ALKENES: STRUCTURE AND

REACTIVITY

Alkene: Hydrocarbon with

Carbon-Carbon Double Bond

2

Also called an olefin but alkene is better

Includes many naturally occurring materials

Flavors, fragrances, vitamins

Why this Chapter?

3

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

Industrial Preparation and Use of Alkenes

4

Ethylene and propylene are the most important organic chemicals

produced.

Calculating Degree of Unsaturation

5

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

Degree of Unsaturation With Other

Elements

6

Organohalogens (X: F, Cl, Br, I)

Halogen replaces hydrogen

C4H6Br2 and C4H8 have one degree of unsaturation

Organooxygen compounds (C,H,O) - if connected by single bonds

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

Organonitrogen Compounds

7

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

Summary - Degree of Unsaturation

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

Naming of Alkenes

9

Name the parent hydrocarbon

Number carbons in chain so that double bond carbons have lowest

possible numbers

Rings have cyclo prefix

Many Alkenes are Known by Common

Names

10

Cis-Trans Isomerism in Alkenes

11

Carbon atoms in a double bond are sp2-hybridized.

Three equivalent orbitals at 120 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.

Rotation of Bond is Prohibitive

12

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

Creates possible alternative structures.

13

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.

Cis, Trans Isomers Require That End

Groups Must Differ in Pairs

14

180rotation superposes

Bottom pair cannot be superposed without

breaking C=C

Sequence Rules: The E,Z Designation

15

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

E,Z Stereochemical Nomenclature

16

Priority rules of Cahn, Ingold, and Prelog

Compare where higher priority groups are with respect to bond

and designate as prefix

E - opposite sides

Z - together on the same side

Ranking Priorities: Cahn-Ingold-Prelog

Rules

17

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

Extended Comparison

18

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.

Dealing With Multiple Bonds

19

RULE 3

Substituent is drawn with connections shown and no double or

triple bonds.

Added atoms are valued with 0 ligands themselves.

Stability of Alkenes

20

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

Comparing Stabilities of Alkenes

21

Evaluate heat given off when C=C is converted to C-C

More stable alkene gives off less heat

trans-Butene generates 4 kJ less heat than cis-butene

Hyperconjugation

22

Stabilizing interaction between an unfilled orbital and a

neighbouring filled the C-H bond on a substituent.

More substituents, greater stabilisation.

Alkyl groups are better than H.

Electrophilic Addition of Alkenes

23

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.

Electrophilic Addition Energy Path

24

Two step process.

First transition state is high energy point.

Electrophilic Addition for Preparations

25

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

HI is generated from KI and phosphoric acid.

Orientation of Electrophilic Addition:

Markovnikovs Rule

26

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 Hs and X attaches to the other end (to the one with the most alkyl substituents).

This is Markovnikovs Rule

Example of Markovnikovs Rule

27

Addition of HCl to

2-methylpropene.

Regiospecific one product forms where two are possible.

If both ends have similar substitution, then not regiospecific.

Markovnikovs Rule (restated)

28

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.

Carbocation Structure and Stability

29

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.

30

A plot of dissociation enthalpy versus substitution pattern for the

gas-phase dissociation of alkyl chlorides to yield carbocations.

More highly substituted alkyl halides dissociate more easily than

less highly substituted ones.

Inductive Stabilization of Cation Species

31

inductive effect is simply the shifting of electrons in a bond in

response to the electronegativity of nearby atoms

The Hammond Postulate

32

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 (G)

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

Transition State Structures

33

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.

Examination of the Hammond Postulate

34

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

Competing Reactions and the Hammond

Postulate

35

Normal Expectation: Faster reaction gives more stable intermediate.

Intermediate resembles transition state.

Mechanism of Electrophilic Addition:

Rearrangements of Carbocations

36

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

Hydride Shifts in Biological Molecules

37

ALKENES: REACTIONS AND

SYNTHESIS

Diverse Reactions of Alkenes

39

Alkenes react with many electrophiles to give useful products by

addition (often through special reagents).

Why this Chapter?

40

To begin a systematic description of major functional

groups.

Begin to focus on general principles and patterns of

reactivity that tie organic chemistry.

Preparation of Alkenes:

A Preview of Elimination Reactions

41

Alkenes are commonly made by

elimination of HX from alkyl

halide (dehydrohalogenation)

Uses heat and KOH

elimination of H-OH from an

alcohol (dehydration)

require strong acids

(sulfuric acid, 50 C)

Addition of Halogens to Alkenes

42

Bromine and chlorine add to alkenes to give 1,2-dihalides, an industrially

important process

F2 is too reactive and I2 does not add

Cl2 reacts as Cl+ Cl-

Br2 is similar

Addition of Br2 to Cyclopentene

43

Addition is exclusively trans

Mechanism of Bromine Addition

44

Br+ adds to an alkene producing a cyclic ion.

Bromonium ion, bromine shares charge with carbon.

Gives trans addition

Bromonium Ion Mechanism

45

Electrophilic addition of

bromine to give a cation is

followed by cyclization to

give a bromonium ion

This bromoniun ion is a

reactive electrophile and

bromide ion is a good

nucleophile

The Reality of Bromonium Ions

46

Bromonium ions were postulated more than 60 years ago to

explain the stereochemical course of the addition (to give the

trans-dibromide from a cyclic alkene.

Olah showed that bromonium ions are stable in liquid SO2

with SbF5 and can be studied directly.

Addition of Hypohalous Acids to

Alkenes: Halohydrin Formation

47

This is formally the addition of HO-X to an alkene to give a

1,2-halo alcohol, called a halohydrin.

