Meso-stilbene dibromide

Purpose To demonstrate the preparation of an alkyne by a double dehydrohalogenation. 

The potassium hydroxide soiution used in this experiment is highly caustic. Do not allow it to come in contact with your skin, if this shouid happen, flood the affected area with water and then thoroughly rinse the area with a solution of dilute acetic acid. Wear latex gloves when preparing and transferring all solutions. 

Preparation Sign in at www.cengage.com/login to answer Pre-Lab Exercises, access videos, and read the MSDSs for the chemicais used or produced in this procedure. Review Sections 2.9, 2.10, and 2.17. 

Apparatus A sand bath, hot piate, 25-mL Erienmeyer fiask, apparatus for vacuum fiitration. 

Setting Up Preheat the sand bath to about 190-200 °C. Piace 800 mg of meso-stiibene dibromide and 5 peiiets (about 400 mg) of commerciai potassium hydroxide in the Erienmeyer fiask. Add 4 mL of triethyiene giycoi and a carborundum boiiing stone to the fiask. 

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Fieser et al. have already found that bromination of trans-stilbene with pyridinium hydrobromide perbromide in acetic acid gave exclusively meso-stilbene dibromide, and have further shown that the agent possesses far greater stereoselectivity than free bromine (ref. 26). Fournier et al. have reported the bromo-addition to double-bond of several alkenes by use of TBA Br3 (ref. 27). Moreover, Bethelot et al. described the bromo-addition to triple-bond of alkynes with TBA Br3 (ref. 28). 

The claims of exclusive formation of rac-stilbene dichloride upon gas-solid addition of chlorine to frans-stilbene (103) [71] and of meso-stilbene dibromide in the gas-solid addition of bromine to trans- or czs-stilbene [54] could not be verified. Scheme 12 shows the results of more detailed studies indicating the mesolrac ratios on the solid-state chlorination and bromination of trflns-stilbene (103) and some variations when the crystal size was changed [58, 60-61]. There is a risk of partial transient liquefaction if the chlorine is added too rapidly, due to initially heavy reaction. But even at the start with a stoichiometric amount of chlorine at 0.1 bar and 0 °C, a persistent product layer forms on the unground crystal powder of 103 that cannot be disintegrated by the ultrasound of a cleaning bath at 20 °C for 60 h (only 7% conversion with mesolrac ratio of 11 89 under these conditions) [22]. It is therefore unavoidable to mill the crystals of 103 to sizes <1 pm in order to overcome these rare diffi-... 

Silver acetate, CH3C02Ag. Mol. wt. 163.90. Suppliers B, F, MCB. In studying the reaction of tu c-dihalides with silver acetate in acetic acid, Winstcin and Buckles discovered that addition of a small amount of water alters the course of the reaction. The observation provided the basis for a simple procedure for the preparation of the otherwise difficultly accessible [Pg.504]

Improvements in the preparation and dehydrohalogenation of w o-stilbene dibromide [Org. Syntheses, Coll. Vol. 3, 350 (1955)] are as follows. ra 5-Stilbene (20 g.) is heated with 400 ml. of acetic acid on the steam bath until dissolved, 40 g. of pyridinium bromide perbromide is added, and the mixture is heated on the steam bath and swirled for 5 minutes, meso-Stilbene dibromide separates at once in pearly white plates. The mixture is cooled to room temperature, and the product is collected and washed with methanol. The yield of dibromide, m.p. 

The rate-determining expulsion of bromide ion through a bridged intermediate requires an anti orientation of the two bromides. The nucleophilic attack of iodide at one bromide enhances its nucleophilicity and permits formation of the bridged ion. The stereochemical preference in noncyclic systems is also anti, as indicated by the fact that meso-stilbene dibromide yields frans-stilbene, whereas /,/-stilbene dibromide gives mainly ds-stilbene under these conditions. " ... 

The mechanisms of dehalogenations have been reviewed by Miller and in a series of papers , the stereoselectivity of the dehalogenation of the stilbene dibromides with a wide variety of reagents has been discussed. The meso-stilbene dibromide always eliminates to give the thermodynamically more stable alkene, namely tra 5-stilbene which is product of apparent a t/-elimina-tion. However, the J/-stilbene dibromide gives both cis- and rm i-stilbenes, and the ratio of these products can provide useful mechanistic information. One-electron reductants, such as chromous ion, give rise to intermediate radical formation in which rotation about the Ca-Cg bond allows thermodynamic control of the reaction. Two-electron reductants, such as iodide ion in dimethyl formamide, induce highly stereoselective a i-elimination. In protic solvents, carbonium ion intermediates were proposed to explain the trend towards thermodynamic control. Miller has proposed a reaction mechanism which embraces elimination, substitution, and electrophilic addition to alkenes. 

