Carbonyl Reactions - Moravian College Chemistry Department

2. Metal Hydrides: CGWW pp. 139-142, 355-356.a. Reagents and Reaction Conditions:Carbonyl Reactions - 11 15(1.) NaBH 4 + H 2 O (2.) LiAlH 4 + (1. Ether 2. H 3 O + / H 2 O)-b. Reactivity: Both react as H: nucleophiles or bases. In both types of hydride reagents the highest energy e-'s are those in the partiallycovalent hydrogen-metal bond. As with Grignard reagents, these e-'s have lower energy than a free localized hydride ion, H: - , but haveconsiderably higher energy than electrons on negatively charged nitrogen or oxygen atoms.Compared to the larger aluminum atom, the smaller boron atom provides more e - -nuclear attraction to lower the energies of the e - 's in theB-H bond so:(1.) In NaBH 4 the B-H bonding e - 's have low enough energy that NaBH 4 can be used in the presence of H 2 O or ROH. The acid-basereaction between NaBH 4 and H 2 O or ROH is slow enough that it doesn't out compete with the NaBH 4 reactions with carbonylcompounds. H 2 O or ROH can then act as proton sources for the alkoxide ion formed in the addition reaction.Example:OCHHH B HHO HCH 3 CH 2O NaHO+ Na+ B HCHHH HH+ CH+HCH 3 CH 2 O BHH(2.) The Al-H bonding e - 's have high enough energy that LiAlH 4 reacts very rapidly with acidic protons of water, alcohols or amines. SoLiAlH 4 addition reactions must be done in the absence of acidic protons (H 2 O, ROH, RNH 2 or RCOOH). The proton for thealkoxide ion formed in the reaction is supplied by acid added in the second step.Example:OCHH1.+ DryH Al H + LiCEtherHHOHLiH+ Al HH2.DiluteH 2 SO 4H 2 OOCHHH+HH AlHOH