Sodium Borohydride Reduction: Diphenylmethanol from Benzophenone

Megan Entwistle, Maria Amos, and Paul Golubic CHEM 0330 Organic Lab 1 Sodium Borohydride Reduction: Diphenylmethanol from Benzophenone 11/16/11 Introduction 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.

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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 rate slow enough to keep the reaction temperature below 25°C.

Higher temperatures may decompose the hydride, and adding the carbonyl compound to the alkaline sodium borohydride solution may cause side reactions of base-sensitive substrates. The amount of solvent is not crucial, but enough should be used to completely dissolve the reactants Hydrogenation is a process that creates hydrogen bonds on carbon molecules, usually a pair of hydrogen atoms. This process is done by treating hydrogen as a reducing chemical in a chemical reaction between hydrogen and another compound. In this hydrogenation process, the chemicals are usually accompanied with a catalyst.

Catalysts are very much needed in this process to make it usable, without the presence of a catalyst this chemical reaction can only be possible at very high temperatures. Thus, in a laboratory setting, it is vital to have catalysts in this reaction. In short, hydrogenation has three components, unsaturated substrate, hydrogen (mostly in gaseous state), and a catalysts. The temperature of the reaction varies depending on the substrate and the activity of the catalyst. The substrate for hydrogenation is almost always alkenes that produce saturated alkanes as the end product.

This chemical process is very selective due to the steric hindrance that plays a role in determining where exactly would the hydrogen atoms be placed. There are few catalysts, namely, platinum, nickel, palladium, rhodium, and ruthenium. These are considered very active catalysts as they are able to operate at lower temperatures. Hydrogenation is a very exothermic reaction, which means a lot of heat is liberated from this chemical reaction. A more specific reduction method involves the hydride (H-) transfer reagents. This process only works for carbonyl groups.

Carbonyl groups are functional groups that have a carbon atom and double-bonded oxygen attached to it (C=O). Carbonyl groups can be reduced by reacting them with hydride reagents. There are two types of hydride reagents, NaBH4 and LiAlH4. However, carbonyl can also be reduced by hydrogen as well, but with the presence of catalysts. Examples of catalysts are copper, chromite, Raney nickel, rhenium, ruthenium and rhodium. Mechanism for hydride transfer reagent: The borohydride anion delivers a hydride ion to the carbonyl compound at the same time that a proton is transferred from the solvent to the carbonyl oxygen.

As the B-H bond breaks, a new bond between the boron and, the oxygen atom of the solvent is formed. An important thing to note is the hydrogen atom that ends up bonded to the carbonyl carbon comes from the NaBH4 while the hydroxyl hydrogen is derived from the solvent. Many reaction mixtures require heat in order to perform reactions at a reasonable rate. Reflux is a heating process that involves the boiling of reaction mixtures in solution. In reflux, a solvent is chosen so that its boiling point coincides with a reaction temperature that is ideal.

Heating a reaction mixture to reflux assures chemists of a constant, appropriate temperature. Reflux is the process of boiling reactants while continually cooling the vapor returning it back to the flask as a liquid. It is used to heat a mixture for extended periods at certain temperatures. If the heating rate has been correctly adjusted, the liquid being heated under reflux will travel only partly up the condenser tube before condensing. Below the condensation point, the solvent will be seen running back into the flask. Above it, the condenser will be dry.

The boundary between the two zones will be clearly demarcated and a reflux ring or a ring of liquid will appear there. In heating a reaction under reflux, the rate of heating should be adjusted so that the reflux ring is no higher than a third to half the distance to the top of the condenser. The temperature of a reaction in a refluxing mixture will be approximately the boiling point of the solvent used for the reaction. The purpose of reflux is to maintain a reaction temperature at the temperature of the boiling solvent and to prevent the solvent from being lost to the atmosphere.

Reagent Table Name/Structure| Molar Mass (g/mol)| Density (g/mL)| Amount in lab (g or mL)| BP and/or MP (? C) | Safety| Benzophenone| 182. 217 g/mol| 1. 11| 5. 51g| BP: 305. 4 ? CMP: 47. 9 ? C| HarmfulSkin, eye and respiratory irritant. | Diphenylmethanol| 184. 23 g/mol| 1. 18| –| BP: 297-298 ? CMP: 65-67 ? C| –| Sodium Borohydride| 37. 83 g/mol| 1. 0740| 1. 07g| BP: 500 ? CMP: 400 ? C| Toxic by ingestion. Risk of serious internal burns if ingested. Harmful if inhaled and in contact with skin. May cause burns or severe irritation in contact with skin or eyes. | Methanol| 32. 04 g/mol| 0. 918 g/mL| 50mL| BP: 65 ? CMP: -98 ? C| Flammable, ToxicToxic by inhalation, ingestion or skin absorption. May be a reproductive hazard. Ingestion may be fatal. Risk of very serious, irreversible damage if swallowed. Exposure may cause eye, kidney, heart and liver damage. Chronic or substantial acute exposure may cause serious eye damage, including blindness. Irritant. Narcotic. | Diethyl ether| 74. 12 g/mol| 0. 7134 g/mL| 150mL| BP: 34. 6 ? CMP: -116. 3 ? C| Extremely Flammable, Harmful| Anhydrous Magnesium Sulfate| 120. 366 g/mol| 2. 66| –| MP: 1124 ? C| Harmful if swallowed. May cause irritation.

