Preparing Alcohols, Including Fermentation – Alcohols and Ethers (Senior 5 Chemistry) | Ganzaa

Alcohols and Ethers

Lessons

Senior 5 · Chemistry

Alcohols and Ethers

5 lessons

Lesson 2 of 5

Preparing Alcohols, Including Fermentation

25 min

Banana beer on the road

On roads across Rwanda you can see bicycles loaded with yellow jerrycans of urwagwa, the traditional banana beer. It is made by pressing the juice from ripe bananas, usually adding roasted sorghum flour, and leaving the mixture for a few days while yeasts turn its sugars into ethanol and carbon dioxide. Sorghum beer, ikigage, is brewed the same way from sorghum grain. Fermentation is the oldest way of making an alcohol, but chemists have several others, each starting from a different family of compounds.

Definition

Fermentation: the breakdown of sugars by enzymes from microorganisms such as yeast, in the absence of air, giving ethanol and carbon dioxide.

Reduction (in organic chemistry): the addition of hydrogen or the removal of oxygen; written with [H] for the hydrogen supplied by a reducing agent.

Atom economy: the percentage of the total mass of the reactants that ends up in the desired product: $\frac{\text{mass of desired product}}{\text{total mass of reactants}} \times 100$ .

Routes from other families

Figure 2.1: Four routes to one secondary alcohol.

Hydration of alkenes (Unit 3): $\mathrm{CH_2{=}CH_2(g) + H_2O(g) \rightleftharpoons C_2H_5OH(g)}$ , with phosphoric acid on silica at about 300 °C and 60–70 atm. This is how most industrial ethanol is made.
Hydrolysis of halogenoalkanes (Unit 4): heat under reflux with aqueous sodium hydroxide: $\mathrm{CH_3CHBrCH_3 + OH^- \rightarrow CH_3CH(OH)CH_3 + Br^-}$ .
Reduction of aldehydes and ketones with sodium tetrahydridoborate(III), $\mathrm{NaBH_4}$ , in water or methanol, or the more powerful lithium tetrahydridoaluminate(III), $\mathrm{LiAlH_4}$ , in dry ether. An aldehyde gives a primary alcohol, a ketone a secondary alcohol: $\mathrm{CH_3COCH_3 + 2[H] \rightarrow CH_3CH(OH)CH_3}$ . $\mathrm{LiAlH_4}$ also reduces carboxylic acids and esters to primary alcohols.
Grignard reagents add to the C=O bond; dilute acid then gives the alcohol. Methanal gives a primary alcohol, other aldehydes a secondary alcohol, and ketones a tertiary alcohol, with a new C–C bond each time: $\mathrm{CH_3MgBr + CH_3CHO \rightarrow (CH_3)_2CHOMgBr}$ , then $\xrightarrow{\mathrm{H_3O^+}} \mathrm{(CH_3)_2CHOH}$ .
Methanol is made industrially from synthesis gas: $\mathrm{CO(g) + 2H_2(g) \rightleftharpoons CH_3OH(g)}$ , over a copper–zinc oxide catalyst at about 250 °C and 50–100 atm.

Fermentation

Starch and sucrose are first broken down to glucose by enzymes (in brewing, the enzymes of germinating grain or of the yeast itself). Then enzymes in yeast, together called zymase, convert glucose into ethanol:

\mathrm{C_6H_{12}O_6(aq) \xrightarrow{\text{yeast enzymes}} 2C_2H_5OH(aq) + 2CO_2(g)}

Figure 2.2: Fermenting glucose. The limewater shows that carbon dioxide is produced and stops air from entering.

Temperature about 25–35 °C: colder is too slow, and above about 40 °C the enzymes are denatured.
No air: with oxygen, yeast respires the sugar fully to $\mathrm{CO_2}$ and water, and bacteria can oxidise ethanol to ethanoic acid (vinegar).
Fermentation stops at about 15% ethanol, when the alcohol kills the yeast. Stronger drinks and fuel ethanol are concentrated by fractional distillation (ethanol boils at 78 °C, water at 100 °C).

Table 2.1: Two ways to make ethanol

	Fermentation	Hydration of ethene
raw material	sugars and starch from crops (renewable)	ethene from crude oil or natural gas (non-renewable)
conditions	yeast, 25–35 °C, no air, about 1 atm	$\mathrm{H_3PO_4}$ catalyst, about 300 °C, 60–70 atm
process	batch: slow (days)	continuous: fast
product	dilute (up to about 15%); must be distilled	almost pure ethanol
atom economy	$\frac{2 \times 46.0}{180} \times 100 = 51.1\%$ ( $\mathrm{CO_2}$ is also formed)	100% (addition reaction)

Worked example

Problem. Suggest two ways to make 2-methylbutan-2-ol, $\mathrm{CH_3CH_2C(OH)(CH_3)_2}$ , using a Grignard reagent and a ketone.

