1CHEM 3013 ORGANIC CHEMISTRY I LECTURE NOTES

CHAPTER 6

1 . The Alkene Functional Group

Alkenes are also called olefins, or unsaturated hydrocarbons, as compared to alkanes whichare referred to as saturated hydrocarbons. In alkenes, there are two or more trivalent carbons with thepresence of one or more carbon-carbon double bonds. They have the general formula CnH2n.

HC

H HC

HEthylene (an alkene)

1s

2s

2p

1s

2p

2sp2mix 2s andtwo 2porbitals

ENERGY

ElementalCarbon

sp2 HybridizedCarbon in Alkenes

H

HH

H

π-bond (two 2p orbitals on carbon

σ-bond (two sp2 orbitals on the carbons

σ-bond (1s orbital

on hydrogen and sp2

orbital on carbon)

Bonding in Alkenes

a. Bonding in Alkenes

Hybrid orbitals of sp2 and a p orbital combine to form a σ-bond and a π-bond, respectively.

It should be noted that a double bond is actually comprised of two two-electron bonds (σ and π); thisis why double bonds are shorter and stronger than single bonds. The difference between a sp3 hybridorbital and a sp2 hybrid orbital is the amount of "s" character (25% vs 33%, respectively); since thelatter has 33% it makes shorter and stronger sigma bonds to the hydrogens.

2

b. Cis and Trans Stereoisomerism in Alkenes

The π-bonding in alkenes has substantial consequences when it comes to the conformations ofthe alkenes. As you will recall from our discussions about rotation around bonds in ethane andbutane, it is relatively easy to rotate around a sp3-sp3 sigma bond (i.e. 3-5 Kcal/mole barrier height). Rotation around double bonds requires that the π-bond first be broken (about 50-60 Kcal/mole)before we can then rotate....this requires very high temperatures and is seldom observed .

This lack of free rotation around a π-bond leads to stereochemical consequences, there nowexists the possibility of two compounds that have the same carbon connectivity, but different 3-Darrangements of the groups. These are stereoisomers of a specific type called geometric isomers.When we have an alkene which has at least two identical atoms or groups on the two doubly-bondedcarbons of the alkene we use the same cis and trans formalism we used to define stereoisomers incycloalkanes. As shown below, the cis stereoisomers has identical groups on the same side of thealkene, whereas the trans isomer has identical groups on opposite sides of the alkene.

C

C

H

H

CH3

H3C

CH3

H3C

H

H

C

C

H

H3C

H

H3C

H

H3C

H

H3C

trans-2-butene(methyls on oppositeside of alkene

cis-2-butene(methyls on sameside of alkene

p orbitals of π-bond areperpendicular to plane of page;all C's in planeBUILD A MODEL!

Very hard to rotate around a double bond; have to break a π-bondto do so

Stereoisomers of 2-butene

2 . Nomenclature in Alkenes

Nomenclature used for alkenes is much like that used for alkanes...except that the endingchanges to ...ene. Thus any compound that ends in ene is an alkene.

3 a. Common Names

Some alkenes and alkenyl groups have common names that can be recognized:

CH2 CH2 CH CH2 C CH2 CH CH2 C CH2H3C H3C

H3C

H3C

CHCH2

CH2

HCH2

H

CH2

CH2

CH3

ethylene propylene isobutylene styrene isoprene

Common alkenes

alkene substituent groups

vinyl allyl isoprenyl

Common Alkenes and Substituent groups

b. IUPAC Nomenclature

1. Name the longest chain i ncluding the double bond.2. Drop the ane suffix of the base name and add ene3. Number from the end nearest the double bond.4. Complete the name as with alkenes

5. Specify the cis, trans or E,Z stereoisomer isomer if indicated in structure. 6. If more than one double bond exists in the compound, it is named as a diene, triene,

1

2

3

4

5

6

7NOT

1

2

3

4

5

6

7

trans-3-hepteneCorrect

trans-4-hepteneIncorrect

1 2

3

4

567

8 NOT

12

3

4

5 6

7

8

1,4-cyclooctadieneCorrect

1,6-cyclooctadieneIncorrect

Numbering Alkenes

4

H2C H2C H2C

CH2

CH2

CH2

CH2

Octane cannot be theprincipal chain if it does not contain bothcarbons of the alkene.

2-propyl-1-hexene

CORRECT

12 3

4

5

6

Inclusion of thedouble bond is moreimportant than thelongest carbon chain.

NOT

12

34

52-butyl-1-pentaneIncorrect

NO! see above

2,3-diethyl-1,3-butadiene CORRECT

1

2

3

4

Lowest numbering of chain with maximum number of double bonds

CHCH2

CH2

CH CH2

1

23

7

4

65

1

23

3-allyl-6-vinyl-1,4-cycloheptadiene

OR

3-(2-propenyl)-6-allyl-1,4-cycloheptadiene

Naming alkenes as substituent groups

c. Naming of Stereoisomers, the E/Z Conventio n

As we have seen, the lack of rotation around the double bond introduces the possibilities of geometric isomerism . Geometric isomers are different compounds with different chemical andphysical properties.. For example cis -1,2-dibromoethylene has a boiling point of 110˚ whereas thetrans isomer has a boiling point of 129 ˚C. Thus, a nomenclature method for differentiating betweenthe different isomers is required. When we have at least two identical atoms (or groups) on the twodoubly -bonded carbons, as in the compound 2-pentene (draw the structure, the alkyl groups aredifferent...but the H's are identical), we can use the cis/trans formalism. But what about a compound

5such as 1-bromo-1-chloro-1-propene? The are now four different substituents on the two carbons ofthe double bond; a new convention is required.

