Solomons • Fryhle • Snyder Organic Chemistry ELEVENTH EDITION Solomons • Fryhle • Snyder Chapter 8 Alkenes and Alkynes II Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

Pseudopolarization A useful technique to figure out how to add ionic or polar particles to pi systems (anything with pi bonds) is to break all of the pi bonds involved in the reaction and replace them with positive and negative charges on the atoms which previously held the pi bonds In extended pi systems (later in the course) positive and negative charges will alternate on the atoms which previously held the pi bonds. Pseudocharges are assigned by asking, “If this atom had to hold a charge, would it be more stable holding a positive charge or a negative one?” Electronegative/negative groups add to pseudopositive charged atoms and vice-versa

Hyperconjugation puts some negative charge on rightmost carbon in double bond Middle tertiary carbon is more stable with positive charge than negative charge (stabilized by some negative charge on neighboring carbons) Like charges on neighboring atoms are very unstable - opposite charges more stable

Add positive group to pseudonegative carbon first and look for carbocation rearrangements when pseudopositive carbon becomes actually positive

The pseudopositive carbon becomes actually positive in this type of reaction, so you need to watch out for carbocation rearrangements

Less electronegative and Lewis acidic B atom adds to pseudonegative carbon More electronegative H atom adds to pseudopositive carbon No rearrangements occur because B and H add together, so no carbocation is formed Hydrogen peroxide turns initial borane product into an alcohol Net result is backwards addition of water (or alcohol) to a double bond

Stereochemical Calculations You can calculate the result of adding two of the same groups to a double bond (ie X2, where X is Cl or Br, or H2) If you start with a cis double bond and add two groups in a syn fashion, then since the like groups on the double bond eclipsed each other, and the groups which add to the double bond also eclipse each other while the addition takes place, the product will have the like groups situated so that they can eclipse each other (high symmetry). A trans double bond gives rise to low symmetry with a syn addition A cis double bond gives low symmetry with an anti addition A trans double bond gives high symmetry with an anti addition The topology rule is symmetry of reactant times symmetry of addition = symmetry of product Cis reactant (+) times syn addition (+) = high symmetry product (+); [(+) x (+) = (+)] Trans reactant (-) times anti addition (-) = high symmetry product (+); [(-) x (-) = (+)]

Halogen is the + polarized group and OH is the – polarized group Pseudopolarize double bond to figure out which way to add X and OH Since OH takes the place of one of the X atoms symmetry calculations are done by pretending that X and OH are the same group In this case a cis double bond (high symmetry) reacts via an anti addition (low symmetry) to yield a low symmetry product (OH is trans to the added Br)

OsO4 and cold KMnO4 add two OH groups in a syn (high symmetry) fashion

To solve these redraw the two carbonyl compounds so that the oyygens face one another, erase the oxygens, and draw a single double bond between the two carbonyl carbons

Lone pairs on halogen atom stabilizes cation (+ charge) on carbon it is attached to via resonance

HBr adds backwards to pi bonds (double or triple) in the presence of peroxides Mechanism is not polar and can’t be rationalized by pseudopolarization This only works for HBr; won’t work for HCl, HF, or HI