8.13 Reaction Stereochemistry: Addition of \(H_{2}O\) to a Chiral Alkene
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After completing this section, you should be able to explain why the addition of H2O to a chiral alkene leads to unequal amounts of diastereomeric products.
As illustrated in the drawing below, in an alkene the \(pi\)-bond fixes the carbon-carbon double bond in a planar configuration, and does not permit free rotation about the double bond itself.
We see then that addition reactions to this function might occur in three different ways, depending on the relative orientation of the atoms or groups that add to the carbons of the double bond: (i) they may bond from the same side, (ii) they may bond from opposite sides, or (iii) they may bond randomly from both sides. The first two possibilities are examples of stereoselectivity, the first being termed syn-addition, and the second anti-addition.
fig
Since initial electrophilic attack on the double bond may occur equally well from either side, it is in the second step (or stage) of the reaction (bonding of the nucleophile) that stereoselectivity may be imposed.
Stereochemistry of Brønsted acid addition
Addition of an acid (such as haloacids or hydronium ion) to an alkene typically results in a mixture of syn and anti addition products. This lack of stereospecifiity is due to the fact that the carbocation intermediate is planar, and can be attacked from either side by the nucleophile.
fig
Stereochemistry of halogen addition
The halogens chlorine and bromine add rapidly to a wide variety of alkenes without inducing the kinds of structural rearrangements (carbocation shifts) noted for strong acids - this is because a discreet carbocation intermediate does not form in these reactions. The stereoselectivity of halogen additions is strongly anti, as shown in many of the following examples.
We can account both for the high stereoselectivity and the lack of rearrangement in these reactions by proposing a stabilizing interaction between the developing carbocation center and the electron rich halogen atom on the adjacent carbon. This interaction delocalizes the positive charge on the intermediate and blocks halide ion attack from the syn-location.
The stabilization provided by the halogen-carbocation bonding makes rearrangement unlikely, and in a few cases three-membered cyclic halonium cations have been isolated and identified as true intermediates. A resonance description of such a bromonium ion intermediate is shown below. The positive charge is delocalized over all the atoms of the ring, but should be concentrated at the more substituted carbon (where positive charge is more stable), and this is the site to which the nucleophile will bond.
Contributors
Dr. Dietmar Kennepohl FCIC (Professor of Chemistry, Athabasca University)
Prof. Steven Farmer (Sonoma State University)
William Reusch, Professor Emeritus (Michigan State U.), Virtual Textbook of Organic Chemistry