Ch 1 · Alcohols · AS & A Level Chemistry 9701
Topic 1 of 1 in Hydroxy compounds — AS & A Level Chemistry 9701 — 62 lessons.
Introduction
Alcohols, containing the -OH functional group, sit at the centre of AS organic chemistry: they are made from alkenes and carbonyl compounds, and can themselves be converted into halogenoalkanes, aldehydes, ketones, carboxylic acids and esters. This lesson covers their preparation, characteristic reactions and the tests used to identify them.
Key ideas — Preparation of alcohols
Alcohols are produced industrially by the catalytic addition of steam to an alkene, using phosphoric(V) acid as catalyst at about 300 °C and high pressure: mathrmCH_2=CH_2 (g) + H_2O (g) arrow CH_3CH_2OH (g) Alcohols are also formed by reduction of carbonyl compounds: aldehydes and ketones are reduced by NaBH_4 to primary and secondary alcohols respectively, and carboxylic acids are reduced by the stronger reducing agent LiAlH_4 in dry ether to primary alcohols. CH_3CHO + 2[H] arrow CH_3CH_2OH
🧑🏫 Teacher's explanation
Think of making an alcohol like building a LEGO structure. You can either take a simple piece (an alkene) and snap a water molecule onto it using heat and pressure, or you can take a piece that already has an oxygen atom (a carbonyl group) and 'add' hydrogen atoms to it to satisfy the bonds.
📖 Definitions
🔍 Worked example — Easy
Calculate the molar mass of ethanol, CH_3CH_2OH, given C=12, H=1, O=16.
🔍 Worked example — Standard
A sample of ethene (C_2H_4) reacts with steam to produce ethanol (C_2H_5OH). If 0.50 moles of ethene react completely, how many moles of ethanol are produced?
🔍 Worked example — Exam-style
Propanal (CH_3CH_2CHO) is reduced using NaBH_4. Write the balanced equation for the formation of the resulting alcohol, assuming [H] represents the hydride ion transfer.
⚠️ Common errors
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🎯 Exam question
Key ideas — Combustion, substitution and dehydration
Alcohols burn completely in excess oxygen to carbon dioxide and water, making them useful fuels: C_2H_5OH (l) + 3O_2 (g) arrow 2CO_2 (g) + 3H_2O (l) The -OH group is substituted by a halogen on heating with a hydrogen halide, phosphorus(V) chloride or phosphorus(III) chloride, forming a halogenoalkane: C_2H_5OH + HBr arrow C_2H_5Br + H_2O Heating an alcohol with an excess of hot concentrated phosphoric(V) acid (or passing the vapour over hot aluminium oxide) eliminates water to give an alkene: dehydration.
🧑🏫 Teacher's explanation
Think of an alcohol molecule like a person wearing a heavy backpack (the -OH group). Sometimes, that person wants to burn energy to run, which is like the combustion reaction where the molecule reacts with oxygen to release heat. Sometimes, they want to swap that backpack for something else, like a halogen atom; this is substitution. Finally, sometimes they just want to get rid of the backpack entirely to become a leaner, more unsaturated molecule; this is dehydration.
📖 Definitions
🔍 Worked example — Easy
Calculate the mass of ethanol (C_2H_5OH) required to produce 44 g of CO_2 upon complete combustion. (A_r: C=12, H=1, O=16)
🔍 Worked example — Standard
A sample of propan-1-ol (CH_3CH_2CH_2OH) is heated with concentrated H_3PO_4. Write the balanced equation for the dehydration product.
🔍 Worked example — Exam-style
Calculate the percentage yield of an alkene produced when 0.100 mol of ethanol is dehydrated. If 0.075 mol of alkene is actually collected, what is the percentage yield?
⚠️ Common errors
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🎯 Exam question
(a) Identify the type of reaction occurring. [1] (b) Write the balanced equation for the formation of the organic product. [2] (c) Explain why the bromine water test confirms the identity of the product. [1] [4 marks]
Key ideas — Oxidation and the classification of alcohols
Acidified potassium dichromate(VI), mathrmCr_2O_7^2-/H^+, or potassium manganate(VII), MnO_4^-/H^+, oxidises alcohols according to their class. A primary alcohol is oxidised first to an aldehyde (distilled off as it forms) and, if heated under reflux with excess oxidant, on to a carboxylic acid. A secondary alcohol is oxidised only as far as a ketone, since there is no second hydrogen on the carbon bearing -OH to remove. A tertiary alcohol has no hydrogen atom on that carbon at all and is not oxidised. This behaviour, together with the colour change of the oxidant (orange to green for dichromate, purple to colourless for manganate), classifies an unknown alcohol as primary, secondary or tertiary from its oxidation product.
🧑🏫 Teacher's explanation
Think of oxidation in organic chemistry as 'stripping away' hydrogen atoms from the carbon that holds the -OH group. Imagine a person (the alcohol) trying to walk through a narrow doorway (the oxidation process). A primary alcohol is like someone carrying a small backpack; they can easily step through to become an aldehyde, and if they keep walking, they eventually lose the backpack and become a carboxylic acid. A secondary alcohol is like someone carrying a heavy suitcase; they can step through once to become a ketone, but they can't step through again because they are too bulky. A tertiary alcohol is like someone wearing a massive, rigid frame; they simply cannot fit through the doorway at all.
