Alkanes and Cycloalkanes: Structure and Formulas

Alkanes are saturated hydrocarbons with only single bonds and the general formula C_nH_2n+2, and cycloalkanes are alkanes whose carbon atoms form a ring, with the general formula C_nH_2n. In brief: an alkane with 8 carbon atoms is C₈H₁₈ (octane), while cyclohexane with 6 carbon atoms is C₆H₁₂.

Alkanes are the simplest organic compounds and the main components of natural gas, petrol, diesel and wax. Their structure and naming form the basis of all organic nomenclature. This article gives the formulas and names of the first ten alkanes, explains the shape of the molecules and the strain in small rings, and describes their typical reactions.

What are alkanes?

Alkanes contain only carbon and hydrogen atoms linked by single (sigma) bonds. They are called saturated because each carbon atom holds as many hydrogen atoms as possible. For an alkane with n carbon atoms, the number of hydrogen atoms is 2n + 2. The C–C bonds are 154 pm long, the C–H bonds are 109 pm long, and all carbon atoms are sp³ hybridized with bond angles of about 109.5°.

The alkanes form a homologous series: each member differs from the next by one CH₂ unit, which has a mass of 14 g/mol. Alkanes can have a straight chain or a branched chain, and compounds with the same formula but a different arrangement are isomers.

The first ten alkanes

The first four alkanes are gases at room temperature, pentane to about hexadecane (C₁₆H₃₄) are liquids and longer ones are waxy solids. The boiling points rise with chain length because the dispersion forces grow stronger. The number of isomers increases rapidly: there are 366,319 for C₂₀H₄₂.

What is the shape of an alkane molecule?

Each carbon atom is at the center of a tetrahedron, so the chains are not straight but zigzag in their most stable form. Rotation around C–C single bonds is easy at room temperature, and the molecules constantly change between conformations. In the stable staggered conformation of ethane, the hydrogens on the two carbon atoms are as far apart as possible. Alkanes are nonpolar, since the C–C bond is nonpolar and the C–H bond only slightly polar.

What are cycloalkanes?

In a cycloalkane the carbon atoms are joined in a ring. The ring has two hydrogen atoms fewer than the open-chain alkane with the same number of carbons, since the two ends are joined together, so the formula is C_nH_2n. They are named by adding the prefix cyclo-: cyclopropane (C₃H₆), cyclobutane (C₄H₈), cyclopentane (C₅H₁₀) and cyclohexane (C₆H₁₂). Cyclohexane is the most important: its carbon atoms are not in a plane but in a puckered chair conformation, in which all the bond angles are close to 109.5°.

Why are small rings strained?

The ideal C–C–C angle of an sp³ carbon is 109.5°. In a small ring the angles are forced to be smaller, and this angle strain, together with other types of strain, makes the molecule less stable and more reactive.

Cyclopropane is the most strained, and its ring opens in addition reactions, for example with bromine, similar to an alkene. Cyclohexane in the chair form is virtually free of strain. Actual cyclobutane and cyclopentane are slightly puckered to relieve some strain.

How do alkanes react?

Alkanes are relatively unreactive, since C–C and C–H bonds are strong and nonpolar. There are two main kinds of reaction:

• Combustion: alkanes burn in excess oxygen to give carbon dioxide and water, with the release of a lot of energy: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. In limited oxygen carbon monoxide or soot is produced.
• Free-radical substitution: in ultraviolet light, halogens replace hydrogen: CH₄ + Cl₂ → CH₃Cl + HCl. The reaction goes through a chain mechanism with initiation, propagation and termination steps, and gives mixtures of products.

Worked examples

Example 1: finding the formula

An alkane has 12 carbon atoms. With n = 12, the number of hydrogens is 2 × 12 + 2 = 26, so the formula is C₁₂H₂₆, called dodecane.

Example 2: finding the number of carbons from the molar mass

An alkane has a molar mass of 100.2 g/mol. With C_nH_2n+2, M = 12.011n + 1.008(2n + 2) = 14.027n + 2.016. Therefore, n = (100.2 − 2.016) / 14.027 = 7.0, and the compound is a heptane, C₇H₁₆.

Example 3: alkane or cycloalkane?

A hydrocarbon has the formula C₅H₁₀. The corresponding alkane would be C₅H₁₂, so there are two hydrogen atoms fewer. The compound is therefore either a cycloalkane (cyclopentane) or an alkene (pentene).

Example 4: energy from combustion

The standard enthalpy of combustion of methane is about −890 kJ/mol. Burning 16.0 g of methane (1.00 mol) therefore releases about 890 kJ of heat.

Where are alkanes used?

• Fuels: methane (natural gas), propane and butane (LPG), octane (gasoline) and longer chains in diesel and jet fuel.
• Lubricants and waxes: long-chain alkanes are used in oils, paraffin wax and petroleum jelly.
• Solvents: pentane, hexane and cyclohexane are nonpolar solvents.
• Chemical feedstock: alkanes are converted into alkenes and other building blocks by cracking and reforming.

Which mistakes are common?

• Using the wrong general formula: an alkane is C_nH_2n+2, a cycloalkane is C_nH_2n.
• Drawing the carbon chain as straight: the bonds are at about 109.5°, so the chain zigzags.
• Assuming that cyclohexane is flat: it exists mostly in the chair form.
• Forgetting that alkanes burn incompletely in limited oxygen: carbon monoxide is poisonous.
• Confusing alkanes with alkenes: alkenes contain a C=C bond and do not follow the 2n + 2 rule.