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Alkene Addition Reactions: Crash Course Organic Chemistry #16
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Alkene Addition Reactions: Crash Course Organic Chemistry #16

Crash Course Organic Chemistry Introduction

In this section, Deboki Chakravarti introduces Crash Course Organic Chemistry and discusses the impact of black pepper on the Age of Discovery.

Crash Course Organic Chemistry App

  • The Crash Course app is available for Android and iOS devices.
  • It allows users to review content from Crash Course Organic Chemistry.

Introduction to Crash Course Organic Chemistry

  • Deboki Chakravarti welcomes viewers to Crash Course Organic Chemistry.
  • She mentions Vasco da Gama's exploration for a sea route to India, driven by the desire to bring back black pepper and other spices to Europe.
  • Piperine, an organic chemical found in black pepper, triggers the "spicy" receptors in our mouths.
  • The E-double bonds in piperine can be converted to cis bonds through a rare isomerization reaction with sunlight.

Focus on Alkene Reactions

  • The episode will focus on chemical reactions involving alkenes.
  • Alkenes are nucleophilic and can attack electrophiles, leading to addition reactions where two sigma bonds form across the double bond.

Chemical Reactions with Alkenes

This section explores the general pattern of reactions involving alkenes and introduces three key questions that help predict reaction products.

General Pattern of Alkene Reactions

  • Alkenes are nucleophilic and can attack electrophiles.
  • Addition reactions occur when two groups are added across the double bond.

Three Key Questions for Predicting Reaction Products

  1. What is being added across the double bond?
  1. Where will the group add on an asymmetrical molecule?
  1. What is the expected stereochemistry of the added groups?

Question 1: What is being added?

  • Examples include hydrogen bromide (HBr) and water (H2O).
  • Addition of HBr is called hydrobromination, while addition of water is called hydration.

Question 2: Where will the group add?

  • Markovnikov's rule states that a proton adds to the carbon with the most hydrogens.
  • This is known as Markovnikov addition.
  • In some cases, groups can add in the opposite orientation, known as anti-Markovnikov addition.

Question 3: What is the expected stereochemistry?

  • Groups can add on the same face (syn addition) or on opposite faces (anti addition).

Halogenation and Halohydrin Formation

This section focuses on two specific addition reactions: halogenation and halohydrin formation.

Halogenation

  • Halogenation involves adding chlorine or bromine across the double bond.
  • The reaction occurs in a non-nucleophilic solvent like carbon tetrachloride.
  • The added groups are anti to each other.

Halohydrin Formation

  • Halohydrin formation also adds chlorine or bromine across the double bond but uses water as a solvent.
  • The final product differs due to the presence of multiple solvent molecules.
  • The added groups may be syn or anti to each other.

Timestamps have been associated with bullet points where available.

Halohydrin Reaction

In this section, we learn about the halohydrin reaction and its regioselectivity.

Halohydrin Reaction

  • The halohydrin reaction involves a water molecule as the nucleophilic attacker.
  • Oxygen's lone pair attacks the side that contributes more to the resonance hybrid, which is the tertiary one.
  • Water adds to the more substituted side of the bond, making this reaction regioselective.

Chlorohydrins Formation

This section discusses chlorohydrins formation and anti addition in question number 3.

Chlorohydrins Formation

  • The chloronium ion blocks syn addition, so water adds anti.
  • A second water molecule deprotonates the oxonium ion, forming an alcohol group in the final products.
  • This reaction is called halohydrin formation and results in chlorohydrins.

Acid-Catalyzed Hydration Comparison

Here we compare acid-catalyzed hydration with other addition reactions discussed earlier.

Acid-Catalyzed Hydration Comparison

  • Acid-catalyzed hydration involves adding water across a double bond with the help of an acid catalyst.
  • This reaction may involve a 1,2 hydride shift but not always.
  • For a detailed mechanism of acid-catalyzed hydration, refer to episode 14.

Oxymercuration/Reduction Reaction

We explore oxymercuration/reduction as another method to add a nucleophile without carbocation rearrangements.

Oxymercuration/Reduction Reaction

  • Oxymercuration involves using a mercury atom to add oxygen across the double bond.
  • The reduction part removes the mercury atom after its role is complete.
  • Mercury acetate helps in adding a nucleophile with an oxygen across a double bond.

Oxymercuration/Reduction Mechanism

This section explains the mechanism of oxymercuration/reduction and the role of mercury (II) acetate.

Oxymercuration/Reduction Mechanism

  • Mercury (II) acetate and water are used in this reaction.
  • Metal complexes like mercury dissociate easily, allowing for attachment to one or two acetate molecules at any time in solution.
  • The first step involves the formation of a mercurinium ion, which stabilizes the positive charge and prevents carbocation rearrangements.
  • Water acts as a nucleophile and attacks the carbon that better stabilizes a positive charge.
  • A second water molecule deprotonates, completing the addition reaction.

Sodium Borohydride Addition

This section discusses sodium borohydride addition as part of oxymercuration/reduction.

Sodium Borohydride Addition

  • Sodium borohydride is used in the second step of oxymercuration/reduction.
  • The details of this metal reaction mechanism are not covered here, but it replaces the mercury atom with hydrogen to form an alcohol group.

Hydroboration Reaction Introduction

We introduce hydroboration as another alkene addition reaction.

