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Reduction of carvone

Objective

The purpose of this laboratory exercise is to carry out the reduction of (-)-carvone using NaBH4 in methanol as the reducing medium.

Background information

The reduction of carbonyl compounds is an important synthetic route to alcohols.

Aldehydes and ketones may be reduced by a variety of reagents to yield their respective alcohol products:

Among the most useful reagents for accomplishing these reductions are complex metal hydrides such as lithium aluminum hydride (LiAlH4 or LAH) and sodium borohydride (NaBH4). The hydrogen atoms in these metal hydrides are reactive in carbonyl group reductions. The two metal hydrides are quite different in their reactivities. Lithium aluminum hydride is highly reactive and will reduce groups such as acid chlorides, esters, carboxylic acids, amides, and nitriles, as well as aldehydes and ketones. Lithium aluminum hydride reacts violently with water and other hydroxylic compounds. Reductions carried out with this reagent must be carried out under nonprotic anhydrous conditions.

Sodium borohydride is less reactive and hence a more selective reagent than lithium aluminum hydride. It will reduce aldehydes and ketones rapidly to their corresponding alcohols, but will not reduce carboxylic acids and amides, and only slowly reduces esters. Sodium borohydride is a safer reagent that can be used in a wide range of solvents, including water and alcohols. The borohydride reacts slowly with water or other hydroxylic compounds to form hydrogen gas. This reaction is accelerated by an increase in temperature or by a decrease in pH.

Mechanism:

In alcohol syntheses using sodium borohydride, the probable reaction mechanism involves hydride transfer from the metal hydride to the electropositive carbon of the carbonyl group to yield an alkoxyborohydride (1). The alkoxyborohydride generated is capable of reducing three further equivalents of ketone or aldehyde (2). Hydrolysis of the borate liberates the alcohol (3).

If an organic chemist was allowed to choose ten aliphatic compounds to be stranded on a desert island with, alcohols would almost certainly be the compounds of choice. Nearly every kind of aliphatic compound can be prepared from an alcohol. In this experiment, you will synthesize carveol by reduction of (-)-carvone using sodium borohydride (NaBH4). The carvone that you isolated last week is not pure enough to use in this experiment, thus you will use commercial (-)-carvone. Treatment of (-)-carvone with sodium borohydride (NaBH4) is expected to yield the following diastereomeric alcohols:

In addition to the desired products, other compounds result from a competing reaction in which the double bond of (-)-carvone is reduced. The structures of these compounds are illustrated below.

Comparison of reactivities of hydride reagents:

Metal-Hydride Reagents Formula Selectivity
Lithium Aluminum Hydride*Very reactive! Pyrophoric![powerful, unselective] LiAlH4 (LAH) acid→1°alcoholester→1°alcoholsamide→aminenitrile→1ºamine
Borane-Tetrahydrofuran Complex*Safer and easier to use than LAH![greater selectivity than LAH] BH3·THF acid→1ºalcoholalkene→alcohol(hydroboration/oxidation)
Sodium HydridePyrophoric! [hydride basicity, vs. nucleophilicity, predominates] NaH {Deprotonationto C=O initiates condensation and alkylation reactions}
Sodium BorohydrideSafer and easier to use than LAH![can be used in aqueous solutions] NaBH4 aldehyde→1ºalcoholketone→2ºalcohol{but doesn’t reduce acids}
Sodium Cyanoborohydride*(reagent of choice for reductive aminations) NaBH3CN imine→amine
Diisobutylaluminum Hydride*(reagent of choice for this partial reduction) AlH(isoBu)2(DIBAL) ester→aldehyde {stops here; doesn’t reduce further to an alcohol}
Lithium Tri-t-butoxyaluminum Hydride*(reagent of choice for this partial reduction) LiAlH(O+)3 acid chloride→aldehyde {stops here; doesn’t reduce to alcohol}

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Source:  OpenStax, Chem 215 spring08. OpenStax CNX. Mar 21, 2008 Download for free at http://cnx.org/content/col10496/1.8
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