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Synthesis of Alcohols

Synthesis of Alcohols. Reduction of Aldehydes and Ketones Common reducing agents and conditions: NaBH 4 ( sodium borohydride ) alcohol, ether, or H 2 O as solvent (1) LiAlH 4 ( lithium aluminum hydride =LAH ) (2) H 3 O + Raney Ni finely divided H 2 -bearing form of Ni

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Synthesis of Alcohols

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  1. Synthesis of Alcohols • Reduction of Aldehydes and Ketones • Common reducing agentsand conditions: • NaBH4 (sodium borohydride) • alcohol, ether, or H2O as solvent • (1) LiAlH4 (lithium aluminum hydride =LAH) (2) H3O+ • Raney Ni • finely divided H2-bearing form of Ni • also reduces C=C to alkane

  2. Synthesis of Alcohols • Reducing agent: • a substance that causes another to be reduced. • LAH • NaBH4 • H2 • Mechanism of Hydride Reduction:

  3. Synthesis of Alcohols • Reduction of Aldehydes and Ketones • 1o (from aldehyde) or 2o (from ketone) alcohol formed as product LiAlH4 or NaBH4 LiAlH4 or NaBH4

  4. Synthesis of Alcohols • Reduction of Acids and Esters using LAH • LAH can reduce not only aldehydes and ketones but also carboxylic acids and esters. Notice: 2 step process

  5. Synthesis of Alcohols • Reduction of Acids and Esters using LAH • Examples:

  6. Synthesis of Alcohols • NaBH4 is a weaker reducing agent than LAH. • NaBH4 reduces only aldehydes and ketones. No reaction

  7. Synthesis of Alcohols Example: Predict the major product formed in each of the following reactions:

  8. Synthesis of Alcohols Example: Predict the major product formed in each of the following reactions:

  9. Synthesis of Alcohols Example: Predict the product.

  10. Synthesis of Alcohols • Nucleophilic Addition of Organometallic Reagents • Organometallic reagent: • An organic compound that contains a covalent bond between a carbon atom and a metal atom • Carbon is more electronegative than most metals • C-M bond is polarized • Carbon is nucleophilic

  11. Synthesis of Alcohols • Two common organometallic reagents for producing alcohols: • Grignard Reagent • Organomagnesium halide • Organolithium compounds

  12. Synthesis of Alcohols • Preparation of Grignard Reagents • Alkyl halide: • 1o, 2o, or 3o • Vinyl, allyl, aryl halides • Ether must be used as a solvent to stabilize the Grignard reagent

  13. Synthesis of Alcohols Examples: Mg (s)/

  14. Synthesis of Alcohols • Preparation of Organolithiums • Alkyl halide: • 1o, 2o, 3o halide • Vinyl, allyl, aryl • Solvent: • ether, alkanes

  15. Synthesis of Alcohols Examples: Phenyl lithium

  16. Synthesis of Alcohols • Grignard reagents and organolithiums are strong nucleophiles • Behave like R - • Attack electrophilic carbons • Aldehydes, ketones • Esters, acid chlorides • Epoxides

  17. Synthesis of Alcohols • Attack of R - on an aldehyde or ketone produces an alkoxide ion which can be protonated to form an alcohol.

  18. Synthesis of Alcohols • Addition of RMgX or RLi to Formaldehyde: • forms primary alcohol

  19. Synthesis of Alcohols • Addition of RMgX or RLi to Aldehydes • forms secondary alcohol

  20. Synthesis of Alcohols • Addition of RMgX or RLi to Aldehydes:

  21. Synthesis of Alcohols • Addition of RMgX or RLi to Ketones • forms tertiary alcohol

  22. Synthesis of Alcohols • Addition of RMgX or RLi to Esters • forms tertiary alcohol with two identical R groups • identical R groups come from the organometallic reagent

  23. Synthesis of Alcohols • The addition of RMgX or RLi to an ester occurs in two steps: • R- displaces an alkoxide group, forming a ketone • A second R- attacks the carbonyl, forming the alcohol (after protonation)

  24. Synthesis of Alcohols • Addition of RMgX or RLi to Acid Chlorides • forms tertiary alcohol with two identical R groups • identical R groups come from the organometallic reagent

  25. Synthesis of Alcohols • The addition of RMgX or RLi to an acid chloride occurs in two steps: • R- displaces the chloride ion, forming a ketone • A second R- attacks the carbonyl, forming the alcohol (after protonation)

  26. Synthesis of Alcohols Examples:

  27. Synthesis of Alcohols • Addition of RMgX or RLi to Epoxides • forms primary alcohol with two more carbons than the original alkyl halide

  28. Synthesis of Alcohols Example: Predict the product of each reaction.

  29. Synthesis of Alcohols • Limitations/Side Reactions of RMgX and RLi • RMgX and RLi are strong nucleophiles and strong bases • React with acidic protons • Incompatible with water, ROH, RSH, R-NH-R’, and RCO2H

  30. Synthesis of Alcohols • Limitations/Side Reactions of RMgX and RLi • RMgX and RLi will react with any multiple bond that contains a strongly electronegative element. • RMgX and RLi will react with C=O, S=O, C=N, N=O and C N present in the solvent, organometallic reagent, or substrate

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