Hydroxy Compounds and Ethers: One Mark Questions, Book Back Answers and Notes

About Hydroxy Compounds and Ethers

Medium ~120 min study

Replace one hydrogen of a hydrocarbon with an –OH group and you get an alcohol. Attach the –OH directly to a benzene ring and you get a phenol. Put an oxygen between two alkyl or aryl groups and you get an ether. That one oxygen atom changes a lot. Alcohols boil far higher than alkanes of similar mass because their molecules hydrogen bond with each other, and the lower alcohols mix with water for the same reason.

Alcohols are also good starting points in synthesis. From ethanol you can reach ethene, ethanal, ethanoic acid, ethyl esters and diethyl ether. To follow these conversions you need what you learnt in Class 11 about hybridisation, carbocation stability and substitution and elimination reactions. You will use the same ideas again for aldehydes, ketones, acids and amines in Units 12 and 13.

Questions from this unit usually test identification and named reactions. Expect to be asked which alcohol gives turbidity first in the Lucas test, what colour a secondary alcohol gives in Victor Meyer's test, which Grignard reagent and carbonyl compound give a particular alcohol, and what products form when an ether is heated with HI.

What you'll learn

• Tell primary, secondary and tertiary alcohols apart using the Lucas and Victor Meyer tests.
• Explain how phenol is prepared from benzene sulphonic acid and from cumene.
• Describe the mechanisms of dehydration and esterification of alcohols.
• Explain why phenol is more acidic than ethanol using resonance in the phenoxide ion.
• Predict the products when an ether is heated with HI or HBr.
• Write IUPAC names for alcohols, phenols and ethers.

Before you start

• Covalent bonding and hybridisation of carbon and oxygen.
• IUPAC naming of alkanes, alkenes and haloalkanes.
• \(S_{N}1\), \(S_{N}2\) and elimination reactions from haloalkanes.

Hydroxy Compounds and Ethers explained

Alcohols, Phenols and Ethers: What the Unit Covers

Classification and Naming

Alcohols are grouped by the number of –OH groups: monohydric (ethanol), dihydric (ethane-1,2-diol) and trihydric (propane-1,2,3-triol, glycerol). A monohydric alcohol is primary, secondary or tertiary depending on how many carbon atoms are bonded to the carbon that carries the –OH group. In IUPAC names, pick the longest chain containing the carbon bearing –OH, number from the end nearer to it, and replace the final -e of the alkane with -ol. Ethers are named as alkoxyalkanes, for example methoxyethane.

Preparation of Alcohols

Alkenes give alcohols by acid-catalysed hydration, which follows Markovnikov's rule. Hydroboration-oxidation of an alkene gives the anti-Markovnikov product instead, so propene gives propan-1-ol. Aldehydes and ketones can be reduced to primary and secondary alcohols. Grignard reagents add to carbonyl compounds and build a new carbon-carbon bond: methanal gives a primary alcohol, any other aldehyde a secondary alcohol, and a ketone a tertiary alcohol.

Reactions of Alcohols

An alcohol reacts either by breaking the C–OH bond or the O–H bond. The Lucas test depends on how fast a carbocation forms. Tertiary alcohols turn cloudy at once, secondary alcohols take about five minutes, and primary alcohols give no turbidity at room temperature. On heating with concentrated sulphuric acid, alcohols lose water to give alkenes, and the ease of dehydration is tertiary > secondary > primary. Oxidation takes a primary alcohol to an aldehyde and then a carboxylic acid, a secondary alcohol to a ketone, while tertiary alcohols resist oxidation under normal conditions.

Phenols

In phenol the –OH group sits directly on the benzene ring. Phenol is more acidic than ethanol because the phenoxide ion is stabilised by resonance. The –OH group strongly activates the ring towards electrophilic substitution at the ortho and para positions, so nitration and bromination go under mild conditions. Bromine water gives a white precipitate of 2,4,6-tribromophenol straight away. In the Kolbe reaction, sodium phenoxide heated with CO₂ under pressure gives sodium salicylate, which is acidified to salicylic acid. In the Reimer-Tiemann reaction, phenol with chloroform and NaOH gives salicylaldehyde.

