215-216 HH W10 notes-Ch 14

215-216 HH W10 notes-Ch 14 - Chem 215-216 HH W10 Notes Dr...

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Chem 215-216 HH W10 Notes – Dr. Masato Koreeda - Page 1 of 11. Date: February 15, 2010 Chapter 14 Aldehydes and Ketones: Addition Reactions at Electrophilic Carbons Overview of Chapter 14 1. Structures of aldehydes and ketones C R R' O R, R' = alkyl, aryl: ketones R = alkyl, aryl; R' = H: aldehydes δ δ electrophilic C Aldehyde C=O carbons are less sterically hindered and more electrophilic compared with the corresponding ketone carbons (i.e., with the same R) lone pair: more basic than C=O π 2. Reactions of aldehydes and ketones with an electrophile and a nucleophile C R' R O σ− framework lone pairs all of these sigma-bonds and lone pairs on the same plane El + with this trajectory π -bonding Highest occupied molecular orbital (HOMO) R' R empty anti-bonding orbitals Lowest unoccupied molecular orbital (LUMO) Nu: trajectory of a nucleophile approach trajectory angle of 107° C R' R O Nu C R' R O Nu sp 3 between sp 2 and sp 3 ; on its way to sp 3 C R R' O El all atoms including El on the same plane (HOMO) C O 3. Activation of RR’C=Z (Z = O and N) with H-A or a Lewis acid activates C=O toward a nucleophilic addition O O H A becomes even more δ ; i.e., more electrophilic O H A H A Lewis acid (L.A.) O L.A. O L.A. O M (if M + is used) or
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Chem 215-216 HH W10 Notes – Dr. Masato Koreeda - Page 2 of 11. Date: February 15, 2010 Chapter 14: Overview (continued) 4. Four categories of nucleophilic addition reactions Two classes of nucleophiles: reversible and irreversible O Reversible Nu: Irreversible Nu: Type 1 H M O H M H 3 O H 2 O HO H Type 2 R O R M HO R RO OR HO OR HO N or e.g., RLi, RMgX (Grignard reagent) Type 3 divalent Nu: e.g., ROH, RSH S N 1 N R H R E1 Type 4 Trivalent Nu: e.g., R-NH 2 H 3 O H 2 O or ============================================================== I. Nucleophilic Addition Reactions of RR’C=Z (Z: electronegative atom) Na B H H H H - mild reducing agent - reduces aldehyde and ketone C=Os and imine C=NRs at room temp, but not ester/amide C=Os. - relatively stable reagent (against moisture, air) electronegativity values: H 2.1; B 2.0; Al 1.5 | Δ e.n.| for B-H: 0.1 | Δ e.n.| for Al-H: 0.6 1. Sodium borohydride (NaBH 4 ) Li Al H H H H - powerful reducing agent - reacts violently with water, ROH to produce H 2 gas - Reactions with LiAlH 4 are usually carried out in a polar aprotic solvent such as anhydrous tatrahydrofuran (THF) and anhydrous (diethyl) ether (CH 3 CH 2 OCH 2 CH 3 ) 2. Lithium aluminum hydride (LiAlH 4 ) more polarized, more on the H a stronger H donor O O THF (diethyl) ether In addition, the difference in the coordination power of Na+ and Li+ (stronger) on the C=O oxygen further contributes to make the reactivity of LiAlH 4 stronger. Reduction with LiAlH 4 requires an aqueous (usually acdic) workup . I-1. Irreversible nucleophiles (H and R ) (1) Hydride reducing agents Also, the larger size difference between Al-H compared with B-H makes dissociation of Al-H much easier.
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215-216 HH W10 notes-Ch 14 - Chem 215-216 HH W10 Notes Dr...

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