Organische Chemie 1B

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Organische Chemie 1B

• Prof. dr. Floris Rutjes

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Organische Chemie 1B
Docent Prof. dr. Floris Rutjes Werkcollege
assistenten Pieter de Witte Bas
Gruijters UL 354 Tel. 53202, rutjes_at_sci.kun.nl B
oek Maitland Jones, Jr., Organic Chemistry, 2nd
ed Aanbevolen Student Version met antwoorden op
vragen uit het boek
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Organische Chemie 1B

• Hoor- en werkcolleges worden door elkaar heen
  gegeven
• College wordt gegeven met PP slides
• PP files worden uitgereikt als dictaat
• Een deel zelfstudie vereist (oefenen van opgaven)
• Te lezen op http//www.sci.kun.nl/chemistry/index
  _nl.html
• Belangrijke mededelingen over dit college zijn
  hier te vinden

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Topics in OC 1B

• Reactions with alkenes (Chapter 8 and 9)
• Reactions with alkynes (Chapter 9)
• Reactions with aromatic compounds (Chapter 11, 12
  and 13)
• Radical reactions (Chapter 10)

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Electronegativity

• The electronegativity increases from left to
  right and from bottom to top in the periodic
  table
• A difference in electronegativity creates a
  dipole, or in other words an electrophilic and a
  nucleophilic part

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The arrow formalism
This is what you have always learned.
This is how you must write it from now on !

• Extremely important
• the arrows represent the flow of electrons !

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Stability of cations

• The stability of the cation increases with the
  presence of additional alkyl sbstituents

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Why is Et more stable than Me ?

• The reason is called hyperconjugation overlap of
  one of the CH-orbitals with the empty 2p orbital.

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Review of resonance (p 317-322)
Make problems 8.31, 8.34 and 8.35.
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The allylic cation (orbitals)
The delocalization can also be explained by
considering the orbital overlap
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Inductive effects

• Inductive effects the influence of
  electron-withdrawing or electron-donating groups
• Inductive effects take place via the CC-bonds

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Inductive vs. resonance effects
Rule of thumb resonance effects are stronger
than inductive effects
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Substitution reactions (SN2)

• SN2 direct substitution of the leaving group by
  the nucleophile
• Inversion of configuration
• Especially with primary and secondary carbon atoms

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Substitution reactions (SN1)

• SN1 substitution proceeds via an intermediate
  carbocation
• Stereochemical information is lost
• Especially at stabilized carbocations, i.e.
  tertiary carbocations or carbocations stabilized
  by resonance structures

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Elimination reactions (E2)

• E2 elimination takes place in a synchronous
  fashion, without the intermediacy of carbocations
• The substituents must be anti or
  anti-periplanar with respect to each other

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Elimination reactions (E1)

• E1 elimination proceeds via the formation of a
  carbocation
• Occurs especially when a stable cation (tertiary,
  or stabilized by resonance) can be formed

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Alkenes in Nature

• Fatty acids

• Terpenes

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Alkene-containing amino acids
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Addition reactions to alkenes
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Nucleophiles
Species N?C HS I HO Br N3 NH3 Cl F CH3O
H H2O
Species cyanide thiolate iodide hydroxide bromid
e azide ammonia chloride fluoride methanol water
Relative nucleophilicity 126,000 126,000 80,000
16,000 10,000 8,000 8,000 1,000 80 1 1
Excellent nucleophiles
Good nucleophiles
Fair nucleophiles
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Other types of nucleophiles

• Nucleophile an atom (or group of atoms) that
  contains free electron pairs such as heteroatoms
• A better definition is an atom (or group of
  atoms) that contains delocalizable electrons
• In other words p-bonds can also act as
  nucleophiles

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An example
Olefins are very weak nucleophiles therefore, a
strong electrophile needs to be present (in this
case H) A relatively stable secondary
carbocation is formed, which is trapped by the
chloride nucleophile
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The carbocation is planar

• The intermediate secondary carbocation is
  sp2-hybridized

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The energy profile
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Question
Which product do you expect for the following
reaction (give the mechanism) ?
1-pentene
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Stability of cations
Which product will be formed in the following
reaction ?
In other words which cation is more stable ?
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Stability of the cation (II)
Explain the outcome of the following reaction
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The allylic cation
An allylic cation is relatively stable due to the
delocalization of the positive charge.
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The rule of Markovnikov in addition reactions
the nucleophile ends up at the more substituted
carbon atom
The Markovnikov Rule
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Problems

• Make problems 8.36, 8.38, 8.41

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The addition of H2O
In the presence of a strong acid (e.g. H2SO4) and
water, the addition reaction also takes place.
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Another example isobutene
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Polymerization
In the presence of only a catalytic amount of H
and a high concentration of the olefin,
polymerization takes place.
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Question
Give the product of the following reaction
styrene
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Anti-Markovnikov addition of H2O

• Anti-Markovnikov addition of water is achieved
  via a hydroboration reaction, i.e. addition of
  BH3 to the double bond.

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What does BH3 look like ?
BH3 exists as a dimer
It is usually sold as a stable complex with THF
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The mechanism (I)
The nucleophilic alkene attacks the electron poor
boron atom
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The mechanism (II)
Hydroboration selective syn-addition to the
double bond !
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The mechanism (III)
The anti-Markovnikov outcome is largely explained
by steric interactions
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BH3 can add to more olefins
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Steric effects determine the outcome
The more substituted the olefin, the less olefins
will fit on one boron atom
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Conversion of B into OH
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The overall result
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Cation rearrangements
How can this outcome be explained?
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A hydride-shift occurs
The molecule rearranges to the thermodynamically
more stable tertiary carbocation via a
hydride-shift
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How to explain this result?
Hint AgI is insoluble in water
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A methyl-shift occurs
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Problems

• Make problems 8.44, 8.45, 8.47, 8.48