7. Alkenes: Structure and 7. Alkenes: Structure and ReactivityReactivity

Why this Chapter?Why this Chapter?

C-C double bonds are present in most organic and biological molecules

To examine consequences of alkene stereoisomerism

To focus on general alkene reaction: electrophilic addition

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Alkene - Hydrocarbon With Carbon-Alkene - Hydrocarbon With Carbon-Carbon Double BondCarbon Double Bond Also called an olefin but alkene is better Includes many naturally occurring materials

◦ Flavors, fragrances, vitamins

3

7.1 Industrial Preparation and 7.1 Industrial Preparation and Use of AlkenesUse of AlkenesEthylene and propylene are the

most important organic chemicals produced

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7.2 Calculating Degree of 7.2 Calculating Degree of UnsaturationUnsaturation Relates molecular formula to possible structures Degree of unsaturation: number of multiple bonds or rings Formula for a saturated acyclic compound is CnH2n+2

Each ring or multiple bond replaces 2 H's

5

Example: CExample: C66HH1010

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

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Degree of Unsaturation With Other Degree of Unsaturation With Other ElementsElements

Organohalogens (X: F, Cl, Br, I) Halogen replaces hydrogen

C4H6Br2 and C4H8 have one degree of unsaturation Organoxygen compounds (C,H,O) - if connected by single

bonds These don't affect the total count of H's

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Organonitrogen compoundsOrganonitrogen 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

8

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

Summary - Degree of Summary - Degree of UnsaturationUnsaturation

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7.3 Naming of Alkenes7.3 Naming of Alkenes

Name the parent hydrocarbon Number carbons in chain so that double bond

carbons have lowest possible numbers Rings have “cyclo” prefix

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Many Alkenes Are Known by Many Alkenes Are Known by Common NamesCommon Names

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7.4 Cis-Trans Isomerism in Alkenes7.4 Cis-Trans Isomerism in Alkenes Carbon atoms in a double bond are sp2-hybridized

◦ Three equivalent orbitals at 120º separation in plane◦ Fourth orbital is atomic p orbital

Combination of electrons in two sp2 orbitals of two atoms forms bond between them

Additive interaction 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

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Rotation of Rotation of Bond Is Prohibitive Bond Is Prohibitive

This prevents rotation about a carbon-carbon double bond (unlike a carbon-carbon single bond).

Creates possible alternative structures

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The presence of a carbon-carbon double bond 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

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

180°rotation superposes Bottom pair cannot be superposed without breaking C=C

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7.5 Sequence Rules: The 7.5 Sequence Rules: The E,ZE,Z DesignationDesignation 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

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E,Z Stereochemical NomenclatureE,Z Stereochemical Nomenclature

Priority rules of Cahn, Ingold, and Prelog

Compare where higher priority groups are with respect to bond and designate as prefix

E -entgegen, opposite sides

Z - zusammen, together on the same side

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Ranking Priorities: Cahn-Ingold-Ranking Priorities: Cahn-Ingold-Prelog RulesPrelog Rules

RULE 1 Must rank atoms that are connected at comparison

point Higher atomic number gets higher priority

◦ Br > Cl > S > P > O > N > C > H

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RULE 2 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 ComparisonExtended Comparison

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RULE 3 Substituent is drawn with connections shown and

no double or triple bonds Added atoms are valued with 0 ligands themselves

Dealing With Multiple Bonds:Dealing With Multiple Bonds:

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7.6 Stability of Alkenes7.6 Stability of Alkenes Cis alkenes are less stable than trans alkenes Compare heat given off on hydrogenation: Ho

Less stable isomer is higher in energy◦ And gives off more heat◦ tetrasubstituted > trisubstituted > disubstituted >

monosusbtituted◦ hyperconjugation stabilizes

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Comparing Stabilities of AlkenesComparing 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

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HyperconjugationHyperconjugation

Electrons in neighboring filled orbital stabilize vacant antibonding orbital – net positive interaction

Alkyl groups are better than H

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7.7 Electrophilic Addition of Alkenes7.7 Electrophilic Addition of 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

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Two step process First transition state is high energy point

Electrophilic Addition Energy PathElectrophilic Addition Energy Path

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Electrophilic Addition for Electrophilic Addition for preparationspreparations

