Vanillyl alcohol - Lecture notes 10 PDF

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Robu Tatiana

Student name: Tatiana Robu Partner name: Allana Gearl Date: 04/16/2021 Title: Borohydride reduction of vanillin Introduction The reduction of carbonyl compounds is an important synthetic method for the generation of alcohols. Compounds one oxidation level above alcohols, such as aldehydes and ketones, can be reduced by a variety of reagents to yield the corresponding alcohols. In general, most synthetic chemists employ one of two reagents for this transformation: sodium borohydride (NaBH 4 ) or lithium hydride (LiAlH4 or LAH). While either of these reagents can be employed for the reduction of aldehydes and ketones, there are vast differences between the reactivities of the LAH and NaBH4. Sodium borohydride is a rather mild reducing agent, reducing aldehydes and ketones selectively in the presence of more highly oxidized functional groups. LAH on the other hand is highly reactive and will readily reduce not only aldehydes and ketones, but esters, carboxylic acids, amides, acyl chlorides and nitriles as well. Aside from the lack of selectivity, LAH reacts violently with water and other hydroxylic compounds, and reducing using this reagent must be carried out under non- protic anhydrous conditions. This not only limits the solvents with which LAH can be used, but it presents greater challenges in the safe handling of LAH. Sodium borohydride on the other hand reacts only slowly with water and alcohols and can be used in a wide range of solvents including water and alcohols, without consequence. It should also be noted that when using water as solvent, sodium borohydride is relatively stable and pH 10 or higher. However, if a weekly acidic

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proton is present sodium borohydride will react with the acid which would thus destroy reagent. The reduction is usually exothermic, but only mildly and can be easily controlled using a cooling bath of ice water. Taken together sodium borohydride is much safer to handle than is LAH making it a sage and less expensive choice whenever the functional group to be reduced is an aldehyde or ketone. In a protic solvent such water or an alcohol, sodium borohydride ionizes tp form Na+ and BH4-. The negatively charged borohydride complex, BH reacts with the carbonyl as illustrated below. The alcohol facilitates the reduction by hydrogen bonding interactions between the carbonyl oxygen and the acidic hydroxyl group of the alcohol. The borate BH3+ OEt -form from the Lewis acid-base reaction OEt and BH3 has three remaining B-H bonds, each of which in turn reduces another molecule of the starting ketone. Note that the theoretical stoichiometry of the overall reaction is the following : 1 mole of NaBH 4 reduces 4 moles of aldehydes or ketones. Thus, this should require 0.25 equivalents of NaBH 4 due to the fact that the reagent does slowly react with the protic solvent that is typically used. At the end of the reaction any excess NaBH 4 must be destroyed or quenched prior to the reaction workup and product isolation. In the following procedure dilute aqueous HCl is added to quench any unreacted NaBH4. In today’s experiment sodium borohydride will be used to reduce the aldehyde in vanillin to the corresponding primary alcohol, vanillyl alcohol. Note that we will run the reaction in 1 M NaOH. We will monitor the reaction process by TLC and once the reaction is complete we will quench the excess NaBH4 with dilute HCl. The IR and H NMR are taken after the recrystallization.

Robu Tatiana

Procedure Part A: Reduction of vanillin with sodium borohydride To a 250 mL Erlenmeyer flask containing a magnetic stir bar transferred 5.065 g of vanillin after was added 0.2 M NaOH solution about 50 mL and methanol 5 mL to dissolve the solid. The solution was stirred. Prepared the ice bath and was placed on the magnetic stir. In a separate 100 mL beaker was added 1.442 g of NaBH 4 and dissolved in a 0.2 M NaOH (3mL) and 3 mL H 2O. Placed a dropping funnel on a ring clamp above the reaction flask and was added NaBH 4 solution to the dropping funnel. Slowly was added drop wise over 10 minutes, the reaction was kept in a ice- water bath through all the addition of the reducing agent because the reaction is exothermic. Part B: Performing a mini work up to analyze the reaction by TLC A solution was prepared in a clean 20 mL scintillation vial by dissolving 10 mg of vanillin in acetone (5mL), prepared the mobile phase in a TLC chamber 6mL hexane and 4mL ethyl acetate. Into a clean 20mL scintillation vial was added 5 mL of DI water then transferred 2mL of HCl, Added 10 drops of reduction mixture and shacked vigorously, and was added 2 mL ethyl acetate to separate the mixtures. Spotted the top layer of TLC place by using the TLC capillary. Placed

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the TlC plate vertically into the chamber, close the lid and waited for to be ready. The reaction is completed. Part C: reaction work-up The reaction is cooled in a ice-water bath. While stirring was added 3 M HCl dropwise to the reaction mixture until no more gas evolves the mixture. The pH was checked. Part D: Isolation of the reaction product by vacuum filtration The vacuum filtration was assembled and left it for 20 minutes the product to dry out. Part E: Recrystallization of crude vanillin alcohol In a 125 mL Erlenmeyer flask was added 75 mL ethyl acetate and placed it on a hot plate to boil. Product was transferred in a 125 mL Erlenmeyer flask next added the boiling ethyl acetate slowly while swirling. The solution was cooled at room temperature then was placed in the ice. The product was isolated. Calculations and results:

Reagent

Molar Mass

Equiv

mol

amount

vanillin

152.15 g/mol

1.00

0.033

5.065 g

NaBH4

38g/mol

1.15

0.038

1.442 g

Vanillin alcohol

154.17g/mol

1.00

0.033

5.088

Moles(vanillin) = 5.065g/ 152.15 g/mol = 0.033 moles Moles (NaBH4) = 1.442g/ 38g/mol=0.038 moles

Robu Tatiana

Limiting reagent = 0.033moles/0.033 moles= 1 0.038 moles/ 0.033 moles= 1.15 The limiting reagent will be Vanillin is consumed first in the reaction. Theoretical mass = 0.033 moles x 153.17 g/mol= 5.088g Actual mass =0.201 g Theoretical yield = 0.201 g/5.088g x100%= 4 % During this process of synthesizing vanillin to vanillyl alcohol it was important to use and an aprotic solvent so that the vanillin would react. If it was a protic solvent then there would be excess H+ that would react instead of the vanillin. One of the biggest reasons of having such a low percent yield would be from having a protic solvent instead of aprotic solvent. That is why we had tp test to see if our reaction was basic before adding the HCl. Another big problem that o faced was that my solution would go through the filter paper during filtration and recrystallization. The percentage was found to be 4% which is extremely low hence this implies that errors might have occurred during this experiment. The Melting point range 113.4-114 o C according to the melting point in literature value for the melting point of vanillyl alcohol is 113

o

C this means that experimental value is right in the

range of the literature value range and the product obtained is absolutely pure. According to the IR spectrum the most characteristic signals are C=O vibrations at the 1602 cm -1 stretches at about 2889 cm -1 this is H-C=O vibrations, O-H stretches appears at about 3440 cm -1.

Robu Tatiana

According to NMR spectrum methoxy group has more shielded effect at ortho/para positions as compared to meta position. 3 sets of multiplet between 6.5 and 7.0 Hz represents aromatic protons. H will be more shielded due to presence of 2 electrons shielded effects of O-CH 3 and -CH2-OH so it has signal at 6.8 Hz and 7.0 Hz.

Robu Tatiana

References McMurry, John (2008). Organic chemistry 7th ed. Belmont: Thomas/Brooks/Cole. Letcher, C.S. Sodium Borohydride Reduction of Vanillin: A low solvent synthesis of Vanillyl alcohol, chemistry, Marion college, 2007. Duff, Jack. Reduction of vanillin with sodium borohydride....