Chamras Glendale Community College Chemistry 106 ...

[Pages:2]Chamras

Glendale Community College

Chemistry 106 Laboratory Experiment: Multi-Step Synthesis of Benzilic Acid

Background Information:

In this experiment you will synthesize benzilic acid via a 3-step, linear synthetic pathway. Organic synthetic pathways can be linear or convergent. In a linear synthesis, as shown in figure 1 below, a single linear sequence of synthetic steps affords the product of interest. On the other hand, a convergent synthetic scheme is made of two or more linear sequences that are eventually combined (converge) to afford the final synthetic target.

Linear Synthesis

A

B

C

D (final synthetic target)

Convergent Synthesis

A

B

C

F (final synthetic target)

D

E

Figure 1. A general depiction of linear and convergent synthetic pathways.

Fluency in carrying out organic laboratory techniques, preparation, and organization will enhance the success and efficiency of your experiment, which will be apparent in the Overall Percent Yield. In addition to percent yield calculations for individual steps, the overall percent yield should be calculated and reported. The overall percent yield is the product of all the individual percent yields. For example, if in a four-step synthesis, the individual yields are calculated to be 80%, 90%, 95%, and 70%, the overall percent yield is equal to:

80 x 90 x 95 x 70 = 4,788,000 = 48% 100 100 100 100 100,000,000

As seen in the numerical illustration above, minor losses in individual steps end up resulting in substantial diminishing of the overall % yield.

1

Introduction:

H

O

Thiamine,

HCl

HO

1

HNO3

O O

2

1. KOH

O

2. HCl(aq)

HO HO

3

O

Figure 3. Linear synthetic scheme for benzilic acid.

As seen in Figure 3 above, a self-condensation reaction between two equivalents of benzaldehyde, catalyzed by thiamine hydrochloride, affords benzoin in part 1. After isolation and purification, benzoin is subjected to oxidation of its hydroxyl group by nitric acid, thus yielding benzyl, a highly symmetrical and fully conjugated

-diketone. Finally, during the third part of the reaction, benzil is converted into potassium benzilate under heat and basic condition.

Procedure:

Part 1: Synthesis and Purification of Benzoin

a) Synthesis: Using the analytical balance, mass approximately 1.5 grams of thiamine hydrochloride to a tared Erlenmeyer flask of appropriate size, equipped with a magnetic stir bar. Add approximately 2 mL of deionized water and stir to dissolve. After complete dissolution of thiamine hydrochloride, add approximately 15 mL of 95% ethanol and stir the solution until it is homogeneous. To this solution, add 4.5 mL of a 2.0M aqueous sodium hydroxide solution, and swirl the flask until the bright yellow color fades to a pale yellow color. With the catalytic reaction medium ready, take your reaction flask to the balance room, tare, and add the mass of 4.50 mL of benzaldehyde. Stir the contents of the flask until they are homogeneous. Cork the flask and allow it to stand in your laboratory locker until the next period.

b) Isolation and Purification: After a few days, crystals of crude benzoin are expected to be visible in the reaction flask. If so, prepare an ice bath and cool the contents of the flask to maximize crystallization. If no crystals have formed, then seed the mixture to initiate crystallization. Once crystals are visible place the flask in ice bath to cool. Separate the crude benzoin crystals from the liquid by vacuum filtration, followed by washing with two 5mL portions of cold deionized water. To purify the crude product, transfer the crystals quantitatively into a clean 125mL Erlenmeyer flask and recrystallize with 95% ethanol. Cool and vacuum filter the recrystallized benzoin crystals. If needed, you may wash the isolated crystals with drops of cold 95% ethanol. Quantitatively transfer the purified benzoin onto a sufficiently large, clean, and tared watch glass, and place in a 100oC oven for 5-7 minutes to dry. After this time period, check the crystals for dryness with a clean microspatula. If dried, measure the mass of the pure benzoin crystals. For your report: 1. Report the % Yield 2. Measure the melting point 3. Obtain the IR spectrum 4. Predict the 1H-NMR spectrum.

2

 ................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download