(c) Alkanes

Learning objectives

4.19Alkane general formulaKnow the general formula for alkanes 4.20Saturated hydrocarbonsExplain why alkanes are classified as saturated hydrocarbons 4.21Alkane structures and isomersUnderstand how to draw the structural and displayed formulae for alkanes with up to five carbon atoms in the molecule, and to name the unbranched-chain isomers 4.22Alkane substitution with halogensDescribe the reactions of alkanes with halogens in the presence of ultraviolet radiation, limited to mono-substitution knowledge of reaction mechanisms is not required

Alkanes form a homologous series with the general formula $\ce{C_nH_{2n+2}}$ , where $n$ is a positive whole number equal to the number of carbon atoms.

$n$	Name	Molecular formula
1	methane	$\ce{CH4}$
2	ethane	$\ce{C2H6}$
3	propane	$\ce{C3H8}$
4	butane	$\ce{C4H10}$
5	pentane	$\ce{C5H12}$

To test a proposed alkane formula, count its carbon atoms, substitute that value for $n$ , and check whether the hydrogen count is $2n+2$ . For example, $n=6$ gives $\ce{C6H14}$ .

The general formula describes molecular formulae for open-chain alkanes. Do not simplify $\ce{C2H6}$ to the empirical formula $\ce{CH3}$ , and do not use the alkene formula $\ce{C_nH_{2n}}$ .

An alkane is a saturated hydrocarbon: it contains carbon and hydrogen only, and every bond between carbon atoms is a single covalent bond.

Classification	Evidence in an alkane
hydrocarbon	the molecule contains only carbon and hydrogen atoms
saturated	there are no carbon–carbon double or triple bonds; the carbon framework carries the maximum number of hydrogens for that open chain

Because an alkane has no $\ce{C=C}$ bond to open, it does not undergo the addition reactions characteristic of alkenes. Its reaction with a halogen is substitution instead.

Saturated does not mean that every carbon atom is bonded to four hydrogen atoms. Carbon makes four bonds in total; in ethane, for example, each carbon also bonds to the other carbon.

A structural formula shows how atoms are grouped along the carbon chain; a displayed formula shows every atom and every covalent bond.

Carbon atoms	Unbranched name	Molecular formula	Condensed structural formula
1	methane	$\ce{CH4}$	$\ce{CH4}$
2	ethane	$\ce{C2H6}$	$\ce{CH3CH3}$
3	propane	$\ce{C3H8}$	$\ce{CH3CH2CH3}$
4	butane	$\ce{C4H10}$	$\ce{CH3CH2CH2CH3}$
5	pentane	$\ce{C5H12}$	$\ce{CH3CH2CH2CH2CH3}$

For a displayed formula, first join the required carbon skeleton using single bonds. Then add hydrogen atoms until every carbon has four bonds and every hydrogen has one. Count the atoms at the end to confirm the molecular formula.

Structural isomers have the same molecular formula but different carbon connectivity. $\ce{C4H10}$ has straight-chain butane and branched methylpropane; $\ce{C5H12}$ has pentane, 2-methylbutane and 2,2-dimethylpropane.

Rotating or bending the same chain does not create a new isomer. Compare which carbon atoms are connected, and use methane, ethane, propane, butane and pentane for the required unbranched-chain names.

In ultraviolet radiation, an alkane reacts with a halogen by substitution: one halogen atom replaces one hydrogen atom in the alkane.

Reactants	Required condition	Products after one substitution
alkane + chlorine	ultraviolet radiation	chloroalkane + hydrogen chloride
alkane + bromine	ultraviolet radiation	bromoalkane + hydrogen bromide

\ce{CH4 + Cl2 ->[UV] CH3Cl + HCl}

\ce{C2H6 + Br2 ->[UV] C2H5Br + HBr}

Keep the carbon skeleton unchanged, replace exactly one H by Cl or Br, and use the other halogen atom to form $\ce{HCl}$ or $\ce{HBr}$ . Check that every atom is conserved.

For this specification, stop after mono-substitution and state ultraviolet radiation. Further substitutions can occur chemically, but they are outside 4.22; reaction mechanisms are also not required. Do not call this addition.

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—(d) AlkenesRead

(c) Alkanes

Learning objectives

Use the alkane general formula

Explain why alkanes are saturated hydrocarbons

Draw and name alkanes up to five carbons

Describe mono-substitution of alkanes by halogens

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