Avoid breathing vapors, or dusts. Use with adequate ventilation. Avoid contact with eyes, skin, and clothes. Wash thoroughly after handling. Keep container closed. | Anhydrous Calcium ChlorideCaCl2| 110. 98 g/mol| 2. 15| –| BP: 1935 ? CMP: 772 ? C| Irritant| 6M Hydrochloric acid| 36. 46 g/mol| | | BP: -85. 06MP: -114. 9| Poisonous and corrosive liquid. Liquid and mist will cause severe burns to all body tissue. May be fatal if swallowed or inhaled. | Water | 18. 01528| 1000 kg/m3| 30 ml| BP: 100MP: 0| N/A| Aldrich Handbook of Fine Chemicals and Laboratory Equipment, Sigma-Aldrich, 2003. ChemFinder: http://www. chemfinder. camsoft. com| Wikipedia. com| Experimental A drying tube was prepared as follows: a loose plug of cotton was placed on the bottom of the tube to keep the CaCl2 from falling into the reaction vessel; the tube was filled with anhydrous CaCl2; another plug of cotton was placed on the top and one end of the drying tube into a thermometer adapter. In a 100mL round bottom flask, 5. 51g of benzophenone was dissolved in 50mL of methanol. A few boiling stones were added and the flask was cooled on ice. 1. 7g of NaBH4 was weighed into a tarred 50mL beaker covered with a watch glass. With the flask on ice, NaBH4 was carefully transferred, in small portions, into the benzophenone solution through a funnel over a period of approximately 10 minutes. When all NaBH4 was added and H2 evolution became noticeably slower, the reflux apparatus was assembled using the longest condenser possible (Figure 1. 1). After the condenser and drying tube were properly attached, the apparatus was heated over a steam bath and refluxed for 20-30 minutes. The steam bath and the drying tube were removed from the apparatus.

The reaction vessel (with condenser still attached) was cooled on ice. Through the condenser, 6mL HCl was slowly added to acidify the solution. The pH was checked with pink litmus paper and HCl was continually added until the litmus paper turned blue. The quenched reaction mixture was transferred to a 250mL Erlenmeyer flask and 50mL of diethyl ether was added. 30-50mL of water was added to dissolve the solid boric acid and/or make two distinct layers. When 2 distinct layers formed, with no solid, both layers were poured into a 500mL separatory funnel.

The organic layer was isolated into another flask. Then the aqueous layer was extracted twice with 50mL portions of ether. After each extraction, the organic layer was added to the beaker for the organic layer. The combined organic layers were then washed with a 50mL portion of water and collected in an Erlenmeyer flask. The ether phase was dried with magnesium sulfate for 10 minutes and decanted into a beaker. A couple of boiling stones were added and a steam bath was used to evaporate away the ether from the diphenylmethanol product. The melted product was removed from steam bath and cooled.

The crude solid from hexanes was recrystallized and the final product was collected using vacuum filtration, which was then set in aside to dry for a week. A week later, the product was weighed and melting point and an IR spectrum was obtained. (Figure 1. 1 Reflux Apparatus) Results IR Spectroscopy | Expected Absorption Wavenumbers (cm-1)| Observed Absorption Wavenumbers (cm-1)| Conc. Alcohols & phenols| 3200-3550| 3383. 14| Aromatic Ring| 1500 & 1600| 1494. 26| C-H (sp3)| 2800-3000| ~3020-3050| Note the lack of a carbonyl absorption between 1680-1750 Theoretical Yield (Equation 1)

Equation 1. Mol of Limiting Reagent*Limiting ReagentSynthesized Product*mol weight product .0302 mol Benzophenone X1 Mol Benzophenone1 Mol Diphenylmethanol X 184. 23gmolDiphenylmethanol =5. 56 g Diphenylmethanol Equation 2 Percent Yield (Equation 2) Percent Yield=Mass Actual Actual YieldMass Theoretical Yield*100 Percent Yield=3. 23 grams5. 56*100=58. 09% Melting Point of Diephenylmethanol| Literature | 65-67°C| Actual| 61-63. 1 °C| Discussion The carbon-oxygen double bond was broken when a hydride ion donated from NaBH4 attacked the carbon within the carbonyl functional group.

