Alcohols

AQA Year 1 · written to the mark scheme

In a nutshell

Alcohols are the organic family with an -OH (hydroxyl) functional group. This subtopic is about three things AQA tests hard: how ethanol is made (by fermentation and by hydrating an alkene), how alcohols are oxidised, and how they are dehydrated back into alkenes.

The marks cluster on precise equations, two mechanisms, and the exact conditions and observations, so this note leads with the words and symbols that earn them.

Assumed knowledge: Alkenes, Introduction to organic chemistry.

Core content

Alcohol production: industrial hydration of an alkene

Industrially, alcohols are made by hydration of an alkene in the presence of an acid catalyst. Hydration is the addition of water to a molecule.

Ethanol is made by reacting ethene with steam:

CH₂=CH₂(g) + H₂O(g) ⇌ CH₃CH₂OH(g)

The conditions are a high temperature (about 300 °C), a high pressure (roughly 60 to 70 atm) and a catalyst of concentrated phosphoric acid, H₃PO₄.

The ethene comes from cracking fractions of crude oil, so it is a finite (non-renewable) feedstock.

	Hydration of ethene	Fermentation
Feedstock	ethene (from crude oil), finite	sugar/glucose, renewable
Rate	fast	slow
Process	continuous	batch
Product purity	pure ethanol	impure, needs distilling
Equipment	high technology, high energy cost	low technology, cheap

Alcohol production: fermentation of glucose

In fermentation, glucose is converted to ethanol and carbon dioxide:

C₆H₁₂O₆(aq) ⟶ 2CH₃CH₂OH(aq) + 2CO₂(g)

You must be able to justify each condition:

Yeast is added: it supplies the enzymes that catalyse the reaction.
A warm temperature (about 30 to 40 °C) is used as a compromise: too cold and the reaction is too slow; too hot and the yeast dies / the enzymes denature.
Anaerobic conditions (no air/oxygen): oxygen would oxidise the ethanol to ethanoic acid (vinegar), lowering the yield.

The mixture made is dilute and impure, so the ethanol is separated by fractional distillation (ethanol boils at 78 °C, below water).

Alcohol production: ethanol as a biofuel

A biofuel is a fuel produced from renewable biological resources (such as plants).

Ethanol made by fermentation can be burned as a biofuel, and is sometimes called carbon neutral. Carbon neutral means an activity that has no net (overall) carbon dioxide emission to the atmosphere.

The argument runs over three equations: the CO₂ released when the ethanol is made and burned equals the CO₂ absorbed when the crop grew.

Photosynthesis (crop grows): 6CO₂ + 6H₂O ⟶ C₆H₁₂O₆ + 6O₂
Fermentation: C₆H₁₂O₆ ⟶ 2CH₃CH₂OH + 2CO₂
Combustion: 2CH₃CH₂OH + 6O₂ ⟶ 4CO₂ + 6H₂O

Per mole of glucose: 6 CO₂ absorbed in step 1, and 2 + 4 = 6 CO₂ released in steps 2 and 3. The totals match, so there is no net CO₂ contribution.

Why the claim is not really valid: the calculation ignores the CO₂ from the fossil fuels used to run the process: making fertilisers, powering farm machinery, transport, and the fractional distillation of the ethanol. Once those are counted, there is a net emission, so the ethanol is not truly carbon neutral.

Environmental and ethical issues of biofuel use: growing fuel crops uses farmland that could grow food, which can raise food prices and cause shortages (the "food versus fuel" problem), and land may be cleared by deforestation. Against this, the feedstock is renewable and reduces reliance on crude oil.

Alcohol production: the mechanism of hydration (electrophilic addition)

The spec requires the mechanism for forming an alcohol from an alkene and steam with an acid catalyst. It is electrophilic addition (the H⁺ catalyst is regenerated at the end).

Figure not available yet (chem-mech-ethene-hydration)

Learn it as four mark points (these earn the marks while the diagram is drawn):

Curly arrow from the C=C double bond to the H of H⁺.
Correct carbocation structure, with the + on the carbon.
Curly arrow from a lone pair on the oxygen of H₂O to the positive carbon.
Curly arrow from the O-H bond of the protonated alcohol, releasing H⁺ (the catalyst is regenerated).

