5 Organic Reactions to discuss

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5 Organic Reactions to discuss.

• Addition Reactions
• Oxidation/Reduction Reactions
• Substitution Reactions
• Esterification Reactions
• Polymerization Reactions
• There are thousands of other organic reactions
  we will not cover!

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1. Addition Reactions

• Must involve a double or triple bond an alkene
  or alkyne!
• As the name suggests, it involves adding a
  compound to the double bond
• Sometimes a catalyst is used to speed up the
  reaction this will be written over the arrow!

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What is the mechanism of these reactions?

• A mechanism visually shows the steps that the
  molecules follow in order to complete the
  reaction
• Lets look at a sample addition reaction..

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• This reaction is what we call stepwise it
  occurs in steps!
• H-Cl breaks into H1 and Cl-1 in solution
• It is an ionic compound with hydrogen acting as
  the metal
• The H1 attacks the double bond
• Why..?

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• The double bond is a great source of electrons!
• Opposites attract!
• This causes the electrons in the double bond to
  drop out and bond with the hydrogen!
• This leaves the other C positive why.?

1
6

• This carbon is missing a bond!
• The formal charge on the carbon on the right is a
  1
• This highly unstable atom is known as a
  carbocation
• This atom will react immediately
• Anything negative will attack it in this case,
  the chlorine!
• The new electrons for the bond come from the
  chlorine atom

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• All addition reactions occur this way
• Something positive (called the electrophile)
  attacks the double bond.
• A carbocation is formed
• Something negative attacks the carbocation
• The positive atom or molecule is called the
  electrophile because it is electron loving, or
  seeking! Opposites attract!

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• We can add a number of substances to double or
  triple bonds
• Halogens Cl2, Br2, I2, F2 can be added with a
  uv light catalyst
• The mechanism follows the same pattern.
• attack of the double bond by something positive
• Formation of a carbocation
• Attack of the carbocation by something negative
• Uv light acts as a catalyst to form two chlorine
  ions one positive, one negative

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• Eventually, both halogens add to the double bond
• This serves to increase the boiling point of the
  compound.
• Why?
• This is useful for solidifying hydrocarbons
• PVC is made this way

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Addition of bromine.
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• Alcohols can be made from water and an alkene or
  alkyne!

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• Water can be forced to break into H1 and OH-1 by
  the double bond.
• The H1 attacks first
• The carbocation forms, and the OH-1 attacks the
  carbocation
• The overall effect is to create an alcohol!

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Catalytic Hydrogenation

• The addition of hydrogen using a catalyst..
• This breaks the double bond, adding two
  hydrogens
• This has the effect of raising the boiling
  point..
• This is how margarine and other fats are created
  from vegetable oil

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• The platinum atom holds the H2 into place
• The hydrogens then split and attack the double
  bond in a similar manner as the other addition
  reactions
• Oils and fats have many double and triple bonds
• Some of these are broken, which is called
  partially hydrogenated
• This has the effect of raising the boiling point
  slightly, changing the substance from a liquid to
  a thicker liquid or even a solid
• The physical state is based on the amount of
  hydrogenation

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• Partial hydrogenation involves breaking some of
  these double or triple bonds
• This changes the texture of the fat, and creates
  trans-fatty acids
• These fats are unhealthy!

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Alkynes can undergo addition
Pt
Pt
In summary an addition reaction eliminates
double or triple bonds, and adds compounds or
elements!
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Does it matter what carbon the positive atom
attacks?

• The molecule in all previous examples, ethene, is
  symmetrical!
• It doesnt matter, because both carbons are
  identical!
• A Russian scientist named Markovnikov found that
  with other alkenes and alkynes that are not
  symmetrical, it does matter!

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• An addition reaction will occur
• Will the H1 attack carbon 1 or carbon 2?
• It matters
• Markovnikovs Rule states that the carbocation
  will form on the more substituted carbon
• What does that mean?
• Lets look at the two possible carbocations that
  can form!

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• Which will form.?
• The one that can be most stabilized will form!
• Carbocations are unstable and must be stabilized
  by.
• Electrons!
• Nearby C-H bonds that have electrons in them will
  flow over to stabilize the carbocation..
• This is called hyperconjugation!

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More hyperconjugation for stability
Less hyperconjugation for stability
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The carbocation (positive carbon) will form on
the atom that has more attached carbons, and
therefore more hyperconjugation for
stabilityThis is called Markovnikovs Rule. The
negative ion then attacks the carbocation
minor
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How about this reaction.?
Would you expect this.Why.?
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This forms instead! Why..?
The entire molecule rearranges to form a more
stable carbocation! This is called rearrangement
of the carbocation.
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The chlorine then attaches at carbon 2, not
carbon 3!
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2. Oxidation/Reduction Reactions

• Oxidation, by definition, can mean three things
• Gaining oxygen
• Losing hydrogen (dehydrogenation) and adding
  double or triple bonds
• Losing electrons
• Reduction, by definition, can mean three things
• Losing oxygen
• Gaining hydrogen (hydrogenation as in the
  addition reaction mentioned before!) and removing
  double or triple bonds
• Gaining electrons