The hydride ion was strongly attracted to the carbon due to its partial positive charge produced by the oxygen atoms electron withdrawing ability. The negatively charged oxygen atom was then readily protonated in the presence of the non-nucleophilic HCL acid. Infrared spectroscopy (IR) produced absorption peaks within the expected values verifying the synthesis of the expected product: there was a broad absorption peak at 3383. 14 wavenumbers indicative of a hydroxyl group; peaks around ~3020-3050 wavenumbers corresponding with the newly formed sp3 C-H bonded orbital; and a peak at 1494. 6 wavenumbers proving the retention of the phenol rings (Experimental spectra). Furthermore, an absorption peak at approximately 1680-1750 wavenumbers corresponding with carbonyl functional groups was no longer present (Benzophenone spectra). The percent yield was relatively high at 58. 09%. The observed melting point was 4-3. 9 ? C lower than expected 65-67 ? C range demonstrating the sample did contain a level of purity. The low melting point indicates that atmospheric pressure in the laboratory as well as a moderate degree of impurities contributed to a lower BLANK.

Diethyl ether was used during the extraction process and was evaporated by heating on a steam bath. Because the product was solvated in ether, and shares a melting point within a close proximity to ethers boiling point, it melted during the evaporation process -causing a loss of product. After the reaction is complete, excess sodium borohydride is decomposed by acidifying the reaction mixture (slowly and while stirring) using aqueous HCl. Hydrogen gas is evolved during this process as the excess sodium borohydride decomposes.

In terms of solvent choice, the solubility of sodium borohydride in water is greater in water (25-88. 5 g/100g of solvent) than in methanol (16. 4 g/100g of solvent). However, the reaction of benzophenone with water poses a problem. Benzophenone is a pure hydrocarbon that is very insoluble in water. In turn, compromising and using methanol as the solvent is favorable although it sacrifices product. NaBH4 Solubility in Water vs Methanol| Solvent| Temperature °C | Solubility (g/100g of Solvent | H2O| 0| 25. 0| | 25| 55. 0| | 60| 88. 5| MeOH| 20| 16. 4| References Carbonyl Reactivity. ” Michigan State University :: Department of Chemistry. Web. 13. Nov. 2011. <http://www2. chemistry. msu. edu/faculty/reusch/VirtTxtJml/aldket1. htm>. Computers in Chemical Education (CCE) Newsletter. Web. 13 Nov. 2011. <http://orgchem. colorado. edu/CCCE/frame/images/handbook. pdf>. http://www. pitt. edu/~bandik/organicweb/exp11text. html Huston, Erica. “Sodium Borohydride Reduction: Diphenylmethanol from Benzophenone. ” Organic Chemistry I Laboratory Manual. Plymouth: Hayden-McNeil, 2012. 92. Print. Padias, Anne B. Making the Connections? A How-To Guide for Organic Chemistry Lab Techniques. 2nd ed. Plymouth, MI: Hayden McNeil, 2011. Print. “Stereospecific Reduction of Benzil with Sodium Borohydride; Determination of the Stereochemistry by NMR Spectroscopy. ” Web. 13 Nov. 2011. <http://www. enc. edu/~timothy. t. wooster/courses/CH322/Lab/2-28-3-14%20Oxidation%20Reduction. pdf>. “The Organic Chemistry Laboratory Web Pages – UW Madison. ” Home | UW Madison – Department of Chemistry. Web. 13 Nov. 2011. <http://www. chem. wisc. edu/areas/organic/orglab/tech/reflux. htm>. Vollhardt, K.

Peter C. , and Neil Eric Schore. Organic Chemistry: Structure and Function. New York: W. H. Freeman, 2011. Print. <http://www. pitt. edu/~cedar>. ——————————————– [ 2 ]. “Carbonyl Reactivity. ” Michigan State University :: Department of Chemistry. Web. 13. Nov. 2011. . [ 3 ]. “Stereospecific Reduction of Benzil with Sodium Borohydride; Determination of the Stereochemistry by NMR Spectroscopy. ” Web. 13 Nov. 2011. . [ 4 ]. Padias, Anne B. Making the Connections? : A How-To Guide for Organic Chemistry Lab Techniques. 2nd ed.

Plymouth, MI: Hayden McNeil, 2011. Print. [ 5 ]. “The Organic Chemistry Laboratory Web Pages – UW Madison. ” Home | UW Madison – Department of Chemistry. Web. 13 Nov. 2011. . [ 6 ]. Computers in Chemical Education (CCE) Newsletter. Web. 13 Nov. 2011. . [ 7 ]. Huston, Erica. “Sodium Borohydride Reduction: Diphenylmethanol from Benzophenone. ” Organic Chemistry I Laboratory Manual. Plymouth: Hayden- McNeil, 2012. 92. Print. [ 8 ]. http://fsl. ne. uiuc. edu/Project%20Presentation/fuel%20cell%20project_files/July%20workshop%20presentations/uiuc-talk-25July2005. pdf

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