2. Qualitative ORGANIC functional group tests in various homologous series

TEST FOR

RCCR + 2Br₂ ==> Br₂RC-CRBr₂

R = H, alkyl or aryl

Saturated alkanes give no fast reaction with bromine.

Hydroxy group R-OH in alcohols and phenols

The first 3 tests (i) - (iii) given on the right are quite general for most alcohols.

(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.

(ii) as for (1) but no ester smell!

(iii) You should get a 'pleasant' characteristic smell of an ester.

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

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.

(ii) Distil with potassium dichromate(VI) and mod. conc. H₂SO_4(aq) 

(ii) If product distilled off immediately an aldehyde odour can be detected and the solution colour changes from orange to green.

(ii) R-CH₂OH + [O] => R-CHO + H₂O or the full works! 3R-CH₂OH + Cr₂O₇^2-  + 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.

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.

(ii) Distil with K₂Cr₂O₇/H₂SO_4(aq) 

(ii) If product distilled off immediately a ketone odour can be detected and the solution colour changes from orange to green.

(ii) R₂CHOH + [O] => R-CO-R + H₂O or the full works! 3R₂CHOH + Cr₂O₇^2-  + 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.

R₃COH, R = alkyl or aryl.

(ii) Distil with K₂Cr₂O₇/H₂SO_4(aq) 

(ii) No aldehyde or ketone readily formed

(iii) Stable to modest oxidation.

RCOOH

(see also salts of aliphatic carboxylic acids below)

ethanoic acid from an ethanoate salt (smell of acetic acid, vinegar) and butanoates release butanoic acid (butyric acid, rancid odour).

Acid or Acyl Chloride

RCOCl

Fumes in air forming HCl_(g)

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

(ii) As above and you may detect a 'pleasant' ester odour.

The acid chloride is hydrolysed to form HCl acid (chloride ions) and the original carboxylic acid.

RCONH₂

e.g. CH₃CH₂CH₂-NH₂

(ii) If a suspected salt of an amine, then add sodium hydroxide solution to free the amine.

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

(ii) The reaction is e.g.

 R-NH₃⁺ + OH^- ==> R-NH₂ + H₂O

Aromatic amines

e.g. C₆H₅-NH₂

(ii) Add a phenol dissolved in dilute sodium hydroxide.

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

(ii) An azo dyestuff molecule is formed in a coupling reaction e.g.

C₆H₅-N=N-C₆H₄-OH

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)?

R₂C=O + (NO₂)₂C₆H₃NHNH₂ ==>

(NO₂)₂C₆H₃NHN=CR₂ + H₂O

(R = H, alkyl or aryl)

All this tells you is its an aldehyde or ketone, but can't distinguish them, read on below!

(b)(ii) simmer with Fehling's or Benedicts solution [a blue complex of Cu²⁺_(aq)]

(ii) A brown or brick red  ppt.

(i) silver(I) ion to silver metal

(ii) 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.

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

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.

(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.

(ii) see the (i) notes as above for more details.

(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).

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 formation of CHI₃, triiodomethane (or old name 'iodoform'.

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.

KS3 SCIENCE * KS4 SCIENCE GCSE IGCSE GCE AS A2 IB CHEMISTRY * SITE PURPOSE EDUCATION - online learning or 'self-private-tuition' using revision notes, quizzes, practice tests involving SCIENCE in the areas of REVISING only the CHEMISTRY-Earth Science-Radioactivity at Doc Brown's Chemistry Clinic via HOMEPAGE in secondary school/schools, 6th form college/colleges, academy/academies or home self-study and may help with 1st year undergraduate university chemistry courses. Hopefully it will encourage interest and understanding of Chemistry, Earth Science and Radioactivity in any country of the world, though the site is written entirely in English. The website is designed to help and unofficially support students/teachers revise-learn/teach the chemistry for modular or co-ordinated examination science courses from UK QCA based AQA, OCR (Oxford and Cambridge) Twenty First (21st) Century and Gateway Science, Edexcel 360Science , Nuffield, Salters, Cambridge International (CIE), London International, WJEC, CCEA exams etc. Also, national award assessments-examinations for GCSE-IGCSE-KS4-O level-BTEC-NVQ applied, additional and chemistry science courses and examinations, Advanced Subsidiary Level GCE-AS-A2-IB-KS5-BTEC-NVQ National Chemistry assessment levels, KS3 SATs Science-biology/chemistry/physics (SAT revision levels 3-5 or 5-7) and covers much of the revising, learning and teaching chemistry of the International Baccalaureate, K12 US grade 6,7,8,9,10,11,12,AP basic level examinations courses for the national curriculum for secondary schools and colleges. The site does not support the content of England, Wales or Northern Ireland primary science KS1 or KS2. The notes should also provide some background theory for a coursework assignment or a project. BUT please note that my on-line revision notes and quizzes are no substitute for good classroom teaching-lecturing and thorough studying of your own notes and textbooks, practicing past papers and a copy of the syllabus which are readily downloaded from the examination board sites, but I hope here and there they will lend a tutoring hand on some topic, unit, module etc. For final revision you have to be intellectually honest about what you don't know or follow, YOU have to take the stuff to pieces, analyse what you do/do not understand and reconstruct it so it all makes sense in the end. There is no other way, there are no magic secrets on how to revise and learn, its mainly down to hard work and just good old fashioned study and employing teach-yourself strategies without the need for extra tutors and tutoring lessons. I also think there is too much hit and miss revision using past papers (which I do NOT supply) and not enough systematic revision. I also hope it will help teachers in planning lessons and developing schemes of work for science-chemistry. There are no lesson plans on the site but there are plenty of quizzes to incorporate into classroom activities whether photocopied or on electronic whiteboard projector for use as self-tuition-assessment purposes and a variety of teaching and learning styles and the images may be used in Microsoft Word documents and powerpoint projections. The site seems to be used by a large number of home study tutors, particularly the revision notes. An individual tutor may printout out the notes for science-chemistry learning teaching-tuition purposes and for background material for assignments and projects. I have no interest or time in producing WORD.doc or xxxx.pdf revision notes files of the notes at the moment. Neither have I time to write up many practical laboratory experiments ('lab'-'labs') at the moment, but the notes contain lots of background information of chemical reactions in terms of observations-balanced equations-reactants-products-theory etc. I also find it difficult to recommend specific exam websites or syllabus textbooks, it depends exactly on what you need, what you have time for, and there are so many of them to choose from and I do not supply past examination papers for classes. The sites resources include revision notes, quizzes and worksheets which provide support for home study or tuition for homework and coursework help e.g. science investigations for any of the key stage courses indicated, but I do not supply lesson plans. Dr W P Brown I 29-11-2007 * KS3 SCIENCE * KS4 SCIENCE GCSE IGCSE GCE AS A2 IB CHEMISTRY

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