐚𝐫𝐤𝐨𝐯𝐧𝐢𝐤𝐨𝐯 𝐑𝐮𝐥𝐞):")`

Markovnikov, a Russian chemist made a generalisation in 1869 thatled Markovnikov to frame a rule called Markovnikov rule.



`color{green}("𝐓𝐡𝐞 𝐫𝐮𝐥𝐞 𝐬𝐭𝐚𝐭𝐞𝐬 𝐭𝐡𝐚𝐭 𝐧𝐞𝐠𝐚𝐭𝐢𝐯𝐞 𝐩𝐚𝐫𝐭 𝐨𝐟 𝐭𝐡𝐞 𝐚𝐝𝐝𝐞𝐧𝐝𝐮𝐦")` `color{green}(" (𝐚𝐝𝐝𝐢𝐧𝐠 𝐦𝐨𝐥𝐞𝐜𝐮𝐥𝐞) 𝐠𝐞𝐭𝐬 𝐚𝐭𝐭𝐚𝐜𝐡𝐞𝐝 𝐭𝐨 𝐭𝐡𝐚𝐭 𝐜𝐚𝐫𝐛𝐨𝐧 𝐚𝐭𝐨𝐦 𝐰𝐡𝐢𝐜𝐡")` `color{green}(" 𝐩𝐨𝐬𝐬𝐞𝐬𝐬𝐞𝐬 𝐥𝐞𝐬𝐬𝐞𝐫 𝐧𝐮𝐦𝐛𝐞𝐫 𝐨𝐟 𝐡𝐲𝐝𝐫𝐨𝐠𝐞𝐧 𝐚𝐭𝐨𝐦𝐬.")`



★ `color{green}("𝐌𝐞𝐜𝐡𝐚𝐧𝐢𝐬𝐦:")`


Hydrogen bromide provides an electrophile, `color{red}(H^+)`, which attacks the double bond to form carbocation as shown below :



(i) The secondary carbocation (b) is more stable than the primary carbocation (a), therefore, the former predominates because it is formed at a faster rate.

(ii) The carbocation (b) is attacked by `color{red}(Br^–)` ion to form the product as follows :

★ In the presence of peroxide, addition of `color{red}(HBr)` to unsymmetrical alkenes like propene takes place contrary to the Markovnikov rule. This happens only with `color{red}(HBr)` but not with `color{red}(HCl)` and `color{red}(HI)`.

★ This addition reaction was observed by M.S. Kharash and F.R. Mayo in 1933 at the University of Chicago. This reaction is known as peroxide or Kharash effect or addition reaction anti to Markovnikov rule.

`color{red}(CH_3 - CH = CH_2 + HBr overset((C_6H_5CO)_2O_2)→ underset("1-Bromopropane")(CH_3 - underset(underset(CH_2 Br)(|))CH_2))` .................(13.43)

★ `color{green}("𝐌𝐞𝐜𝐡𝐚𝐧𝐢𝐬𝐦 :")`
Peroxide effect proceeds via free radical chain mechanism as given below:

(i) `color{red}(underset("Benzoyl peroxide")(C_6H_5 - overset(overset(O)(||))C - O - O - overset(overset(O)(||))C - C_6H_5) overset("Homolysis")→ 2C_6H_5 - overset(overset(O)(||))C - underset(. .)overset(.)O : → 2 overset(.)C_6 H_5 + 2CO_2)`

(ii) `color{red}(overset(•)C_6H_5 + H - Br overset(" Homolysis")→ C_6H_6 + overset(•)Br)`




The secondary free radical obtained in the above mechanism (step iii) is more stable than the primary

`color{green}("★ 𝐏𝐞𝐫𝐨𝐱𝐢𝐝𝐞 𝐞𝐟𝐟𝐞𝐜𝐭 𝐢𝐬 𝐧𝐨𝐭 𝐨𝐛𝐬𝐞𝐫𝐯𝐞𝐝 𝐢𝐧 𝐚𝐝𝐝𝐢𝐭𝐢𝐨𝐧 𝐨𝐟 𝐇𝐂𝐥 𝐚𝐧𝐝 𝐇𝐈.")`

This may be due to the fact that the `color{red}(H–Cl)` bond being stronger (`color{red}(430.5 kJ mol^(–1))` ) than `color{red}(H–Br)` bond `color{red}((363.7 kJ mol^(–1))` ), is not cleaved by the free radical, whereas the `color{red}(H–I)` bond is weaker (`color{red}(296.8 kJ mol^(–1))` ) and iodine free radicals combine to form iodine molecules instead of adding to the double bond.

★ `color{green}("𝐀𝐝𝐝𝐢𝐭𝐢𝐨𝐧 𝐨𝐟 𝐬𝐮𝐥𝐩𝐡𝐮𝐫𝐢𝐜 𝐚𝐜𝐢𝐝 :")` Cold concentrated sulphuric acid adds to alkenes in accordance with Markovnikov rule to form alkyl hydrogen sulphate by the electrophilic addition reaction.



★ `color{green}("𝐀𝐝𝐝𝐢𝐭𝐢𝐨𝐧 𝐨𝐟 𝐰𝐚𝐭𝐞𝐫 :")` In the presence of a few drops of concentrated sulphuric acid alkenes react with water to form alcohols, in accordance with the Markovnikov rule.

(a)Alkenes on reaction with cold, dilute, aqueous solution of potassium permanganate (Baeyer’s reagent)
produce vicinal glycols. Decolorisation of `color{red}(KMnO_4)` solution is used as a test for unsaturation.


(b) Depending upon the nature of the alkene and the experimental conditions acidic potassium
permanganate or acidic potassium dichromate oxidises alkenes to ketones and/or acids.

`color{red}(underset("2 - Methlypropene")((CH_3)_2 C = CH_2) overset(KMnO_4 // H^(+) )→ underset(" Propan - 2 -one ")( (CH_3)_2 C = O ) + CO_2 + H_2O)` ........(13.49)

`color{red}(underset("But - 2 - ene ")(CH_3 - CH = CH - CH_3) overset(KMnO_4//H^(+))→ underset("Ethanoic acid ")(2CH_3COOH))` ..............(13.50)

★ Ozonolysis of alkenes involves the addition of ozone molecule to alkene to form ozonide, and then cleavage of the ozonide by `color{red}(Zn-H_2O)` to smaller molecules. This reaction is highly useful in detecting the position of the double bond in alkenes or other unsaturated compounds.

★ Polythene is obtained by the combination of large number of ethene molecules at high temperature, high pressure and in the presence of a catalyst. The large molecules thus obtained are called polymers. This reaction is known as polymerisation. The simple compounds from which polymers are made are called monomers.



★ `color{green}("𝐔𝐒𝐄𝐒 𝐎𝐅 𝐏𝐎𝐋𝐘𝐌𝐄𝐑𝐒:")`

★ Polymers are used for the manufacture of plastic bags, squeeze bottles, refrigerator dishes, toys, pipes, radio and T.V. cabinets etc.

★Polypropene is used for the manufacture of milk crates, plastic buckets and other moulded articles.