Ethers

In the Williamson synthesis an alkoxide ion attacks an alkyl halide by an \(S_{N}2\) mechanism, giving symmetrical or unsymmetrical ethers. Ethers are fairly unreactive, which is why diethyl ether is a common solvent, but concentrated HI cleaves them. With methyl or primary groups the iodide ion attacks the less hindered carbon by \(S_{N}2\). When one group is tertiary, the reaction goes by \(S_{N}1\) and the iodide ends up on the tertiary carbon.

Common mistakes to avoid

• Reversing the Lucas test order. Tertiary alcohols go cloudy immediately; primary alcohols show no turbidity at room temperature because a primary carbocation is too unstable to form.
• Mixing up the nitration products of phenol. Dilute nitric acid gives a mixture of o-nitrophenol and p-nitrophenol; concentrated nitric acid gives 2,4,6-trinitrophenol (picric acid).
• Putting the halogen on the wrong group in ether cleavage. With methyl or primary groups the iodide attacks the smaller group, so methoxyethane gives iodomethane and ethanol. If one group is tertiary, the \(S_{N}1\) path puts iodine on the tertiary carbon.
• Stopping primary alcohol oxidation at the aldehyde when a strong oxidant is used. Acidified K2Cr2O7 or KMnO4 takes it on to the carboxylic acid; only a mild reagent like PCC stops at the aldehyde.

Test yourself on these with the practice test, then check the worked reasoning in the solved MCQs.

Frequently asked questions

How can I tell primary, secondary and tertiary alcohols apart?

Use the Lucas test or Victor Meyer's test. With Lucas reagent at room temperature, a tertiary alcohol turns cloudy at once, a secondary alcohol in about five minutes, and a primary alcohol not at all. In Victor Meyer's test, the final alkaline solution is blood red for a primary alcohol, blue for a secondary alcohol and colourless for a tertiary alcohol.

Why is phenol more acidic than ethanol?

When phenol loses a proton, the negative charge on the phenoxide ion spreads into the benzene ring through resonance, which stabilises the ion. The ethoxide ion has no such resonance, and the electron-releasing ethyl group actually concentrates the charge on oxygen. A more stable conjugate base means a stronger acid, so phenol gives up its proton more easily.

What is the Reimer-Tiemann reaction used for?

It puts a –CHO group on the ring of phenol, mainly at the ortho position. Phenol is heated with chloroform and aqueous sodium hydroxide, and acidifying the product gives salicylaldehyde (2-hydroxybenzaldehyde). Dichlorocarbene formed from chloroform is the electrophile that attacks the ring.

How are Grignard reagents used to make alcohols?

A Grignard reagent adds to the C=O group and the adduct is hydrolysed with dilute acid. Methanal gives a primary alcohol, other aldehydes give secondary alcohols and ketones give tertiary alcohols. For example, CH3MgBr with propanone gives 2-methylpropan-2-ol. To make a given alcohol, work backwards to the carbonyl compound it needs.

What happens when alcohols are dehydrated?

Heated with concentrated sulphuric acid, an alcohol loses a water molecule and forms an alkene. Since the reaction goes through a carbocation, tertiary alcohols dehydrate most easily. Ethanol needs about 443 K to give ethene; at a lower temperature of about 413 K with excess ethanol, diethyl ether forms instead.

What is the Williamson ether synthesis?

It is an \(S_{N}2\) reaction between a sodium alkoxide and an alkyl halide. To make an unsymmetrical ether, choose the combination in which the halide is methyl or primary. A tertiary halide would react with the strongly basic alkoxide by elimination and give an alkene rather than an ether.