Hydroboration Reaction Introduction

  • Hydroboration involves adding water across a double bond using boron as a reagent.
  • Borane is added initially and then replaced with an OH group.
  • Hydroboration is similar to adding water across a double bond.

Hydroboration Mechanism

This section explains the mechanism of hydroboration and its regioselectivity.

Hydroboration Mechanism

  • The nucleophile, 1-methylpent-1-ene, attacks the boron-containing reagent BH3.
  • A concerted reaction occurs where all bonds break and form simultaneously.
  • The boron atom adds to the less hindered carbon, resulting in anti-Markovnikov addition.
  • The B and H atoms add to the same side of the ring due to their attachment during the reaction, leading to syn addition.

Oxidation in Hydroboration Reaction

This section discusses oxidation as the second step in hydroboration.

Oxidation in Hydroboration Reaction

  • Peroxide (H2O2) and sodium hydroxide act as referees in this process, breaking off the boron and leaving behind the alcohol group.
  • Further details about oxidation will be covered later.

Comparison of Alkene Addition Reactions

We compare acid-catalyzed hydration, oxymercuration/reduction, and hydroboration reactions.

Comparison of Alkene Addition Reactions

  • Acid-catalyzed hydration has Markovnikov regioselectivity but can have carbocation rearrangements.
  • Oxymercuration has Markovnikov regioselectivity and blocks carbocation rearrangements.
  • Hydroboration has anti-Markovnikov regioselectivity, syn-stereochemistry, and no carbocation rearrangements.
Video description

Like a trendy dance, a fighting combo, or a secret handshake, organic reactions can be broken down into simpler steps. In this episode of Crash Course Organic Chemistry, we’ll specifically be looking at alkene addition reactions, and with each new reaction ask ourselves three questions to help us puzzle through the mechanism and understand what’s going on. Episode Sources: Le Couteur, P, Burreson, J. Napoleon’s Buttons: 17 molecules that changed history., Penguin, New York, 2004, Chapter 1. Whipps, H., “How the Spice Trade Changed the World,” https://www.livescience.com/7495-spice-trade-changed-world.html Last accessed, 3/28/2020. Henriques, Martha, “How spices changed the ancient world,” http://www.bbc.com/future/bespoke/made-on-earth/the-flavours-that-shaped-the-world/ Last accessed, 3/28/2020. Hashimoto, K., Yaoi, T., Koshiba, H., Yoshida, T., Maoka, T., Fujiwara, Y., Yaamoto, Y, Mori, K., “Photochemical Isomerization of Piperine, a Pungent Constituent of Pepper”, Food Sci. Technol., Int. 1996, 2 (1), 24-29. Series Sources: Brown, W. H., Iverson, B. L., Ansyln, E. V., Foote, C., Organic Chemistry; 8th ed.; Cengage Learning, Boston, 2018. Bruice, P. Y., Organic Chemistry, 7th ed.; Pearson Education, Inc., United States, 2014. Clayden, J., Greeves, N., Warren., S., Organic Chemistry, 2nd ed.; Oxford University Press, New York, 2012. Jones Jr., M.; Fleming, S. A., Organic Chemistry, 5th ed.; W. W. Norton & Company, New York, 2014. Klein., D., Organic Chemistry; 1st ed.; John Wiley & Sons, United States, 2012. Louden M., Organic Chemistry; 5th ed.; Roberts and Company Publishers, Colorado, 2009. McMurry, J., Organic Chemistry, 9th ed.; Cengage Learning, Boston, 2016. Smith, J. G., Organic chemistry; 6th ed.; McGraw-Hill Education, New York, 2020. Wade., L. G., Organic Chemistry; 8th ed.; Pearson Education, Inc., United States, 2013. *** Watch our videos and review your learning with the Crash Course App! Download here for Apple Devices: https://apple.co/3d4eyZo Download here for Android Devices: https://bit.ly/2SrDulJ Crash Course is on Patreon! You can support us directly by signing up at http://www.patreon.com/crashcourse Thanks to the following patrons for their generous monthly contributions that help keep Crash Course free for everyone forever: Vincent LaBarca, Bjartur Thorlacius, Scott Harrison, Alexander Thomson, Mark & Susan Billian, Junrong Eric Zhu, Alan Bridgeman, Jennifer Smith, Matt Curls, Tim Kwist, Jonathan Zbikowski, Jennifer Killen, Sarah & Nathan Catchings, Brandon Westmoreland, team dorsey, Trevin Beattie, Yasenia Cruz, Eric Koslow, Indika Siriwardena, Khaled El Shalakany, Shawn Arnold, Siobhán, Ken Penttinen, Nathan Taylor, William McGraw, Andrei Krishkevich, Sam Ferguson, Jirat, Brian Thomas Gossett, SR Foxley, Ian Dundore, Jason A Saslow, Jessica Wode, Caleb Weeks, Mark, Patty Laqua, Stephen Saar, John Lee, Eric Prestemon __ Want to find Crash Course elsewhere on the internet? Facebook - http://www.facebook.com/YouTubeCrashCourse Twitter - http://www.twitter.com/TheCrashCourse Tumblr - http://thecrashcourse.tumblr.com Support Crash Course on Patreon: http://patreon.com/crashcourse CC Kids: http://www.youtube.com/crashcoursekids