📖 Definitions
🔍 Worked example — Easy
Identify the class of alcohol with the formula CH_3 extCH_2 extOH and predict its products when reacted with excess acidified K_2Cr_2O_7/H^+ under reflux.
🔍 Worked example — Standard
An unknown alcohol is reacted with acidified K_2Cr_2O_7/H^+. The orange solution turns green. The organic product is a ketone. Classify the alcohol.
🔍 Worked example — Exam-style
A sample of 2.00 g of an alcohol is oxidized using excess acidified KMnO_4/H^+. The reaction is successful, but the alcohol is found to be tertiary. How many moles of alcohol were present if the M_r of the alcohol is 60?
⚠️ Common errors
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🎯 Exam question
(a) Identify the class of alcohol used. [1] (b) Write the structural formula of the carboxylic acid produced. [1] [2 marks]
Key ideas — Esterification
An alcohol condenses with a carboxylic acid, warmed with a few drops of concentrated sulfuric acid as catalyst, in a reversible reaction forming an ester and water: CH_3COOH + C_2H_5OH leftharpoons CH_3COOC_2H_5 + H_2O
🧑🏫 Teacher's explanation
Imagine you are trying to join two separate Lego bricks together. In esterification, the alcohol and the carboxylic acid are like two specialized bricks that want to snap together to form a new, larger structure called an ester. However, they need a 'helper' to make the connection happen more quickly, which is why we add concentrated sulfuric acid.
📖 Definitions
🔍 Worked example — Standard
A student reacts 0.20 mol of ethanoic acid with 0.50 mol of ethanol using H_2SO_4 as a catalyst. Calculate the maximum yield of ethyl ethanoate in moles, assuming the reaction goes to completion.
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🎯 Exam question
Key ideas — The tri-iodomethane (iodoform) test
Warming a compound with iodine and aqueous sodium hydroxide detects the CH_3CH(OH)- group (the alkaline iodine first oxidises this group to CH_3CO-, which then reacts further). A positive result is a pale yellow precipitate of tri-iodomethane, CHI_3, with a characteristic antiseptic smell. Ethanol and propan-2-ol give a positive test; methanol does not.
🧑🏫 Teacher's explanation
Think of the iodoform test as a 'chemical fingerprinting' tool. Just as a specific key only fits a specific lock, this test only reacts with molecules that have a very specific structural arrangement: a methyl group attached to a carbon that also holds a hydroxyl group (the CH_3 extCH(OH)- group).
📖 Definitions
🔍 Worked example — Easy
Identify which of the following alcohols will give a positive iodoform test: Ethanol (CH_3 extCH_2 extOH), Methanol (CH_3 extOH), and Propan-1-ol (CH_3 extCH_2 extCH_2 extOH).
🔍 Worked example — Standard
A sample of an unknown alcohol is reacted with I_2 and NaOH. A yellow precipitate is formed. If the alcohol is a straight-chain alcohol with three carbons, identify it.
🔍 Worked example — Exam-style
Calculate the mass of CHI_3 produced when 0.10 mol of propan-2-ol reacts completely with excess iodine in aqueous NaOH. (Given: A_r of C=12, extH=1, extI=127)
⚠️ Common errors
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🎯 Exam question
(a) Identify the structure of X. [2] (b) Write the chemical equation for the formation of the precipitate from the carbonyl intermediate. [1] [3 marks]
Key ideas — Acidity of alcohols compared with water
Alcohols are weakly acidic: sodium reacts with them to release hydrogen gas and form a sodium alkoxide, 2ROH + 2Na arrow 2RONa + H_2. Alcohols are, however, less acidic than water, because the electron-donating alkyl group increases electron density on the oxygen atom, making the O-H bond less polar and destabilising the alkoxide ion relative to the hydroxide ion; alcohols therefore do not react with aqueous sodium carbonate, unlike carboxylic acids.
🧑🏫 Teacher's explanation
Think of an acid as a molecule that is willing to 'give away' a hydrogen ion (H^+). For a molecule to be a strong acid, it needs to be very stable once that hydrogen is gone. In water (H_2O), once the H^+ leaves, you are left with the hydroxide ion (OH^-), which is quite stable because oxygen is very electronegative and holds its negative charge well.
📖 Definitions
🔍 Worked example — Easy
Write a balanced equation for the reaction between ethanol (C_2H_5OH) and sodium metal (Na).
🔍 Worked example — Standard
Explain why methanol (CH_3OH) is a stronger acid than ethanol (C_2H_5OH).
🔍 Worked example — Exam-style
Compare the acidity of water (H_2O) and ethanol (C_2H_5OH). Which is more acidic and why?
⚠️ Common errors
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🎯 Exam question
Worked example
Question. Compound Q, molecular formula mathrmC_4H_10O, reacts with sodium to give a gas but does not decolourise acidified potassium dichromate(VI) even under prolonged reflux. Compound Q gives a negative tri-iodomethane test. (a) Deduce the class of alcohol Q belongs to, explaining your reasoning. [2] (b) Suggest a structure for Q consistent with this molecular formula. [1] (c) Explain why Q shows no colour change with acidified dichromate(VI). [2]
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