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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

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

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Tests for primary, secondary and tertiary alcohols

Adapted from All the tests for organic functional groups

diagram structure of alcohol functional group general structural formula advanced level organic chemistry 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 ROH 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) ROH + CH3COCl ==> CH3COOR + HCl

An ester and hydrogen chloride are formed

(ii) ROH + PCl5 ==> RCl + 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. H2SO4(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) RCH2OH + [O] => RCHO + H2O or the full works!

3RCH2OH + Cr2O72  + 8H+ ==>

2Cr3+ + 3RCHO + 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/H2SO4(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] => RCOR + H2O or the full works!

3R2CHOH + Cr2O72  + 8H+ ==>

2Cr3+ + 3RCOR + 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/H2SO4(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 119oC.

This reaction is given by the primary alcohol ethanol CH3CH2OH and all secondary alcohols with the 2ol structure CHOHCH3

BUT, it is also given by the aldehyde ethanal CH3CHO and all ketones with the 2one structure RCOCH3  ('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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