Stereochemistry of halogenation

Interpretations of stereochemistry have focused attention on the role played by cyclic halonium ions  

If the addition of Br to the alkene results in a bromonium ion, the anti stereochemistry can be readily explained. Nucleophilic ring-opening by bromide ion would occur by back-side attack at carbon, with rupture of one of the C-Br bonds, giving overall anti addition  

On the other hand, a freely rotating open carbonium ion would be expected to give both syn and anti addition. If the principal intermediate were an ion pair that collapsed faster than rotation about the C-C bond, syn addition could predominate. 

The stereochemistry of chlorination can be explained in similar terms. Chlorine would be expected to be a somewhat poorer bridging group than bromine. Comparison of the data for cw- and trans-1-phenylpropenc in bromination and chlorination confirms that it is. Although anti addition is dominant for bromination, syn addition is preferred for chlorination. For nonconjugated alkenes, however, stereospecific anti addition is usually observed with both halogens. 

There is direct evidence for the existence of halonium ions such as those that are invoked in bromination and chlorination reactions. They can be formed from dihalides by ionization with neighboring-group participation and can be observed by NMR measurements in nonnucleophilic solvents  

LEARN the skill Predict the major product obtained upon radical bromination of 2,2,4-trimethylpentane  

Identify all possible positions and determine the most stable location for a radical. 

Analyze all possible positions that can possibly undergo bromination, and identify each position as primary, secondary, or tertiary  

Make sure to avoid selecting a quaternary position, as quaternary positions do not possess a C—H bond. Bromination cannot occur at a quaternary site. The major product is expected to result from bromination at the tertiary position  

Draw the product of bromination at the location of the most stable radical. 

Problem 10.21 Draw the transition state for each step in the general mechanism for the halogenation of an alkene. 

Problem 10.22 Draw the products of each reaction, including stereochemistry. 

How does the proposed mechanism invoking a bridged halonium ion intermediate explain the observed trans products of halogenation For example, chlorination of cyclopentene affords both enantiomers of rram-l,2-dichlorocyclopentane, with no cis products. 

Initial addition of the electrophile CF (from CI2) occurs from either side of the planar double bond to form the bridged chloronium ion. In this example, both modes of addition (from above and below) generate the same achiral intermediate, so either representation can be used to draw the second step. 

I Addition of Cb occurs from I both sides of the double bond. i 

In the second step, nucleophilic attack of CP must occur from the back side— Ihat is, from the side of the flve-membered ring opposite to the side having the bridged chloronium ion. Because the nucleophile attacks from below in this example and the leaving group departs from above, the two Cl atoms in the product are oriented trans to each other. Backside attack occurs with equal probability at either carbon of the three-membered ring to yield an equal amount of two enantiomers—a racemic mixture. 

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The bromonium ion postulate, made more than 75 years ago to explain the stereochemistry of halogen addition to alkenes, is a remarkable example of deductive logic in chemistry. Arguing from experimental results, chemists were able to make a hypothesis about the intimate mechanistic details of alkene electrophilic reactions. Subsequently, strong evidence supporting the mechanism came from the work of George Olah, who prepared and studied stable... 

The mechanism and stereochemistry of halogenation, physical methods / 273 Dihalogenation / 276 The use of A-halo compounds / 280 The use of other reagents / 282 The oxidation of halohydrins / 283 Vinylogous a-halo ketones / 284... 

The issue of the role of bridged radicals in the stereochemistry of halogenation has recently been examined computationally and a new interpretation offered. The structure, rotational barriers, and for halogen atom abstraction for P-haloethyl radicals were studied. For the reactions where X=C1 or Br, the halogen atom abstraction reaction shows a preference for a trans TS. 

Stereochemistry of Alkene Hydrogenation 230 6.15 Stereochemistry of Halogen Addition 250... 

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