Doc Brown's GCE Chemistry organic reaction mechanisms

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 >Cδ+=Oδ 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' (Cδ+) to form a C–Nuc bond. A comparison of electrophilic addition to alkenes with nucleophilic addition to aldehydes/ketones is included in these notes.

 


10.6.2 Nucleophilic addition of hydrogen cyanide to aldehydes or ketones to give hydroxy–nitriles

The organic synthesis of hydroxynitriles from the reaction of cyanide with aldehydes and ketones

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

    • (i) aldehydes and ketones nomenclature (c) doc b + HCN ==>

      • ethanal + hydrogen cyanide ==> 2-hydroxypropanenitrile

    • (ii) aldehydes and ketones nomenclature (c) doc b + 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).

organic reaction mechanisms

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

  • [mechanism 7 above] The >Cδ+=Oδ 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 >Cδ+=Oδ 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 (R2Cδ+=Oδ) 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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