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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 alcoholsROH, 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)

some associated revising links

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 * 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 –C C– (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.

RCOO–Na+ * 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) SO3

2– * sulphur dioxide gas SO2 * Tollen's Reagent *

unsaturated/saturated * water H2O * zinc ion Zn2+ *

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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

2. Qualitative ORGANIC functional group tests in various homologous series

CHEMICAL TEST FOR TEST METHOD OBSERVATIONS TEST CHEMISTRY and comments

ALKENE or alkynechemical test i.e. anynon–aromaticunsaturated

hydrocarbon with a double ortriple carbon–carbon bond.

Bubble gas through, or addliquid to, a solution of brominein hexane or water.

The orange/brown brominerapidly decolourises, as asaturated colourless organicbromo–compound is formed.

R2C=CR2 + Br2 ==> BrR2C–CR2Br

RC CR + 2Br2 ==> Br2RC–CRBr2

R = H, alkyl or aryl

Saturated alkanes give no fast reaction with bromine.

Hydroxy group R–OHchemical test inalcohols and phenols(in 'dry' conditions*)

The first 3 tests (i) – (iii) givenon the right are quite generalfor most alcohols.

(i) Mix it with a few drops ofethanoyl chloride, test fumeswith litmus and silver nitrate (*note ethanoyl chloride reactswith water, phenols and aminestoo!).

(ii) Mix it with a littlephosphorus(V) chloride and testas above.

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

(i) Litmus turns red and a whiteprecipitate with silver nitrate(aq)

(drop on end of glass rod), if themixture is poured into water youmay detect a 'pleasant' esterodour, can test for HCl but waterand 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

Primary alcohol chemical test

RCH2OH, R = H, alkyl or aryl

(NOT a phenol).

(ii) is not a good test on itsown, since so many otherreadily reducible organiccompounds will give the samereaction, though following it upby testing for an aldehyde givesit much more validity.

(i) Lucas test – shake a fewdrops with cold zinc chloride inconc. HCl(aq)

(ii) Distil with potassiumdichromate(VI) and mod. conc.H2SO4(aq)

(i) Solution remains clear.

(ii) If product distilled offimmediately an aldehyde odourcan be detected and thesolution colour changes fromorange 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 greenchromium(III) ion. If the organic product is collected you couldtest for an aldehyde.

Secondary alcohol chemicaltest

R2CHOH, R = alkyl or aryl.

(ii) is not a good test on itsown, since so many otherreducible organic compoundswill give the same reaction,though following it up by testingfor a ketone gives it much morevalidity.

(i) Lucas test.

(ii) Distil withK2Cr2O7/H2SO4(aq)

(i) Solution may cloud veryslowly or remains clear (hit andmiss)

(ii) If product distilled offimmediately a ketone odourcan be detected and thesolution colour changes fromorange to green.

(i) May be reactive enough to slowly form an insoluble secondaryhalogenoalkane: 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 greenchromium(III) ion. If the organic product is collected you couldtest for an aldehyde.

Tertiary alcohol chemical test

R3COH, R = alkyl or aryl.

(i) Lucas test.

(ii) Distil withK2Cr2O7/H2SO4(aq)

(i) Goes cloudy very quickly.

(ii) No aldehyde or ketonereadily formed

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

(iii) Stable to modest oxidation.

Phenols (OH group isattached directly toaromatic ring)

chemical test. R–OH, where Ris aryl e.g. C6H5OH

Add a few drops of iron(III)chloride solution to a little of thephenol in water.

Usually gives a purple colour. (see also test for primary aromatic amines – use it in reversestarting with a known primary aromatic amine!)

Carboxylic acids chemicaltest

RCOOH

Mix the carboxylic acid withwater and add a little sodiumhydrogencarbonate solid orsolution.

fizzing, colourless gas giveswhite 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

Add a little dilutehydrochloric/sulfuric acid to a

The solid or solution shouldhave no strong odour, but after

The stronger acid, HCl/H2SO4 displaces the weaker aliphatic

carboxylic acid which have strong–pungent characteristic

(RCOO–)2Mg etc. suspected salt of an aliphaticcarboxylic acid.

adding the mineral acid youshould get a pungent odour ofthe original aliphatic acid. If itsthe salt of an aromaticcarboxylic acid, you get littleodour and maybe a whitecrystalline precipitate.

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 Chloridechemical test

RCOCl

Fumes in air forming HCl(g)

(i) Add a few drops to water,test with litmus and silver nitratesolution.

(ii) Add to a little ethanol andpour the mixture into water.

(i) Litmus turns red and a whiteprecipitate with silver nitrate.

(ii) As above and you maydetect 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 withdilute sodium hydroxidesolution, see in inorganic forammonia tests.

ammonia evolved on boiling (noheat required to form ammonia,if it was an ammonium salt)

RCONH2 + NaOH ==> RCOONa + NH3

Aliphatic amines (primary,where R = alkyl) chemical testR–NH2

e.g. CH3CH2CH2–NH2

(i) Lower members soluble inwater but a very fishy smell! testwith red litmus and conc.HCl(aq) fumes.

