Doc Brown's GCE Chemistry

Revising Advanced Level Organic Chemistry

GCE A Level Revision Notes PART 10 Summary of organic reaction mechanisms

A mechanistic introduction to organic chemistry and explanations of different types of organic reactions

10.6 Aldehydes–Ketones

10.6.2 Reaction with hydrogen cyanide

Examples are explained of the organic chemistry mechanisms for aldehydes and ketones undergoing nucleophilic substitution, nucleophilic addition reactions are described with diagrams and full explanation revision notes. Carbonyl compounds – ALDEHYDES and KETONES – introduction, Nucleophilic addition of hydrogen cyanide to form a hydroxy–nitrile. The revision notes include full diagrams and explanation of the mechanisms and the 'molecular' equation and reaction conditions and other con–current reaction pathways for these reactions of aldehydes and ketones and products are also explained.

10.6 Carbonyl compounds – ALDEHYDES and KETONES

10.6.1 Introduction to aldehyde and ketone reactivity

Aldehydes and ketones readily undergo nucleophilic attack because of the highly polar carbonyl bond >C^δ+=O^δ– caused by the big difference in the electronegativity between carbon (2.5) and oxygen (3.5). An electron pair donating nucleophile (Nuc:), will therefore attack the 'positive carbon' (C^δ+) to form a C–Nuc bond. A comparison of electrophilic addition to alkenes with nucleophilic addition to aldehydes/ketones is included in these notes.

• Examples of nucleophilic addition of hydrogen cyanide to aldehydes and ketones to give hydroxynitriles

  • (i) + HCN ==>

    • ethanal + hydrogen cyanide ==> 2-hydroxypropanenitrile

  • (ii) + HCN ==>

    • butanone + hydrogen cyanide ==> 2-hydroxy-2-methylbutanenitrile

• What is the mechanism for the addition of hydrogen cyanide to the carbonyl group of an aldehyde or ketone?

• e.g. RR'C=O + HCN ==> RR'C(OH)CN   [see mechanism 7 below]

• The reaction involves mixing an aldehyde (R = H, R' = H or alkyl) or ketone (R and R' are either alkyl or aryl, but NOT H) with buffered potassium cyanide solution to provide a source of negative cyanide ions, the nucleophile.

  The product of the nucleophilic addition of hydrogen cyanide is a hydroxynitrile (a cyanohydrin).

mechanism 7 – nucleophilic addition of cyanide ion to an aldehyde or ketone

• [mechanism 7 above] The >C^δ+=O^δ– bond is highly polarised because of the great difference in electronegativity between carbon (2.1) and oxygen (3.5).

  • Step (1) The nucleophilic electron pair donating cyanide ion attacks the positive carbon of the polarised C=O bond, forming a C–C bond. The Π electron pair of the original C=O bond moves onto the oxygen to give it a whole negative charge. The cyanide ion is the nucleophile - donating an electron pair to a partially positive carbon atom.

  • Step (2) The intermediate formed, RR'C(CN)O^–, is a strong conjugate base and will abstract a proton from water to give the hydroxynitrile product and a hydroxide ion.

• FURTHER COMMENTS

  • Why do alkenes react by electrophilic addition and carbonyl compounds by nucleophilic addition?

    • In alkenes, the electron pair ('rich') donating double bond, is much more likely to react with an electron pair accepting electrophile (Lewis acid) like a positive ion. Electron pair donating nucleophiles, especially if negative (e.g. X^– or OH^–) will tend to be repelled by the high electron density of the Π bond.

    • However, in carbonyl compounds, the highly polar >C^δ+=O^δ– bond, will be susceptible to attack at the positive carbon by electron pair donating nucleophiles

  • In this nucleophilic addition reaction, at the functional group centre of the reaction (>C=O), you change from an unsaturated trigonal planar situation to a saturated tetrahedral bond network about the carbon atom. This carbon atom is, in most cases a chiral carbon and the product therefore can exhibit optical isomerism (R/S isomerism). However the product is usually a 50:50 mixture of the enantiomers (non–superimposable mirror–image forms) i.e. a racemic mixture.

    • Why is the product an optically inactive racemate even if the product is an asymmetric molecule with a chiral carbon?

    • The reason can be clearly argued by considering the mechanism 7 above. The nucleophile attacks the carbon of the polarised carbonyl group (R₂C^δ+=O^δ–) in a trigonal planar bonding situation which changes to a tetrahedral on formation of the C–Nucleophile bond. Quite simply, there is a 50:50 chance of which side of the carbonyl group the nucleophile attacks and therefore a 50:50 chance of which optical isomer is formed as the configuration about the carbon atom changes.

