1. Chapter 6 Alkenes and Alkynes

2. Alkenes and Alkynes Alkene (or “olefin”) Alkyne
Hydrocarbon that contains a carbon-carbon double bond
Present in most organic and biological molecules
β-Carotene
Alkyne
Hydrocarbon that contains a carbon-carbon triple bond
Rarely occur in biological molecules or pathways

3. 6.1 Calculating a Degree of Unsaturation
Because of its double bond an alkene has fewer hydrogens than an alkane with the same number of carbons
Alkane Alkene
CnH2n+2 CnH2n
C2H6 C2H4
(Saturated) (Unsaturated)

4. Calculating a Degree of Unsaturation
Number of rings and/or multiple bonds present in the molecule
Unknown hydrocarbon of formula C6H10with molecular weight of 82 has two fewer pairs of hydrogens (H14 – H10 = H4 = 2H2) than a saturated hydrocarbon
Degree of unsaturation is two
Possible structures for unknown

5. Calculating a Degree of Unsaturation
Calculations for compounds containing other elements in addition to carbon and hydrogen:
Organohalogen compounds (C,H,X, where X = F, Cl, Br, or I)
Add number of halogens and hydrogens to arrive at an equivalent hydrocarbon formula

6. Calculating a Degree of Unsaturation
Organooxygen compounds (C,H,O) oxygen does not affect the formula of an equivalent hydrocarbon.

7. Calculating a Degree of Unsaturation
Organonitrogen compounds (C,H,N)
Has one more hydrogen than a related hydrocarbon
Must subtract the number of nitrogens from the number of hydrogens for equivalent hydrocarbon formula

8. 6.2 Naming Alkenes and Alkynes
Similar to the naming rules for alkanes
Step 1 - Name the parent hydrocarbon
Find longest carbon chain containing the double bond
Step 2 – Number the carbon atoms in the chain
Double-bond carbons should receive lowest possible numbers

9. Naming Alkenes and Alkynes
Step 3 - Write the full name using suffix -ene in place of -ane
If more than one double bond indicate position and use suffixes -diene, -triene, and so on

10. Naming Alkenes and Alkynes
Cycloalkenes
Number cycloalkene so double bond is between C1 and C2 and first substituent has lowest number possible

11. Naming Alkenes and Alkynes
Common names that are often used and are recognized by IUPAC:

12. Naming Alkenes and Alkynes
Named just like alkenes using suffix -yne
Number main chain so triple bond receives as low a number as possible
Alkyl, alkenyl, and alkynyl groups:

13. 6.3 Cis-Trans Isomerism in Alkenes
Carbon-carbon double bond description
Valence bond language
Carbons are sp2 hybridized
Three equivalent hybrid orbitals that lie in a plane at angles of º to one another
Carbons form a σ bond by head-on overlap of sp2 orbitals and a p bond by sideways overlap of unhybridized p orbitals oriented perpendicular to sp2 plane

14. Cis-Trans Isomerism in Alkenes
Molecular orbital language
Interaction between p orbitals leads to one bonding and one antibonding p molecular orbital
p bonding MO contains no node between nuclei
p antibonding MO contains a node between nuclei resulting from combination of lobes with different algebraic signs

15. Cis-Trans Isomerism in Alkenes
Free rotation is not possible around a double bond
The barrier to double bond rotation must be at least as great as the strength of the p bond itself (~ 350 kJ/mol)

16. Cis-Trans Isomerism in Alkenes
Disubstituted alkene
Two substituents other than hydrogen are attached to the double-bond carbons
But-2-ene
Two isomers cannot interconvert spontaneously
Methyl groups are cis- or trans- to each other
Bond rotation cannot occur – the two but-2-enes are cis-trans stereoisomers

17. Cis-Trans Isomerism in Alkenes
Cis-trans isomerism occurs when both double-bond carbons are bonded to two different groups
Carbons bonded to two identical groups cannot exist as cis-trans isomers

18. 6.4 Sequence Rules: The E,Z Designation
E,Z system
Sequence rules used to assign priorities to the substituent groups on the double-bond carbons (alkenes)
Z – for German zusammen “together”
High priority groups on each carbon are on same side of the double-bond
E – for German entgegen “opposite”
High priority groups on each carbon are on opposite sides of the double-bond

