Addition of Bromine to an Alkene

As a bromine molecuie approaches an aikene, the electron density of the aikene ir bond repeis electron density in the cioser bromine, polarizing the bromine molecule and making the closer bromine atom electrophilic. The aikene donates a pair of electrons to the closer bromine, causing displacement of the distant bromine atom. As this occurs, the newly bonded bromine atom, due to its size and polarizability, donates an electron pair to the carbon that would otherwise be a carbocation, thereby stabilizing the positive charge by delocalization. The result is a bridged bromonium ion intermediate. 

A bromide anion attacks at the back side of one carbon (or the other) of the bromonium ion in an S, 2 reaction, causing the ring to open and resulting in the formation of a v/c-dibromide. 

This process is shown for the addition of bromine to cyclopentene below. 

Attack at either carbon of the cyclopentene bromonium ion is equally likely because the cyclic bromonium ion is symmetric. It has a vertical plane of symmetry passing through the bromine atom and halfway between carbons 1 and 2. The tram-dibromide, therefore, is formed as a racemic mixture. 

The mechanisms for addition of CI2 and I2 to alkenes are similar to that for Br2, involving formation and ring opening of their respective halonium ions. 

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In reaction with an alkene, initially a three-membered ring Lewis acid/Lewis base-complex 5 is formed, where the carbon-carbon double bond donates r-electron density into the empty p-orbital of the boron center. This step resembles the formation of a bromonium ion in the electrophilic addition of bromine to an alkene ... 

Displacement Mechanisms. In these reactions the organic substrate uses its electrons to cause displacement on an electrophilic oxidizing agent. One example is the addition of bromine to an alkene (15-37). 

The addition of halogens, halogen acids, water and hypohalides proceeds according to the trans scheme. As an example we take the addition of bromine to an alkene which proceeds in the following steps. 

The second piece of evidence against the mechanism of Figure 10-7 is that bromine addition reactions carried out in the presence of more than one nucleophilic reagent usually give mixtures of products. Thus the addition of bromine to an alkene in methanol solution containing lithium chloride leads not only to the expected dibromoalkane, but also to products resulting from attack by chloride ions and by the solvent ... 

Reaction (a) is an electrophilic addition of bromine to an alkene the appropriate reagent is bromine in carbon tetrachloride. 

Bromonium ion (Section 11.4) A three-membered ring containing a positively charged bromine atom an intermediate formed in the addition of bromine to an alkene. 

The addition of bromine to an alkene in the presence of water can lead to the formation of a 1,2-bromo-alcohol (or bromohydrin) in addition to a 1,2-dibromide. This is because water can act as a nucleophile and compete with the bromide ion for ring-opening of the bromonium ion. Even though Br is a better nucleophile than H20, if H20 is present in excess, then the 1,2-bromo-alcohol will be formed. 

For an elementary reaction (involving a single step), the overall order of the reaction is the same as the molecularity (the number of reacting molecules in the rate-limiting step). Reactions composed of two or more elementary reactions are called complex reactions. A complex reaction may yield a complicated rate expression that includes concentration terms in the denominator or sums of terms that are similar to the right-hand side of equation 6.17. In such cases the overall order of the reaction is not defined. For example, we will see in Chapter 9 that the kinetic expression for the addition of bromine to an alkene in CCI4 solution may take the form... 

One of the classic reactions of organic chemistry is the addition of bromine to an alkene. We will first summarize the evidence that led to the textbook mechanism (shown in Figme 9.2), and then we will survey experimental data that reveal the complexities underlying this simple mechanistic representation. 

Addition of bromine to an alkene is highly stereospecific and gives products with stereochemistry consistent with an anti addition mechanism. For example, addition of bromine to cyclohexene produces fraMS-l,2-dibromocyclohexane. Addition of bromine to cis-2-butene produces ( )-2,3-dibromobutane (equation 9.2), while addition to the trans isomer gives the meso product (equation 9.3). ° ... 

Addition of bronrine to an alkene is much more complicated than the simple representation in Figure 9.2 would suggest. The classical bromonium ion description of electrophilic addition of bromine to an alkene is useful only as a beginning point to describe the mechanistic options. The structure of the intermediate, the kinetics of the reaction, and both the stereochemistry and the regiochemistry of the products are all complex functions of the nature and concentration of tiie brominating agent, the solvent, any added nucleophiles, and the structure of the alkene. 

It has been proposed that there is a close analogy between the SnI reaction and the electrophilic addition of bromine to an alkene. Compare and contrast the two reactions in terms of (a) gross mechanistic features (especially charge development), (b) substituent effects, and (c) solvent effects. 

A vicinal dihalide (abbreviated rTtc-dihalide) is a compound bearing the halogens on adjacent carbons vicinus, Latin adjacent). Vicinal dihalides are also called 1,2-dihalides. A vicinal dibromide, for example, can be synthesized by addition of bromine to an alkene (Section 8.1). The w c-dibromide can then be subjected to a double dehydrohalogenation reaction with a strong base to yield an alkyne. 

The addition of bromine and chlorine to a cycloalkene gives a trans dihalocycloal-kane. For example, the addition of bromine to cyclohexene gives trawfrl,2-dibromocyclo-hexane the cis isomer is not formed. Thus, the addition of a halogen to a cycloalkene is stereoselective. A stereoselective reaction is a reaction in which one stereoisomer is formed or destroyed in preference to all others that might be formed or destroyed. We say that addition of bromine to an alkene occurs with anti stereoselectivity. 

Addition of bromine to an alkene results In almost immediate loss of the red-brown color of Br2 as the reaction takes place. 

We can easily see the evidence for the addition of bromine to an alkene. Bromine is red-orange. It reacts with alkenes to give a colorless product. Hence, the disappearance of the red-orange color of bromine can be used to determine if a compound is unsaturated. If the bromine color disappears when Br is added to a compound, the compound is unsaturated. 

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