The reaction is successful with HCl and with HI as well as HBr

HI is generated from KI and phosphoric acid

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7.8 Orientation of Electrophilic Addition: 7.8 Orientation of Electrophilic Addition: Markovnikov’s RuleMarkovnikov’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

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Addition of HCl to 2-methylpropene Regiospecific – one product forms where two are possible If both ends have similar substitution, then not

regiospecific

Example of Markovnikov’s RuleExample of Markovnikov’s Rule

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Markovnikov’s Rule (restated)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

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7.9 Carbocation Structure and Stability7.9 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

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Inductive stabilization of cation speciesInductive stabilization of cation species

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7.10 The Hammond Postulate7.10 The Hammond Postulate If 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

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Transition State StructuresTransition 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

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Examination of the Hammond Examination of the Hammond PostulatePostulate

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

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Competing Reactions and the Competing Reactions and the Hammond PostulateHammond Postulate Normal Expectation: Faster reaction gives more

stable intermediate Intermediate resembles transition state

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7.11 Mechanism of Electrophilic Addition: 7.11 Mechanism of Electrophilic Addition: Rearrangements of CarbocationsRearrangements 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

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Hydride shifts in biological Hydride shifts in biological moleculesmolecules

What is the degree of What is the degree of unsaturation for a compound with unsaturation for a compound with the formula Cthe formula C66HH1010NBr?NBr?

1. 02. 13. 24. 35. 4

Learning Check:Learning Check:

What is the degree of What is the degree of unsaturation for a compound with unsaturation for a compound with the formula Cthe formula C66HH1010NBr?NBr?

1. 02. 13. 24. 35. 4

Solution:Solution:

Which of the following Which of the following cannotcannot have a triple bond?have a triple bond?

1. C6H12

2. C10H16

3. C10H18

4. C4H6O

5. C4H4O

Learning Check:Learning Check:

Which of the following Which of the following cannotcannot have a triple bond?have a triple bond?

1. C6H12

2. C10H16

3. C10H18

4. C4H6O

5. C4H4O

Solution:Solution:

What is the IUPAC name for the What is the IUPAC name for the following compound?following compound?

1. (E)-2,5-dimethylhex-2-ene2. (Z)-2,5-dimethylhex-2-ene 3. 1-isobutyl-2-methylprop-1-ene4. 1-isopropyl-2-methylbut-2-ene5. 2,5-dimethylhex-2-ene

Learning Check:Learning Check:

What is the IUPAC name for the What is the IUPAC name for the following compound?following compound?

1. (E)-2,5-dimethylhex-2-ene2. (Z)-2,5-dimethylhex-2-ene 3. 1-isobutyl-2-methylprop-1-ene4. 1-isopropyl-2-methylbut-2-ene5. 2,5-dimethylhex-2-ene

Solution:Solution:

Which of the following molecules is Which of the following molecules is transtrans-4-ethyl-5-methylhex-2-ene?-4-ethyl-5-methylhex-2-ene?

1. 2.

3. 4. 5.

Learning Check:Learning Check:

Which of the following molecules is Which of the following molecules is transtrans-4-ethyl-5-methylhex-2-ene?-4-ethyl-5-methylhex-2-ene?

1. 2.

3. 4. 5.

Solution:Solution:

How many isomers exist for CHow many isomers exist for C22HH22BrBr22??

1. 12. 23. 34. 45. 5

Learning Check:Learning Check:

How many isomers exist for CHow many isomers exist for C22HH22BrBr22??

1. 12. 23. 34. 45. 5

Solution:Solution:

What is the major product of the What is the major product of the following reaction?following reaction?

HBr

1. 2.

3. 4. 5.

Learning Check:Learning Check:

What is the major product of the What is the major product of the following reaction?following reaction?

HBr

1. 2.

3. 4. 5.

Solution:Solution:

Which of the following carbocations is Which of the following carbocations is most stable?most stable?

1. 2.

3.

4. 5.

Learning Check:Learning Check:

Which of the following carbocations is Which of the following carbocations is most stable?most stable?

1. 2.

3.

4. 5.

Solution:Solution:

What is the best starting material for What is the best starting material for the following reaction?the following reaction?

+ HBr

Br

?

1. 2.

3. 4. 5.