Oxidation of alcohols: primary, secondary and tertiary

Alcohols are classified by how many carbon atoms are attached to the carbon that carries the -OH group:

Primary (1°): the carbon bonded to -OH is attached to one other carbon atom (methanol, with none, counts as primary). Example: propan-1-ol.
Secondary (2°): that carbon is attached to two other carbon atoms. Example: propan-2-ol.
Tertiary (3°): that carbon is attached to three other carbon atoms. Example: 2-methylpropan-2-ol.

Figure not available yet (chem-alcohol-classification)

Oxidation of alcohols: the reactions and controlling the product

The oxidising agent is acidified potassium dichromate(VI) (K₂Cr₂O₇ with dilute H₂SO₄). When it oxidises an alcohol it is itself reduced, so the observation is the solution turning from orange (Cr₂O₇^2-) to green (Cr³⁺).

AQA accepts the oxidising agent written as [O] in equations. What forms depends on the class of alcohol and, for a primary alcohol, on the conditions.

Primary alcohol ⟶ aldehyde ⟶ carboxylic acid. The two stages need different conditions:

To stop at the aldehyde: use a limited amount of oxidising agent, warm gently and distil the aldehyde off as it forms, so it leaves before it can be oxidised further (the aldehyde has no hydrogen bonding, so it has the lowest boiling point and distils out first).
To go all the way to the carboxylic acid: use excess oxidising agent and heat under reflux, so the aldehyde stays in the flask and is fully oxidised.

Still don't get it? · why distilling gives the aldehyde but reflux gives the acid

Think of the aldehyde as a bus passenger who gets on, then can get off at the next stop (the carboxylic acid) if the bus keeps going.

Distillation is you pulling the passenger off the bus at the first stop: you warm the flask gently so the aldehyde, which boils lowest, evaporates and travels down the condenser into the collection flask the moment it forms. Once it has left the reaction mixture it can no longer meet the oxidising agent, so it never reaches the acid.

Reflux is the opposite: the vertical condenser turns the vapour back to liquid and returns it to the flask, so nothing escapes. The aldehyde is kept in with excess oxidising agent and is oxidised a second time, all the way to the carboxylic acid.

In the exam: "distil off / limited [oxidising agent]" is the aldehyde answer; "reflux / excess" is the carboxylic acid answer. Naming the apparatus and the amount of oxidant is what earns the marks.

Secondary alcohol ⟶ ketone. Heat under reflux. A ketone is not oxidised further by acidified dichromate(VI).

Tertiary alcohol ⟶ no reaction. Tertiary alcohols are not easily oxidised: there is no hydrogen atom on the carbon bonded to the -OH group for the oxidising agent to remove, so the dichromate stays orange.

Worked oxidation equations are in the Worked examples section below.

Figure not available yet (chem-alcohol-oxidation-pathways)

Required practical 5: distillation of a product from a reaction

Distillation separates a product (such as an aldehyde) from a reaction mixture as it forms, using the difference in boiling points.

Key features of the apparatus and method:

The thermometer bulb sits level with the side-arm of the still head (the T-junction), so it reads the boiling point of the vapour passing into the condenser.
A Liebig condenser is used with water in at the bottom and out at the top, so it runs full and cools against the flow of vapour.
Anti-bumping granules are added to give smooth, even boiling (they do not change the reaction).
Collect only the distillate at the boiling point of the desired product (for an aldehyde, well below the boiling point of the alcohol), so the collected liquid is pure.

Figure not available yet (chem-distillation-apparatus)

Oxidation of alcohols: distinguishing aldehydes from ketones

An aldehyde can be oxidised further (to a carboxylic acid) but a ketone cannot. Two gentle oxidising tests use this difference: the aldehyde is oxidised and the reagent is reduced (giving a visible change), while the ketone gives no change.

