(e) Alcohols

Learning objectives

429C Alcohol functional groupKnow that alcohols contain the functional group -OH 430C Alcohol structures and namesUnderstand how to draw structural and displayed formulae for methanol, ethanol, propanol (propan-1-ol only) and butanol (butan-1-ol only), and name each compound the names propanol and butanol are acceptable 431C Oxidation of ethanolKnow that ethanol can be oxidised by:• burning in air or oxygen (complete combustion)• reaction with oxygen in the air to form ethanoic acid (microbial oxidation)• heating with potassium dichromate(VI) in dilute sulfuric acid to form ethanoic acid 432C Manufacture of ethanolKnow that ethanol can be manufactured by:• reacting ethene with steam in the presence of a phosphoric acid catalyst at a temperature of about 300 ºC and a pressure of about 60-70 atm• the fermentation of glucose, in the absence of air, at an optimum temperature of about 30 ºC and using the enzymes in yeast 433C Fermentation conditionsUnderstand the reasons for fermentation, in the absence of air, and at an optimum temperature

Alcohols contain the hydroxyl functional group $\ce{-OH}$ , with the oxygen bonded to a carbon atom and to a hydrogen atom.

In methanol, $\ce{CH3OH}$ , and ethanol, $\ce{CH3CH2OH}$ , writing $\ce{OH}$ at the end makes the functional group visible. A displayed formula must show the separate $\ce{C-O}$ and $\ce{O-H}$ bonds.

Do not identify any molecule containing oxygen as an alcohol. The required group is $\ce{-O-H}$ attached to the carbon framework; a carboxylic acid contains $\ce{-COOH}$ and belongs to a different functional group.

The required alcohols have an unbranched carbon chain with one terminal $\ce{-OH}$ group. Their names end in “ol”.

Carbon atoms	Required name	Structural formula
1	methanol	$\ce{CH3OH}$
2	ethanol	$\ce{CH3CH2OH}$
3	propan-1-ol (propanol accepted)	$\ce{CH3CH2CH2OH}$
4	butan-1-ol (butanol accepted)	$\ce{CH3CH2CH2CH2OH}$

To draw a displayed formula, join the carbon atoms with single bonds, attach $\ce{-O-H}$ to the end carbon, then add hydrogens until every carbon has four bonds, oxygen has two and hydrogen has one.

A displayed formula must show the $\ce{O-H}$ bond; writing an unconnected OH label is incomplete. For this objective, propanol means propan-1-ol and butanol means butan-1-ol—not propan-2-ol or butan-2-ol.

Ethanol can be oxidised in three required ways: complete combustion, microbial oxidation in air, and heating with acidified potassium dichromate(VI).

Route	Conditions / oxidant	Main products
complete combustion	burn in air or oxygen	carbon dioxide and water
microbial oxidation	oxygen in air; microorganisms	ethanoic acid
laboratory oxidation	heat with potassium dichromate(VI) in dilute sulfuric acid	ethanoic acid

\ce{C2H5OH + 3O2 -> 2CO2 + 3H2O}

\ce{C2H5OH + O2 -> CH3COOH + H2O}

During oxidation with acidified potassium dichromate(VI), the dichromate colour changes from orange to green. The required acid is dilute sulfuric acid, and the mixture is heated.

Combustion does not produce ethanoic acid: it oxidises ethanol completely to $\ce{CO2}$ and $\ce{H2O}$ . For the dichromate route, do not replace dilute sulfuric acid with phosphoric acid even if a legacy mark scheme once allowed it.

Ethanol is manufactured either by hydrating ethene with steam or by fermenting glucose with enzymes in yeast.

Method	Reactants	Required conditions
hydration	ethene + steam	phosphoric acid catalyst; about $300\,^{\circ}\mathrm{C}$ ; about $60$ – $70\,\mathrm{atm}$
fermentation	glucose	enzymes in yeast; absence of air; optimum temperature about $30\,^{\circ}\mathrm{C}$

\ce{C2H4 + H2O -> C2H5OH}

\ce{C6H12O6 -> 2C2H5OH + 2CO2}

Hydration produces ethanol as its only product in the equation. Fermentation produces both ethanol and carbon dioxide, so both products and their coefficient 2 must be retained when balancing.

Do not exchange the conditions: phosphoric acid, high temperature and high pressure belong to ethene hydration; yeast enzymes, no air and about $30\,^{\circ}\mathrm{C}$ belong to fermentation.

Fermentation is carried out without air and near the enzymes’ optimum temperature so that glucose is converted to ethanol at a useful rate without losing the desired product.

Condition	Why it is needed
absence of air	prevents ethanol being oxidised to ethanoic acid and prevents aerobic respiration from replacing ethanol production
about $30\,^{\circ}\mathrm{C}$	gives yeast enzymes a fast working rate without denaturing them
not too cold	enzyme-controlled reactions become slow
not too hot	enzymes denature, so their active sites no longer catalyse fermentation

temperature too low → insufficient particle movement and slow enzyme activity; optimum temperature → fastest effective enzyme action; temperature too high → enzyme structure changes → fermentation stops

Saying only that ‘yeast dies’ does not explain the temperature condition. The required explanation concerns enzyme activity and denaturation; absence of air is a separate chemical condition.

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—(f) Carboxylic acidsRead

(e) Alcohols

Learning objectives

Recognise the alcohol functional group

Draw and name the first four alcohols

Compare the three oxidation routes for ethanol

Compare the two ways to manufacture ethanol

Explain the conditions needed for fermentation

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