Chapter 7 Alkenes: Structure and Reactivity

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Chapter 7Alkenes Structure and Reactivity

• Chapter 7Alkenes Structure and Reactivity

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7.2 Calculating Degree of Unsaturation

• Relates molecular formula to possible structures
• Degree of unsaturation number of multiple bonds
  or rings
• Formula for a saturated acyclic compound is
  CnH2n2
• Alkene has fewer hydrogens than an alkane with
  the same number of carbons CnH2n because of
  double bond
• Each ring or multiple bond replaces 2 H's

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Example C6H10

• Saturated is C6H14
• therefore 4 H's are not present
• This has two degrees of unsaturation
• Two double bonds?
• or triple bond?
• or two rings?
• or ring and double bond?

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Degree of Unsaturation With Other Elements

• Organohalogens (X F, Cl, Br, I)
• Halogen replaces hydrogen
• C4H6Br2 and C4H8 have one degree of unsaturation

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Degree of Unsaturation (Continued)

• Organoxygen compounds (C,H,O) Oxygen forms 2
  bonds
• these don't affect the formula of equivalent
  hydrocarbons
• May be ignored in calculating degrees of
  unsaturation

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Organonitrogen Compounds

• Nitrogen has three bonds
• So if it connects where H was, it adds a
  connection point
• Subtract one H for equivalent degree of
  unsaturation in hydrocarbon

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Summary - Degree of Unsaturation

• Count pairs of H's below CnH2n2
• Add number of halogens to number of H's (X
  equivalent to H)
• Ignore oxygens (oxygen links H)
• Subtract N's - they have two connections

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Cis-Trans Isomerism in Alkenes

• Rotation of ? bond is prohibitive
• This prevents rotation about a carbon-carbon
  double bond (unlike a carbon-carbon single bond).

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7.6 Stability of Alkenes

• Cis alkenes are less stable than trans alkenes
• Less stable isomer is higher in energy

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Stability of Alkenes (Continued) Comparing
Stabilities of Alkenes

• Evaluate heat given off when CC is converted to
  C-C
• More stable alkene gives off less heat
• trans-Butene generates 5 kJ less heat than
  cis-butene

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7.6 Stability of Alkenes

• Less stable isomer is higher in energy
• tetrasubstituted gt trisubstituted gt disubstituted
  gt monosusbtituted

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Hydrogenation Data Helps to Determine Stability
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7.7 Electrophilic Addition of Alkenes

• General reaction mechanism of electrophilic
  addition
• Attack on electrophile (such as HBr) by ? bond of
  alkene
• Produces carbocation and bromide ion
• Carbocation is an electrophile, reacting with
  nucleophilic bromide ion

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Electrophilic Addition of Alkenes (Continued)
Electrophilic Addition Energy Path

• Two step process
• First transition state is high energy point
• First step is slower than second

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Electrophilic Addition of Alkenes (Continued)

• The reaction is successful with HCl and with HI
  as well as HBr
• HI is generated from KI and phosphoric acid

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7.8 Orientation of Electrophilic Additions
Markovnikovs Rule

• In an unsymmetrical alkene, HX reagents can add
  in two different ways, but one way may be
  preferred over the other
• If one orientation predominates, the reaction is
  regioselective
• Markovnikov observed in the 19th century that in
  the addition of HX to alkene, the H attaches to
  the carbon with more Hs and X attaches to the
  other end (to the one with more alkyl
  substituents)
• This is Markovnikovs rule

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Example of Markovnikovs Rule

• Addition of HCl to 2-methylpropene
• Regiospecific one product forms where two are
  possible
• If both ends have similar substitution, then not
  regiospecific

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Markovnikovs Rule (restated)

• More highly substituted carbocation forms as
  intermediate rather than less highly substituted
  one
• Tertiary cations and associated transition states
  are more stable than primary cations

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Markovnikovs Rule (restated)
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Definitions

• Regioisomers two constitutional isomers that
  could result from an addition reaction.
• Regioselective both regioisomers are formed,
  but one is formed in preference.
• Regiospecific only one regiosisomer forms at
  the expense of the other.

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7.9 Carbocation Structure and Stability

• Carbocations are planar and the tricoordinate
  carbon is surrounded by only 6 electrons in sp2
  orbitals
• the fourth orbital on carbon is a vacant
  p-orbital
• the stability of the carbocation (measured by
  energy needed to form it from R-X) is increased
  by the presence of alkyl substituents

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Carbocation Structure and Stability (Continued)

• A plot of DH dissociation shows that more highly
  substitued alkyl halides dissociate more easily
  than less highly substituted ones

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Carbocation Structure and Stability (Continued)

• A inductive stabilized cation species

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Competing Reactions and the Hammond Postulate

• Normal Expectation Faster reaction gives more
  stable intermediate
• Intermediate resembles transition state

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7.11 Evidence for the Mechanism of Electrophilic
Addition Carbocation Rearrangments

• Carbocations undergo structural rearrangements
  following set patterns
• 1,2-H and 1,2-alkyl shifts occur
• Goes to give most stable carbocation

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2.4 Resonance

• Some molecules are have structures that cannot be
  shown with a single representation
• In these cases we draw structures that contribute
  to the final structure but which differ in the
  position of the ? bond(s) or
• lone pair(s)
• Such a structure is delocalized and is
  represented by resonance forms
• The resonance forms are connected by a
  double-headed arrow

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Resonance Hybrids

• A structure with resonance forms does not
  alternate between the forms
• Instead, it is a hybrid of the resonance forms,
  so the structure is called a resonance hybrid
• For example, benzene (C6H6) has two resonance
  forms with alternating double and single bonds
• In the resonance hybrid, the actual structure,
  all its C-C bonds
• are equivalent, midway between double and single

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2.5 Rules for Resonance Forms

• Individual resonance forms are imaginary - the
  real structure is a hybrid (only by knowing the
  contributors can you visualize the actual
  structure)
• Resonance forms differ only in the placement of
  their ? or nonbonding electrons
• Different resonance forms of a substance do not
  have to be equivalent
• Resonance forms must be valid Lewis structures
  the octet rule generally applies
• The resonance hybrid is more stable than any
  individual resonance form would be

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Curved Arrows and Resonance Forms

• We can imagine that electrons move in pairs to
  convert from one resonance form to another
• A curved arrow shows that a pair of electrons
  moves from the atom or bond at the tail of the
  arrow to the atom or bond at the head of the arrow

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2.6 Drawing Resonance Forms

• Any three-atom grouping with a p orbital on each
  atom has two resonance forms

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Different Atoms in Resonance Forms

• Sometimes resonance forms involve different atom
  types as well as locations
• The resulting resonance hybrid has properties
  associated with both types of contributors
• The types may contribute unequally
• The enolate derived from acetone is a good
  illustration, with delocalization between carbon
  and oxygen

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2,4-Pentanedione

• The anion derived from 2,4-pentanedione
• Lone pair of electrons and a formal negative
  charge on the central carbon atom, next to a CO
  bond on the left and on the right
• Three resonance structures result