hydride

Examples of hydride in the following topics:

• Binary Hydrides

  • The energy carrier NADH reacts as a hydride donor or hydride equivalent.
  • Hydrides can be characterized as ionic, covalent, or interstitial hydrides based on their bonding types.
  • Covalent hydrides refer to hydrogen centers that react as hydrides, or those that are nucleophilic.
  • Transition metal hydrides also include compounds that can be classified as covalent hydrides.
  • Some are even classified as interstitial hydrides and other bridging hydrides.

• Reduction

  • This hydride addition is shown in the following diagrams, with the hydride-donating moiety being written as AlH4(–).
  • Lithium aluminum hydride reduces nitriles to 1º-amines, as shown in the following equation.
  • Two such reagents will be mentioned here; the reactive hydride atom is colored blue.
  • Each of these reagents carries one equivalent of hydride.
  • The first (LtBAH) is a complex metal hydride, but the second is simply an alkyl derivative of aluminum hydride.

• Irreversible Addition Reactions

  • Fortunately, metal derivatives of these alkyl, aryl and hydride moieties are available, and permit their addition to carbonyl compounds.
  • Two practical sources of hydride-like reactivity are the complex metal hydrides lithium aluminum hydride (LiAlH4) and sodium borohydride (NaBH4).
  • The first three reactions illustrate that all four hydrogens of the complex metal hydrides may function as hydride anion equivalents which bond to the carbonyl carbon atom.
  • If the saturated alcohol is the desired product, catalytic hydrogenation prior to (or following) the hydride reduction may be necessary.
  • In contrast to the metal hydride reagents, diborane is a relatively electrophilic reagent, as witnessed by its ability to reduce alkenes.

• Reductions & Oxidations of Carboxylic Acids

  • Reduction to a 1º-alcohol takes place rapidly on treatment with the powerful metal hydride reagent, lithium aluminum hydride, as shown by the following equation.
  • One third of the hydride is lost as hydrogen gas, and the initial product consists of metal salts which must be hydrolyzed to generate the alcohol.
  • These reductions take place by the addition of hydride to the carbonyl carbon, in the same manner noted earlier for aldehydes and ketones.
  • Reduction of a 1-degree Alcohol when treated with lithium aluminum hydride.

• Aluminum

  • The important aluminum hydride is lithium aluminum hydride (LiAlH4), which is used as a reducing agent in organic chemistry.
  • It can be produced from lithium hydride and aluminum trichloride:

• Overview of Reducing Agents

  • Note that Lithium Aluminum Hydride (LiAlH4) is the strongest reducing agent listed, and it reduces all the substrates.

• Addition to Carbonyl Double Bonds

  • Selective reduction of 4-tert-butylcyclohexanone (I) to a 10:1 mixture of trans- and cis-4-tert-butylcyclohexanol by LiAlH4 is an example of diastereoselectivity, reflecting a preference for hydride attack at the more hindered axial face of the carbonyl group.
  • This selectivity can be reversed by using a larger hydride reagent, such as Li(sec-butyl)3BH; in which case severe hindrance to axial approach diverts the reaction to the equatorial face.

• Models for Addition to Acyclic Substrates

  • In the following illustration, two hydride reductions of chiral methyl ketones are shown.
  • Curiously, hydride addition occurs without any diastereoselectivity.
  • Equations 6 & 7 are additional examples of steric control by intramolecular hydride transfer (see equation 3).
  • The intramolecular hydride transfer mechanism noted above serves as a model for achieving enantioselective reduction.
  • Coordination of the ketone oxygen with the Lewis acidic boron orients, and activates the carbonyl group for hydride transfer to its si-face.

• Boranes: Boron-Hydrogen Compounds

  • The development of the chemistry of boron hydrides led to new experimental techniques and theoretical concepts.
  • Boron hydrides have been studied as potential fuels, for rockets, and for automotive uses.

• Hydrogen Bonding

  • Most of the simple hydrides of group IV, V, VI & VII elements display the expected rise in boiling point with molecular mass, but the hydrides of the most electronegative elements (nitrogen, oxygen and fluorine) have abnormally high boiling points for their mass.