Hydroxy compounds

Syllabus points

• Classify alcohols as primary, secondary or tertiary.
• Explain the physical properties of alcohols in terms of hydrogen bonding.
• Describe the reactions of alcohols: combustion, with sodium, esterification, oxidation, dehydration and with halogenating agents.
• Describe the oxidation products of primary, secondary and tertiary alcohols, and the conditions used.
• Describe the reactions of phenol and explain why it is more reactive than benzene.

Classifying alcohols

The class is decided by how many carbon atoms are attached to the carbon bearing the OH group.

The classification is worth getting right at the start of any question, because the oxidation behaviour depends entirely on it.

Physical properties

Alcohols contain O-H, so they form hydrogen bonds with one another. That gives them much higher boiling points than alkanes of similar relative molecular mass: ethanol boils at 78 °C while propane, of almost the same mass, boils at -42 °C.

The shorter alcohols are soluble in water, again because they hydrogen bond with it. Solubility falls as the chain lengthens, because the non-polar hydrocarbon part grows and comes to dominate the molecule.

Reactions of alcohols

With sodium

2CH₃CH₂OH + 2Na → 2CH₃CH₂ONa + H₂

Effervescence, and the sodium dissolves. The reaction is like sodium with water but gentler, which shows the alcohol is a very weak acid.

Combustion

Alcohols burn completely in plenty of oxygen to carbon dioxide and water. Ethanol is used as a fuel, and because it can be made by fermenting sugar from crops it is described as carbon neutral in principle, since the carbon dioxide released was taken from the air by the plant. In practice the fuel used in growing, harvesting and distilling reduces that benefit, and land used for fuel crops is land not used for food.

Esterification

An alcohol with a carboxylic acid, warmed with a few drops of concentrated sulfuric acid as catalyst, gives an ester and water:

CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O

The reaction is reversible and slow, and the sweet smell of the ester is the observation.

Dehydration

Passing the vapour over hot aluminium oxide, or heating with concentrated sulfuric or phosphoric acid, removes water and gives an alkene:

CH₃CH₂OH → CH₂=CH₂ + H₂O

This is an elimination reaction, and it is the reverse of the industrial hydration of ethene.

With halogenating agents

The OH group can be replaced by a halogen:

• With PCl₅, giving a chloroalkane, HCl and POCl₃. The steamy fumes of HCl make this a test for the OH group.
• With HBr, made in situ from NaBr and concentrated sulfuric acid.
• With red phosphorus and iodine, giving an iodoalkane.

Oxidation

This is the heart of the chapter. The oxidising agent is acidified potassium dichromate(VI), and it turns from orange to green when it acts, because chromium goes from +6 to +3.

The conditions matter as much as the alcohol:

• Distilling removes the aldehyde as it forms, before it can be oxidised further. The aldehyde boils lower than the alcohol because it cannot hydrogen bond to itself.
• Refluxing returns the vapour to the flask, so the aldehyde stays and is oxidised on to the acid.

A tertiary alcohol is not oxidised, because the carbon bearing the OH has no hydrogen on it, and oxidation here means removing hydrogen. The dichromate stays orange, which makes this a way of identifying a tertiary alcohol.

Distinguishing the three classes

1. Warm each with acidified potassium dichromate(VI). No colour change means tertiary.
2. For the two that turn green, distil off the product and test it with Tollens' reagent. A silver mirror means an aldehyde was formed, so the alcohol was primary; no reaction means a ketone, so it was secondary.

The tri-iodomethane test

Warming with iodine and sodium hydroxide gives a pale yellow precipitate of CHI₃ with a distinctive smell.

It is positive for compounds containing the CH₃CH(OH) group, so ethanol and propan-2-ol give it, and methanol and propan-1-ol do not. It also detects the methyl ketone group CH₃CO, so it is a useful structural test rather than a test for alcohols as a class.

Phenol

Phenol has the OH group attached directly to a benzene ring, and that changes its chemistry.

Phenol is more acidic than an alcohol: it reacts with sodium hydroxide to give a salt, which ethanol does not. The reason is that the negative charge on the phenoxide ion is delocalised into the ring, which stabilises it, so the O-H bond ionises more readily. Phenol is still a weak acid, and weaker than carbonic acid, so it does not react with sodium carbonate.

Phenol is also more reactive than benzene towards electrophiles. A lone pair on the oxygen is delocalised into the ring, increasing the electron density there, so the ring attracts electrophiles more strongly.

The evidence is that phenol reacts with bromine water at room temperature with no catalyst, decolourising it and giving a white precipitate of 2,4,6-tribromophenol. Benzene needs a halogen carrier catalyst and gives only single substitution.

Common mistakes

• Saying a tertiary alcohol resists oxidation because it is bulky. It has no hydrogen on the carbon bearing the OH.
• Giving the carboxylic acid as the product of distillation. Distilling gives the aldehyde; reflux gives the acid.
• Forgetting that the dichromate colour change is orange to green, or giving it the wrong way round.
• Saying phenol reacts with sodium carbonate. It is too weak an acid for that.
• Explaining phenol's reactivity by the OH group being electron withdrawing. The lone pair is donated into the ring.
• Using the tri-iodomethane test as a general alcohol test. It needs the CH₃CH(OH) or CH₃CO group.