6. Alkenes: Structure and Reactivity Based on McMurry’s Organic Chemistry, 6 th edition

2 Alkene - Hydrocarbon With Carbon- Carbon Double Bond Includes many naturally occurring materials Flavors, fragrances, vitamins Important industrial products These are feedstocks for industrial processes

3 Alkenes

4 6.2 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 C n H 2n+2 Each ring or multiple bond replaces 2 H's

5 Example: C 6 H 10 Saturated is C 6 H 14 Therefore 4 H's are missing This has two degrees of unsaturation Two double bonds? or triple bond? or two rings or ring and double bond

6 Other Examples of C 6 H 10 :

7 Degree of Unsaturation With Other Elements Organohalogens (X: F, Cl, Br, I) Halogen replaces hydrogen C 4 H 6 Br 2 and C 4 H 8 have one degree of unsaturation Oxygen atoms: these don't affect the total count of H's C4H8O3C4H8O3

8 Degree of Unsaturation With Other Elements

9 If C-N Bonds Are Present Nitrogen has three bonds So if it connects where H was, it adds a connection point, and there an extra H Subtract one H for equivalent degree of unsaturation in hydrocarbon

Prob. 6.3: Valium--C 16 H ?? ClN 2 O 10

11 Count pairs of H's below C n H 2n+2 Add number of halogens to number of H's (X equivalent to H) Don't count oxygens (oxygen does not affect the # of H’s) Subtract N's - they have three connections Summary - Degree of Unsaturation

Naming of Alkenes Find longest continuous carbon chain for root of the name Number carbons in chain so that double bond carbons have lowest possible numbers Double bond carbons must be numbered consecutively

13 Alkene Nomenclature: longest chain must contain the C=C

14 Alkene Nomenclature

15 Cycloalkene nomenclature

16 Cycloalkene nomenclature 1-methylcyclohexene 3-methylcyclohexene 4-methylcyclohexene

17 Problem 6.4 (Page 178)

18 Problem 6.6 (Page 178)

Many Alkenes Are Known by Common Names 19

20 Alkene Group Names

Electronic Structure of Alkenes Carbon atoms in a double bond are sp 2 - hybridized Three equivalent orbitals at 120º separation in plane Fourth orbital is an unhybridized p orbital Combination of an electron pair in an orbital formed by the overlap of two sp 2 orbitals of two atoms forms  bond between them

Electronic Structure of Alkenes Additive interaction (overlap) of p orbitals creates a  bonding orbital Subtractive interaction creates a  anti- bonding orbital Occupied  orbital prevents rotation about  -bond Rotation prevented by  bond - high barrier, about 268 kJ/mole in ethylene

23 Rotation of  Bond Is Energetically Costly This prevents rotation about a carbon-carbon double bond (unlike a carbon-carbon single bond). Creates possible geometric isomers (cis/trans)

Cis-Trans Isomerism in Alkenes The presence of a carbon-carbon double can create two possible structures cis isomer - two similar groups on same side of the double bond trans isomer similar groups on opposite sides Each carbon must have two different groups for these isomers to occur

Cis, Trans Isomers Require That End Groups Must Differ in Pairs 25 X

Sequence Rules: The E,Z Designation Neither compound is clearly “cis” or “trans” Substituents on C1 are different than those on C2 We need to define “similarity” in a precise way to distinguish the two stereoisomers Cis, trans nomenclature only works for disubstituted double bonds

27 Develop a System for Comparison of Priority of Substituents Assume a valuation system If Br has a higher “value” than Cl If CH 3 is higher than H Then, in A, the higher value groups are on opposite sides In B, they are on the same side Requires a universally accepted “valuation”

28 Ranking Priorities: Cahn-Ingold- Prelog Rules Must rank atoms that are connected at comparison point Higher atomic number gets higher priority In this case,The higher priority groups are opposite: (E )-1-bromo-1-chloro-propene

29 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

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31 2-chloro-2-butenes

32 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 Extended Comparison

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

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36 Some Examples:

