10.6 Reduction of aldehydes/ketones with hydrides

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 - Reduction with hydrides

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 – via reduction with LiAlH4 or NaBH4 to give alcohols.

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.

10.6.3 Nucleophilic addition of a hydride ion in the reduction of aldehydes/ketones to primary/secondary alcohols

The organic synthesis of alcohols by the reduction of aldehydes and ketones

• Examples of nucleophilic addition of hydride to aldehydes and ketones to reduce them to alcohols

  • (i) + 2[H] ==>

  • butanal + [hydrogen] ==> butan-1-ol

  • On reduction, aldehydes give primary alcohols

  • The hydrogen is derived from the reducing agent e.g.

    • NaBH₄ sodium tetrahydridoborate(III)

    • LiAlH₄ lithium tetrahydridoaluminate(III)

  • (ii) + 2[H] ==>

  • butanone + [hydrogen] ==> butan-2-ol

  • On reduction, ketones give secondary alcohols

• What is the mechanism for the reduction of aldehydes and ketones with sodium tetrahydridoborate(III) or lithium tetrahydridoaluminate(III)?

• Lithium tetrahydridoaluminate(III) (lithium aluminium hydride) or sodium tetrahydridoborate(III) (sodium tetraborohydride) reduce aldehydes to primary alcohols and ketones to secondary alcohols.

  •  very simply the reaction is: RR'C=O + 2[H] ==> RR'CHOH

  • Aldehyde: R = H, R' = H or alkyl) or ketone: R and R' are either alkyl or aryl, but NOT H.

mechanism 40 – nucleophilic addition of a hydride ion (via NaBH₄ or LiAlH₄) to an aldehyde or ketone

• [mechanism 40 above] is a considerable simplification of the full mechanism.

  • In step (1) the AlH₄^– or BH₄^– ion act as nucleophiles and donate the 'equivalent' of a  nucleophilic hydride ion :H^– to the positive carbon of the polarised carbonyl bond. A hydride ion is effectively the nucleophile - donating an electron pair to a partially positive carbon atom (though the full mechanism is quite complicated).

  • In step (2) the intermediate ion is a strong conjugate base and reacts with any proton donor e.g. water (but can be an alcohol ROH or acid H₃O⁺) to form the alcohol product.

  • The real mechanism involves a step–wise replacement of the hydrogen atoms on the reducing reagent with alkoxide groups (RR'CH–O–, e.g. ethoxy CH₃CH₂–O– from reduced ethanal CH₃CHO). This happens because all the intermediates are themselves nucleophilic agents. In the sequence X = B or Al and R₂ =RR' for simplicity)

  • XH₄^– + R₂C=O => [H₃XO–CHR₂]^– = R₂C=O => [H₂X(O–CHR₂)₂]^– = R₂C=O => [HX(–OCHR₂)₃]^– = R₂C=O => [X(–OCHR₂)₄]^– 

  • Then the alkoxide complex reacts with any proton donor (depending on reagent/reaction conditions e.g. the water/ethanol/ acid) to free the alcohol.

    • [X(OCR₂)₄]^– + water/acid/alcohol => 4R₂CHOH + compound of X

keywords phrases: reaction conditions formula intermediates organic chemistry reaction mechanisms nucleophilic addition elimination nucleophilic substitution  NaBH4 sodium tetrahydridoborate(III) LiAlH4 lithium tetrahydridoaluminate(III) (OH)CN RR'C=O + 2[H] ==> RR'CHOH XH4– + R2C=O => [H3XO–CHR2]– = R2C=O => [H2X(O–CHR2)2]– = R2C=O => [HX(–OCHR2)3]– = R2C=O => [X(–OCHR2)4] – [X(OCR2)4]– + water/acid/alcohol => 4R2CHOH

APPENDIX -  COMPLETE MECHANISM and Organic Synthesis INDEX (so far!)