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Doc Brown's Chemistry Qualitative
Methods of Analysis Revision Notes
GCSE/IGCSE
Revision Qualitative Analysis Quiz on chemical test methods for
identifying ions, gases and compounds
Quantitative analysis:
acid-base, silver nitrate-chloride, EDTA titrations
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2. Qualitative ORGANIC functional group tests in various homologous series |
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TEST FOR |
TEST METHOD | OBSERVATIONS | TEST CHEMISTRY and comments |
 ALKENE or alkyne i.e. any non-aromatic unsaturated
hydrocarbon with a double or triple carbon-carbon bond. |
Bubble gas through, or add liquid to, a solution of bromine in hexane or water. | The orange/brown bromine rapidly decolourises, as a saturated colourless organic bromo-compound is formed. |
R2C=CR2
+ Br2 ==>
BrR2C-CR2Br
RC R = H, alkyl or aryl Saturated alkanes give no fast reaction with bromine. |
|
Hydroxy
group R-OH
in alcohols and phenols
(in 'dry'
conditions*)
The first 3 tests (i) - (iii) given on the right are quite general for most alcohols. |
(i)
Mix
it 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 it 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. |
(i)
Litmus
turns red and a white precipitate with silver nitrate(aq)
(drop on end of glass rod), if the mixture is
poured into water you may detect a 'pleasant' ester odour, can test for
HCl but water and amines produce it too!
(ii) as for (1) 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
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Primary
alcohol RCH2OH, R = H, alkyl or aryl (NOT a phenol). (ii) is not a good test on its own, since so many other readily reducible 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) |
(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 + Cr2O72- + 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 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) |
(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 + Cr2O72- + 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 R3COH, R = alkyl or aryl. |
(i)
Lucas test.
(ii) Distil with K2Cr2O7/H2SO4(aq) |
(i)
Goes cloudy very quickly.
(ii) No aldehyde or ketone readily formed |
(i)
Reactive enough to immediately form an insoluble
tertiary halogenoalkane R3COH + HCl => R3CCl
+ H2O
(iii) Stable to modest oxidation. |
 Phenols
(OH group is attached directly to aromatic ring). R-OH, where R is aryl
e.g. C6H5OH |
Add a few drops of iron(III) chloride solution to a little of the phenol in water. | Usually gives a purple colour. |
(see
also test for primary aromatic amines - use it in reverse starting with
a known primary aromatic amine!)
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Carboxylic
acids
RCOOH |
Mix the carboxylic acid with water and add a little sodium hydrogencarbonate solid or solution. | fizzing, colourless gas gives white precipitate with limewater |
RCOOH
+ NaHCO3 ==> RCOONa + H2O + CO2 (see also salts of aliphatic carboxylic acids below) |
| Salts of aliphatic carboxylic acids e.g. RCOO-Na+ or (RCOO-)2Mg etc. | Add a little dilute hydrochloric/sulfuric acid to a suspected salt of an aliphatic carboxylic acid. | The solid or solution should have no strong odour, but after adding the mineral acid you should get a pungent odour of the original acid. |
The stronger acid, HCl/H2SO4
displaces the weaker aliphatic carboxylic acid which have
strong-pungent characteristic odours e.g. ethanoic acid from an ethanoate salt (smell of acetic acid, vinegar) and butanoates release butanoic acid (butyric acid, rancid odour).
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Acid
or Acyl Chloride
RCOCl Fumes in air forming HCl(g) |
(i) Add a few drops to water, test with litmus and
silver nitrate solution. (ii) Add to a little ethanol and pour the mixture into water. |
(i)
Litmus turns red and a white precipitate with silver nitrate. (ii) As above and you may detect a 'pleasant' ester odour. |
(i) RCOCl
+ H2O ==> RCOOH + HCl The acid chloride is hydrolysed to form HCl acid (chloride ions) and the original carboxylic acid. (ii) CH3CH2OH + RCOCl ==> RCOOCH2CH3 + HCl, an ethyl ester and hydrogen chloride are formed |
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Acid
Amide
RCONH2 |
Boil the suspected amide with dilute sodium hydroxide solution, see in inorganic for ammonia tests. | ammonia evolved on boiling (no heat required to form ammonia, if it was an ammonium salt) | RCONH2 + NaOH ==> RCOONa + NH3 |
|
Aliphatic
amines (primary, where R = alkyl) R-NH2
e.g. CH3CH2CH2-NH2 |
(i)
Lower members soluble
in water but a very fishy smell! test with red litmus and conc. HCl(aq)
fumes. (ii) If a suspected salt of an amine, then add sodium hydroxide solution to free the amine. |
(i)
A
fishy
odour, litmus turns blue, white clouds with HCl. (ii) The above is not observed until after adding the alkali. |
(i)
Unless
its a liquid or solid, only
the more fishy odour distinguishes it from ammonia.
(ii) The reaction is e.g. R-NH3+ + OH- ==> R-NH2 + H2O
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Aromatic
amines
(where R = aryl with the amine
or amino group directly attached to an aromatic ring) R-NH2
e.g. C6H5-NH2 |
(i)
Dissolve the primary aromatic amine in dilute hydrochloric acid at 5oC and mix with sodium
nitrite solution.
