Alkanes

Lessons

Senior 5 · Chemistry

Alkanes

4 lessons

Lesson 3 of 4

Preparing Alkanes in the Laboratory

22 min

Making a pure alkane on purpose

Crude oil gives alkanes by the tonne, but always as mixtures. A research laboratory that needs one pure alkane, or a chemist who wants to prove the structure of a new compound by making it, needs reactions that build a specific skeleton. In Senior 3 you made methane by heating sodium ethanoate with soda lime. That is one of five standard routes, and each one changes the carbon count in its own way.

Definition

Hydrogenation: the addition of hydrogen across a C=C or C≡C bond, using a metal catalyst.

Decarboxylation: the removal of carbon dioxide from a carboxylic acid or its salt; the alkane formed has one carbon fewer.

Kolbe electrolysis: electrolysis of a concentrated solution of a carboxylate salt, in which two alkyl groups join at the anode.

Wurtz reaction: the joining of two alkyl groups by heating a halogenoalkane with sodium metal in dry ether.

Grignard reagent: an organomagnesium halide, RMgX, made from a halogenoalkane and magnesium in dry ether.

Five routes

Figure 3.1: Five laboratory routes to one alkane. Each suits a different starting material.

1. Hydrogenation of alkenes and alkynes. Hydrogen adds across the multiple bond over finely divided nickel at about 150 °C, or over platinum or palladium at room temperature:

\mathrm{CH_3CH{=}CH_2(g) + H_2(g) \xrightarrow{\text{Ni, 150 °C}} CH_3CH_2CH_3(g)}

The carbon skeleton is unchanged. An alkyne takes two molecules of hydrogen: $\mathrm{HC{\equiv}CH(g) + 2H_2(g) \rightarrow C_2H_6(g)}$ .

2. Decarboxylation. A dry sodium carboxylate is heated with soda lime (sodium hydroxide mixed with calcium oxide, which keeps it dry and less corrosive to glass):

\mathrm{RCOONa(s) + NaOH(s) \xrightarrow{\text{heat}} RH(g) + Na_2CO_3(s)}

The –COONa carbon leaves as carbonate, so the alkane has one carbon fewer than the salt: sodium propanoate gives ethane.

3. Kolbe electrolysis. At the anode each carboxylate ion loses an electron and then $\mathrm{CO_2}$ ; the alkyl radicals pair up:

\mathrm{2RCOO^-(aq) \rightarrow R{-}R(g) + 2CO_2(g) + 2e^-}

Two alkyl groups join, so the product has twice the carbons of R: sodium propanoate (R = $\mathrm{C_2H_5}$ ) gives butane.

Figure 3.2: Kolbe electrolysis. Two alkyl groups from two carboxylate ions join at the anode.

4. The Wurtz reaction. A halogenoalkane is warmed with sodium in dry ether:

\mathrm{2CH_3CH_2Br(l) + 2Na(s) \xrightarrow{\text{dry ether}} CH_3CH_2CH_2CH_3(g) + 2NaBr(s)}

It doubles the alkyl group, so it suits symmetrical alkanes with an even number of carbons. It cannot make methane, and a mixture of two different halogenoalkanes gives a mixture of three alkanes.

5. Grignard reagents. A halogenoalkane reacts with magnesium in dry ether, and the Grignard reagent is then decomposed by water:

\mathrm{CH_3CH_2Br + Mg \xrightarrow{\text{dry ether}} CH_3CH_2MgBr} \qquad \mathrm{CH_3CH_2MgBr + H_2O(l) \rightarrow CH_3CH_3(g) + Mg(OH)Br(s)}

The bromine is replaced by hydrogen; the carbon count is unchanged. Water must be kept out until the last step, because it destroys the reagent.

Table 3.1: Choosing a route

Route	Starting material	Carbons in product	Example
hydrogenation	alkene or alkyne	same	propene → propane
decarboxylation	sodium carboxylate, $\mathrm{C_n}$	$n - 1$	sodium butanoate → propane
Kolbe electrolysis	sodium carboxylate, $\mathrm{C_n}$	$2 (n - 1)$	sodium ethanoate → ethane
Wurtz reaction	halogenoalkane, $\mathrm{C_n}$	$2 n$	bromoethane → butane
Grignard, then water	halogenoalkane, $\mathrm{C_n}$	same	2-bromopropane → propane

In industry, alkanes come from the fractional distillation of crude oil and natural gas, and from cracking (Lesson 4), not from these reactions.