Solution. A tertiary alcohol comes from a ketone. The –OH carbon carries $\mathrm{CH_3}$ , $\mathrm{CH_3}$ and $\mathrm{C_2H_5}$ ; one of these comes from the Grignard reagent, the other two were on the ketone.

Propanone + ethylmagnesium bromide: $\mathrm{(CH_3)_2CO + C_2H_5MgBr}$ , then dilute acid.
Butanone + methylmagnesium bromide: $\mathrm{CH_3COC_2H_5 + CH_3MgBr}$ , then dilute acid.

Worked example

Problem. (a) Calculate the maximum mass of ethanol that 1.00 kg of glucose can give. (b) A brewer obtains 0.409 kg. What is the percentage yield? ( $M$ : glucose 180, ethanol 46.0 $\mathrm{g\,mol^{-1}}$ )

Solution. (a) $n(\text{glucose}) = \frac{1000}{180} = 5.56$ mol, giving $2 \times 5.56 = 11.1$ mol of ethanol: $11.1 \times 46.0 = 511$ g. (b) Yield $= \frac{409}{511} \times 100 = 80.0\%$ .

Warning

Fermentation in a sealed bottle can burst it: carbon dioxide builds up. Home-distilled spirits can contain dangerous amounts of methanol and other toxic alcohols (Lesson 4). In calculations, remember that one glucose gives TWO ethanol molecules.

Tip

Reduction or Grignard? Count the carbons. Reduction keeps the skeleton (propanone → propan-2-ol); a Grignard reagent adds its alkyl group to the carbonyl carbon (ethanal + $\mathrm{CH_3MgBr}$ → propan-2-ol).

FuseSchool: Making Alcohols by Fermentation and from Ethene (4 min; the video's title says "ethane", but it is about ethene). Watch for the conditions of each method and the advantages and disadvantages of each.

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Practice

Write an equation, with conditions, for the industrial hydration of ethene.
Name the alcohol formed when (a) butanal and (b) butanone are reduced with $\mathrm{NaBH_4}$ . Classify each.
Which Grignard reagent and which carbonyl compound would you use to make butan-2-ol? Give one pair.
Explain why fermentation is carried out (a) at about 35 °C; (b) without air.
Calculate the atom economy of making ethanol by fermentation, and explain why it is less than 100%.
Give two advantages of making ethanol by hydration of ethene and one advantage of fermentation.
Write equations for the hydrolysis of 1-chlorobutane to butan-1-ol and the industrial synthesis of methanol.

Show answersHide answers

$\mathrm{C_2H_4(g) + H_2O(g) \rightleftharpoons C_2H_5OH(g)}$ ; phosphoric acid on silica, about 300 °C, 60–70 atm.
(a) butan-1-ol, primary; (b) butan-2-ol, secondary.
Ethanal + ethylmagnesium bromide (or propanal + methylmagnesium bromide), then dilute acid.
(a) Fast enough, but below the temperature at which the yeast enzymes are denatured. (b) With air, the sugar is respired fully to $\mathrm{CO_2}$ and water, and ethanol can be oxidised to ethanoic acid.
51.1%; carbon dioxide is formed as a by-product, so not all the reactant mass ends up in ethanol.
Hydration: fast, continuous, pure product, 100% atom economy (any two). Fermentation: uses renewable raw materials (and low temperature and pressure).
$\mathrm{C_4H_9Cl + OH^- \rightarrow C_4H_9OH + Cl^-}$ (NaOH(aq), reflux); $\mathrm{CO + 2H_2 \rightleftharpoons CH_3OH}$ .

Sources and credits

Written by Ganzaa. Background on banana beer: Banana beer, Wikipedia contributors, licensed under CC BY-SA 4.0, adapted by Ganzaa.
Image: Banana beer transportation.jpg by Rogerirakoze, via Wikimedia Commons, licensed under CC BY-SA 4.0.
Video: Making Alcohols By Fermentation & From Ethane, FuseSchool – Global Education, embedded from YouTube.
The text, diagrams, table, worked examples and Practice items were written and drawn by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Banana beer on the road

2Routes from other families

3Fermentation

Banana beer on the road

Routes from other families

Fermentation