Chapter 4 Alkenes

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Chapter 4 Alkenes Alkynes
Perception of light cis vs. trans
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Reaction of alkene
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Alkene Addition Reactions
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4.1 addition of HX to alkenes

• Hydrohalogenation

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4.2 Orientation of Alkene Addition Reaction

• Regiospecific Rxn.
• Markovnikov observed in the 19th century that in
  the addition of HX to alkene, the H attaches to
  the carbon with the most Hs and X attaches to
  the other end (to the one with the most alkyl
  substituents)
• This is Markovnikovs rule

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Example of Markovnikovs Rule

• Addition of HCl to 2-methylpropene
• Regiospecific
• If both ends have similar substitution, then not
  regiospecific

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Example of Markovnikovs Rule
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Example of Markovnikovs Rule
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Example of Markovnikovs Rule

• Mixture products

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Energy of Carbocations and Markovnikovs Rule

• More stable carbocation forms faster
• Tertiary cations and associated transition states
  are more stable than primary cations

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Mechanistic Source of Regiospecificity in
Addition Reactions

• If addition involves a carbocation intermediate
• and there are two possible ways to add
• the route producing the more alkyl substituted
  cationic center is lower in energy
• alkyl groups stabilize carbocation

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The basis of Markovnikovs rule
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Problem
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4.3 Carbocation Structure Sability

• Electronic structure of carbocation
• Stability

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Problem
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4.4 Addition of H2O

• Hydration of an alkene is the addition of H-OH
  to to give an alcohol
• Acid catalysts are used in high temperature
  industrial processes ethylene is converted to
  ethanol

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Mechanism of the acid-catalyzed hydration
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4.5 Addition of X2

• Halogenation

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The stereochemistry of the addition reaction of
Br2?anti stereochemistry

• Fig 4-1,2,3

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Mechanism
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4.6 Addition of H2

• Hydrogenation
• Syn. stereochemistry

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mechanism of alkene hydrogenation
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4.7 Oxidation of Alkenes Hydroxylation and
Cleavage
Hydroxylation in basic solution, addition of one
or more OH group to a molecule.
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Cleavage in acidic solution
No H on C CO One H on C COOH two Hs on C CO2
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Alkene Cleavage Ozone

• Ozone, O3, adds to alkenes to form molozonide
• Reduce molozonide to obtain ketones and/or
  aldehydes

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Examples of Ozonolysis of Alkenes
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Examples of Ozonolysis of Alkenes

• Cleavage products reveal an alkenes structure

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4.8 Alkene polymers

• A polymer is a very large molecule consisting of
  repeating units of simpler molecules, formed by
  polymerization
• Alkenes react with radical catalysts to undergo
  radical polymerization
• Ethylene is polymerized to poyethylene, for
  example

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Free Radical Polymerization Initiation

• Initiation - a few radicals are generated by the
  reaction of a molecule that readily forms
  radicals from a nonradical molecule
• A bond is broken homolytically

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Polymerization Propagation

• Radical from intiation adds to alkene to generate
  alkene derived radical
• This radical adds to another alkene, and so on
  many times

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Polymerization Termination

• Chain propagation ends when two radical chains
  combine
• Not controlled specifically but affected by
  reactivity and concentration

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PRACTICE PROBLEM 4.5
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4.9 Conjugate Dienes

• Conjugated dienes they are more than one double
  separated by only one single bond and their
  orbitals interact
• H2CCHCHCH2 H2CCHCH2CHCH2
• 1,3-Butadiene 1,4-Pentadien
• (conjugatedalternating (nonconjugated
  nonalterinating
• double and single bonds) double and single
  bonds)
• Conjugated dienes are somewhat more stable than
  nonconjugated dienes

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1. electrophilic addition
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2. 1,2-addition 1,4-addition
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Carbocations from Conjugated Dienes

• Addition of H leads to delocalized secondary
  allylic carbocation

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4.10 Stability

• why are conjugated dienes so stable?
• orbital hybridization
• p orbital overlap

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Stability of Allylic Carbocation

• Resonance

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The Diels-Alder Cycloaddition Reaction

• Conjugate dienes can combine with alkenes to form
  six-membered cyclic compounds
• The formation of the ring involves no
  intermediate (concerted formation of two bonds)
• Discovered by Otto Paul Hermann Diels and Kurt
  Alder in Germany in the 1930s

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Diels-Alder cycloaddition
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4.11 Drawing and Interpreting Resonance Forms
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• Resonance forms are imaginary
• Resonance forms differ only in the placement of
  their p or non-bonding electrons.

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Different resonance forms of a substance dont
have to be equivalent.

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Resonance forms must be valid Lewis structures
and obey normal rules of valency.

Resonance leads to stability.

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4.12 Alkynes Reactions

• Naming Alkynes
• General hydrocarbon rules apply wuith -yne as a
  suffix indicating an alkyne

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1. Reduction of Alkynes

• Addition of H2 using chemically deactivated
  palladium on calcium carbonate as a catalyst (the
  Lindlar catalyst) produces a cis alkene
• The two hydrogens add syn (from the same side of
  the triple bond)

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2. Reactions of Alkynes Addition of HX and X2

• Addition reactions of alkynes are similar to
  those of alkenes
• Intermediate alkene reacts further with excess
  reagent
• Regiospecificity according to Markovnikov

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Addition of Bromine and Chlorine

• Initial addition gives trans intermediate
• Product with excess reagent is tetrahalide

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3. Hydration of Alkynes

• Alkynes do not react with aqueous protic acids
• Mercuric ion (as the sulfate) is a Lewis acid
  catalyst that promotes addition of water in
  Markovnikov orientation
• The immediate product is a vinylic alcohol, or
  enol, which spontaneously transforms to a ketone

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Keto-enol

• Enols rearrange to the isomeric ketone by the
  rapid transfer of a proton from the hydroxyl to
  the alkene carbon
• The keto form is usually so stable compared to
  the enol that only the keto form can be observed

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Hydration of Unsymmetrical Alkynes

• If the alkyl groups at either end of the C-C
  triple bond are not the same, both products can
  form and this is not normally useful
• If the triple bond is at the first carbon of the
  chain (then H is what is attached to one side)
  this is called a terminal alkyne
• Hydration of a terminal always gives the methyl
  ketone, which is useful

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4. Oxidative Cleavage of Alkynes

• Strong oxidizing reagents (O3 or KMnO4) cleave
  internal alkynes, producing two carboxylic acids
• Terminal alkynes are oxidized to a carboxylic
  acid and carbon dioxide
• Neither process is useful in modern synthesis
  were used to elucidate structures because the
  products indicate the structure of the alkyne
  precursor

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5. Alkyne Acidity Formation of Acetylide Anions

• Terminal alkynes are weak Brønsted acids (alkenes
  and alkanes are much less acidic (pKa 25)
• Reaction of strong anhydrous bases with a
  terminal acetylene produces an acetylide ion

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Alkylation of Acetylide Anions

• Acetylide ions can react as nucleophiles as well
  as bases
• Reaction with a primary alkyl halide produces a
  hydrocarbon that contains carbons from both
  partners, providing a general route to larger
  alkynes