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Yes, this is an addition reaction it is also a
reduction reaction, because it involves adding
hydrogens, and removing double and triple
bonds.hydrogenation
Yes, this is the reverse of an addition reaction
it is an oxidation reaction, because it
involves removing hydrogens, and adding double
and triple bondsdehydrogenation.
Pt
Pt
Notice the chemical written over the arrow this
is known as a catalyst
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• Oxidation reactions require a catalyst.
• Look for common catalysts in oxidation reactions
• K2Cr2O7
• H2SO4
• KMnO4
• Hydrocarbons follow a predictable pattern when
  oxidized
• An alcohol is produced then an aldehyde then
  a carboxylic acid and eventually water and
  carbon dioxide

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1. Substitution reactions.

• A substitution reaction involves one molecule or
  atom substituting another in a hydrocarbon!
• It has the general form

• Nu-1 is called a nucleophile. It means nucleus
  loving, or seeking! They like positive charge
  because they are negative!

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• Carbon atoms have a tetrahedral shape around
  them
• This is crucial in determining the type of
  substitution reaction you will have
• Two types
• SN1 substitution
• SN2 substitution
• Lets look at SN2 substitution first

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• What motivates the leaving group to actually
  leave.?
• The incoming nucleophile!
• The carbon, if it is going to bond to something
  new, has to eject something
• That something is called the leaving group!
• What type of atoms would leave?
• Ones that are highly electronegative, and would
  want to leave, pulling electrons off with it!
• In this case, the chlorine!
• Why is the nucleophile attacking from the back..?
• The chlorine is in the way in the front!
• The nucleophile cant attack from the front!

SN2 substitution.
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• The leaving group takes the electrons with it
• The incoming group repels the electrons that are
  being shared with the hydrogens on the carbon
• Like charges repel!
• This causes the atoms to reverse direction, or
  invert shape!
• This is called inversion of configuration.
• This whole process happens all at once, because
  the only thing that would promote the Cl leaving
  would be something coming in from the other side
  to bond with the carbon!
• This is called a concerted reaction meaning,
  all at once!

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• This reaction also occurs with backside attack,
  meaning that the nucleophile attacks from the
  backside of the molecule!
• It is called SN2 because two molecules must
  collide to promote the reaction!
• In summary, SN2 reactions occur
• Backside attack
• Concerted
• Inversion of configuration

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SN1 substitution

• This occurs when backside attack is not possible
• Why would backside attack not be possible?
• Too many carbons in the way!
• This only occurs when you have a tertiary carbon
  a carbon that is attached to three other
  carbons! ONLY THREE CARBONS CAN EFFECTIVELY
  BLOCK BACKSIDE ATTACK BACKSIDE ATTACK CAN OCCUR
  WITH ONE OR TWO CARBONS ATTACHED!
• Backside attack is still possible with a
  secondary carbon meaning, a carbon with two
  other carbon groups attached to it!

Tertiary carbon
Not possible!
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SN1 substitution

• So could frontside attack take place?
• NO!
• There is a chlorine in the front in the way!
• So we have to remove the chlorine

Must use something to remove the chlorine.
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SN1 substitution

• Normally, a solvent is used to remove the
  chlorine
• Ag1 works well to pull off the chlorine
• The molecule now has a trigonal planar geometry
  around it, allowing the OH-1 to attack from
  either side!
• However, a carbocation forms.!
• But arent carbocations UNSTABLE?

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• There are nearby carbon-hydrogen bonds to
  stabilize the carbon!
• This is why the carbocation can form
• What would have happened if we had a primary
  (meaning no or one carbon attached) carbon?
• There is no stabilization!
• The molecule would go back to square one
• No reaction.

No stabilization!
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• This is why primary carbons wont occur frontside
  attack
• The solvent would pull off the leaving group, but
  there wouldnt be enough nearby carbons to
  stabilize the carbocation
• Only secondary (2 carbons attached to the
  carbocation) or tertiary carbons can occur SN1.
• And you HAVE TO HAVE A SOLVENT TO PULL OF THE
  LEAVING GROUP!

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• Both products are formed in an SN1 substitution
• Both frontside and backside attack can take
  place
• This can either invert the shape (if backside
  occurs) or retain the shape (if frontside occurs)
• The reaction occurs in steps, or stepwise,where
  SN2 occurs all at once, or concerted
• Both products are produced 50-50!

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This reaction is called SN1 because the speed of
the reaction is dependent on only one step!
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SN1 versus SN2 substitution http//www.colby.edu/
chemistry/OChem/DEMOS/Substitution.html
SN1 versus SN2 substitution http//www.colby.edu/
chemistry/OChem/DEMOS/Substitution.html

• SN1 Substitution
• Must have a solvent to pull off leaving group
• Must have a secondary or tertiary carbon to
  stabilize the carbocation
• Can occur frontside or backside attack
• Can retain or invert configuration
• Occurs stepwise

• SN2 Substitution
• Occurs with primary or secondary carbons
• Only occurs backside attack
• Always inverts configuration
• Always occurs concertedly