Learning Check:Learning Check:

What is the best starting material for What is the best starting material for the following reaction?the following reaction?

+ HBr

Br

?

1. 2.

3. 4. 5.

Solution:Solution:

Norethindrone, one of the first oral Norethindrone, one of the first oral contraceptives, has the structure shown contraceptives, has the structure shown below. What is its degree of below. What is its degree of unsaturation?unsaturation?

1. 52. 63. 74. 85. 9

O

H3C OH

H

norethindrone

Learning Check:Learning Check:

Norethindrone, one of the first oral Norethindrone, one of the first oral contraceptives, has the structure shown contraceptives, has the structure shown below. What is its degree of below. What is its degree of unsaturation?unsaturation?

1. 52. 63. 74. 85. 9

O

H3C OH

H

norethindrone

Solution:Solution:

Consider the reaction between HCl and ethene Consider the reaction between HCl and ethene (the reactants). The carbocation intermediate (the reactants). The carbocation intermediate and the Cland the Cl–– may revert to the reactants or go on may revert to the reactants or go on to form the product, chloroethane. Which to form the product, chloroethane. Which statement about this process is correct?statement about this process is correct?

1. The formation of chloroethane from the carbocation and Cl– is the rate limiting process.

2. The structure of the transition state for conversion of the carbocation to chloroethane resembles the product.

3. The carbocation intermediate (and Cl–) has energy that is intermediate between the reactants and the products.

4. The intermediate may bond with Cl– on either of its two carbons.

5. Based on the Hammond postulate, the carbocation and Cl– form the chloroethane faster than they revert to reactants.

Learning Check:Learning Check:

Consider the reaction between HCl and ethene Consider the reaction between HCl and ethene (the reactants). The carbocation intermediate (the reactants). The carbocation intermediate and the Cland the Cl–– may revert to the reactants or go on may revert to the reactants or go on to form the product, chloroethane. Which to form the product, chloroethane. Which statement about this process is correct?statement about this process is correct?

1. The formation of chloroethane from the carbocation and Cl– is the rate limiting process.

2. The structure of the transition state for conversion of the carbocation to chloroethane resembles the product.

3. The carbocation intermediate (and Cl–) has energy that is intermediate between the reactants and the products.

4. The intermediate may bond with Cl– on either of its two carbons.

5. Based on the Hammond postulate, the carbocation and Cl– form the chloroethane faster than they revert to reactants.

Solution:Solution:

What is(are) the double bond What is(are) the double bond configuration(s) in the following configuration(s) in the following compound?compound?

1. E2. Z3. E, Z4. E, E5. Z, Z

NC

CN

COOH

H2N

OH

Learning Check:Learning Check:

What is(are) the double bond What is(are) the double bond configuration(s) in the following configuration(s) in the following compound?compound?

1. E2. Z3. E, Z4. E, E5. Z, Z

NC

CN

COOH

H2N

OH

Solution:Solution:

Which of the alkenes has the most Which of the alkenes has the most negative heat of hydrogenation?negative heat of hydrogenation?

1. 2.

3. 4. 5.

Learning Check:Learning Check:

Which of the alkenes has the most Which of the alkenes has the most negative heat of hydrogenation?negative heat of hydrogenation?

1. 2.

3. 4. 5.

Solution:Solution:

Which statement best describes Which statement best describes the energetics of the reaction the energetics of the reaction shown below?shown below?CC22HH44 + H + H22 → C → C22HH66

1. exothermic with positive activation energy

2. exothermic with negative activation energy

3. endothermic with positive activation energy

4. endothermic with negative activation energy

5. cannot be determined without knowing BDE’s

Learning Check:Learning Check:

Which statement best describes Which statement best describes the energetics of the reaction the energetics of the reaction shown below?shown below?CC22HH44 + H + H22 → C → C22HH66

1. exothermic with positive activation energy

2. exothermic with negative activation energy

3. endothermic with positive activation energy

4. endothermic with negative activation energy

5. cannot be determined without knowing BDE’s

Solution:Solution:

1. A and B2. A and E3. B and E4. C and D5. C and E

Which of the following Which of the following σσ-bonds-bonds participate in hyperconjugation?participate in hyperconjugation?

