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Synthesis of 2º Alcohols

=>. Synthesis of 2º Alcohols. Grignard + aldehyde yields a secondary alcohol. =>. Synthesis of 3º Alcohols. Grignard + ketone yields a tertiary alcohol. =>. How would you synthesize…. Grignard Reactions with Acid Chlorides and Esters. Use two moles of Grignard reagent.

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Synthesis of 2º Alcohols

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  1. => Synthesis of 2º Alcohols Grignard + aldehyde yields a secondary alcohol.

  2. => Synthesis of 3º Alcohols Grignard + ketone yields a tertiary alcohol.

  3. => How would you synthesize…

  4. Grignard Reactions with Acid Chlorides and Esters • Use two moles of Grignard reagent. • The product is a tertiary alcohol with two identical alkyl groups. • Reaction with one mole of Grignard reagent produces a ketone intermediate, which reacts with the second mole of Grignard reagent. =>

  5. Grignard + Acid Chloride (1) • Grignard attacks the carbonyl. • Chloride ion leaves. Ketone intermediate =>

  6. Grignard and Ester (1) • Grignard attacks the carbonyl. • Alkoxide ion leaves! ? ! Ketone intermediate =>

  7. => Second step of reaction • Second mole of Grignard reacts with the ketone intermediate to form an alkoxide ion. • Alkoxide ion is protonated with dilute acid.

  8. => How would you synthesize... Using an acid chloride or ester.

  9. => Grignard Reagent + Ethylene Oxide • Epoxides are unusually reactive ethers. • Product is a 1º alcohol with 2 additional carbons.

  10. Limitations of Grignard • No water or other acidic protons like O-H, N-H, S-H, or -C—C-H. Grignard reagent is destroyed, becomes an alkane. • No other electrophilic multiple bonds, like C=N, C—N, S=O, or N=O. =>

  11. Reduction of Carbonyl • Reduction of aldehyde yields 1º alcohol. • Reduction of ketone yields 2º alcohol. • Reagents: • Sodium borohydride, NaBH4 • Lithium aluminum hydride, LiAlH4 • Raney nickel =>

  12. => Sodium Borohydride • Hydride ion, H-, attacks the carbonyl carbon, forming an alkoxide ion. • Then the alkoxide ion is protonated by dilute acid. • Only reacts with carbonyl of aldehyde or ketone, not with carbonyls of esters or carboxylic acids.

  13. => Lithium Aluminum Hydride • Stronger reducing agent than sodium borohydride, but dangerous to work with. • Converts esters and acids to 1º alcohols.

  14. Comparison of Reducing Agents • LiAlH4 is stronger. • LiAlH4 reduces more stable compounds which are resistant to reduction. =>

  15. => Catalytic Hydrogenation • Add H2 with Raney nickel catalyst. • Also reduces any C=C bonds.

  16. Thiols (Mercaptans) • Sulfur analogues of alcohols, -SH. • Named by adding -thiol to alkane name. • The -SH group is called mercapto. • Complex with heavy metals: Hg, As, Au. • More acidic than alcohols, react with NaOH to form thiolate ion. • Stinks! =>

  17. => Thiol Synthesis Use a large excess of sodium hydrosulfide with unhindered alkyl halide to prevent dialkylation to R-S-R.

  18. => Thiol Oxidation • Easily oxidized to disulfides, an important feature of protein structure. • Vigorous oxidation with KMnO4, HNO3, or NaOCl, produces sulfonic acids.

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