Under the same reaction conditions, f/zreo-2-bromo-l,2-diphenylethanol was also found to produce frans-stilbene and meso-stilbene dibromide. It... 

In this reaction, both ( )-stilbene and (Z)-stilbene produce l,2-dibromo-l,2-diphenylethane. However, bromination of the (Z) isomer results in a racemic mixture of DL-stilbene dibromide, while the bromination of an ( ) isomer results in a majority of meso-stilbene dibromide along with small amount of DL-enantiomers. 

It was demonstrated that a complexation of trans-stilbene with cyclodextrin led to a decrease in stereoselectivity of additive bromination and to a significant yield of DL-stilbene dibromide in contrast to the formation of meso-stilbene dibromide in nonpolar solvents [98]. The authors suggested that this reversal of stereoselectivity was attributed to the polar environment provided by the secondary hydroxyl groups of... 

The reaction of bromine with (E)-stilbene (47) to give meso-stilbene dibromide (48) as the major product (Eq. 10.21) is another example of an electrophilic addition reaction of alkenes. The addition of bromine to many alkenes is a stereospecific reaction that proceeds by anti addition to the double bond. However, the addition of bromine to 47 is not stereospecific because small amounts of dl-stilbene dibromide (49) are also formed in this reaction. The formation of wreso-stilbene dibromide presumably occurs via the nucleophilic attack of bromide on the intermediate cyclic bromonium ion, 50. The possible interconversion of 50 and the acyclic carbocation 51 (Eq. 10.22) is one possible way to account for the presence of dl-stilbene dibromide in the product. 

Demonstrate that attack of bromide ion, Br , on the carbocation 51 can provide both dl- and meso-stilbene dibromide. 

Discuss the differences observed in the IR (Fig. 10.36) and NMR spectra of (E)-stilbene and meso-stilbene dibromide that are consistent with addition of... 

Setting Up Equip the 10-mL pressure-rated tube with the stirbar and add 0.40 g of meso-stilbene dibromide, about 0.2 g (two pellets) of solid potassium hydroxide, and 2 mL of methanol. Cap the pressure-rated tube and gently shake it or place it on a magnetic stirrer to facilitate initial mixing of its contents. Place the tube in the cavity of the microwave apparatus. 

Write three-dimensional structures of meso-stilbene dibromide and one enantiomer of dZ-stilbene dibromide. 

The E2 reaction of most compounds is known to proceed preferentially by removal of a proton anti-periplanar to the leaving group. Based upon this generalization, predict the geometry of the l-bromo-l,2-diphenylethylene that is produced as the intermediate in the double dehydrobromination of meso-stilbene dibromide. 

If meso-stilbene dibromide is treated with KOH in ethanol, it is possible to isolate the l-bromo-l,2-diphenylethylene that is formed from the first dehydrobromination. The E2 elimination of the second molecule of hydrogen bromide from this intermediate alkene to give diphenylacetylene has a higher activation enthalpy than the first elimination and thus requires a higher reaction temperature. Explain. 

The IR and NMR spectra of meso-stilbene dibromide are provided in Figures 10.36 and 10.37, respectively. 

The b series of synthetic experiments also begins with benzaldehyde, which is converted in Experiment [Alb] into (E)-stilbene. (E)-Stilbene is the precursor to meso-l,2-dibromo-l,2-diphenylethane (meso-stilbene dibromide) prepared in Experiment [A2b]. This dibromide is in turn converted... 

Common names meso-stilbene dibromide, meso-l,2-dibromo-l,... 

Weigh the meso-stilbene dibromide and calculate the percent yield. Determine the evacuated melting point, and compare your result with the literature value. Obtain IR and NMR spectra and compare them with those reported in the literature The Aldrich Library of IR Spectra, The Aldrich Library of NMR Spectra, and/or the corresponding spectral data available online (e.g., SciFinder Scholar)). 

If meso-stilbene dibromide is treated with KOH in ethanol solvent, it is possible to isolate the monodehydrohalogenation product, the bromoalkene. 

Reagents and Equipment. Weigji and place 400 mg (1.2 mmol) of meso-stilbene dibromide and 387 mg (6.9 mmol) of KOH flakes in a 10-mL Erlenmeyer flask containing a magnetic stir bar. Using a graduated cylinder, measure and add 2 mL of triethylene glycol to the flask. 

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