1. Chapter 7 Alkenes: Structure and Reactivity

2. 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 CnH2n+2
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

3. 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?

4. Degree of Unsaturation With Other Elements
Organohalogens (X: F, Cl, Br, I)
Halogen replaces hydrogen
C4H6Br2 and C4H8 have one degree of unsaturation

5. 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

6. 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

7. Summary - Degree of Unsaturation
Count pairs of H's below CnH2n+2
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

8. 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).

9. 7.6 Stability of Alkenes
Cis alkenes are less stable than trans alkenes
Less stable isomer is higher in energy

10. Stability of Alkenes (Continued): Comparing Stabilities of Alkenes
Evaluate heat given off when C=C is converted to C-C
More stable alkene gives off less heat
trans-Butene generates 5 kJ less heat than cis-butene

11. 7.6 Stability of Alkenes Less stable isomer is higher in energy
tetrasubstituted > trisubstituted > disubstituted > monosusbtituted

12. Hydrogenation Data Helps to Determine Stability

13. 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

14. 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

15. 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

16. 7.8 Orientation of Electrophilic Additions: Markovnikov’s 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 H’s and X attaches to the other end (to the one with more alkyl substituents)
This is Markovnikov’s rule

17. Example of Markovnikov’s Rule
Addition of HCl to 2-methylpropene
Regiospecific – one product forms where two are possible
If both ends have similar substitution, then not regiospecific

18. Markovnikov’s 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

19. Markovnikov’s Rule (restated)

20. 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.

21. 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

22. 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

23. Carbocation Structure and Stability (Continued)
A inductive stabilized cation species

24. Competing Reactions and the Hammond Postulate
Normal Expectation: Faster reaction gives more stable intermediate
Intermediate resembles transition state

25. 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

26. 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

27. 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

28. 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

29. 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

30. 2.6 Drawing Resonance Forms
Any three-atom grouping with a p orbital on each atom has two resonance forms

31. 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

32. 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 C=O bond on the left and on the right
Three resonance structures result