H

H

A

B

C

D

E

CH3

Learning Check:Learning Check:

Which of the following Which of the following σσ-bonds-bonds participate in hyperconjugation?participate in hyperconjugation?

1. A and B2. A and E3. B and E4. C and D5. C and E

H

H

A

B

C

D

E

CH3

Solution:Solution:

Which of the following bromides is the major product Which of the following bromides is the major product of the reaction shown below, assuming that there are of the reaction shown below, assuming that there are no carbocation rearrangements?no carbocation rearrangements?

1. 2.

3. 4. 5.

+ HBr

(1 equivalent)

C13H17Br

Learning Check:Learning Check:

Which of the following bromides is the major product Which of the following bromides is the major product of the reaction shown below, assuming that there are of the reaction shown below, assuming that there are no carbocation rearrangements?no carbocation rearrangements?

1. 2.

3. 4. 5.

+ HBr

(1 equivalent)

C13H17Br

Solution:Solution:

How many degrees of How many degrees of unsaturation are there in the unsaturation are there in the molecule of prizmane?molecule of prizmane?

1. 12. 23. 34. 45. 5

Learning Check:Learning Check:

How many degrees of How many degrees of unsaturation are there in the unsaturation are there in the molecule of prizmane?molecule of prizmane?

1. 12. 23. 34. 45. 5

Solution:Solution:

The heat of formation of 2-methylpropane is –The heat of formation of 2-methylpropane is –32 kcal/mol. The heat of formation of butane is 32 kcal/mol. The heat of formation of butane is –30 kcal/mol. The heat of hydrogenation of 2-–30 kcal/mol. The heat of hydrogenation of 2-methylpropene is –28 kcal/mol. What is the methylpropene is –28 kcal/mol. What is the heat of formation of 2-methylpropene?heat of formation of 2-methylpropene?

1. 4 kcal/mol2. 2 kcal/mol3. –2 kcal/mol4. –4 kcal/mol5. –60 kcal/mol

Learning Check:Learning Check:

The heat of formation of 2-methylpropane is –The heat of formation of 2-methylpropane is –32 kcal/mol. The heat of formation of butane is 32 kcal/mol. The heat of formation of butane is –30 kcal/mol. The heat of hydrogenation of 2-–30 kcal/mol. The heat of hydrogenation of 2-methylpropene is –28 kcal/mol. What is the methylpropene is –28 kcal/mol. What is the heat of formation of 2-methylpropene?heat of formation of 2-methylpropene?

1. 4 kcal/mol2. 2 kcal/mol3. –2 kcal/mol4. –4 kcal/mol5. –60 kcal/mol

Solution:Solution:

Hyperconjugation is most Hyperconjugation is most important in stabilization of which important in stabilization of which kind of species?kind of species?

1. alkanes2. alkenes3. carbocations4. carbanions5. carbon radicals

Learning Check:Learning Check:

Hyperconjugation is most Hyperconjugation is most important in stabilization of which important in stabilization of which kind of species?kind of species?

1. alkanes2. alkenes3. carbocations4. carbanions5. carbon radicals

Solution:Solution:

If D = If D = 22H reacts in the same way as H reacts in the same way as 11H, what H, what would be the most likely product of the following would be the most likely product of the following addition run under conditions that favor addition run under conditions that favor rearrangements?rearrangements?D Cl+ ?

1. 2.

3.

4. 5.

Learning Check:Learning Check:

If D = If D = 22H reacts in the same way as H reacts in the same way as 11H, what H, what would be the most likely product of the following would be the most likely product of the following addition run under conditions that favor addition run under conditions that favor rearrangements?rearrangements?D Cl+ ?

1. 2.

3.

4. 5.

Solution:Solution:

Which of the marked bonds in the carbocation Which of the marked bonds in the carbocation shown below is the most likely to migrate to shown below is the most likely to migrate to generate another carbocation?generate another carbocation?

1. A2. B3. C4. D5. E

H3C

H

H

A

B

C

DE

Learning Check:Learning Check:

Which of the marked bonds in the carbocation Which of the marked bonds in the carbocation shown below is the most likely to migrate to shown below is the most likely to migrate to generate another carbocation?generate another carbocation?

1. A2. B3. C4. D5. E

H3C

H

H

A

B

C

DE

Solution:Solution: