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Tests for aldehydes

Aldehydes (RCHO) 

Aldehydes are reducing agents; they may be oxidised to carboxylic acids (in acidic solution) or their salts (in alkaline solution). Positive tests for aldehydes depend on this fact.

i) Acidified potassium dichromate

Add 1cm^3 of dilute sulfuric acid to 1cm^3 of potassium dichromate solution. 

Add 1cm^3 of the unknown and heat to 50C in a water bath.

If the colour changes from orange to green, the dichromate(VI) has been reduced to chromium(III) and an aldehyde is confirmed. 

e.g. 3 RCHO + Cr2O7^2-  + 8H+ ----------------->  3 RCOOH + 2 Cr^3+ + 4 H2O

                                                   HEAT

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Tests for aldehydes

ii) Tollens' reagent (silver mirror test) 

Add a few drops of aqueous sodium hydroxide to 1cm^3 of silver nitrate solution to produce a precipitate of silver oxide. 

Add just enough aqueous ammonia to redissolve this precipitate and form a colourless solution. 

Add 1cm^3 of the unknown and heat in a water bath at 50C for a few minutes. 

If a 'silver mirror' is formed on the inside of the test tube, this confirms the presence of an aldehyde. 

e.g. RCHO + 2 Ag(NH3)2 +  + 3OH-  ----------------> RCOO- + Ag + 4 NH3 + 2 H2O 

                                                         HEAT

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Tests for aldehydes

iii) Fehling's (or Benedict's) solution

Both Fehling's and Benedict's solutions contain complexed copper(II) ions in an alkaline solution. 

To 1cm^3 of freshly prepared Fehling's (or Benedict's) solution add 1cm^3 of the unknown and heat in a water bath at 50C for a few minutes.

If a red precipitate of copper(I) oxide is formed, this confirms the presence of an aldehyde. 

e.g. RCHO + 2 Cu(L-L)2 2+  + 5 OH-  -----------------> RCOO- + Cu2O + L-L + 3 H2O

                                                           HEAT 

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Tests for ketones

Ketones (R2CO)

Ketones are not reducing agents; they may not be oxidised to carboxylic acids. If a carbonyl compound gives negative results with any of the tests above it confirms the presence of a ketone. 

i) Methyl ketones (RCOCH3)- the iodoform (triiodomethane) reaction 

Methyl ketones undergo the iodoform reaction. 

The production of a pale yellow crystalline precipitate confirms the presence of a methyl secondary alcohol or a methyl ketone. 

e.g. RCOCH3 + 4 OH-  + 3 I2 ------------------> CHI3 + RCOO- + 3I- + 3 H2O 

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Tests for carboxylic acids

Carboxylic acids (RCOOH)

There is no single test for a carboxylic acid, but they may be confirmed by a combination of tests. 

i) Reaction with phosphorus pentachloride 

Carboxylic acids contain a hydroxyl (-OH) group. They therefore react with PCl5. 

Add a spatula measure of PCl5 solid to 1cm^3 of the liquid to be tested. 

An exothermic reaction which evolves steamy fumes (HCl) which turn damp litmus red and form a white 'smoke' with ammonia gas confirms the presence of an -OH group in the molecule. 

As this also gives a positive result with water and alcohols, you must also be sure that the unknown is dry and acidic before you can confirm it is a carboxylic acid. 

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Tests for carboxylic acids

ii) Reaction with water 

Carboxylic acids are weak acids, i.e. they are only partially ionised/ dissociated in aqueous solution.

Most are reasonably soluble, giving a solution with a pH in the range 3-5.

Some long-chain carboxylic acids may be only sparingly soluble, giving a solution with a pH marginally below 7.

e.g. RCOOH + H2O <----------------------> RCOO- + H3O+

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Tests for carboxylic acids

iii) Reaction with sodium carbonate (or hydrogencarbonate) 

Carboxylic acids are weak acids, but generally acidic enough to react with both sodium carbonate and sodium hydrogencarbonate to produce carbon dioxide gas. 

So, add 1cm^3 of the unknown to 1cm^3 of sodium carbonate solution.

If effervescence occurs, collect a sample of the gas in a disposable pipette and bubble through 1cm^3 of lime water. 

If the lime water turns cloudy this confirms CO2 has been evolved and the unknown is an acid. 

This distinguishes carboxylic acids from weaker acids, such as phenol, which do not react with sodium carbonate.

e.g. 2 RCOOH +Na2CO3 -----------------> 2 RCOO- Na+ + CO2 +H2O 

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Tests for carboxylic acids

iv) Reaction with an alcohol (esterification)

To 1cm^3 of ethanol in a test tube add 1cm^3 of the unknown followed by a few drops of concentrated sulfuric acid (catalyst). 

