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Preparation of Alkenes

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Estimated time: 3 minutes
Maharashtra State Board: Class 12

Key Points: Preparation of Alkenes

Method Example
Dehydration of alcohol \[\ce{CH3CH2CH2OH ->[Conc H2SO4][160°C] CH3CH = CH2 + H2O}\]
Dehydrohalogenation of alkyl halide \[\begin{array}{cc} 
\phantom{}\ce{CH3 - CH2 ->[Alc KOH][Δ] CH2 = CH2 + KBr + H2}\phantom{}\\
|\phantom{.............................................}\\
\ce{Br}\phantom{...........................................}
\end{array}\]
Dehalogenation of vicinal dihalide image (1) 3.jpg
From alkyne (partial reduction — gives cis)

\[\ce{CH ≡ CH + H2 ->[Pd/C] \underset{cis-alkene}{CH2 = CH2}}\]
From alkyne (gives trans) \[\begin{array}{cc} 
\phantom{..........................................}\ce{R}\phantom{...........}\ce{H}\phantom{.}\\
\phantom{..........................................}\backslash\phantom{........}/\phantom{.}\\
\phantom{}\ce{R - C ≡ C - R ->[Na][Liquid NH3] C = C}\phantom{}\\
\phantom{..........................................}/\phantom{........}\backslash\phantom{.}\\
\phantom{..........................................}\underset{\text{trans-alkene}}{\ce{R}\phantom{...........}\ce{R}}\phantom{.}
\end{array}\]
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