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Advanced A/AS Level Organic Chemistry: Chemical Tests for Alcohols

Part 4. The chemistry of ALCOHOLS

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry

Part 4.8 Chemical tests for alcohols

(primary, secondary or tertiary)

INDEX of notes on ALCOHOLS chemistry

All Advanced Organic Chemistry Notes

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Tests for primary, secondary and tertiary alcohols

Adapted from All the tests for organic functional groups

Aliphatic alcohols

Reminders: You need to know the structures of the sub-classes of alcohols - primary, secondary and tertiary.

There is no simple test that defines the presence of an alcohol or its sub-class i.e. primary, secondary or tertiary alcohol.

You need to do several tests and match the observations with an alcohol and its sub-class.

Hydroxy group R–OH chemical test in alcohols and phenols (in 'dry' conditions*)

The first 3 tests (i) – (iii) given on the right are quite general for most alcohols AND other substances too!

(i) Mix the dry sample with a few drops of ethanoyl chloride, test fumes with litmus and silver nitrate (* note ethanoyl chloride reacts with water, phenols and amines too!).

(ii) Mix the dry sample with a little phosphorus(V) chloride and test as above.

(iii) Warm with a little ethanoic acid and a few drops of conc. sulphuric acid. Pour into water and stir gently.

(i) Fumes turn litmus red, and a white precipitate with silver nitrate_(aq) (drop on end of glass rod), ammonia fumes give a white cloud of ammonium chloride, if the mixture is poured into water you may detect a 'pleasant' ester odour, can test for HCl but water and amines produce HCl too!

(ii) as for (i) but no ester smell!

(iii) You should get a 'pleasant' characteristic smell of an ester.

(i) R–OH + CH₃COCl ==> CH₃COOR + HCl

An ester and hydrogen chloride are formed

(ii) R–OH + PCl₅ ==> R–Cl + POCl₃ + HCl

a chloro compound and hydrogen chloride are formed.

(i) and (ii) Ag⁺_(aq) + Cl^–_(aq) ==> AgCl_(s) from the hydrogen chloride fumes dissolved in water.

(iii) CH₃COOH + ROH ==> CH₃COOR + H₂O

Note: Alcohols are distinguished from carboxylic acids (pH < 7) by being neutral pH 7 - alcohols do not change blue litmus red, or create effervescence with a pinch of sodium hydrogencarbonate.

Primary alcohol chemical tests

RCH₂OH, R = H, alkyl or aryl (NOT a phenol).

(ii) is not a good test on its own, since so many other readily oxidisable organic compounds will give the same reaction, though following it up by testing for an aldehyde gives it much more validity.

(i) Lucas test – shake a few drops with cold zinc chloride in conc. HCl_(aq)

(ii) Distil with potassium dichromate(VI) and mod. conc. H₂SO_4(aq)

Not very convenient !!!

(i) Solution remains clear.

(ii) If product distilled off immediately an aldehyde odour can be detected and the solution colour changes from orange to green.

(i) Not usually reactive enough to form a primary halogenoalkane

(ii) R–CH₂OH + [O] => R–CHO + H₂O or the full works!

3R–CH₂OH + Cr₂O₇^2– + 8H⁺ ==>

2Cr³⁺ + 3R–CHO + 7H₂O

The orange dichromate(VI) ion is reduced to the green chromium(III) ion. If the organic product is collected you could test for an aldehyde.

Secondary alcohol chemical tests

R₂CHOH, R = alkyl or aryl.

(ii) is not a good test on its own, since so many other reducible organic compounds will give the same reaction, though following it up by testing for a ketone gives it much more validity.

(i) Lucas test.

(ii) Distil with K₂Cr₂O₇/H₂SO_4(aq)

Not very convenient !!!

(i) Solution may cloud very slowly or remains clear (hit and miss)

(ii) If product distilled off immediately a ketone odour can be detected and the solution colour changes from orange to green.

(i) May be reactive enough to slowly form an insoluble secondary halogenoalkane: R₂CHOH + HCl => R₂CHCl + H₂O

(ii) R₂CHOH + [O] => R–CO–R + H₂O or the full works!

3R₂CHOH + Cr₂O₇^2– + 8H⁺ ==>

2Cr³⁺ + 3R–CO–R + 7H₂O

The orange dichromate(VI) ion is reduced to the green chromium(III) ion. If the organic product is collected you could test for an aldehyde.

Tertiary alcohol chemical tests

R₃COH, R = alkyl or aryl.

(i) Lucas test.

(ii) Distil with K₂Cr₂O₇/H₂SO_4(aq) Not very convenient !!!

(i) Goes cloudy very quickly.

(ii) No aldehyde or ketone readily formed, no change from orange to green.

(i) Reactive enough to immediately form an insoluble tertiary halogenoalkane R₃COH + HCl => R₃CCl + H₂O

(iii) Stable to modest oxidation.

Iodoform test

The formation of CHI₃, triiodomethane (or old name 'iodoform'.

NaOH_(aq) is added to a solution of iodine in potassium iodide solution until most of the colour has gone. The organic compound is warmed with this solution.

A yellow solid is formed with the smell of an antiseptic, CHI₃,

triiodomethane, melting point 119^oC.

This reaction is given by the primary alcohol ethanol CH₃CH₂OH and all secondary alcohols with the 2–ol structure –CHOH–CH₃

BUT, it is also given by the aldehyde ethanal CH₃CHO and all ketones with the 2–one structure R–CO–CH₃ ('methyl ketones')

Its a combination of halogenation and oxidation and is not a definitive test for anything, it just indicates a possible part of a molecules structure.

There is no simple test that defines the presence of an alcohol or its sub-class i.e. primary, secondary or tertiary alcohol.

You need to do several tests and match the observations with an alcohol and its sub-class

All the tests for organic functional groups

Doc Brown's Advanced Level Chemistry Revision Notes

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INDEX of notes on ALCOHOLS chemistry

All Advanced Organic Chemistry Notes

Index of GCSE/IGCSE Oil - Useful Products Chemistry Revision Notes

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