(ii) If a suspected salt of anamine, then add sodiumhydroxide solution to free theamine.

(i) A fishy odour, litmus turnsblue, white clouds with HCl.

(ii) The above is not observeduntil after adding the alkali.

(i) Unless its a liquid or solid, only the more fishy odourdistinguishes it from ammonia.

(ii) The reaction is e.g.

R–NH3+ + OH– ==> R–NH2 + H2O

Aromatic amineschemical test (whereR = aryl with the

amine or amino group directlyattached to an aromatic ring) R–NH2

e.g. C6H5–NH2

(i) Dissolve the primary aromaticamine in dilute hydrochloric acid

at 5oC and mix with sodiumnitrite solution.

(ii) Add a phenol dissolved indilute sodium hydroxide.

(i) It should be a clear solutionwith few, if any, brown fumes.

(ii) A coloured precipitate [red –brown – yellow etc.]

(i) If a primary aromatic amine, a 'stable' diazonium salt isformed. Diazonium salts from aliphatic amines decomposerapidly 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) todistinguish from ketones(R2C=O, R = alkyl or aryl) and

also reducing sugars.

Note

(1) Test (b)(i) and (ii) can beused to distinguish aldehydes(reaction) and ketones (noreaction).

(2) Aromatic aldehydes do NOTgive a positive result with (b)(ii)Benedict's or Fehling's reagent).

(3) Reducing sugars may alsogive a positive test with (b)(i)/(ii)reagent e.g. glucose(aldohexose) but not fructose?(ketohexose)?

(a) Add a few drops of thesuspected carbonyl compoundto Brady's reagent (2,4–dinitrophenylhydrazinesolution)

(a) A yellow–orange precipitateforms with both types ofcarbonyl 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'tdistinguish them, read on below!

(b)(i) warm a few drops of thecompound with Tollens'reagent [ammoniacal silvernitrate]

(b)(ii) simmer with Fehling's orBenedicts solution [a blue

complex of Cu2+(aq)]

(b) Only the aldehyde produces(i) A silver mirror on the side ofthe test tube.

(ii) A brown or brick red ppt.

Aldehydes are stronger reducing agents than ketones and reducethe 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 insolublecopper(I) oxide Cu2O.

RCHO + 2Cu2+ + 2H2O ==> RCOOH + Cu2O + 4H+

With (b)(i)/(ii) no reactions with ketones.

Halogenoalkanes(haloalkanes) chemical test R–X where R = alkyl, X = Cl, Bror I

The halide is covalently bound(C–X bond), so the halogen Xcannot react with the silver ion

(i) Warm a few drops of thehaloalkane with aqueousethanolic silver nitrate solution,the ethanol increases thesolubility of the immisciblehaloalkanes.

(ii) Gently simmering a fewdrops with aqueous NaOH (may

(i) Observe colour of precipitateand the effect of ammoniasolution on it (for rest of detailssee the (i) notes for chloride,bromide and iodide testsabove in inorganic)

(ii) see the (i) notes as abovefor more details.

(i) AgNO3 + RX ==> R–NO3? + AgX(s)

(ii) The sodium hydroxide converts the halogen atom into theionic halide ion in a hydrolysis reaction.

RX(aq) + NaOH(aq) ==> ROH(aq) + NaX(aq)

then Ag+(aq) + X–

(aq) ==> AgX(s)

to form the ionic Ag+X–(s)

precipitate until it is converted to

the 'free' X– ionic form. Note thataromatic halogen compoundswhere the X is directly attachedto the ring, do NOT readilyhydrolyse in this way and noAgX ppt. will be seen. AromaticC–X is a stronger bond thanaliphatic C–X.

need to add ethanol to increasesolubility and reaction rate). Adddilute nitric acid followed byaqueous silver nitrate solution.

The addition of dilute nitric acid prevents the precipitation of othersilver 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 anester. Usually a colourlessliquid with a pleasant 'odour'.

The ester can be reacted withsaturated ethanolichydroxylamine hydrochloride +20% methanolic KOH andgently heated until boiling. Thenmixture acidified with 1M HCl(aq)

and FeCl3(aq) added dropwise.

Deep red or purple colourformed. The test depends on theformation of a hydroxamic acidR–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 iodidesolution until most of the colourhas gone. The organiccompound is warmed with thissolution.

A yellow solid is formed with thesmell 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 adefinitive test for anything, it just indicates a possible part of amolecules structure.

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 forunsaturation? 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 primaryalcohol? 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 isthe 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 doyou 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 fora 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 analdehyde 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 areducing 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 fora 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 theiodoform test? (triiodomethane formation) How do you do the iodoform test? What does the iodoform reaction test for?

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