Mechanisms of Alkenes

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Mechanisms of Alkenes

• Electrophilic Addition Reactions

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Understanding the basics

• Mechanisms are the most mind-boggling part of
  organic chemistry.
• Students, generally speaking, have spent their
  time memorizing their way through science
  courses.
• Mechanisms require a student to actually
  UNDERSTAND the fundamentals of electron flow

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• Everyone knows that electrons are negatively
  charged.
• Everyone knows that electrons are attracted to
  things with positive charges.
• Yet, the understanding of a mechanism remains
  elusive to many students
• Lets review the basics

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• Electron flow is always from the electron-rich to
  the electron-poor species.
• The electron-rich species is a Lewis Base (must
  have a lone pair) and is called the
  nucleophile.
• The electron-poor species is a Lewis Acid (must
  have empty orbital) and is called the
  electrophile.

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• SO electron flow is always from Nucleophile to
  Electrophile.
• Watch the direction of your arrows
• from lone pairs to carbocation
• or anion to cation
• or anion to partial positive charge
• or alkene pi bond to cation or partial positive
  charge

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• When working through a mechanism, the goal is NOT
  to memorize the steps of a mechanism OF A
  SPECIFIC MOLECULE when you do that, typically
  you become too focused on the structures provided
  in one example.
• When that happens, you get confused when the next
  mechanism problem has a DIFFERENT structure.

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• What you want to do is break down the steps of
  the mechanism, into little parts or steps.
• These basic little steps can be memorized.
• By knowing the steps, you know how the mechanism
  progresses, regardless of the structure you are
  given to work with.
• SO break them down

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Dehydration of Alcohols

• Identify this mechanism Starts with alcohol,
  ends with alkene

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Dehydration of Alcohols

• Steps Involved
• Convert OH to H2O
• Loss of H2O and Carbocation formation
• Removal of H, resulting in formation of pi bond
  to complete conversion to alkene

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Dehydration of Alcohols

• Step 1 Convert OH to H2O

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Dehydration of Alcohols

• Step 2 Loss of H2O (spontaneous dissociation)
  to form carbocation

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Dehydration of Alcohols

• Step 3 Removal of H, resulting in formation of
  pi bond to complete conversion to alkene

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Acid-Catalyzed Hydration

• Identify this mechanism Starts with alkene,
  ends with alcohol

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Acid-Catalyzed Hydration

• Steps Involved
• Reaction of pi bond with H (acid cat.) resulting
  in Carbocation formation
• Addition of H2O
• Removal of extra proton (H) to finish formation
  of OH.

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Acid-Catalyzed Hydration

• Step 1 Reaction of pi bond with H (acid cat.)
  resulting in Carbocation formation

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Acid-Catalyzed Hydration

• Step 2 Addition of H2O

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Acid-Catalyzed Hydration

• Step 3 Removal of extra proton (H) to finish
  formation of OH.

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Addition of H-X

• Identify this mechanism Starts with alkene,
  ends with single halide

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Addition of H-X

• Steps Involved
• Reaction of pi bond with H (of H-X), concurrent
  separation of X-, and formation of carbocation
  intermediate.
• Attack of X- to finish formation of product.

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Addition of H-X

• Step 1 Reaction of pi bond with H (of H-X),
  concurrent separation of X-, and formation of
  carbocation intermediate.

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Addition of H-X

• Step 2 Attack of X- to finish formation of
  product.

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Addition of X2

• Identify this mechanism Starts with alkene,
  ends with two halides

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Addition of X2

• Steps Involved
• Attack by pi bond on polarized X-X with Halonium
  Ion formation
• Attack of X- to pop open three-membered ring and
  finish formation of product.

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Addition of X2

• Step 1 Attack by pi bond on polarized X-X with
  Halonium Ion formation

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Addition of X2

• Step 2 Attack of X- to pop open three-membered
  ring and finish formation of product.

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Addition of X2

• Of course, you can attack the other end of the
  halonium and open in the other direction

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Addition of X2 and H2O

• Identify this mechanism Starts with alkene,
  ends with one alcohol and one halide

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Addition of X2 and H2O

• Steps Involved
• Attack by pi bond on polarized X-X with Halonium
  Ion formation
• Attack by H2O to pop open three-membered ring.
• Removal of extra proton (H) by X- to complete
  the formation of OH.

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Addition of X2 and H2O

• Step 1 Attack by pi bond on polarized X-X with
  Halonium formation

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Addition of X2 and H2O

• Step 2 Attack by H2O to pop open three-membered
  ring.

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Addition of X2 and H2O

• Step 3 Removal of extra proton (H) by X- to
  complete the formation of OH.

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AND REMEMBER

• If you UNDERSTAND the basic steps to these
  mechanisms, it wont matter what the double bond
  in the molecule looks like...
• Every alkene reacts the same way, every time,
  regardless of whats attached

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Finally

• Track the pieces you need to add or subtract
  overall See where you are starting and where
  you are ending
• Dont memorize a specific molecule but go ahead
  and memorize the sequence of steps involved it
  wont matter what the alkene looks like if you
  approach it this way