(ii) Add a phenol dissolved in dilute sodium hydroxide. |
(i)
It should be a clear solution with few, if any, brown fumes.
(ii) A coloured precipitate [red - brown - yellow etc.] |
(i)
If a primary aromatic amine, a 'stable' diazonium salt is formed.
Diazonium salts from aliphatic amines decompose rapidly evolving
colourless nitrogen.
(ii) An azo dyestuff molecule is formed in a coupling reaction e.g. C6H5-N=N-C6H4-OH |
| Aldehydes
(R-CHO, R = H, alkyl or aryl) to distinguish from ketones (R2C=O, R = alkyl or
aryl) and also
reducing sugars. Note (1) Test (b)(i) and (ii) can be used to distinguish aldehydes (reaction) and ketones (no reaction). (2) Aromatic aldehydes do NOT give a positive result with (b)(ii) Benedict's or Fehling's reagent). (3) Reducing sugars may also give a positive test with (b)(i)/(ii) reagent e.g. glucose (aldohexose) but not fructose? (ketohexose)? |
(a) Add a few drops of the suspected carbonyl compound to Brady's reagent (2,4-dinitrophenylhydrazine solution) | (a) A yellow-orange precipitate forms with both types of carbonyl compound. |
The
aldehyde or ketone 2,4-dinitrophenylhydrazone is formed
R2C=O + (NO2)2C6H3NHNH2 ==> (NO2)2C6H3NHN=CR2 + H2O (R = H, alkyl or aryl) This tells you its an aldehyde or ketone, but can't distinguish them, read on below! |
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(b)(i) warm a few drops of the compound with Tollens' reagent [ammoniacal
silver nitrate] (b)(ii) simmer with Fehling's or Benedicts solution [a blue complex of Cu2+(aq)] |
(b) Only the aldehyde produces
(i) A silver mirror on the side of the test tube. (ii) A brown or brick red ppt. |
Aldehydes are stronger reducing agents than
ketones and reduce the metal ion and are oxidised in the process i.e. RCHO + [O] ==> RCOOH (i) reduction of silver(I) ion to silver metal RCHO + 2Ag+ + H2O ==> RCOOH + 2Ag + 2H+ (ii) reduction of copper(II) to copper(I) i.e. the blue solution of the Cu2+ complex changes to the brown/brick red colour of insoluble copper(I) oxide Cu2O. RCHO + 2Cu2+ + 2H2O ==> RCOOH + Cu2O + 4H+ With (b)(i)/(ii) no reactions
with ketones. |
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Halogenoalkanes
(haloalkanes) R-X where R = alkyl, X = Cl, Br or I
The halide is covalently bound (C-X bond), so the halogen X cannot react with the silver ion to form the ionic Ag+X-(s) precipitate until it is converted to the 'free' X- ionic form. Note that aromatic halogen compounds where the X is directly attached to the ring, do NOT readily hydrolyse in this way and no AgX ppt. will be seen. Aromatic C-X is a stronger bond than aliphatic C-X. |
(i)
Warm
a few drops of the haloalkane with aqueous ethanolic silver nitrate
solution, the ethanol increases the solubility of the immiscible
haloalkanes.
(ii) Gently simmering a few drops with aqueous NaOH (may need to add ethanol to increase solubility and reaction rate). Add dilute nitric acid followed by aqueous silver nitrate solution. |
(i)
Observe
colour of precipitate and the effect of ammonia solution on it
(for rest of details see the (i) notes for chloride,
bromide and iodide tests
above in
inorganic)
(ii) see the (i) notes as above for more details. |
(i)
AgNO3
+ RX ==> R-NO3? + AgX(s)
(ii) The sodium hydroxide converts the halogen atom into the ionic halide ion in a hydrolysis reaction. RX(aq) + NaOH(aq) ==> ROH(aq) + NaX(aq) then Ag+(aq) + X-(aq) ==> AgX(s) The addition of dilute nitric acid prevents the precipitation of other silver salts or silver oxide (e.g. Ag2O forms if solution alkaline).
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Esters RCOOR' R = H, alkyl or aryl R' = alkyl or aryl There is no simple test for an ester. Usually a colourless liquid with a pleasant 'odour'. |
The ester can be reacted with saturated ethanolic hydroxylamine hydrochloride + 20% methanolic KOH and gently heated until boiling. Then mixture acidified with 1M HCl(aq) and FeCl3(aq) added dropwise. | Deep red or purple colour formed. The test depends on the formation of a hydroxamic acid R-C(=NOH)OH which forms coloured salts with Fe3+(aq) ion. | The reaction is also given by acid chlorides and acid anhydrides, and phenols give a purple colour with iron(III) chloride, so frankly, the test is not that good. This test is not likely to be expected |
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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, tri-iodomethane, melting point 119oC. |
This
reaction is given by the alcohol ethanol CH3CH2OH
and all alcohols with the 2-ol structure -CHOH-CH3 and 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. |
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