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Alkenes are hydrocarbons that contain at least one double bond between carbons. This sets them apart from alkanes, which have only single bonds.

As with alkanes, alkenes can have branched chains of carbon. There are more possibilities for different structures in alkenes because the position of the double bond may vary.

These two alkenes with four carbon atoms are isomers.
These two alkenes with four carbon atoms are isomers.

All alkenes with a single double bond have the general formula $$\text{C}_\text{n}\text{H}_{2\text{n}},$$ no matter where the double bond is found and regardless of whether the alkenes are branched or unbranched.

Like alkanes, alkenes can undergo combustion.

Complete combustion occurs when there is sufficient (or excess) oxygen. In a complete combustion reaction, alkenes react to form carbon dioxide and water: $$$\ce{CH2=CH2 + 3O2 -> 2CO2 + 2H2O}$$$

Incomplete combustion occurs when there is insufficient oxygen. The reaction produces carbon monoxide ($$\ce{CO}$$) or carbon soot ($$\ce{C}$$) instead of $$\ce{CO2}$$.

Incomplete combustion of ethene involves two different reactions: $$$\ce{CH2=CH2 + 2O2 -> 2CO + 2H2O}$$$ $$$\ce{CH2=CH2 + O2 -> 2C + 2H2O}$$$

Incomplete combustion (and hence a sooty flame) is more common with alkenes than with alkanes.

This is because alkenes have a higher ratio of carbon to hydrogen than what is found in alkanes.

Some of the carbon will not be oxidised properly, forming soot.

Complete combustion gives a clean flame (left) while incomplete combustion produces sooty flames (right) which give off black smoke.
Complete combustion gives a clean flame (left) while incomplete combustion produces sooty flames (right) which give off black smoke.

Alkanes and alkenes have similar physical properties. They can only be distinguished by chemical means.

Alkanes burn in oxygen with a clean flame while alkenes usually produce a sooty flame. This is, however, only a rough test.

The main test to distinguish alkenes from alkanes uses bromine water, a reddish-brown solution containing bromine ($$\ce{Br2}$$). Alkanes do not react with bromine water at room temperature but alkenes do.

The bromine water becomes colourless when alkenes are added. By contrast, bromine water does not change colour when alkanes are added.

$$$\ce{CH2=CH2 (g) + Br2(aq) -> CH2Br-CH2Br(g)}$$$

Bromine water may not become fully decolourised. It may lighten only slightly if there are not enough alkenes to react with all the bromine.

The double bond in alkenes is relatively reactive, so alkenes participate in several different reactions.

In an addition reaction, a small molecule is added to carbon atoms connected by a double bond.

The double bond is converted into a single bond, the other molecule is split into two parts and the two alkene carbon atoms bond with these parts.

Hydrogenation is the addition of hydrogen to alkenes, turning them into alkanes. It requires high temperatures and a catalyst (usually nickel).

The hydrogenation of propene to produce propane is shown:

$$$\ce{CH2=CH-CH3 + H2 -> CH3-CH2-CH3}$$$

Like alkanes, alkenes can undergo combustion. The availability of oxygen in the environment determines the type of combustion.

Cracking is the process of breaking up long-chained hydrocarbons into smaller ones.

If the original hydrocarbon is an alkane, the products of the reaction are one alkane, one alkene and hydrogen gas.

$$$\ce{C10H22 -> C7H16 + C3H6}$$$

$$$\ce{C2H6 -> C2H4 + H2}$$$

Cracking is commonly used in the petroleum industry because long-chained alkanes have only limited uses. Short-chained alkanes have wider applications.

Cracking of alkanes requires high temperature and pressure to convert them to gaseous phase.

A catalyst (such as aluminium oxide, silicon dioxide or zeolite) is needed to aid in $$\ce{C-C}$$ bond-breaking.

The cracking process affects carbon bonds randomly. It is not possible to force the breaking of $$\ce{C-C}$$ to occur at a specific location in the carbon chain.

As a result, the cracking of a large alkane can give rise to numerous different products.

Hydrocarbons can be classified according to the types of bonds between the carbon atoms:

  • Saturated hydrocarbons have only single bonds between carbon atoms.
  • Unsaturated hydrocarbons have at least one double or triple bond between carbon atoms.

Alkanes are saturated hydrocarbons. Alkenes (which have at least one double bond) and alkynes (which have at least one triple bond) are unsaturated.

Saturated Unsaturated

Polyunsaturated molecules have multiple double or triple bonds between carbon atoms.

Many oils (like olive oil or sunflower oil) are polyunsaturated organic compounds.

Hydrogenation is the process that converts unsaturated carbon chains (which have at least one double bond) into saturated chains.

This process involves adding hydrogen atoms across $$\ce{C=C}$$ or $$\ce{C#C}$$ bonds. Hydrogenation is important for converting oils into fats.

Fats (like butter) contain carbon chains that have few double bonds between carbons. They are solid at room temperature and are derived mostly from animals.

Oils (like olive oil) are similar in structure to fats but they have more double bonds (less saturated). They are liquid at room temperature.

Margarine - a product of hydrogenation of vegetable oil.
Margarine - a product of hydrogenation of vegetable oil.

To convert oil into a solid fat, the oil is heated and hydrogen gas is pumped through it. The reaction requires a catalyst (frequently nickel, $$\ce{Ni}$$).

The hydrogenated product is margarine (a substitute for butter).