Mechanisms of Alkenes
Electrophilic Addition Reactions
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…
• 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…
• Let’s review the basics…
• 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”.
• 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…
• 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.
• 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…
Dehydration of Alcohols
Identify this mechanism – Starts with
alcohol, ends with alkene…
OH
H+
+ H2O
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
Dehydration of Alcohols
Step 1: Convert –OH to H2O
OH
H
H+
O
H
Dehydration of Alcohols
Step 2: Loss of H2O (“spontaneous
dissociation”) to form carbocation
H
O
H
+ H-O-H
Dehydration of Alcohols
Step 3: Removal of H+, resulting in
formation of pi bond to complete
conversion to alkene
+ H-O-H
H
+ H-O-H
H
Acid-Catalyzed Hydration
Identify this mechanism – Starts with
alkene, ends with alcohol…
OH
H+, H2O
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.
Acid-Catalyzed Hydration
Step 1: Reaction of pi bond with H+ (acid
cat.) resulting in Carbocation formation
H+
H
Acid-Catalyzed Hydration
Step 2: Addition of H2O
H
O
H
+ H-O-H
H
H
Acid-Catalyzed Hydration
Step 3: Removal of extra proton (H+) to
finish formation of –OH.
H
O
H
+ H-O-H
OH
+ H-O-H
H
H
Addition of H-X
Identify this mechanism – Starts with
alkene, ends with single halide…
X
H-X
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.
Addition of H-X
Step 1: Reaction of pi bond with H+ (of HX), concurrent separation of X-, and
formation of carbocation intermediate.
H
X
H
+ X
Addition of H-X
Step 2: Attack of X- to finish formation
of product.
X
H
X
H
Addition of X2
Identify this mechanism – Starts with
alkene, ends with two halides…
X2
X
X
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.
Addition of X2
Step 1: Attack by pi bond on polarized XX with Halonium Ion formation
X
X
+ X
X
Addition of X2
Step 2: Attack of X- to pop open threemembered ring and finish formation of
product.
X
X
X
X
Addition of X2
Of course, you can “attack” the other end
of the halonium and open in the other
direction…
X
+ X
X
X
Addition of X2 and H2O
Identify this mechanism – Starts with
alkene, ends with one alcohol and one
halide…
X2
H2O
OH
X
+ HX
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 threemembered ring.
– Removal of extra proton (H+) by X- to
complete the formation of –OH.
Addition of X2 and H2O
Step 1: Attack by pi bond on polarized XX with Halonium formation
X
X
+ X
X
Addition of X2 and H2O
Step 2: Attack by H2O to pop open threemembered ring.
H
X
O
H
+ X
+ H-O-H
X
Addition of X2 and H2O
Step 3: Removal of extra proton (H+) by
X- to complete the formation of –OH.
H
O
H
H
O
+ X
X
+ H-X
X
AND REMEMBER…
If you UNDERSTAND the basic steps to
these mechanisms, it won’t matter what
the double bond in the molecule looks
like...
Every alkene reacts the same way, every
time, regardless of what’s attached…
Finally…
Track the pieces you need to add or
subtract overall… See where you are
starting and where you are ending…
Don’t memorize a specific molecule but go
ahead and memorize the sequence of
steps involved… it won’t matter what
the alkene looks like if you approach it
this way…