Alkenes' Thermodynamics and Kinetics Chapter 3

1
Alkenes. Thermodynamics and KineticsChapter 3

2
Contents of Chapter 3

• General Formulae and Nomenclature of Alkenes
• Cis-Trans Isomerism
• Reactivity Considerations
• Thermodynamics and Kinetics

3
General Molecular Formula for Alkenes

• General molecular formula for acyclic alkanes is
  CnH2n2

CH3CH2CH2CH2CH3
an alkane
C5H12 CnH2n2
4
General Molecular Formula for Alkenes

• Each ? bond introduced, reduces the H content by 2

CH3CH2CH2CHCH2
an alkene
C5H10 CnH2n
5
General Molecular Formula for Alkenes

• Each ring also reduces the H content by 2

a cyclic alkane
C5H10 CnH2n
6
General Molecular Formula for Alkenes

• Generalization The molecular formula for a
  hydrocarbon is CnH2n2 minus 2 hydrogens for
  every ? bond and/or ring present in the molecule
• Each ? bond or ring is considered a unit of
  unsaturation.

C5H8 CnH2n-2
a cyclic alkene with 2 units of unsaturation
7
Saturated and Unsaturated Hydrocarbons

• Alkanes or saturated hydrocarbons contain the
  maximum number of carbon-hydrogen bonds

CH3CH2CH2CH2CH3
a saturated hydrocarbon
8
Saturated and Unsaturated Hydrocarbons

• Alkenes contain fewer than the maximum number of
  carbon-hydrogen bonds and are therefore referred
  to as unsaturated hydrocarbons

CH3CH2CH2CHCH2
an unsaturated hydrocarbon
9
Nomenclature of Alkenes
IUPAC names of alkenes are based on the
corresponding alkane with ane replaced by ene
10
IUPAC Rules for Alkene Nomenclature

• The longest chain containing the functional group
  (the double bond) is numbered such that the
  double bond is the lowest possible number

11
IUPAC Rules for Alkene Nomenclature

• If there are substituents, the chain is still
  numbered in a direction that gives the double
  bond the lowest number

12
IUPAC Rules for Alkene Nomenclature

• If chain has more than one substituent, they are
  cited in alphabetical (not numerical) order.
• Rules for alphabetizing are the same as for
  alkanes

13
IUPAC Rules for Alkene Nomenclature

• If the same number for the double bond is
  obtained in both directions, number in the
  direction that gives lowest locant number to a
  substituent.

14
IUPAC Rules for Alkene Nomenclature

• In cyclic compounds, a number is not needed to
  denote the position of the functional group
• The double bond is assumed to be between carbons
  1 and 2

15
IUPAC Rules for Alkene Nomenclature

• If both directions yield same low number for a
  functional group and for one substituent, number
  in the direction which yields the lower number
  for one of the remaining substituents

16
IUPAC Rules for Alkene Nomenclature

• Two groups containing double bonds that are used
  as names for substituents are the vinyl group and
  the allyl group

17
IUPAC Rules for Alkene Nomenclature

• The sp2 carbons of an alkene are called vinylic

An sp3 adjacent carbon is called allylic
18
The E, Z System of Nomenclature
Which isomer is cis and which is trans?
A more definitive nomenclature is needed!
19
The E, Z System of Nomenclature

• First prioritize the groups bonded to the two sp2
  carbons
• If the higher priority group for each carbon is
  on the same side of the double bond, it is the Z
  isomer (for Zusammen, German for together)
• If the higher priority group for each carbon is
  on the opposite side of the double bond, it is
  the E isomer (for Entgegen, German for opposite)

20
The E, Z Prioritization Rules

• Relative priorities depend first on the atomic
  number of the atom (not the formula weight of the
  group) bonded to the sp2 carbon
• In the case of a tie, the atomic numbers of the
  atoms bonded to the tied atoms are considered
  next (e.g. C, C, H beats C, H, H)

21
The E, Z Prioritization Rules

• If an atom is doubly bonded to another atom, the
  system treats it as if it were bonded to two such
  atoms
• In the case of isotopes, the isotope with the
  greater mass number has the higher priority

22
Reactivity Considerations

• Electrophiles react with nucleophiles
• An alkene has electron density above and below
  the ? bond making it electron-rich and therefore
  a nucleophile
• Therefore alkenes react with electrophiles

23
Reaction Mechanisms

• We use curved arrows to indicate the movement of
  pairs of electrons as two molecules, ions or
  atoms interact

24
Reaction Mechanisms

• Curved arrows are drawn only from the
  electron-rich site to the electron deficient site

25
Reaction Mechanisms

• An arrow with a single barb represent the
  movement of a single unpaired electron

26
Thermodynamics

• When ?G is negative the reaction is exergonic

27
Thermodynamics

• When ?G is positive the reaction is endergonic

28
Kinetics

• Knowing the ?G of a reaction will not tell us
  how fast it will occur or if it will occur at all
• We need to know the rate of reaction
• The rate of a reaction is related to the height
  of the energy barrier for the reaction, ?G, the
  free energy of activation

29
Free Energy of Activation
30
Rate-Determining Step

• Formation of the carbocation intermediate is the
  slower of the two steps
• It is the rate-determining step

31
Rate-Determining Step

• Carbocation intermediates are consumed by bromide
  ions as fast as they are formed
• The rate of the overall reaction is determined by
  the slow first step

32
Transition States and Intermediates

• It is important to distinguish between a
  transition state and a reaction intermediate
• A transition state
• is a local maximum in the reaction coordinate
  diagram
• has partially formed and partially broken bonds
• has only fleeting existence

33
Transition States and Intermediates

• An intermediate
• is at a local minimum energy in the reaction
  coordinate diagram
• may be isolated in some cases