    • Apart from explaining the formation of a racemic mixture, you can also argue, in turn, that the lack of optical activity in the product is itself evidence for an initial attack of the nucleophile at the carbon of the carbonyl group and you might reasonably expect a 2nd order rate expression.

      • rate = k[aldehyde/ketone][CN^–]

      • though I don't know if the kinetics are actually this simple for what seems to be an initial bimolecular rate determining step mechanism!

  • –

nucleophilic addition  RR'C=O + HCN ==> RR'C(OH)CN

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APPENDIX

  COMPLETE MECHANISM and Organic Synthesis INDEX (so far!)

• PART 10 An Introduction to organic chemical reaction mechanisms and technical terms explained

• Organic Synthesis - MECHANISMS INDEX

• Part 10.2 ALKANES - an introduction to their chemistry

  • Free radical chlorination/bromination to give halogenoalkanes (haloalkanes, alkyl halides)

  • Free radical mechanism for cracking hydrocarbons to give shorter alkanes and alkenes

  • Ionic mechanism for cracking hydrocarbons

• Part 10.3 ALKENES - introduction to their chemistry

  • Electrophilic addition of hydrogen bromide [HBr_{(conc. aq)} and HBr_{(g/non-polar solvent)}] to form halogenoalkanes

  • Electrophilic addition of bromine with pure bromine or in non-polar solvent (non-aqueous Br_2(l/solvent)) to give dibromoalkanes AND addition using bromine water [aqueous Br_2(aq)] to give bromo-alcohols

  • Electrophilic addition of sulphuric acid AND electrophilic addition of water [acid catalyst] to form alcohols

  • Free radical polymerisation to give poly(alkene) polymers e.g. ethene ==> poly(ethene)

  • Hydrogenation to give saturated alkanes

• Part 10.4 HALOGENOALKANES - introduction to the chemistry of haloalkanes/alkyl halides

  • Nucleophilic substitution by water/hydroxide ion [S_N1 or S_N2, hydrolysis to give alcohols]

    • with extra notes on kinetics, rds, molecularity, rate expression, activated complex etc.

  • Nucleophilic substitution by cyanide ion to give a nitrile [S_N1 or S_N2]

  • Nucleophilic substitution by ammonia/primary amine to give primary/secondary amines etc. [S_N1 or S_N2]

  • Elimination of hydrogen bromide to form alkenes [E1 and E2]

• Part 10.5 ALCOHOLS - introduction to their chemistry

  • Conversion of an alcohol to a halogenoalkane

  • Elimination of water from an alcohol to give an alkene [acid catalysed, E1 and E2]

• Part 10.6 Carbonyl compounds - ALDEHYDES and KETONES - introduction to their chemistry

  • Nucleophilic addition of hydrogen cyanide to form a hydroxy-nitrile

  • Addition of hydrogen - reduction with LiAlH₄ or NaBH₄ to give alcohols

  • The iodination of ketones e.g. a 2-one like propanone (a methyl ketone) to give iodo-ketones

• Part 10.7 Carboxylic Acids and ACID CHLORIDES - introduction - all nucleophilic addition-elimination reactions

  • Hydrolysis of acid chlorides with water to give a carboxylic acid

  • Esterification of acid chlorides with alcohols to give an ester

  • Amide formation from reaction of acid chlorides with ammonia or primary amines

• Part 10.8 AROMATIC HYDROCARBONS - introduction to arene electrophilic substitutions

  • Nitration to give nitro-aromatics like nitrobenzene

  • Chlorination to chloro-aromatics like chlorobenzene

  • Alkylation to give alkyl-aromatics like methylbenzene [Friedel-Crafts reaction]

  • Acylation to give aromatic ketones [Friedel-Crafts reaction]

  • Sulphonation/sulfonation to give a sulphonic/sulfonic acid like benzenesulphonic acid/benzenesulfonic acid

  • The orientation of products in aromatic substitution (2,4,6 or 3,5 positions, ortho, para & meta substitution products)

• Other important associated links

  • Isomerism - chain isomerism, positional isomerism, functional group isomerism & Tautomerism
  • Stereoisomerism introduction, E/Z isomerism ('ex' Geometric/Geometrical cis/trans Isomerism)
  • R/S Optical Isomerism and chiral auxiliary synthesis
  • All Advanced Organic Chemistry Notes
  • Summary of Organic Functional Groups
  • Quiz on Organic Structure Recognition
  • Summary of organic chemistry functional group tests
  • The shapes and bond angles of simple organic molecules
  • Aliphatic structure & nomenclature m/c BUMPER QUIZ
  • Type in name aliphatic structure and nomenclature BUMPER QUIZ

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