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CHEMICAL TESTS FOR IDENTIFYING ORGANIC FUNCTIONAL GROUPS

Doc Brown's Chemistry  Qualitative Methods of Analysis Revision Notes

 Doc Brown's Chemistry  Revising Advanced Organic Chemistry

Chemical identification TESTS Part 2 Qualitative tests to identify organic molecule functional groups

Functional group tests for alkenes RCH=CHR, hydroxy group functional group – primary, secondary & tertiary alcohols ROH, phenols, carboxylic group RCOOH, acyl/acid amide group RCONH2, acyl/acid chloride group RCOCl, aliphatic amine group R–NH2, aryl/aromatic amine, aldehyde functional group RCHO, ketone functional group RCOR, halogenoalkanes/haloalkanes (chloroalkane, bromoalkane, iodoalkane) R–Cl, R–Br, RI, esters RCOOR and the iodoform test CHI3

also = Part 11 Qualitative Analysis Test for and identify organic functional groups notes

 Part 1 Introduction

 Part 2 Qualitative tests to identify organic molecule functional groups of homologous series (this page)

 Part 3 Metal cations (positive ions), metal carbonates, ammonium ion and hydrogen ions (acids)

 Part 4 Gases, water and non–metallic elements

 Part 5 Anions (negative ions) including hydroxide (alkalis)

APPENDIX 1. IDENTIFYING ELEMENTS from LINE SPECTRA (non-chemical test method)

some associated revising links

GCSE/IGCSE Revision QUIZ on chemical tests for identifying ions, gases and compounds

A Level Quantitative analysis: acid–base, silver nitrate–chloride, EDTA titrations

A Level Quantitative analysis: Redox titrations

Full list of KEYWORDS for inorganic/organic identification methods in alphabetical order e.g. test/reagent for: * acid ==> H+ * acid/acyl chloride RCOCl * alcohols – general ROH/prim RCH2OH/sec R2CHOH/tert R3COH) * aldehyde RCHO * prim aliphatic amine R–NH2 * aliphatic/aromatic carboxylic acids * alkali ==> OH * alkane/alkene >C=C</alkyne –CC– (saturated versus unsaturated) * aluminium/aluminum ion Al3+ * amide RCONH2 * prim aliphatic amines R–NH2 * ammonia gas NH3 * ammonium ion NH4+ * prim aromatic amine C6H5–NH2 etc. * barium ion Ba2+ * Benedict's solution * Brady's reagent * bromide ion Br * bromine Br2 * caesium ion Cs+ * calcium ion Ca2+ by flame or hydroxide ppt. * carbonate CO32–/hydrogencarbonate HCO3 with acid or effect of heating metal carbonate e.g. MCO3 * carbon dioxide gas CO2 * carboxylic acid RCOOH * carboxylic acid (aliphatic) salts e.g. RCOONa+ * chloride ion Cl * chlorine gas Cl2 * Chomate(VI) ion CrO42– * copper(II) ion Cu2+ by flame or hydroxide ppt. * 24DNPH (for aldehydes/ketones test) * esters RCOOR * Fehlings test/solution * flame test for metal ions * fluoride ion F * haloalkanes/halogenoalkanes R–X * hydrogen gas H2 * hydrogen sulphide H2S * hydrogen ion, acids H+ * hydrogen bromide gas/hydrobromic acid HBr * hydrogen chloride gas/hydrochloric acid HCl * hydrogen iodide gas/hydriodic acid HI * hydroxide ion, alkali OH * hydroxy/alcohol/phenol (organic) * iodide ion I * iodine I2 * iodoform test – formation of CHI3 * iron(II) ion Fe2+ * iron(III) ion Fe3+ * ketone R2C=O * lead(II) ion Pb2+ * lithium ion Li+ * lime water Ca(OH)2(aq) * magnesium ion Mg2+ * metal carbonates–heating e.g. MCO3 * metal ions via hydroxide precipitate * nitrate or nitrate(V) NO3 * nitrite or nitrate(III) NO2 * nitrogen dioxide or nitrogen(IV) oxide NO2 * oxygen gas O2 * phenols C6H5OH etc. * potassium ion K+ * rubidium ion Rb+ * reducing sugars * saturated/unsaturated * silver nitrate AgNO3 (see chloride, bromide, iodide tests) * sugars (reducing) * sodium ion Na+ * strontium Sr+ * 'sulphate/sulfate' or sulphate(VI) SO42– * sulphide S2 * 'sulphite/sulfite' or sulphate(IV) SO32– * sulphur dioxide gas SO2 * Tollen's Reagent * unsaturated/saturated * water H2O * zinc ion Zn2+ *


