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Alkenes - Chemical Properties of Alkenes

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  • Key Points: Chemical Properties of Alkenes
Maharashtra State Board: Class 12

Key Points: Chemical Properties of Alkenes

Alkenes undergo mainly electrophilic addition reactions due to the π bond (electron-rich site).

Reaction Example
Addition of hydrogen (Hydrogenation)

\[\ce{H2C = CH2 ->[H2/Ni, Pt or Pd][523-573K] H3C - CH3}\]
Addition of halogen \[\begin{array}{cc}
\phantom{}\ce{H3CCH = CH2 + Cl — Cl ->[CCl4] CH3CH - CH2}\phantom{}\\
\phantom{....................................................}|\phantom{.........}|\phantom{}\\
\phantom{.....................................................}\ce{Cl}\phantom{.......}\ce{Cl}\phantom{}
\end{array}\]
Addition of HX (Markovnikov's rule) \[\begin{array}{cc}
\phantom{..............................................................}\ce{Br}\phantom{}\\
\phantom{............................................................}|\phantom{}\\
\phantom{}\ce{\underset{(For unsymmetrical allkene-Markownikoff’s rule)}{H3CCH = CH2 + HBr} -> H3C - CH - CH3}\phantom{}
\end{array}\]
Addition of HBr (Anti-Markovnikov / Kharasch effect)

\[\begin{array}{cc}
\phantom{}\ce{\underset{(In presence of peroxide-reverse of Markownikoff’s rule)}{H3CCH = CH2 + HBr} ->[Peroxide] CH3CH2CH2Br}\phantom{}\\
\end{array}\]
Hydration (addition of H₂SO₄/H₂O) \[\begin{array}{cc} 
\ce{O}\phantom{..}\\ ||\phantom{..}\\ 
\phantom{}\ce{CH2 = CH2 + H - O - S - O - H -> C2H5HSO4}\phantom{}\\ ||\phantom{..}\\ \ce{O}\phantom{..} 
\end{array}\]
Oxidation (KMnO₄/H⁺) \[\begin{array}{cc} \phantom{..........................}\ce{O}\phantom{}\\ \phantom{..........................}||\\ \phantom{}\ce{H3C — CH = CH2 ->[{[O]}][KMnO4, {[H^{+}]}] H3C - C - OH + CO2 + H2O}\phantom{} \end{array}\]
Hydroxylation \[\begin{array}{cc}
\phantom{}\ce{H2C = CH2 + H2O + [O] ->[Dil.KMnO4][273K] CH2 - CH2}\\
\phantom{.....................................................}|\phantom{..........}|\phantom{}\\
\phantom{........................................................}\ce{OH}\phantom{.....}\ce{OH}\phantom{}
\end{array}\]
Ozonolysis \[\begin{array}{cc}
\phantom{.....}\ce{H3C}\phantom{....................................}\ce{H3C}\phantom{............................}\\
\phantom{.....}\backslash\phantom{.........................................}\backslash\phantom{.....................}\\
\phantom{..........}\ce{C = CH2 + O3 ->[Zn/H2O] \phantom{.......}C = O + HCHO}\phantom{}\\
\phantom{......}/\phantom{..........................................}/\phantom{.....................}\\
\phantom{...............}\ce{H3C}\phantom{......................................}\ce{H3C}\phantom{......................................}
\end{array}\]
Polymerisation
Hydroboration-oxidation \[\begin{array}{cc} 
\phantom{...............}\ce{H}\phantom{....}\ce{H}\phantom{.............................................}\ce{H}\phantom{....}\ce{H}\phantom{..........................}\ce{H}\phantom{....}\ce{H}\phantom{............................}\\
\phantom{.............}|\phantom{......}|\phantom{..............................................}|\phantom{......}|\phantom{............................}|\phantom{......}|\phantom{..........................}\\
\phantom{}\ce{6(H - C = C - H) + (BH3)2 ->[THF] 2(H - C - C)3 - B ->[H2O2][OH^Θ] H - C - C - H + B(OH)3}\phantom{}\\
\phantom{..........................................}|\phantom{......}|\phantom{............................}|\phantom{......}|\\
\phantom{..............................................}\ce{H}\phantom{.....}\ce{H}\phantom{..........................}\ce{H}\phantom{.....}\ce{OH}\phantom{.}
\end{array}\]
CISCE: Class 10

Example: Ethene

Ethene undergoes addition reactions:

  • With hydrogen → ethane
  • With halogens → dihaloalkanes
  • With steam (acid catalyst) → ethanol

Forms poly(ethene) (polythene) through polymerisation.

Burns with a sooty flame.

On oxidation with KMnO₄, forms ethylene glycol.

Related QuestionsVIEW ALL [19]

Arrange the following hydrogen halides in order of their decreasing reactivity with propene.

The addition of HBr to 1-butene gives a mixture of products A, B and C

(A)
(B)
(C) CH3 – CH2 – CH2 – CH2 – Br

The mixture consists of:

Write a chemical equation for combustion reaction of the following hydrocarbon:

Toluene

Write a chemical equation for combustion reaction of the following hydrocarbon: 

Pentene 

An alkene ‘A’ contains three C – C, eight C – H σ bonds and one C – C π bond. ‘A’ on ozonolysis gives two moles of an aldehyde of molar mass 44 u. Write the IUPAC name of ‘A’.

In the presence of peroxide, HCl and HI do not give anti-Markovnikov's addition of alkenes because ______.

An alkene ‘A’ contains three C – C, eight C – H σ bonds and one C – C π bond. ‘A’ on ozonolysis gives two moles of an aldehyde of molar mass 44 u. Write IUPAC name of ‘A’.

Identify the correct reagents that would bring about the following transformation.

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