Test	Reagent	With an aldehyde	With a ketone
Tollens' reagent	ammoniacal silver nitrate, containing [Ag(NH₃)₂]⁺; warm gently	Ag⁺ reduced to silver: a silver mirror forms	no change
Fehling's solution	blue solution containing Cu²⁺; warm gently	Cu²⁺ reduced to copper(I) oxide: a brick-red precipitate (Cu₂O)	stays blue, no change

To classify an unknown alcohol this way, you must oxidise it first (warm with acidified dichromate(VI)) and, for a primary alcohol, distil off the aldehyde, then test that product. Testing Tollens' or Fehling's on the alcohol itself does nothing.

Elimination: dehydrating an alcohol to an alkene

Heating an alcohol with a concentrated acid removes water and forms an alkene. Dehydration is the removal of a water molecule from a molecule; the reaction type is acid-catalysed elimination.

Reagent: concentrated sulfuric acid or concentrated phosphoric acid.
Conditions: warm (under reflux). The acid is a catalyst, not used up.

For example, ethanol gives ethene:

CH₃CH₂OH ⟶ CH₂=CH₂ + H₂O

Some secondary and tertiary alcohols can give more than one alkene, because the C=C can form between different pairs of carbons. Butan-2-ol gives both but-1-ene and but-2-ene (but-2-ene is the major product, and it also exists as E and Z isomers).

Because the alkenes made this way can be turned into addition polymers, dehydration offers a route to polymers without using monomers from crude oil.

Elimination: the acid-catalysed elimination mechanism

Figure not available yet (chem-mech-ethanol-dehydration)

Learn it as four mark points:

Curly arrow from a lone pair on the oxygen of the -OH to an H⁺ from the acid.
Correct protonated alcohol with the + charge on the oxygen.
Curly arrow from the C-O bond to the oxygen, so water leaves and a carbocation forms (+ on carbon).
Curly arrow from an adjacent C-H bond into the C-C bond, forming C=C and releasing H⁺.

Still don't get it? · why this is not the same as cracking a halogenoalkane with base

A halogenoalkane elimination is like pulling two people apart: a hydroxide ion grabs a hydrogen off one carbon while a halide ion is pushed off the next, and a base (OH-) does the grabbing.

Dehydration of an alcohol is a different story. There is no good leaving group to start with: an -OH will not just fall off. So the concentrated acid first bolts an H+ onto the oxygen, turning the stubborn -OH into -OH2+, which can now leave as a neutral water molecule. That leaves a positively charged carbon (a carbocation). Finally an H+ is pushed out from the next-door carbon to make the double bond.

The exam trap is to reach for ethanolic KOH or a hydroxide ion here, as if it were a halogenoalkane. It is not. The reagent is a concentrated acid, the first thing that happens is protonation of the oxygen, and the reaction is called elimination (the word "dehydration" on its own is not enough when the mechanism is asked for).

Worked examples

Example 1: writing oxidation equations (using [O]).

Oxidising butan-1-ol, a primary alcohol. First to the aldehyde (distil off, limited oxidant):

CH₃CH₂CH₂CH₂OH + [O] ⟶ CH₃CH₂CH₂CHO + H₂O

Then fully to the carboxylic acid (reflux, excess oxidant), which needs 2[O]:

CH₃CH₂CH₂CH₂OH + 2[O] ⟶ CH₃CH₂CH₂COOH + H₂O

For a secondary alcohol, propan-2-ol, the product is a ketone and no further oxidation occurs:

CH₃CH(OH)CH₃ + [O] ⟶ CH₃COCH₃ + H₂O

Where marks are lost: leaving out the [O] and/or the H₂O, so the equation does not balance (only about one in six candidates wrote a complete oxidation equation); and writing an aldehyde as "COH" instead of the correct CHO (as in CH₃CH₂CH₂CHO).

Example 2: showing ethanol is "carbon neutral" (and why the claim fails).

Set out the three equations and count the carbon dioxide per mole of glucose.

