★ 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 :
★ 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 :