Download Mechanisms of Alkenes

Mechanisms
Some Basic Orgo I 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
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”.
• Examples of Nucleophiles:
H
H
X
O
H
O
• Examples of Electrophiles:
Cl
X-X
H
H+
O
H
• SO – electron flow is always from
nucleophile to electrophile, electronrich to electron-poor…
• Watch the direction of your arrows
– from lone pairs to carbocation…
H
O
H
– from anion to cation…
X
– from anion to partial positive charge…
Br
+ Br
– from alkene pi bond to cation or partial
positive charge…
H+
Br-Br
• 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 a single example.
• If that happens, you will get confused
when the next mechanism problem has a
DIFFERENT structure.
• What you want to do is make a game
plan - break down the steps of the
mechanism, into little parts or steps.
• The basic little steps are easier to
remember.
• 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… losing water…
OH
H+
+ H2O
Dehydration, Acid-catalyzed…
sulfuric or phosphoric acid…
H+ from
Dehydration of Alcohols
Steps Involved:
– Convert –OH to H2O (use that acid!)
– Loss of H2O and carbocation formation
– Removal of H+, resulting in formation of pi
bond to complete the conversion to alkene
– E1 mechanism – think Zaitsev and Trans!
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
Dehydration of Alcohols - Again
Try another example:
OH
H2SO4
heat
+ H3O+
Alcohol to alkene (using acid) =
dehydration (make water, lose water,
form alkene)
Dehydration of Alcohols - Again
Step 1: Convert –OH to H2O
H
O
H+
H
O
H
Remember the soul purpose for the acid
is to turn the –OH into a water
molecule. Now it wants to leave…
Dehydration of Alcohols - Again
Step 2: Make the leaving group leave –
“spontaneous dissociation” occurs:
H
O
H
+
H
O
H
Dehydration of Alcohols - Again
Step 3: Form the pi bond, make an alkene
H
H
O
H
+ H O H
H
Find the beta-H on the more substituted
side (Zaitsev!)… use your water as a
Lewis Base and pull that H+ off, forming
a pi bond… Done!
Dehydration of Alcohols – And Again
Try one more example:
OH
H3PO4
heat
+ H3O+
Do the steps… and check that your arrows
and intermediates look like those you’re
about to see…
Dehydration of Alcohols – And Again
Step 1: Convert –OH to H2O
H
O
H+
H
O
H
Always draw the arrow from electron rich
(lone pairs!) to electron poor (positive
charge!)
Dehydration of Alcohols – And Again
Step 2: Spontaneous Dissociation…
H
O
H
+
Next step? Form the pi bond…
H
O
H
Dehydration of Alcohols – And Again
Step 3: Form the pi bond, make an alkene
H
H
O
H
+ H O H
H
Find the beta-H on the more substituted
side that has an H… use your water and
pull it off, forming a pi bond… Done!
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 catalyst!)
resulting in Markovnikov carbocation
formation
– Addition of H2O (this is where the OH is
coming from)
– 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
Acid-Catalyzed Hydration - Again
Try Again… Identify the mechanism –
alkene to alcohol, using acid and water…
H2O
H2SO4
OH
Acid-Catalyzed Hydration - Again
Step 1: React the pi bond with H+ and
form that carbocation:
H+
H
We don’t have to show this new H but
make sure you are drawing the
Markovnikov carbocation!
Acid-Catalyzed Hydration - Again
Step 2: Add the H2O
H
H
O
H
H
O
H
(the green H is still there, just didn’t
show it in the second structure)
Now - Finish it off… pull the extra
proton…
Acid-Catalyzed Hydration - Again
Step 3: Removal of extra proton (H+) to
finish formation of –OH.
H
O
H
H
O
H
H
O
+
H
O
H
H
Acid-Catalyzed Hydration – And Again
‘Third time’s the charm”… Try one more
example:
H2O
H2SO4
OH
+ H3O+
Do the steps… and check that your arrows
and intermediates look like those you’re
about to see…
Acid-Catalyzed Hydration – And Again
Step 1: React that nucleophilic pi bond
with the proton H+:
H+
H
We don’t have to show this new H but
make sure you are drawing the
Markovnikov carbocation!
Acid-Catalyzed Hydration – And Again
Step 2: Add the H2O
H
O
H
H
H
O
H
H
Finish it off… pull the extra proton…
Acid-Catalyzed Hydration – And Again
Step 3: Removal the extra proton (H+) to
finish formation of –OH.
H
O
H
H
O
H
OH
+
H
H O
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 Markovnikov carbocation intermediate.
– Attack on carbocation by 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 H-X - Again
Try it again… Identify the mechanism…
Adding an Br and an H to an alkene…
H-Br
Br
Addition of H-X - Again
Step 1: Reaction of pi bond with H+ (of HX), concurrent separation of X-, and
formation of carbocation intermediate.
H Br
H
+ Br
Addition of H-X - Again
Step 2: Attack of X- to finish formation
of product.