Heat to 50C in a water bath for five minutes. 

Pour the product into a beaker containing 20cm^3 of sodiumcarbonate solution (to neutralise any excess acid). 

If the product smells of 'glue' or 'pear drops' an ester has been formed and the unknown is confirmed as a carboxylic acid.

e.g. ROOH + HOR1  <------------------------> ROOR1 +H2O

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Tests for alkenes

Alkenes (Carbon-Carbon double bond)

i) Reaction with bromine water (electrophilic addition)

Shake 1cm^3 of the compound with 1cm^3 of bromine water (or bubble it through if a gas) in the dark. If the orange bromine is decolourised, this confirms an alkene

e.g. H2C=CH2  +  Br2  ---------------->  Br-CH2-CH2-Br

ii) Reaction with acidified potassium manganate (VII) (oxidative addition)

Shake 1cm^3 of the compound with 1cm^3 of potassium manganate(VII) solution acidified with a little dilute sulfuric acid (or bubble it through if a gas). If the purple manganate(VII) is decolourised, this confirms an alkene

e.g. H2C=CH2  + [O] + H2O --------------------------->  HO-H2C-CH2-OH 

                                          KMnO4, H+ /OH- 

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Tests for Haloalkanes

Haloalkanes (R-X)

Reaction with aqueous sodium hydroxide (nucleophilic substitution) 

Add 1cm^3 of the compound to 1cm^3 of aqueous sodium hydroxide in a tes tube and heat to 50C in a water bath; neutralise excess hydroxide with dilute nitric acid; add 1cm^3 of aqueous silver nitrate 

White ppt of AgCl ===> R-Cl; Cream ppt of AgBr ===> R-Br; yellow ppt of AgI ===> R-I  

e.g. R-X + OH- -------------------------------> R-OH  + X- 

                      aqueous KOH, warm 

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Tests for alcohols

Alcohols (R-OH) 

Reaction with phosphorus pentachloride 

Add a spatula measure of PCl5 1cm^3 of the liquid to be tested.

An exothermic reaction which evolves steamy fumes (HCl) which turn damp litmus red and form a white 'smoke' with ammonia gas confirms the presence of an -OH group in the molecule.

As this also gives a positive result with water (H-OH) and carboxylic acids (RCOOH), you must also be sure that the unknown is dry and neutral before you can confirm it is an alcohol

e.g. R-OH + PCl5 ------------------> R-Cl + POCl3 +HCl 

Once you have confirmed an alcohol is present, you may need to identify it as primary, secondary or tertiary.


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Tests for alcohols

a) Primary alcohols (R-Ch2-OH) 

Oxidise with potassium dicromate acidified with a little dilute sulfuric acid, distilling off the product. If the product shows a positive test for an aldehyde the unknown must be a primary alcohol

e.g. R-CH2-OH + [O] ------------------------------------------------> R-C(=O)-H + H2O

                                K2Cr2O7, H+, distill off product 

b) Secondary alcohols (R2CH-OH) 

Oxidise with potassium dichromate acidified with a little dilute sulfuric acid, distilling off the product. If the product shows a positive test for a carbonyl compound but a negative test for an aldehyde the product must be a ketone and the unknown must be a secondary alcohol

e.g. R2CH-OH +  [O] -----------------------------------------------> R-C(=O)-R  + H2O

                                K2Cr2O7, H+, distill off product 

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Tests for alcohols

c) Tertiary alcohols (R3C-OH)

Attempt to oxidise by warming potassium dichromate acidifed with a little dilute sulfuric acid. If the potassium dichromate remains orange, no oxidation has taken place and the unknown must be a tertiary alcohol 

e.g. R3C-OH + [O] --------------------------> no reaction 

                               K2Cr2O7, H+ 

d) Compounds containing -CH(CH3)OH- the iodoform (triiodomethane) reaction

To 2cm^3 of aqueous sodium hydroxide add 1cm^3 of the unknown followed by a few drops of iodine solution, stopper and shake well. The production of a pale yellow crystalline ppt (of triiodomethane) confirms the presence of the -CH(CH3)OH group (present in secondary methyl alcohols or ethanol) or a methyl ketone which must be distinguished by additional tests. 

e.g. RCH(CH3)OH  + 6 OH-  + 4 I2 -----------------------> CHI3  + RCOO- + 5 I-  + 5 H2O

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