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Revision:Alkenes

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TSR Wiki > Study Help > Subjects and Revision > Revision Notes > Chemistry > Alkenes

Alkenes have the general formula CnH2n. They contain 1 or more double bonds, and are saturated hydrocarbons with a C=C double bond, consisting of a single sigma bond and a pi bond. The C=C bond is unable to rotate due to the pi bond, and as such there are geometric (E-Z) isomers of alkenes, when there are different branches on the two carbons either side of the double bond. (Explain E-Z naming system, and why it's needed over cis- and trans-).

Addition Reactions

Hydrogen:

Alkenes can have hydrogen atoms added across the double bond to form alkanes. This is done with a nickel catalyst at 150 degrees C (or with a platinum catalyst at rtp).

\mathrm{C_2H_4 + H_2 \longrightarrow C_2H_6}

Halogens:

Similar to addition of hydrogen, no catalyst required.

\mathrm{C_2H_4 + Br_2 \longrightarrow CH_2BrCH_2Br}

Because of the reactivity of the double bond with halogens, a test for alkenes is the addition of bromine water, which will change from orange to colourless with alkenes. The reaction is more complicated than the above however, due to the presence of water:

\mathrm{CH_3CHCH_3 + Br_2 + H_2O \longrightarrow CH_3CH(OH)CH_2Br + HBr}

Halogen Halides:

Similar to previous reaction. Markovnikov's Rule states that the hydrogen will be added to the carbon with the most hydrogens on:

\mathrm{CH_3CHCH_2 + HBr \longrightarrow CH_3CHBrCH_3}

rather than \mathrm{CH_3CHCH_2 + HBr \longrightarrow CH_3CH_2CH_2Br}

Oxidation:

By potassium manganate(VII), to produce a diol:

\mathrm{CH_2CH_2 + [O] + H2O \longrightarrow CH_2OHCH_2OH}

Mechanisms

(Mechanism diagram and evidence for electrophilic addition of bromine and hydrogen bromide to ethene and propene)

Polymerisation

Alkenes can also form polymers when their double bond opens up. (Diagram, incl. repeat unit)

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