Use the alphabetical test list above for identifying anions, cations, gases, molecules etc. to find what you require! for your KS3–KS4 Science–GCSE–IGCSE– Chemistry and GCE–AS–A2–IB–US grades 9–12 K12 advanced subsidiary chemistry course etc. and help you to identify unknown inorganic and organic compounds–molecules for qualitative analysis.

EMAIL query?comment?test missing? * Gas Preparations * Hazard warning signs/symbols–examples of labelling

  Advanced Chemistry Page Index and Links

2. Qualitative ORGANIC functional group tests in various homologous series

CHEMICAL TEST FOR

TEST METHOD OBSERVATIONS  TEST CHEMISTRY and comments
ALKENE  or alkyne chemical test 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

RCCR + 2Br2 ==> Br2RC–CRBr2

R = H, alkyl or aryl

Saturated alkanes give no fast reaction with bromine.

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.

(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

Advanced Chemistry Page Index and Links

Primary alcohol chemical test

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 chemical test

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 chemical test

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) chemical test. 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!)

Advanced Chemistry Page Index and Links

Carboxylic acids  chemical test

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. RCOONa+ 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 aliphatic acid. If its the salt of an aromatic carboxylic acid, you get little odour and maybe a white crystalline precipitate. 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).

 

Acid or Acyl Chloride chemical test

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
Acid Amide  chemical test

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) chemical test 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

 

Advanced Chemistry Page Index and Links

Aromatic amines chemical test (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 chemical test (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!

(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.Advanced Chemistry Page Index and Links

Halogenoalkanes (haloalkanes) chemical test 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).

 

Esters chemical test 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 
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.Advanced Chemistry Page Index and Links

       

 

Keywords & phrases: What is the test for alkenes? How do you test for an alkene functional group? What is the test for unsaturation? How do you test for unsaturation? What is the test for alcohols? How do you test for an alcohol functional group? What is the test for a primary alcohol? How do you test for a primary alcohol? What is the test for a secondary alcohol? how do you test for a secondary alcohol? What is the test for a tertiary alcohol? How do you test for a tertiary alcohol? What is the test for a phenol? How do you test for a phenol? What is the test for a phenolic group? How do you test for a phenolic functional group? What is the test for a carboxylic acid? How to you test for a carboxylic acid functional group? What is the test for a salt of a carboxylic acid? how do you test for the salt of a carboxylic acid? What is the test for an acid/acyl chloride? How do you test for an acid/acyl chloride functional group? What is the test for an acid amide? how do you test for an acid amide? What is the test for an aliphatic amine? How do you test for aliphatic amines? What is the test for a primary amine? How do you test for a primary amine functional group? What is the test for a secondary amine? How do you test for a secondary amine? What is the test for a tertiary amine? How do you test for a tertiary amine? What is the test for an aromatic amine? How do you test for an aromatic amine? What is the test for aldehydes? How do you test for an aldehyde functional group? What is the test for ketones? How do you test for a ketone functional group? What is the test for reducing sugars? How do you test for a reducing sugar? What is the test for a haloalkane? (tests for halogenoalkanes, 'alkyl halides') How do you test for a haloalkane functional group? What is the test for a primary haloalkane? How do you test for a primary haloalkane? What is the test for a secondary haloalkane? How do you test for secondary haloalkane? What is the test for a tertiary haloalkane? How do you test for a tertiary haloalkane? What is the test for an ester? How do you test for the ester functional group? What is the iodoform test? (triiodomethane formation) How do you do the iodoform test? What does the iodoform reaction test for?

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