Updated: Oct 31, 2023
Chapter 6: Reactions of Alkenes
Table of contents of main topics:
Electrophilic Additions
Hydroboration-Oxidation
Oxidation
Reduction
Electrophilic Additions
1. Addition of Hydrogen Halides
The reagent for this reaction is HX, where X is either Cl, Br or I.
Product: Turn double bond into single and add X (halogen) to the MOST substituted carbon from the double bond (Markovnikov rule). Warning: Rearrangements are possible!
2. Addition of Water: Acid Catalyzed Hydration Of Alkenes
The reagent is H2O with any catalytic acid such as H2SO4. Examples are: H2O/H2SO4, H3O+
Product:Turn double bond into single and add OH to the MOST substituted carbon from the double bond (Markovnikov rule). The other carbon gets a hydrogen. Warning: Rearrangements are possible!
Carbocation Rearrangements:
Carbocation rearrangements happen in mechanisms that have a carbocation intermediate such as alkene with HX or alkene with H2O/H2SO4. Carbocation rearrangement will happen if the carbocation made has a more substituted neighboring carbon. A hydrogen or an alkyl group can shift during a carbocation rearrangement.
Hydride Shift
First, the double bond grabs a hydrogen from HCl. The least substituted carbon from the double bond het an H (not shown) and the most substituted becomes a carbocation (one bond is missing). The carbocation is secondary, while its neighboring carbon on the left is tertiary. This means a hydride shift will happen. Hydrogen from the left carbon moves to the right, resulting a new tertiary carbocation. Now Cl- can add to it for the final product.
Alkyl Shift (methyl shift)
First, the double bond grabs a hydrogen from HCl. The least substituted carbon from the double bond het an H (not shown) and the most substituted becomes a carbocation (one bond is missing). The carbocation is secondary, while its neighboring carbon on the left is quaternary. This means a methyl shift will happen. Methyl group from the left carbon moves to the right, resulting a new tertiary carbocation. Now Cl- can add to it for the final product.
3.Halogenation: Addition of Bromine And Chlorine
The reagent is Br2 or Cl2 with a solvent such as (CH2Cl2).
Product: Turn double bond into single and add two bromines (or chlorines) one on each carbon from the double bond. Stereochemistry: ANTI ( one is on a wedge and another is on a dash).
Mechanism of Halogenation
The mechanism involves bromonium ion intermediate. When the second bromine attacks, it does so via an SN2 reactions which results in trans stereochemistry of the product.
4. Addition of HOCl and HOBr
The reagent is Br2 or Cl2 with H2O.
Product: Turn double bond into single and add OH to the most substituted carbon from the double bond and Br (or Cl) to the less substituted carbon from the double bond. Stereochemistry: TRANS
5. Oxymercuration-Reduction
The reagent is 1.Hg(OAc)2, H2O 2.NaBH4
Product: Turn double bond into single and add OH to the most substituted carbon from the double bond. The other carbon gets the hydrogen. No Rearrangements!
This reaction is very useful. Both acid catalyzed alkene hydration and oxymercuration give the same product, but the difference is that oxymercuration can not have any rearrangements. Therefore, for synthesis, it might be a better choice for the reagent.
Oxymercuration - Reduction Mechanism
6. Hydroboration Oxidation
The reagent is 1.BH3, THF 2. H2O2, NaOH or 1.B2H6,diglyme 2.H2O2, NaOH
Product: Turn double bond into single and add OH to the LEAST substituted carbon from the double bond (Anti Markovnikov Rule) and hydrogen on the most substituted carbon from the double bond. OH and H add syn (same side, both are dash or wedge)
Hydroboration is a very useful reaction because it is rare for reactions to go the anti Markovnikov way.
If we notice OH on the less substituted carbon in the product in a synthesis problem, hydroboration most likely led to that product.
Mechanism of Hydroboration Oxidation of alkenes
OXIDATION
Oxidation is the loss of hydrogens and/or gain of oxygens
Reduction is the gain of hydrogens and/or loss of oxygens
7.OsO4—Oxidation of an Alkene to a Glycol
The reagent is 1.OsO4 2. NaHSO3/H2O or H2O2
Product: Turn double bond into a single bond and add two OHs, one on each carbon from the double bond. Stereochemistry is cis (both are on a wedge or a dash)
8. Ozonolysis With Reductive Workup
The reagent is 1.O3 2. (CH3)2S (also called DMS) OR Zn, H2O
Product: Break the double bond in two and add two oxygens, one on each end of the double bond broken.
9. Ozonolysis With Oxidative Workup
The reagent is 1.O3 2.H2O2
Product: Break the double bond in two and add two oxygens, one on each end of the double bond broken. If you made an aldehyde (C=O connected to H), you have to change H into OH to make carboxylic acid.
10. Catalytic Hydrogenation Of Alkenes
The reagent is H2 with a metal catalyst such as Pt, Pd, or Ni
Product: Turn double bond into single by adding 2 hydrogens ,one on each carbon from the double bond. Hydrogen add syn (both wedge or both dash)
SUMMARY: Alkene Reaction Cheat Sheet for Organic Chemistry by Brown, Iverson, Anslyn, Foote