Alkenes and Alkynes: Unsaturated Hydrocarbons

Alkenes are hydrocarbons with at least one carbon–carbon double bond (general formula C_nH_2n), and alkynes are hydrocarbons with at least one carbon–carbon triple bond (general formula C_nH_2n−2); both are called unsaturated because they can add more atoms. In brief: ethene is CH₂=CH₂ (C₂H₄), ethyne is HC≡CH (C₂H₂), and both decolorize bromine water by addition reactions.

Alkenes and alkynes are much more reactive than alkanes because of their multiple bonds. They are some of the most important raw materials of the chemical industry: ethene is the starting material for polyethylene and many other products. This article describes their bonding and structure, compares the three types of carbon–carbon bond, covers the main reactions and shows how to work out the degree of unsaturation.

How are double and triple bonds formed?

In an alkene, each carbon atom of the double bond is sp² hybridized. The two carbon atoms are connected by a sigma bond, formed by overlap of sp² orbitals, and a pi bond, formed by sideways overlap of the unhybridized p orbitals above and below the plane of the molecule. Each carbon is bonded to three atoms in a plane with angles of about 120°.

In an alkyne, each carbon atom of the triple bond is sp hybridized. The triple bond consists of one sigma bond and two pi bonds at right angles to each other. The geometry around each alkyne carbon is linear, with an angle of 180°.

The pi electrons are further from the nuclei and are more exposed than the sigma electrons, so they are more easily attacked by electrophiles. This is the reason for the reactivity of unsaturated hydrocarbons. Rotation around a C=C bond is not possible at room temperature without breaking the pi bond, which leads to cis–trans isomerism in alkenes.

Comparison of C–C, C=C and C≡C

The bond energy of the double bond is less than twice that of the single bond, because the pi bond (about 268 kJ/mol) is weaker than the sigma bond. This is why a double bond can be broken in addition reactions while the sigma bond remains.

How are alkenes and alkynes named?

The suffix is changed from -ane to -ene for alkenes and -yne for alkynes. The chain is numbered from the end that gives the lowest number to the first carbon of the multiple bond, and this number is placed before the suffix: but-1-ene (CH₂=CHCH₂CH₃), but-2-ene (CH₃CH=CHCH₃), pent-2-yne (CH₃C≡CCH₂CH₃). Ethene and ethyne are also commonly known as ethylene and acetylene.

How do they react?

The characteristic reaction is addition, in which the pi bond breaks and two new sigma bonds form:

• Hydrogenation: CH₂=CH₂ + H₂ → CH₃CH₃, with a nickel, platinum or palladium catalyst. This process is used to convert liquid vegetable oils into solid fats.
• Halogenation: CH₂=CH₂ + Br₂ → CH₂BrCH₂Br.
• Addition of hydrogen halides: CH₂=CH₂ + HBr → CH₃CH₂Br. With an unsymmetrical alkene such as propene, the hydrogen adds to the carbon that already has more hydrogen atoms (Markovnikov’s rule).
• Hydration: CH₂=CH₂ + H₂O → CH₃CH₂OH, with an acid catalyst, which is an industrial route to ethanol.
• Polymerization: many ethene molecules join to form polyethylene.

Alkynes can add two molecules of reagent: HC≡CH + 2H₂ → CH₃CH₃. Both alkenes and alkynes burn, but alkynes burn with a sooty flame since their carbon content is higher. The combustion of ethyne with oxygen in an oxyacetylene torch gives a flame temperature of over 3000 °C, enough to cut and weld steel.

How can you test for unsaturation?

Add a few drops of bromine water (orange-brown) to the sample and shake. If the sample contains a C=C or C≡C bond, the bromine is consumed by an addition reaction and the color disappears. An alkane does not react in the dark, so the color remains. A similar test uses a cold, dilute, acidified potassium permanganate solution: purple turns to colorless (or brown MnO₂) with alkenes.

How do you calculate the degree of unsaturation?

The degree of unsaturation (DoU) is the number of double bonds plus rings, with a triple bond counting as two. For a compound C_cH_h, DoU = (2c + 2 − h) / 2.

Worked example

For C₅H₈: DoU = (2 × 5 + 2 − 8) / 2 = 2. The compound could contain a triple bond (pent-1-yne), two double bonds (penta-1,3-diene), one double bond and a ring (cyclopentene) or two rings. For C₄H₈: DoU = (10 − 8) / 2 = 1, and the compound is a butene or a cyclobutane or methylcyclopropane.

Where are they used?

• Plastics: ethene gives polyethylene, propene gives polypropylene, and chloroethene gives PVC.
• Ripening of fruit: ethene is a plant hormone, and fruit is ripened in storage with controlled amounts of it.
• Chemical industry: ethene is one of the largest-volume organic chemicals, produced at well over 100 million tonnes per year worldwide, mostly by steam cracking.
• Welding: ethyne in oxyacetylene torches.
• Natural products: carotenoids and terpenes contain many C=C bonds.

How were they discovered?

Ethene was first prepared in 1795 by a group of Dutch chemists, who called it “oil-forming gas” because it formed an oily liquid with chlorine. Ethyne was discovered by Edmund Davy in 1836, and Marcellin Berthelot made it from its elements in 1860 and gave it the name acetylene. The industrial production of acetylene from calcium carbide and water began in the 1890s. Today, ethyne is largely replaced by ethene as a chemical feedstock.

Which mistakes are common?

• Giving alkenes the formula of alkanes: an alkene with n carbons has 2n hydrogens, not 2n + 2.
• Numbering from the wrong end: the double or triple bond must get the lowest number.
• Forgetting the position of the double bond in the name: but-1-ene and but-2-ene are different compounds.
• Thinking that a double bond is twice as strong as a single bond: the pi bond is weaker than the sigma bond.
• Assuming that cycloalkanes decolorize bromine water: only the strained ones, such as cyclopropane, react to some extent.