CHEM 0330 Organic Lab 1
Sodium Borohydride Reduction: Diphenylmethanol from Benzophenone 11/16/11
Redox (shorthand for REDuction-OXidation) reactions are chemical reactions in which the oxidation state (or oxidation number) of atoms has changed. Oxidation can be observed through the loss of electrons or an increase in oxidation state by an atom, ion or molecule. Reduction describes the gain of electrons or decrease in oxidation state of an atom, ion or molecule. However, there are many processes that are classed as redox even though no electron transfer occurs, for example those reactions that involves covalent bonds. Reduction reactions can be determined through three features. The first is a loss of oxygen from a bond or loss of a bond to oxygen as in the case of carbon-oxygen double bond to a carbon-oxygen single bond. The second is the addition of hydrogen to a bond and thirdly, the replacement of a more electronegative atom with carbon or hydrogen. In redox reactions, the reductant (or reducing agent) loses electrons and is oxidized while the oxidant (or oxidizing agent) gains electrons and is reduced. The reducing agent transfers electrons to another substance. The agent reduces other substances and so, the agent itself is oxidized. The reductant is also called an electron donor as it donates electrons. The electron donors can also form charge transfer complexes with electron acceptors. Examples of good reducing agents are electropositive metal elements such as lithium, sodium, iron, aluminium, zinc, iron, magnesium and carbon. These metals donate electrons readily. In organic chemistry, there are very diverse reductants. For example, in a catalytic reduction to reduce carbon-carbon double or triple bonds, the main reductant would be hydrogen gas (H2) coupled with Lindlar’s catalysts (palladium, platinum or nickel). Hydrogenation reduces most types of multiple bonds. Another method of reduction would be using hydride (H-) transfer reagents such as NaBH4 and LiAlH4 to reduce carbonyl compounds to alcohols. These transfer reagents are inorganic compounds. The LiAlH4 compound is a highly reactive, extremely powerful reducing agent. It is able to reduce the carbonyl group in aldehydes, ketones, carboxylic acids, esters, amides, and acid halides. It reacts violently with water, alcohols, and other acidic groups with the evolution of hydrogen gas. In LiAlH4 reductions, the resulting alkoxide salts are insoluble and need to by hydrolyzed before the target alcohol product can be isolated. These reductions must be carried out under non-protic, anhydrous conditions. In NaBH4 reduction the hydroxylic solvent system achieves this hydrolysis automatically. On the other hand, NaBH4 is a less reactive and more selective reagent. It will convert aldehydes and ketones into alcohols, but it will not reduce carboxylic acids, esters, or amides. NaBH4 reacts only slowly with water and alcohols. It can be used in a wide range of solvents and is much safer to handle than LiAlH4. However by itself, NaBH4 and LiAlH4 cannot reduce carbon-carbon double or triple bonds.
There are various methods of reduction. The more common ones are hydrogenation and hydride transfer reagents. Hydride is the isolated atomic hydrogen anion, H-, or any compound containing hydrogen and another more electropositive element or group. Hydride consists of a singly charged positive nucleus and two electrons of which one electron is weakly held and readily able to be donated. Hydrides are highly reactive, strongly basic and powerfully reducing in synthetic reactions. They are important reducing agents in industrial reactions though they are easily destroyed in the relatively acidic compound water (H2O). In most reactions with sodium borohydride, the aldehyde or ketone is dissolved in the reaction solvent and a solution of sodium borohydride is added, with external cooling if necessary, at a...