Carbonylic Reduction of Vanillin via Sodium Borohydride PDF

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Carbonylic Reduction of Vanillin via Sodium Borohydride Introduction Vanillin is a naturally-occurring version of natural vanilla extract and is widely synthesized and utilized as a substitute due to its low production cost compared to natural vanilla. It is the main component of both natural and artificially produced vanilla extracts 1. Vanillin can be reduced through sodium borohydride (NaBH 4), as well as lithium aluminum hydride (LiAlH4), to produce vanillyl alcohol—a molecule that is widely used in the flavoring of different foods. Sodium borohydride is the preferred method in many laboratory settings due to its decreased reactivity—LiAlH4 has been known to react violently with reagents such as water and alcohols. The following diagram illustrates the balanced reaction scheme for this reduction; the hydride ions released from sodium borohydride in solution act as the nucleophiles that will ultimately reduce the carbonyl group of vanillin.

4

+ NaBH4 + 4 H2O  4 Vanillin

+ H3BO3 + NaOH Vanillyl Alcohol

Ethanol is the primary solvent in this reaction as it does not react as strongly with sodium borohydride as many other solvents containing acidic functional groups would. The reaction is also performed in dilute sodium hydroxide to prevent the nucleophilic hydride ions from reacting with excess protons that would be present in an acidic solution, which would produce hydrogen gas. Because of the ability of sodium borohydride to further react with other 1 Horger, Jacob. “Preparation of Vanillyl Alcohol by Sodium Borohydride Reduction.” Course notes. Organic Chemistry Lab 1. Department of Chemistry, University of North Carolina at Charlotte.

components of the mixture (such as the solvent), a large excess of sodium borohydride is utilized in this experiment to ensure the reaction proceeds and to increase the overall reaction rate. Sodium borohydride serves as a source of four hydride ions rather than one. Therefore, adding one molar equivalent of sodium borohydride to the vanillin solution means there are four nucleophilic hydride ions present per molecule of vanillin, which is to be reduced by a single hydride ion. In the acid workup step following the reaction, concentrated hydrochloric acid is added to the mixture to react with and decompose the remaining sodium borohydride in solution. This process produces large amounts of hydrogen gas and must be contained under the fume hood.

Image 1: sodium borohydride has two practical uses in reduction reactions, reduction of aldehydes (like in this reaction) and ketones. The reactivity of sodium borohydride is too low to reduce carboxylic acids, amides, and esters2. Although NaBH4 and LiAlH4 are the most common reducing agents used in the lab, many other reducing agents exist that differ in their structure and allow for greater selectivity of products. One example of this is a superhydride with a chemical formula of LiBHEt 3. This

2J ames." ReagentFr i day :Sodi um Bor ohydr i de( NaBH4) . "Mast erOr gani cChemi st r y.Web.12Dec.2015. .

reagent is bulked up with three ethane substituents in place of three hydrogen atoms. This creates an inductive effect that increases reducing power, making this superhydride one of the most powerful and selective (even for diastereomer selectivity) reducing agents in organic chemistry3.

Experimental To prepare the reduction reaction, 2.5 mL of 8% sodium hydroxide was mixed into 3.5 mL of distilled water in a small test tube. In a separate 50 mL beaker, 2.53 grams of vanillin was stirred homogenously with 5.0 mL of ethanol, followed by a subsequent addition of 1.0 mL of the NaOH solution, and placed in an ice bath with continuous stirring. 0.63 grams of sodium borohydride was dissolved into the remaining sodium hydroxide solution, which was pipetted very slowly into the cold vanillin solution over a period of 10-12 minutes. Afterwards, the new solution was warmed to room temperature for approximately five minutes with constant stirring to ensure a homogenous mixture. After the reaction had come to completion, concentrated hydrochloric acid (HCl) was added dropwise to the reaction mixture with constant swirling until no more hydrogen gas was observed as being produced by the reaction. This was verified by a pH strip that determined the acidity of the solution at an approximate pH of 6.0 and confirmed neutralization of the mixture. The mixture was once again cooled in an ice bath for ten minutes with constant stirring until a white precipitate visibly formed at the bottom of the beaker. The suction filtration technique was utilized to separate the solid precipitate from the liquid components. 3 Rowl ands,Gar et h." Reduct i ons. "Reduct i onandOxi dat i on( 2002) :4558.Web.12Dec.2015. .

Subsequently, portions of ice cold water were used to wash the solid obtained to rid it of impurities. The solid precipitate was kept in the top section of the funnel, covered with parafilm, and allowed to dry for a period of two weeks. After this period, the weight of the solid produced was determined and recorded with a scale. Afterwards, a sample of the vanillyl alcohol was inserted into a closed capillary tube and brought to a melting point machine in order to determine the melting point range of the compound isolated.

Image 2: sample of vanillyl alcohol used to determine the melting point of the final product.

Results Amount

Observed Melting

Actual Melting

Point Range

Point Range4

4 "Search ChemSpider." ChemSpider. GGA. Web. 03 Dec. 2015. http://www.chemspider.com/.

Vanillin 2.53 grams used -----------------------81° C – 83° C Vanillyl Alcohol 3.29 grams obtained 106.7° C – 108.1° C 113° C – 115° C Table 1: displays amount of vanillin used and the amount of vanillyl alcohol obtained from the reaction, as well as both the observed melting point range of vanillyl alcohol and the actual melting point ranges acquired from ChemSpider database4.

Percent Yield =

3.29 grams x 100% = 128.34% 2.56 grams

Image 3: final product obtained from the reduction reaction—primarily composed of vanillyl alcohol plus some other undesired impurities.

Discussion Because one mole of vanillyl alcohol was expected to be produced per mole of vanillin, a perfect reaction would have yielded 2.56 grams of vanillyl alcohol and resulted in a 100% percent yield. However, there was an additional 0.73

grams in the final product, indicating that additional reagents remained in the compound following the acid workup step. It is possible that not all of the sodium borohydride (as well as sodium hydroxide) used in this reaction was decomposed/neutralized by hydrochloric acid and ultimately remained in the mixture as part of the compound. Additionally, it is probable that not all of the vanillin used as starting reagents was converted to vanillyl alcohol. This appears to be the case because the melting point range observed for vanillyl alcohol was 7-9° C less than the values obtained from primary literature. This is a direct indicator of impurity in the sample and, because vanillin has a significantly lower melting point than vanillyl alcohol, it is likely that excess vanillin remained in the overall mixture. It is unlikely that any boric acid found its way into the final mixture because, 1) it is soluble in water and would not have appeared in the final precipitate, and 2) it has a melting point of 170.9° C and likely would have raised the observed melting point for vanillyl alcohol. This is a fairly “green” reaction with the desired product (vanillyl alcohol) having an atom economy in this reaction of 85.83%. The calculations for this can be found in the observations section. This indicates that most of the atoms in the reagents, with the obvious exception of boron, -OH groups produced from water deprotonation, and sodium ions, find their way into the final product in this mechanism and produces relatively little excess waste. Additonally, a benign solvent (ethanol) is used in this reaction and reacts very slowly with sodium borohydride in basic solutions.

Image 4: illustrates the main reaction scheme that occurs to reduce the aldehyde in vanillin to an alcohol group, converting the molecule from vanillin to vanillyl alcohol. Due to the negatively charged alkoxide anion, either a molecule of water or a hydronium ion could be used as a source of protons. In addition, the acid workup step that decomposes the remaining sodium borohydride is also displayed.

Notebook/Observations:...