The actual reagent is the dihalogen (Br2 or Cl2 in the presence

of water).

Mechanism of Formation of a Bromohydrin

48

Br2 forms bromonium

ion, then water adds

Orientation toward

stable C+ species

Aromatic rings do not

react

An Alternative to Bromine

49

Bromine is a difficult reagent to use for this reaction.

N-Bromosuccinimide (NBS) produces bromine in organic

solvents and is a safer source.

Addition of Water to Alkenes:

Oxymercuration

50

Hydration of an alkene is the addition of H-OH to give

an alcohol.

Acid catalysts are used in high temperature industrial

processes: ethylene is converted to ethanol.

Oxymercuration Intermediates

51

For laboratory-scale hydration of an alkene.

Use mercuric acetate in THF followed by sodium borohydride.

Markovnikov orientation via mercurinium ion.

Addition of Water to Alkenes:

Hydroboration

52

Herbert Brown (HB) invented hydroboration (HB)

Borane (BH3) is electron deficient (Lewis acid)

Borane adds to an alkene to give an organoborane

Non-Markonikov product

Hydroboration-Oxidation Forms an

Alcohol from an Alkene

53

Addition of H-BH2 (from BH3-THF complex) to three alkenes

gives a trialkylborane.

Oxidation with alkaline hydrogen peroxide in water produces

the alcohol derived from the alkene.

Orientation in Hydration via Hydroboration

54

Regiochemistry is opposite to Markovnikov orientation.

OH is added to carbon with most Hs

H and OH add with syn stereochemistry, to the same

face of the alkene (opposite of anti addition).

Mechanism of Hydroboration

55

Borane is a Lewis acid.

Alkene is a Lewis base.

Transition state involves anionic development on B.

The components of BH3 are added across C=C.

More stable carbocation is also consistent with steric preferences.

Addition of Carbenes to Alkenes

56

The carbene functional group is half of an alkene.

Carbenes are electrically neutral with six electrons in the

outer shell.

They add symmetrically across double bonds to form

cyclopropanes.

Formation of Dichlorocarbene

57

Base removes

proton from

chloroform.

Stabilized

carbanion remains.

Unimolecular

elimination of Cl

- gives electron

deficient species,

dichlorocarbene

Reaction of Dichlorocarbene

58

Addition of dichlorocarbene is stereospecific cis

Simmons-Smith Reaction

59

Equivalent of addition of CH2:

Reaction of diiodomethane with zinc-copper alloy produces a carbenoid

species.

Forms cyclopropanes by cycloaddition.

Reduction of Alkenes: Hydrogenation

60

Addition of H-H across C=C.

Reduction in general is addition of H2 or its equivalent.

Requires Pt or Pd as powders on carbon and H2.

Hydrogen is first adsorbed on catalyst.

Reaction is heterogeneous (process is not in solution).

Hydrogen Addition - Selectivity

61

Selective for C=C. No reaction with C=O, C=N

Polyunsaturated liquid oils become solids

If one side is blocked, hydrogen adds to other

Sensitive to steric environment

Mechanism of Catalytic Hydrogenation

62

Heterogeneous

reaction between

phases

Addition of H-H is

syn

Oxidation of Alkenes:

Epoxidation and Hydroxylation

63

Epoxidation results in a cyclic ether with an oxygen atom

Stereochemistry of addition is syn

Osmium Tetroxide Catalyzed Formation

of Diols

64

Hydroxylation - converts to syn-diol.

Osmium tetroxide, then sodium bisulfate.

Via cyclic osmate di-ester.

Oxidation of Alkenes:

Cleavage to Carbonyl Compounds

65

Ozone, O3, adds to alkenes to form molozonide.

Reduce molozonide to obtain ketones and/or aldehydes.

Ozonolysis of Alkenes

66

Result of ozonolysis : C=C bond cleaved and oxygen is doubly

bonded to each of the original alkene carbons.

Permangante Oxidation of Alkenes

67

Oxidizing reagents other than ozone also cleave alkenes.

Potassium permanganate (KMnO4) can produce carboxylic

acids and carbon dioxide if Hs are present on C=C

Cleavage of 1,2-diols

68

Reaction of a 1,2-diol with periodic (per-iodic) acid, HIO4, cleaves

the diol into two carbonyl compounds.

Sequence of diol formation with OsO4 followed by diol cleavage is a

good alternative to ozonolysis.

Addition of Radicals to Alkenes:

Polymers

69

A polymer is a very large molecule consisting of repeating units of simpler molecules, formed by polymerization.

Alkenes react with radical catalysts to undergo radical polymerization.

For example, ethylene is polymerized to polyethylene.

Free Radical Polymerization: Initiation

70

Initiation - a few radicals are generated by the reaction of a

molecule that readily forms radicals from a nonradical molecule.

A bond is broken homolytically.

Polymerization: Propagation

71

Radical from initiation adds to alkene to generate alkene

derived radical.

This radical adds to another alkene, and so on many

times.

Polymerization: Termination

72

Chain propagation ends when two radical chains

combine.

Not controlled specifically but affected by reactivity and

concentration.

Other Polymers

73

Other alkenes give other common polymers

Biological Additions of Radicals to

Alkenes

74

Severe limitations to the usefulness of radical addition reactions in the lab.

In contrast to electrophilic additions, reactive intermediate is not

quenched so it reacts again and again uncontrollably.

Biological Reactions

75

Biological reactions different than in the laboratory.

One substrate molecule at a time is present in the active

site of an enzyme.

Biological reactions are more controlled, more specific

than other reactions.