1. Alkenes : Structure and Reactivity

2. Alkene - Hydrocarbon With Carbon-Carbon Double Bond
Includes many naturally occurring materials (beta carotene)
The general formula of an alkene with 1 double bond is CnH2n

3. 6.2 Degree of Unsaturation
Relates molecular formula to possible structures
Degree of unsaturation: number of multiple bonds or rings
Formula for saturated a acyclic compound is CnH2n+2
Each ring or multiple bond replaces 2 H's

4. Summary - Degree of Unsaturation

5. Example: C6H10 Saturated is C6H14 This has two degrees of unsaturation
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

6. Degree of Unsaturation With Other Elements
Organohalogens (X: F, Cl, Br, I)
Halogen replaces hydrogen
C4H6Br2 and C4H8 have one degree of unsaturation
Oxygen atoms - if connected by single bonds
These don't affect the total count of H's
C4H6O3 should be C4H8O3

7. Degree of Unsaturation and Variation
Compounds with the same degree of unsaturation can have many things in common and still be very different

8. If C-N Bonds Are Present
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

9. Geometric Isomerism in Alkenes
Geometric Isomers or Stereoisomers are isomers that differ only in the spatial orientation of different atoms or groups of atoms. They do not differ in the connectivity.
Alkenes exhibit geometric isomerism when the two atoms or groups of atoms attached to each of the 2 C atoms of the double bond are different.
When a H atom attached to each of the 2 C atoms of ethylene is substituted by a different atom or groups of atoms, a disubstituted alkene is obtained.
When 3 of the 4 H atoms of ethylene is substituted a different atom or groups of atoms, a trisubstituted alkene is obtained.
When all of the 4 H atoms of ethylene is substituted a different atom or groups of atoms, a tetraisubstituted alkene is obtained. .
In a disubstituted alkene, when the 2 substituents are on the same C of the double bond, it is called a geminal disubstituted alkene
Two geometric isomers (cis isomer & trans isomer) exist for disubstituted alkenes with substituents on both C’s
The cis isomer has H atoms on the same side of the double bond while the trans isomer has H atoms on opposite sides of the double bond.

10. Disubstituted alkenes areclassified as being geminal or cis or trans.
But for trisubstituted and tetrasubstituted alkenes, the terms cis and trans are replaced by Z (instead of cis) and E (instead of trans) respectively.
The E isomer has SUBSTITUTENTS OF THE SAME PRIORITY on OPPOSITE SIDES of the double bond
The Z isomer has SUBSTITUTENTS OF THE SAME PRIORITY on the SAME SIDE of the double bond
The Cahn Ingold Prelog rules are used for assigning priorities for the substituents attached to the double bond Carbons. One substituent is assigned the lower priority and the other higher priority.
The E isomer has groups of lower priority (and hence those of higher priority as well) on opposite sides of the double bond.
The Z isomer has groups of lower priority (and hence those of higher priority as well) on the same side of the double bond.

11. Cahn-Ingold-Prelog Rules for Naming Geometric Isomers –
E and Z isomers
We first examine the immediate atoms attached to the C atoms of the double bond. Higher the atomic number, higher the priority.
If the atoms directly attached to the double bond C are identical, examine the 2nd or the 3rd or the 4th and so on atoms until a difference is found. The substituent carrying the higher atomic number is assigned the higher priority.
Multiple bonded atoms are treated as the same number of single-bonded atoms. That is, C=C is treated as if each C atom is bonded to 2 C atoms and CC is treated as if each C atom is attached to 3 C atoms. Therefore a triple bond substituent is assigned higher priority than a double bond substituent and a double bond substituent is assigned higher priority than a single bond substituent

12. 6.3 Naming of Alkenes
Find longest continuous carbon chain containing the double bond – parent chain. The name of this chain is similar to the name of the corresponding alkane with the same number of C atoms but with ending being ‘ene’ instead of ane. The position of the double bond is specified by the first C of the double bond encountered while numbering from end closest to double bond.
2. When there is more than one double bond in the compound, the ending diene , triene, tetraene etc are used. Numbers are required for all the double bonds
3. Number carbons in chain so that double bond carbons have lowest possible numbers.
4. When the double bond is in a cyclic alkene, the ring is numbered so that the double bond is between Carbons 1 and 2. Numbering is then clockwise or
counterclockwise to assign lowest numbers to substituents.
5. Name substituents as you did for the alkanes and specify the numbers of the C atoms on the parent chain to which they are attached.
6. If alkene can exhibit geometric isomerism, use prefixes E or Z to specify arrangement of substituents relative to double bonds. If more than 1 double bond is present, the stereochemistry about each double bond should be specified.

