Experiment 8. Brominating Alkenes CHEM 3710

R2

R1

H

cinnamic acid, R1 = H, R2 = CO2H

trans-stilbene, R1 = H, R2 = Ph

cis-stilbene, R1 = Ph, R2 = H

Ph

HR1

R2

Br

Br-

+

Br-or

C5H5NBr3

CH3CO2H+

Ph

H

R1

R2Br

BrPh

H

R1

R2

Br

Br

Experimental Notes:

Please read and be familiar with the experimental procedure, p. 95-100, in the lab manual. You will analyze your reaction product by m.p. determination. The source of Br2 in this experiment is pyridinium bromide. This solid reagent is safer and easier to use than Br2

(l); however, it is a lachrymator (causes eyes to water) and corrosive. Wear gloves at all times during this experiment, if your gloves are contaminated, take them off and replace them. After weighing out the pyridinium bromide, place it immediately into your round bottom flask—don’t leave it out on the bench!

Lab Write-Up (75 pts) (2 pts) Title (3 pts) Objective Reaction Scheme (omit for now) (3 pts) Table of Physical Constants (include known hazards) (7 pts) Outline of Procedure Use a bulleted outline, 2-column format. We will have trans-stilbene. Use ethanol to

wash the product, not methanol! ************************************************************************************************** (25 pts) Observations and Calculations

(15 pts) Note any observations you make during the entire course of the experiment. (10 pts) Determine the theoretical and percent yield for the total combined mass of all your elimination products. Determine the melting point for your product.

(30 pts) Results and Discussion

(10 pts) How does the melting point help you identify your product? Run a TLC of your reaction (20 pts) Prepare a table showing the bromination product, the bromonium ion intermediate, and the products (both

enantiomers) resulting from addition of Br- to the bromonium ion. Label each stereocenter in the products (R-) or (S-). Include cis-stilbene; even though you have not done this experiment, you should be able to give an educated guess. Draw your structures/reactions using the template that follows:

+

Br

Br-

+

Br-or

Alkene Bromonium Ion Intermediate Anti-Addition Product(s) Expected MP trans-stilbene (show structure) cis-stilbene (show structure)

Potential Products m.p. (1S, 2S)/(1R, 2R)- 1,2-dibromo-1,2-diphenylethane, (racemate) 110 ºC

(1R, 2S)/(1S, 2R)-1,2-dibromo-1,2-diphenylethane, (meso) 238 ºC (5 pts) Conclusion

Green Bromination of trans-Stilbene with Pyridinium Tribromide The halogenation of alkenes is an important reaction in the chemical industry. For example, over 8 million tons of 1,2-dichloroethane per year are produced by the addition of chlorine (Cl2) to ethylene. This product is used both as a solvent and in the preparation of polyvinyl chloride, PVC, a common organic polymer used in household plumbing. The products obtained from alkene halogenation are called vicinal dihalides because the two halogen substituents are attached to adjacent carbon atoms. When the halogen used is either bromine (Br2) or chlorine (Cl2) halogenation of alkenes occurs rapidly at room temperature, and the resulting vicinal dihalides are stable. Fluorination is a violent reaction that is difficult to control and is accompanied by several side reactions. Iodination is an endothermic process, resulting in vicinal diiodides that tend to revert to alkenes. Consequently, the most common applications of alkene halogenation are chlorination and bromination. Typically, alkenes undergo reactions through electrophilic addition, a process in which the alkene pi (π) bond is replaced with two sigma bonds. The general mechanism of electrophilic addition involves two steps as shown below.

C CH

H H

H E Nu E

HH

H

HNu

H

HH

Nu

HH

The first step involves reaction of an electron deficient species, called an electrophile (E+), with the electron-rich π-bond of the alkene. The two electrons of the π-bond shift toward the electrophile, forming a new carbon-electrophile sigma bond. This step results in formation of a positively charged intermediate. In most instances, the positive charge centers on a carbon atom, so the electron-deficient intermediate is called a carbocation. In the second step, the electrophilic carbocation reacts with an electron-rich species called a nucleophile (Nu-). The nucleophile donates an electron pair to the positively charged intermediate forming a carbon-nucleophile sigma bond. Alkene bromination follows the same general mechanism with a few important modifications, as shown below. In the first step, the proximity of the π-bond of the alkene to Br2 polarizes the bromine-bromine bond. This polarization induces a bond dipole that allows Br2 to act as an electrophile. Electrons flow from the π-bond to the polarized Br2, forming a carbon-bromine bond and breaking the bromine-bromine bond, producing a bromide ion (Br-). The positively charged intermediate in bromination is not a carbocation, but a bromonium ion. This cyclic intermediate results from a nonbonding electron pair from bromine that stabilizes the positive charge on carbon. The bromonium ion is more stable than a simple carbocation because all atoms of the bromonium ion have an octet of electrons.

In the second step, Br- acts as a nucleophile, attacking the electron-deficient bromonium ion. The second carbon-bromine sigma bond of the vicinal dihalide forms in this step. An important feature of this step is the resulting stereochemistry. Bromide ion adds to the side opposite the carbon-bromine bonds of the bromonium ion. This process is called anti-addition. Anti-addition occurs because Br- is blocked from is blocked from one face of the bromonium ion by the bromine atom. The consequence of anti-addition of Br2 to cyclic alkenes is the formation of trans-vicinal dihalides. Chiral carbon atoms, or chiral centers, are generated in many organic reactions. A chiral carbon is a carbon atom that is bonded to four different substituents. As a consequence, two different configurations are possible for a chiral center: rectus (R) or sinister (S). Anti-addition of Br- to cis- and trans-bromonium ions results in two different stereochemical outcomes. The cis-bromonium ion leads to the formation of a pair of enantiomers, but the anti-addition of Br- to the trans-bromonium ion leads to the formation of a meso compound. In this experiment, you will brominate an alkene using pyridinium tribromide, a comparatively safe, convenient source of bromine. You will characterize your product by measuring its melting point. You will determine the relative stereochemistry of your product by the melting point measurement because the stereoisomers' melting points differ significantly.