Reactions of alkenes

In a nutshell

Alkenes have a reactive $C=C$ bond which enables them to undergo electrophilic addition reactions. Some reactions need to be carried out in the presence of a catalyst and/or under specific conditions.

Equations

These are general chemical equations. You will need to be able to use these to write equations with specific substances.

$\begin{aligned}alkene \space + \space halogen &\rightarrow dihaloalkane \\\\ alkene \space + \space hydrogen &\xrightarrow[150\degree C]{Ni} alkane \\\\ alkene \space + \space steam &\xleftrightharpoons[300\degree C, \space 60atm]{H_3PO_4} alcohol \\\\alkene &\xrightarrow {H^+/MnO_4^-} \, diol \\\\alkene \space + hydrogen \space halide &\rightarrow haloalkane \\\\alkene \space + \space water &\xrightarrow{H_2SO_4} alcohol\end{aligned}$

Reactivity of alkenes

The presence of $C=C$ bond makes alkenes reactive - going from carbon-to-carbon double bond $(C=C)$ to a carbon-to-carbon single bond ( $C-C$ ). They undergo electrophilic addition reactions whereby another molecule is able to add across the $C=C$ bond, resulting in one product.

The products formed are saturated molecules because there are no $C=C$ bonds present.

Halogenation

Halogenation is when a halogen, such as chlorine or bromine, is 'added' across the $C=C$ bond to give a dihaloalkane (dihalogenoalkane).

$alkene \space + \space halogen \rightarrow dihaloalkane$

Chemistry; Organic chemistry I; KS5 Year 12; Reactions of alkenes

Example

$ethene \space + \space iodine \rightarrow diiodoethane \\ \ \\C_2H_4 \space +\space I_2 \rightarrow C_2H_4I_2$

General Mechanism

Chemistry; Organic chemistry I; KS5 Year 12; Reactions of alkenes

Bromine water

Bromine water is orangey-brown. When an alkene is added to bromine water it becomes colourless. This indicates a reaction between bromine and alkene molecules has occurred. As bromine is used up it leads to the solution to become colourless.

1. Bromine water: orangey-brown solution.
2. Bromoalkane: colourless solution.

Hydrogenation

Hydrogenation is when hydrogen is 'added' across the $C=C$ bond to give an alkane, in the presence of a catalyst such as nickel ( $Ni$ ).

$alkene \space + \space hydrogen \xrightarrow[150\degree C]{Ni} alkane$

Example

Chemistry; Organic chemistry I; KS5 Year 12; Reactions of alkenes

Hydration

Hydration is the addition of a gaseous water molecule (steam) across the $C=C$ bond to form an alcohol, in the presence of a catalyst, such as solid phosphoric ( $V$ ) acid $(H_3PO_4)$ .

$alkene \space + \space steam \xleftrightharpoons[300\degree C, \space 60atm]{H_3PO_4} alcohol$

Example

$ethene \space + \space steam \xleftrightharpoons[300\degree C, \space 60atm]{H_3PO_4} ethanol$

$C_2H_4(g)\space + \space H_2O (g) \xleftrightharpoons[300\degree C, \space 60atm]{H_3PO_4} C_2H_5OH(g)$

Oxidation

Acidified potassium manganate ( $VII)$ is a purple reagent. An alkane can react with this reagent leading to the solution to become decolourised. The product is a diol.

$alkene \xrightarrow {H^+/MnO_4^-} \, diol$

Example

$propene \xrightarrow {H^+/MnO_4^-} \, propane-1,2-diol \\ \ \\C_3H_6 \xrightarrow {H^+/MnO_4^-} \, C_3H_6(OH)_2$

Hydrogen halides

Hydrogen halides can add across the double bond to form a haloalkane.

$alkene \space + hydrogen \space halide \rightarrow haloalkane$

However, there is a possibility of two different products forming if it reacts with an unsymmetrical alkene. The quantity of each product is dependant on the stability of the carbocation intermediate. The greater the number of alkyl groups attached to the carbocation the more stable the carbocation is. This is because alkyl groups inductively donate electrons to the carbocation, stabilising the positive charge.

Chemistry; Organic chemistry I; KS5 Year 12; Reactions of alkenes — 1. Primary carbocation; 2. Secondary carbocation; 3. Tertiary carbocation.

In other words the major product is where the hydrogen is added to the carbon with the most hydrogens attached and the halide adds to the carbon with the least hydrogens attached. This is known as Markownikoff's rule.

Example

Below is a mechanism for the reaction between hydrogen chloride and propene.

Chemistry; Organic chemistry I; KS5 Year 12; Reactions of alkenes

Note: 1. is $2-chloropropane \ (major \ product)$ and 2. is $1-chloropropane \ (minor \ product)$

Formation of alcohol using $\bf{H_2SO_4}$

Alcohols can be formed via the reaction between alkenes and cold concentrated sulfuric acid. An alkyl hydrogen sulfate is one of the intermediates formed during the reaction. Upon the formation of this intermediate, add cold water and warm the reaction mixture to form the alcohol.

$alkene \space + \space water \xrightarrow{H_2SO_4} alcohol$

As with hydrogen halides, you form a mixture of major and minor products if the alkene is unsymmetrical. The most stable carbocation intermediate will go onto form the major product.

Example

$C_2H_4 \space + \space H_2O \xrightarrow{H_2SO_4} C_2H_5OH$ 

Below is the mechanism for the reaction between ethene and cold concentrated sulfuric acid.

Chemistry; Organic chemistry I; KS5 Year 12; Reactions of alkenes