13. Endocyclic double bonds have both carbons in the ring and exocyclic double bonds have only one carbon as part of the ring.
Methylene cyclopentane & cyclohexene are constitutional isomers with the molecular formula C6H10.
Methylene cyclopentane has an exocyclic double bond while cyclohexene has an endocyclic double bond.
Methylene cyclopentane:
Additional substituents to remember:
Methylene: =CH2
Vinyl: -CH=CH2
Allyl: -CH2CH=CH2

15. 6.6 Sequence Rules: The E,Z Designation

16. E,Z Stereochemical Nomenclature
Priority rules of Cahn, Ingold, and Prelog
Compare where higher priority group is with respect to bond and designate as prefix
E -entgegen, opposite sides
Z - zusammen, together on the same side

17. Extended Comparison
If atomic numbers are the same, compare at next connection point at same distance
Compare until something has higher atomic number
Do not combine – always compare

18. Dealing With Multiple Bonds
Substituent is drawn with connections shown and no double or triple bonds
Added atoms are valued with 0 ligands themselves

19. Alkene Stability
Zaitsev’s rule or Saytzeff’s rule: More substituted double bonds are more stable.
Therefore, Tetrasubstituted > Trisubstituted > geminal disubstituted, trans disubstituted > cis disubstituted > monosubstituted > ethylene
One reason for this in the number of sp3C-sp2 C bonds. The more sp3C-sp2 C bonds, the more stable the alkene. The more substituted alkenes have more sp3C-sp2 C bonds. A sp3C-sp2 C bond is stronger than a sp3C-sp3 C bond.
The other reason is the greater degree of Hyperconjugation in more substituted alkenes. Hyperconjugation is the overlap between the C=C pi bond and adjacent C-H sigma bonds.
The more stable the alkene, the less exothermic its heat of hydrogenation.
The heat of hydrogenation is the heat released per mole of H2 added to form the corresponding alkane.

20. 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

21. Hyperconjugation
Electrons in neighboring filled  orbital stabilize vacant antibonding  orbital – net positive interaction
Alkyl groups are better than H

22. 6.8 Electrophilic Addition of HX to Alkenes
General reaction mechanism: electrophilic addition
Attack of electrophile (such as HBr) on  bond of alkene
Produces carbocation and bromide ion
Carbocation is an electrophile, reacting with nucleophilic bromide ion

23. Electrophilic Addition Energy Path
Two step process
First transition state is high energy point

24. Example of Electrophilic Addition
Addition of hydrogen bromide to 2-Methyl-propene
H-Br transfers proton to C=C
Forms carbocation intermediate
More stable cation forms
Bromide adds to carbocation

25. Electrophilic Addition for preparations
The reaction is successful with HCl and with HI as well as HBr
Note that HI is generated from KI and phosphoric acid

26. 6.9 Orientation of Electrophilic Addition: 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 regiospecific
Markovnikov observed in the 19th century that in the addition of HX to alkene, the H attaches to the carbon with the most H’s and X attaches to the other end (to the one with the most alkyl substituents)
This is Markovnikov’s rule

27. 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

28. Energy of Carbocations and Markovnikov’s Rule
More stable carbocation forms faster
Tertiary cations and associated transition states are more stable than primary cations

29. 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

30. 6.10 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
Therefore stability of carbocations: 3º > 2º > 1º > +CH3

31. Regioselectivity of Electrophilic Addition Reactions and The Hammond Postulate
The transition state is more similar in structure to the species to which it is more similar in energy.
In an exergonic reaction, the transition state is more similar to the reactants and in an endergonic reaction, it is more simlar to the products.
The formation of cations in the electrophilic addition to alkenes is an endergonic process. The more stable carbocation will have a lower free energy.
The formation of the corresponding transition state will have lower activation energy Therefore the more stable carbocation will be formed faster than the less stable carbocation.
In an electrophilic addition reaction, the electrophile ( in the case of HX, electrophile = H+) will add to the less substituted C of a double bond which will result in the formation of the more substituted and more stable carbocation intermediate: Markvnikov’s rule.
A reaction in which the formation of one constitutional isomer forms preferentially is called a regioselective reaction.

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

33. 6.12 Mechanism of Electrophilic Addition: Rearrangements of Carbocations
Carbocations undergo structural rearrangements following set patterns
1,2-H and 1,2-alkyl shifts occur
Goes to give more stable carbocation
Can go through less stable ions as intermediates