Borohydride Reduction of a Ketone Hydrobenzoin from Benzil PDF

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Charlie Lin 989176678 Ochem II L-10 Experiment: Borohydride Reduction of a Ketone: Hydrobenzoin from Benzil Discussion: The purpose of this lab experiment was to reduce the ketone benzil using sodium borohydride to form hydrobenzoin, an alcohol product. A reduction reaction entails a decrease in oxygen content or an increase in hydrogen content of an organic starting material to form the product. Compared to other reducing agents, Sodium Borohydride is considered to be both a mild and selective reducing agent, unlike lithium aluminum hydride, meaning it can easily reduce certain carbonyl groups such as ketones or aldehydes to either primary or secondary alcohols, but not reduce esters, amines, or carboxylic acids for example. Another quality to sodium borohydride is that it is a source of hydride anions that is stable in water and soluble in methanol or ethanol. Boron, as a molecule, is inherently satisfied with a valency of three and is therefore an exception to the octet rule. Thus, the hydride anion can stabilize the carbocation in the reaction mechanism. In the reaction mechanism, both ketones on the benzil are reduced to an alcohol. The first step after sodium borohydride dissociates is electronegative oxygen pulling the pi electron density from the carbonyl double bond which gives the carbonyl carbon a positive formal charge. The sodium will dissociate at room temperature to be a spectator ion that doesn’t participate in the reaction of the solvent ethanol, which will later protonate the oxygen as it is a source of protons. But for now, the mechanism will then involve the hydride anion from the borohydride acting as a strong base/ nucleophile which will stabilize the positive formal charge on the

carbocation. Forming the major meso product requires the boron atom to lock into the two oxygens on the ketone benzil to form a borate ester intermediate that is later hydrolyzed via reflux/boiling with the addition of water. Next, the oxygen with a negative formal charge will be protonated by the solvent ethanol, a source of protons that are positively charged (not negative) forming the hydroxyl alcohol group to the ketones on benzil that forms the alcohol hydrobenzoin. The product will form a predominant meso compound (1R,2S)mesohydrobenzoin as the major product but also a minor, enantiomeric racemic mixture of (1R,1R) and (1S,2S)-mesohydrobenzoins. By definition, a meso compound is structurally symmetrical, achiral, and thus cannot rotate plane polarized light due to its optical inactivity, whereas the racemic mixture can have equal amounts of the racemic (1R,2R) and (1S,2S) diol compounds that are enantiomers to each other, have chiral centers, and are optically active (can rotate plane polarized light). As for the experimental procedure, 0.5g of benzil was weighed and combined with 5ml of 95% ethanol solvent that will also serve as a source of protons, not hydrides. Benzil was particularly favorable to use as a reactant due to its yellow color of which its color change can be monitored when the reaction occurs when forming the product. Its crystals were somewhat course in texture and the granules slowly dissolved. Then, 0.1g of sodium borohydride reducing agent was added. A reducing agent will lose electrons and gets oxidized in the reduction reaction when benzil gets reduced. It produced fizzy gas bubbles when it was dissolving in the solution and there was a slight warming in temperature due to the proton transfer. This observation indicates that the transfer of hydrides in the reaction was taking place and thus is an exothermic process at room temperature. The bottom of the flask was run under the faucet tap to control this temperature change by cooling it down. About 10 minutes after, 5ml of water was added before

heating the solution to boiling on a hot plate to help the reaction equilibrium shift to the right faster. This is where the bonds on the borate esters breaks when water was added via hydrolysis which would explain the color change when the yellow color finally disappeared to the cloudy product in solution. After about 5 minutes, the solution was removed from heat to allow the product to crystalize at room temperature, which took around 15 minutes total. The procedure included diluting to saturation where 10ml of water was added to help induce the crystals to precipitate out of solution when the solubility was changed. The crude solution was suction filtered to isolate the product. The crystals of hydrobenzoin appeared clear and was allowed to dry in the locker before melting point and IR analysis. The dry pure product that was isolated gave an experimental yield of 0.32g. The theoretical yield of hydrobenzoin obtained from 0.5g of benzil would be calculated to be 0.510grams. Thus, the percent yield is as illustrated: (0.32/0.51) x100= 63% yield. The average melting point range was 135.6C which is well within the literature melting point of hydrobenzoin of 135C-136C. A Digitemp was used instead of a thermometer, so a temperature calibration factor curve was not used. The IR spectrum that was received immediately displayed a broad peak at ~3400 cm-1 which is characteristic to the presence of a hydroxyl group. The C=C sp2 hybridized aromatic ring had a stretch peak at around 1400-1600 cm-1. There was not peak at 1700 cm-1 indicating the absence of a carbonyl group, thus the ketone benzil is not present and all of the starting material had reacted to completion. Also, a C-H stretch peak was also apparent at around ~2850 cm-1 denoting the carbon and hydrogen atoms in the middle of the hydrobenzoin product. Conclusion: The reduction of benzil a ketone to the hydrobenzoin alcohol via sodium borohydride reduction was successful when the experimental product obtained a melting point within the melting point range of hydrobenzoin....