19. Sequence Rules: The E,Z Designation
Cahn-Ingold-Prelog rules
Rule 1 – look at the two atoms directly attached to each double-bond carbon and rank them according to atomic number

20. Sequence Rules: The E,Z Designation
Rule 2 – If a decision cannot be reached by ranking the first atom in the substituent, look at the second, third, and fourth atoms away from the double-bond carbons until the first difference is found

21. Sequence Rules: The E,Z Designation
Rule 3 – multiple bonded atoms are equivalent to the same number of single-bonded atoms

22. Worked Example 6.1: Assign E and Z Configurations to Substituted Alkenes
Assign E or Z Configuration to the double bond in the following compound:

23. Worked Example 6.1: Assign E and Z Configurations to Substituted Alkenes
Strategy
Look at the two substituents connected to each double-bond carbon, and determine their priorities using the Cahn-Ingold-Prelog rules. Then see whether the two high-priority groups are on the same or opposite sides of the double bond.

24. Worked Example 6.1: Assign E and Z Configurations to Substituted Alkenes
Solution

25. 6.5 Stability of Alkenes Stability of cis- and trans isomers
Interconversion does not occur spontaneously
Interconversion can be brought about by treating the alkene with a strong acid
Cis isomers are less stable than trans isomers because of the steric strain between the two larger substituents on the same side of the double-bond

26. Stability of Alkenes
Determining relative stabilities of alkene isomers
Establish a cis-trans equilibrium through treatment with strong acid
Hydrogenate alkene through a hydrogenation reaction to give corresponding alkane using H2 gas in the presence of a catalyst such as palladium or platinum

27. Stability of Alkenes
Energy Diagram for Hydrogenation of cis- and trans- But-2-ene
Cis isomer is higher energy than the trans isomer
More energy released in the hydrogenation of the cis isomer than the trans isomer

28. Stability of Alkenes
Alkenes become more stable with increasing substitution
Stability order due to two factors:
Hyperconjugation
Stabilizing interaction between the C=C p bond and adjacent C-H σ bonds on substituents
The more substituents there are the greater the stabilization of the alkene

29. Stability of Alkenes Bond strength
A bond between an sp2 carbon and an sp3 carbon is somewhat stronger than a bond between two sp3 carbons

30. 6.6 Electrophilic Addition Reactions of Alkenes
Carbon-carbon double bonds behave as nucleophiles (Lewis bases) in polar reactions by donating a pair of electrons to an electrophile (Lewis acid)
Successful with not only with H2O but also with HBr, HCl, and HI (addition reactions of halogen acids is not common in living organisms)

31. Electrophilic Addition Reactions of Alkenes
Mechanism of the acid-catalyzed electrophilic addition of H2O to 2-methylpropene to give the alcohol 2-methylpropan-2-ol

32. Electrophilic Addition Reactions of Alkenes
Alkynes also undergo electrophilic addition reactions
Reactivity is substantially less than that of alkenes

33. 6.7 Orientation of Electrophilic Addition: Markovnikov’s Rule
Regiospecific reactions - reactions where only one of two possible orientations occurs
Markovnikov’s Rule (Vladimir Markovnivov,1869)
In the addition of HX to an alkene, the H attaches to the carbon with fewer alkyl substituents and the X attaches to the carbon with more alkyl substituents

34. Orientation of Electrophilic Addition: Markovnikov’s Rule (Restated)
In the addition of HX to an alkene, the more highly substituted carbocation is formed as the intermediate rather than the less highly substituted one
When both double-bond carbon atoms have the same degree of substitution, a mixture of addition products results.

35. Worked Example 6.2: Predicting the Product of an Electrophilic Addition Reaction
What product would you expect from reaction of HCl with 1-ethylcyclopentene?

