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Chemistry Stack Exchange is a question and answer site for scientists, academics, teachers, and students in the field of chemistry. It only takes a minute to sign up. If you put sodium in ethanol or any protic solvent , the ethanol is reduced and hydrogen gas is released. I can't see how the presence of a ketone for example will create a more favorable reaction. For once, it doesn't seem thermodynamically favorable - reduction of the ethanol will release hydrogen gas, largely increasing the entropy, while reduction of the ketone produces no gas.
Even if it is thermodynamically favorable, however, reduction of the ethanol seems much more favorable kinetically, since there are much more solvent molecules than substrate molecules. What is going on here?
Oxidation and Reduction in Organic Chemistry
The key thing is that this is transfer of a single electron from Na initially. Where would this go onto ethanol? Clearly, it is faster to transfer an electron to an unsaturated compound i. The carbonyl compound.
20.3 Aldehydes, Ketones, Carboxylic Acids, and Esters
The origin of diastereoselectivity in reductions of chiral ketones has been extensively analyzed and modeled. Transition state III is favored for reductions of alkyl ketones in which R M is an electron-withdrawing group, because the nucleophile and electron-withdrawing substituent prefer to be as far away from one another as possible. Diastereoselectivity in reductions of cyclic ketones has also been studied.
Conformationally flexible ketones undergo axial attack by the hydride reagent, leading to the equatorial alcohol.
Alcohols, Epoxides and Ethers
Rigid cyclic ketones, on the other hand, undergo primarily equatorial attack to afford the axial alcohol. Preferential equatorial attack on rigid ketones has been rationalized by invoking "steric approach control"—an equatorial approach of the hydride reagent is less sterically hindered than an axial approach.
The transition state for equatorial attack V suffers from torsional strain between the incoming hydride reagent and adjacent equatorial hydrogens. The difference between these two strain energies determines which direction of attack is favored, and when R is small, torsional strain in V dominates and the equatorial alcohol product is favored.
Oxidation and Reduction Reactions in Organic Chemistry -
Alkoxyaluminum and closely related hydride reagents reduce a wide variety of functional groups, oftentimes with good selectivity. This section, organized by functional group, covers the most common or synthetically useful methods for alkoxyaluminum hydride reduction of organic compounds. Many selective reductions of carbonyl compounds can be effected by taking advantage of the unique reactivity profiles of metal alkoxylaluminum hydrides.
For instance, lithium tri- tert -butoxy aluminum hydride LTBA reduces aldehydes and ketones selectively in the presence of esters, with which it reacts extremely slowly. Use of relatively unhindered lithium trimethoxyaluminum hydride results in nearly quantitative direct addition to the carbonyl group Eq. Ether cleavage is difficult to accomplish with most hydride reagents.
Results and discussion
Epoxides are generally attacked by alkoxyaluminum hydrides at the less substituted position. A nearby hydroxyl group may facilitate intramolecular delivery of the hydride reagent, allowing for selective opening of 1,2-disubstituted epoxides at the position closer to the hydroxyl group. Unsaturated carbonyl compounds may be reduced either to saturated or unsaturated alcohols by alkoxyaluminum hydride reagents.