37 Practice problem 6.1 p. 183

38 Solution

39 Problem 6.11, p. 184

40 Problem 6.42 (p. 208): E or Z?

Alkene Stability Cis alkenes are usually less stable than trans alkenes

42

Alkene Stability Compare heat given off on hydrogenation:  H o Less stable isomer is higher in energy a nd gives off more heat tetrasubstituted > trisubstituted > disubstituted > monosusbtituted

44 Comparing Stabilities of Alkenes Evaluate heat given off when C=C is converted to C-C (catalytic hydrogenation) More stable alkene gives off less heat (  H)

45 Heats of hydrogenation of butenes

46

47 Equilibration of 2-butenes

48 Alkene Stability Compare heat given off on hydrogenation:  H o Less stable isomer is higher in energy a nd gives off more heat tetrasubstituted > trisubstituted > disubstituted > monosusbtituted

49

50 Alkene Stabilities from  H’s:

51 Hyperconjugation Electrons in neighboring filled  orbital stabilize vacant antibonding  orbital – net positive interaction Alkyl groups are more stabilizing than H

52 Bond strengths/hybridization effects sp 3 -sp 3 bond is weaker than sp 3 -sp 2, sp 2 -sp 2

53 Name each; which is more stable? Problem 13, p. 188

Electrophilic Addition Reactions of Alkenes General reaction mechanism: electrophilic addition Attack of electrophile (such as HBr) on  bond of alkene produces carbocation and bromide ion Carbocation is itself an electrophile, reacting with nucleophilic bromide ion

Examples: 55

56 Writing Organic Reactions No established convention – shorthand Can be formal kinetic expression Not necessarily balanced Reactants can be before or on arrow Solvent, temperature, details, on arrow

For Example: 57

Addition of HBr to 2- methylpropene 58

Electrophilic Addition Energy Diagram: 59

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

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 19 th 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

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

63 Examples:

64 But:

65 Practice Problem 6.2 (p. 193):

66 Solution:

67 Practice Problem 6.3 (p. 194):

68 Solution:

69 Problem 6.14: Major products?

Problem 6.15: Which alkene would you add HX to? 70

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

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74 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

Carbocation Structure and Stability Carbocations are planar The positively charged carbon is surrounded by only 6 electrons in three sp 2 orbitals The fourth orbital on carbon is a vacant p-orbital

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Carbocation Structure and Stability 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 > + CH 3

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79 Heterolytic bond dissociations energies:

80 Stabilizing Carbocations:

81

82 Problem 6.16 : carbocation structure?

The Hammond Postulate If one carbocation intermediate is more stable than another, why is the reaction through the more stable one faster? The relative stability of the intermediate is related to an equilibrium constant (  Gº) The relative stability of the transition state (which describes the size of the rate constant) is the activation energy (  G ‡ ) The transition state is transient and cannot be examined

84 Transition State Structures A transition state is the highest energy species in a reaction step By definition, its structure is not stable enough to exist for one vibration But the structure controls the rate of reaction So we need to be able to guess about its properties in an informed way We classify them in general ways and look for trends in reactivity – the conclusions are in the Hammond Postulate

85 Statement of the Hammond Postulate A transition state should be similar to an intermediate that is close in energy Sequential states on a reaction path that are close in energy are likely to be close in structure - G. S. Hammond carbocation G Reaction In a reaction involving a carbocation, the transition states look like the intermediate

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

87 “Non-Hammond” Behavior More stable intermediate from slower reaction Conclude: transition state and intermediate must not be similar in this case – not common

88 Transition State for Alkene Protonation Resembles carbocation intermediate Close in energy and adjacent on pathway Hammond Postulate says they should be similar in structure

Transition State resembles cation 89

Energy Diagrams for Markovnikov & Anti-Markovnikov Addition 90

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

Carbocation rearrangements: 92

Hydride Shifts 93

Alkyl (methyl) shifts 94

Cholesterol Biosynthesis: 95

96 Problem 6.47: What will be the rearranged cations?

97 Problem 6.19: mechanism?

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99 Problem 6.48: mechanism?

100 Problem 6.49: mechanism?

101

102 Terpenes

103 Terpenes

104 Terpene Biosynthesis

105

106 Limonene biosynthesis