Worked example

Problem. Name the organic product of: (a) heating sodium butanoate with soda lime; (b) Kolbe electrolysis of sodium propanoate; (c) the Wurtz reaction of 1-bromopropane.

Solution. (a) $\mathrm{CH_3CH_2CH_2COONa}$ loses its carboxylate carbon: $\mathrm{CH_3CH_2CH_3}$ , propane. (b) R = $\mathrm{C_2H_5}$ ; two join: $\mathrm{C_2H_5{-}C_2H_5}$ , butane. (c) R = $\mathrm{CH_3CH_2CH_2}$ ; two join: $\mathrm{CH_3(CH_2)_4CH_3}$ , hexane: $\mathrm{2CH_3CH_2CH_2Br + 2Na \rightarrow C_6H_{14} + 2NaBr}$ .

Worked example

Problem. A student plans to make propane by a Wurtz reaction between bromomethane and bromoethane. Evaluate the plan and suggest a better route.

Solution. Sodium joins any two alkyl groups it meets, so three products form: $\mathrm{CH_3{-}CH_3}$ (ethane), $\mathrm{CH_3{-}C_2H_5}$ (propane) and $\mathrm{C_2H_5{-}C_2H_5}$ (butane). The yield of propane is low and it must be separated from the others. Better routes give propane alone: hydrogenation of propene over nickel, or decarboxylation of sodium butanoate with soda lime.

Warning

Safety. Sodium reacts violently with water: keep it under oil and use only dry apparatus. Ether is extremely flammable: no flames in the room. Soda lime and sodium hydroxide are corrosive. If they reach the skin or eyes, remove contaminated clothing and rinse with plenty of water for at least 15 minutes, then get medical help (CDC guidance).

Tip

Count carbons to choose the route: same (hydrogenation, Grignard), one fewer (decarboxylation), double (Wurtz from RX; Kolbe from R–COO⁻, so double what is left after losing $\mathrm{CO_2}$ ).

Najam Academy: Preparation of Alkanes (23 min). Watch for the conditions and the change in carbon number for hydrogenation, decarboxylation, Kolbe electrolysis and the Wurtz reaction.

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Practice

Write a balanced equation, with state symbols, for the hydrogenation of but-1-ene over nickel.
Which sodium salt gives butane when heated with soda lime? Write the equation.
Write the anode half-equation for the Kolbe electrolysis of sodium propanoate, and name the alkane formed.
Name the product of a Wurtz reaction of 2-bromopropane, and write its structural formula.
Explain why the Wurtz reaction cannot be used to make methane.
4.80 g of sodium propanoate ( $M = 96.0\ \mathrm{g\,mol^{-1}}$ ) is heated with excess soda lime. Calculate the maximum volume of ethane at room temperature and pressure (molar volume $24.0\ \mathrm{dm^3\,mol^{-1}}$ ).
Suggest a route from 1-bromobutane to butane, and give the conditions.

Show answersHide answers

$\mathrm{CH_2{=}CHCH_2CH_3(g) + H_2(g) \rightarrow CH_3CH_2CH_2CH_3(g)}$ , Ni, about 150 °C.
Sodium pentanoate: $\mathrm{CH_3(CH_2)_3COONa(s) + NaOH(s) \rightarrow C_4H_{10}(g) + Na_2CO_3(s)}$ .
$\mathrm{2C_2H_5COO^-(aq) \rightarrow C_4H_{10}(g) + 2CO_2(g) + 2e^-}$ ; butane.
2,3-dimethylbutane, $\mathrm{(CH_3)_2CHCH(CH_3)_2}$ .
The reaction joins two alkyl groups, so its product always has at least two carbons.
$n = \frac{4.80}{96.0} = 0.0500$ mol of salt gives 0.0500 mol of ethane; $V = 0.0500 \times 24.0 = 1.20\ \mathrm{dm^3}$ .
Make the Grignard reagent with magnesium in dry ether, then add water: $\mathrm{C_4H_9Br \rightarrow C_4H_9MgBr \rightarrow C_4H_{10}}$ .

Sources and credits

Written by Ganzaa.
First aid: Medical Management Guidelines for Sodium Hydroxide, Agency for Toxic Substances and Disease Registry (CDC).
Video: Preparation of Alkanes, Najam Academy, embedded from YouTube.
The text, diagrams, table, worked examples and Practice items were written and drawn by Ganzaa.

1Making a pure alkane on purpose

2Five routes

Making a pure alkane on purpose

Five routes