Absorbed by photosynthesis: 6CO₂ + 6H₂O ⟶ C₆H₁₂O₆ + 6O₂ gives 6 CO₂ in.
Released by fermentation: C₆H₁₂O₆ ⟶ 2CH₃CH₂OH + 2CO₂ gives 2 CO₂ out.
Released by combustion: 2CH₃CH₂OH + 6O₂ ⟶ 4CO₂ + 6H₂O gives 4 CO₂ out.

Total out = 2 + 4 = 6 CO₂, equal to the 6 absorbed, so there is no net emission.

The claim fails once you count the CO₂ from the fossil fuels burned to farm, transport and distil the ethanol.

Where marks are lost: forgetting that two moles of ethanol burn, so writing only 4 CO₂ out in total and concluding wrongly; and thinking a balanced equation with equal carbons on each side is what "carbon neutral" means.

Example 3: predicting the products of a dehydration.

Butan-2-ol is dehydrated with concentrated sulfuric acid. The -OH is on carbon 2, and water can be lost towards carbon 1 or carbon 3, giving two alkenes:

CH₃CH(OH)CH₂CH₃ ⟶ CH₂=CHCH₂CH₃ (but-1-ene) + H₂O

CH₃CH(OH)CH₂CH₃ ⟶ CH₃CH=CHCH₃ (but-2-ene) + H₂O

But-2-ene is the major product and shows E-Z isomerism (E-but-2-ene and Z-but-2-ene).

Where marks are lost: drawing the "same" but-2-ene twice and labelling one E and one Z incorrectly; using the wrong stem ("butan" or "bute") in the name; and forgetting the acid must be concentrated.

Common exam mistakes

Leaving [O] or H₂O out of an oxidation equation so it does not balance. Both are needed, for example CH₃CH₂CH₂OH + [O] ⟶ CH₃CH₂CHO + H₂O.
Writing an aldehyde as "COH" instead of "CHO". A condensed aldehyde is written CH₃CHO, not CH₃COH.
Testing Tollens' or Fehling's directly on an alcohol. You must oxidise the alcohol first (and distil off the aldehyde from a primary alcohol); the reagents distinguish aldehydes from ketones, not the alcohols themselves.
Explaining a tertiary alcohol's resistance vaguely as "more stable". The mark needs "no hydrogen atom on the carbon bonded to the -OH group". References to a "carbonyl carbon" are wrong.
Forgetting the acid must be concentrated for dehydration. "Sulfuric acid" or "phosphoric acid" on its own does not earn the conditions mark.
Using ethanolic KOH or a hydroxide ion for the dehydration mechanism. That is halogenoalkane elimination. Dehydration begins with the acid protonating the oxygen of the -OH group.
Omitting the lone pair on oxygen or the + charge when drawing the mechanism, or (in the hydration mechanism) missing the arrow that breaks the O-H bond.
Confusing hydration with hydrolysis. Making ethanol from ethene is hydration (adding H₂O across the C=C); do not call it hydrolysis, and do not write it as adding H, H₂ or H⁺.
Saying fermentation needs "excess oxygen". It is anaerobic. Also, "infinite resource" is not accepted for renewable, and "impure" or "a batch process" are only credited with some qualification.
Omitting "precipitate" in the Fehling's result. The answer is a red precipitate (Cu₂O), not just "it goes red".
Drawing the condenser sealed, or the water circulation tubes blocked, in distillation or reflux diagrams; the water jacket must have open inlet and outlet, and the apparatus must never be sealed.

Key definitions

Hydration - the addition of water to a molecule.
Dehydration - the removal of a water molecule from a molecule.
Biofuel - a fuel produced from renewable biological resources (such as plants).
Carbon neutral - an activity that has no net (overall) carbon dioxide emission to the atmosphere.
Primary (1°) alcohol - an alcohol in which the carbon bonded to the -OH group is attached to one other carbon atom (methanol counts as primary).
Secondary (2°) alcohol - an alcohol in which the carbon bonded to the -OH group is attached to two other carbon atoms.
Tertiary (3°) alcohol - an alcohol in which the carbon bonded to the -OH group is attached to three other carbon atoms.

Specification

Alkenes
Introduction to organic chemistry
Organic analysis
Aldehydes and ketones (3.3.8, A-level)

now prove it

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