Br
H
Br
H
Addition of HX – Once Again
Try it again… Identify the mechanism…
Adding a chloride (and an H, not drawn
in) to an alkene…
H-Cl
Cl
Addition of HX – Once Again
Step 1: React the pi bond with H+ (of HX), separate off the X-, form the more
substituted carbocation intermediate.
H
H Cl
+
Cl
Addition of HX – Once Again
Step 2: Attack of X- to finish formation
of product.
H
Cl
Cl
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 (and loss
of X-)
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
Regardless, electrons must always flow
towards the positive charge…
Addition of X2 - Again
Identify this mechanism: Starts with
alkene, ends with two halides… Yes, this
reaction occurs “Anti”
Cl-Cl
Cl
Cl
Addition of X2 - Again
Step 1: Attack by pi bond on polarized XX with Halonium Ion formation
Cl-Cl
Cl
+ Cl
The halonium ion has to form on one face
or the other… whichever you want…
Addition of X2 - Again
Step 2: Attack of X- to pop open threemembered ring and finish formation of
product.
Cl
Cl
+ Cl
Cl
As with the last example, remember the
flow of electrons is towards the positive
charge…
Addition of X2 – And Again
Draw the mechanism steps for the
example below and then check to see if
you drew the arrows “flowing” correctly:
Br
Br-Br
Br
Addition of X2 – And Again
Step 1: Attack by pi bond on X-X followed
by Halonium Ion formation
Br
Br-Br
The halonium ion has to form on one face
or the other… random choice for “up” on
this problem…
Addition of X2 – And Again
Step 2: Attack of X- to pop open threemembered ring and finish formation of
product.
Br
Br
+ Br
Br
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 on more substituted side to
pop open three-membered 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
Markovnikov addition means the water is
attacking the more substituted side
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
Again, electron flow towards the positive
charge…
Addition of X2 and H2O - Again
Identify the mechanism – Starts with
alkene, adding an OH and an X…
Markovnikov…
Cl2, H2O
OH
Cl
Addition of X2 and H2O - Again
Step 1: Attack by pi bond on polarized XX with Halonium formation
Cl-Cl
Cl
+ Cl
Halonium ion forms on one face of the
alkene…
Addition of X2 and H2O - Again
Step 2: Attack by H2O to pop open threemembered ring.
H
Cl
H
O
H
Cl
O
H
Note the inversion on the methyl group
from the backside attack of the water…
Addition of X2 and H2O - Again
Step 3: Removal of extra proton (H+) by
X- to complete the formation of –OH.
H
H
O
Cl
H
+ Cl
O
+ H-Cl
Cl
Again, electron flow towards the positive
charge…
Formation of 3º halides from alcohols
Identify this mechanism – Starts with
alcohol, ends with a halide…
OH
H-Br
Br
Formation of 3º halides:
Steps Involved:
– Tertiary OH must turn into a better leaving
group by picking up a H+ to form water
– Loss of H2O and Carbocation formation
– Attack of halide to form tertiary halide
Formation of 3º halides:
Step 1: Convert –OH to H2O
H
O
H
H-Br
O H
+ X
Good leaving group can now leave…
Formation of 3º halides
Step 2: Loss of H2O (“spontaneous
dissociation”) to form carbocation
H
O H
+
H
O
H
Formation of 3º halides
Step 3: Halide attacks the carbocation
Br
+ Br
Formation of 3º Alcohols
Identify the mechanism – Starts with 3º
halide, ends with a 3º alcohol…
H2O
Br
OH
Of course, the E1 elimination happens
simultaneously but we’ll deal with that
separately.
Formation of 3º alcohols:
Steps Involved:
– Loss of Leaving group and carbocation
formation
– Attack by water
– Removal of extra proton to complete
formation of –OH group
Formation of 3º alcohols:
Step 1: Spontaneous dissociation of –Br to
form carbocation
+ Br
Br
Formation of 3º alcohols
Step 2: Addition of H2O
H
O
H
H
O
H
Formation of 3º alcohols
Step 3: Loss of extra proton to finish
formation of –OH group:
H
O
+ Br
H
+ HBr
OH
Again, flow of electrons towards positive
charge, not a direct attack of it…
E1 elimination of 3º alkyl halide
Identify the mechanism – Starts with 3º
halide, ends with an alkene, weak base…
H2O
Br
OH
Happening at the same time as the SN1 we
just looked at…
E1 Elimination of 3º halide
Steps Involved:
– Loss of Leaving group and carbocation
formation
– Attack by water on beta-H to form alkene
E1 Elimination of 3º halides
Step 1: Spontaneous dissociation of –Br to
form carbocation
+ Br
Br
E1 Elimination of 3º halides
Step 2: Addition of H2O to remove betaH and form pi bond
H
H
O
H
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… get a feel for how to
approach the problems… develop the
instinct…
Thanks for walking through yet another
really long power point… hope it helped…
let me know what you think… if there’s
something else that you think should be
added in, I’ll be happy to try to fill in
any other missing blanks…
Dr.Discordia
[email protected]