36. Worked Example 6.2: Predicting the Product of an Electrophilic Addition Reaction
Strategy
Begin by looking at functional group(s) in the reactants
Decide what kind of reaction occurs
The reactant is an alkene that will probably undergo an electrophilic addition reaction with HCl
Use knowledge of electrophilic addition reactions to predict product
Electrophilic addition reactions follow Markovnikov’s rule

37. Worked Example 6.2: Predicting the Product of an Electrophilic Addition Reaction
Solution
The expected product is 1-chloro-1-ethylcyclopentane

38. Worked Example 6.3: Synthesizing a Specific Compound
What alkene would you start with to prepare the following alkyl halide?
(There may be more than one possibility.)

39. Worked Example 6.3: Synthesizing a Specific Compound
Strategy
Always work backward
Look at product, and identify the functional group(s) it contains
The product is a tertiary alkyl chloride
Find all ways of preparing functional group
Can be prepared by reaction of an alkene with HCl
Draw and evaluate all possibilities

40. Worked Example 6.3: Synthesizing a Specific Compound
Solution
There are three possibilities, any one of which could give the desired product

41. 6.8 Carbocation Structure and Stability
Why Markovnikov’s Rule works
Structure of carbocations
Carbocations are planar
The trivalent carbon is sp2 hybridized and has a vacant p orbital perpendicular to the plane of the carbon
Carbon has three attached groups

42. Carbocation Structure and Stability
Thermodynamic measurements show that the stability of carbocations increases with increasing substitution.

43. Carbocation Structure and Stability
More highly substituted carbocations are more stable than less highly substituted ones
Inductive effect
Results from the response to the electronegativity of nearby atoms
The more alkyl groups that are attached to the positively charged carbon, the more electron density shifts toward the charge and the more inductive stabilization of the cation occurs

44. Carbocation Structure and Stability
Hyperconjugation
the stabilizing interaction between a p orbital and properly oriented C-H σ bonds on neighboring carbons
The more alkyl groups that are on the carbocation the more stable the carbocation

45. 6.9 The Hammond Postulate Electrophilic addition reactions
Electrophilic addition to an unsymmetrically substituted alkene gives the more highly substituted carbocation intermediate
A more highly substituted carbocation is more stable than a less substituted one
There is an intuitive relationship between the stability of a carbocation intermediate and the rate of its formation
Transition states represent energy maxima whose structures are similar to the structure of the nearest stable species

46. The Hammond Postulate
Stability of the carbocation intermediate affects the rate of reaction
When comparing two similar reactions the more stable intermediate forms faster than the less stable one

47. The Hammond Postulate Hammond postulate(1955)
The structure of a transition state resembles the structure of the nearest stable species. Transition states for endergonic steps structurally resemble products, and transition states for exergonic steps structurally resemble reactants.

48. The Hammond Postulate
Hammond postulate applied to electrophilic addition reactions
Formation of carbocation by protonation of an alkene is an endergonic step
Transition state of alkene protonation should structurally resemble the carbocation intermediate
Any factor that stabilizes the carbocation should also stabilize the nearby transition state
More stable carbocations form faster because their greater stability is reflected in the lower-energy transition state leading to them

49. The Hammond Postulate
The transition state is stabilized by hyperconjugation and inductive effects
The more alkyl groups that are present the greater the extent of stabilization and the faster the transition state forms

50. 6.10 Carbocation Rearrangements
Evidence to support carbocation mechanism for electrophilic addition reactions of alkenes
Structural rearrangements often occur during the reaction of HX with an alkene
HCl with 3-methylbut-1-ene

51. Carbocation Rearrangements
Hydride shift
The shift of a hydrogen atom and its electron pair (a hydride ion, :H-) between neighboring carbons
The secondary carbocation intermediate formed by protonation of 3-methylbut-1-ene rearranges to a more stable tertiary carbocation through a hydride shift

52. Carbocation Rearrangements
Alkyl shift
The shift of an alkyl group with its electron pair between neighboring carbons
A secondary carbocation rearranges to a more stable tertiary carbocation by the shift of a methyl group

53. Carbocation Rearrangements
In both cases a group (:H- or :CH3-) moves to an adjacent positively charged carbon taking its bonding electron pair with it
A less stable carbocation rearranges to a more stable ion
Rearrangements are a common feature of carbocation chemistry and are particularly important in biological pathways by which steroids and related substances are synthesized
Hydride shift in biosynthesis of cholesterol