CAHN, INGOLD, PRELOG SEQUENCE RULES FOR E,Z NOMENCLATURE

1. Give the groups on each carbon of a double bond, an order of priority with preference given to the groups with the heavier atom attached to the double bond.

2. If the first atoms of two groups on a carbon are the same, then the group which has the heavier atom attached to the first atom has higher priority.

3. If the second atoms are identical, keep moving out along the group until a difference is found.4. Multiple bonds count as two, three, etc., bonds to the atom attached.5. If the groups of highest priority are on the same side of the double bond, name the isomer Z

(Zusammen).6. If the groups of highest priority are on the opposite side of the double bond, name the isomer

E (Entgegen).

C C

high priority high priority

C C

high priority

high prioritylow prioritylow priority

low priority

low priority

Z (zusammen)....together E (entgegen)....across

The E/Z Nomenclature of Alkene Stereoisomers

6

C CH3C

H3C C2H5

ClC C

H3C

H3C C2H5

Cl

C CH

H3C Br

Cl

C CH

H3C CH2CH2CH2

CH2CH2CH2 H

CH3

IdenticalCompounds

Exactly the same groups;therefore E/Z meaningless

Case 1: 3-chloro-2-pentane

LOW

HIGH(at# Br > Cl)

LOW

Case 2: Z-1-bromo-1-chloro-1-propene

HIGH(at# C > H)

Case 3: Z-3-propyl-2-hepteneSAME

SAME

LOW

HIGH

LOW

HIGH

E/Z Priority Rules

C C

HC C

X

X

H

X

X C C

C C X C C

C C

X

CC

E/Z priority assignments of alkenes and alkynes

d. Relative Stabilities of Alkene Geometric Isomers

Trans isomers are more stable than cis isomers because of steric destabilization of the cisisomer. In 2-butene this destabilization favors the trans isomer by 0.69 Kcal/mole, but in 2,2,6,6-tetramethyl-3-hexene (draw the structure of both isomers) the trans isomer is favored by 6.3Kcal/mole. Additional alkyl substituents on the double bond of the alkene stabilizes thealkene...unless steric effects become too severe.

7

C CH H

C CH

H

H

CH

HHC

HH

H

C

H

H

H

CH

H

BIG stericinteraction

CIS TRANS

SMALL stericinteraction

Steric destabilization of cis-alkenes

3 . Addition of Hydrogen Halides to Alkenes( Electrophilic Addition Reactions )

Polar molecules add across double bonds in a predictable fashion . The electrons in a π-bondare relatively weakly held by the carbon atoms (π-bond strength ~50-60 Kcal/mole). Thus they areavailable to react with a suitable electron-deficient species; alkenes are weak nucleophiles. When thisoccurs a new sigma bond is formed, and an electron deficient carbon (a carbocation or carbonium ion)is formed.

When an alkene reacts with an electrophile ( in the case of a hydrogen halide HX...a proton),donation of π-electrons from the alkene creates a new C-H bond and an intermediate carbocation(empty p orbital). This then reacts with a nucleophile ( in the case of hydrogen halides... a halideanion) to form the observed product.

`

C C C CX

C C

X Y

Alkeneacting as a Nucleophile;a source ofelectron density

X+

Electrophile;an electron poorspecies

IntermediateThe empty porbital is nowan electrophile

Y-

An electron rich species (a nucleophile)

NOTE directionof curved arrows.

ProductNet addition ofboth X and Y tothe two ends ofthe alkene

The basic mechanism of electrophillic addition to alkenes

a. Markovnikov's Rule

Unsymmetrical alkenes often give a highly regioselective reaction (selective addition of anunsymmetrical reagent to an unsymmetrical alkene). The favored product results from the addition ofthe electrophile (proton, in the case of hydrogen halides) to the carbon which has the least alkyl

8 substitution or greatest hydrogen substitution. This is often referred to as the Markovnikov Rule aftera Russian Chemist who first observed this effect in 1869. The rule is often stated as a slogan :

Them that has gets. Them being carbons bearing the greatest number of hydrogen atoms...and gets being the

proton. Markovnikov's Rule is an empirical rule (based on observations of great numbers ofexamples, but it is based on a simple theoretical explanation. Markovnikov's rule favors the productwhich passes through the most stable carbocation intermediate .

H2CCH

H2CCH

H2CCH

H

H

Br

BrHBr 2˚ carbocation

1˚ carbocation

Br-

Br-

observedproduct

not obtained

Regiochemistry of addition of HBr to an unsymmetrical alkene

b. Carbocation Stability

Overlap between the empty p orbital of a carbonium ion center and the sp3 orbital of anadjacent alkyl group creates a stabilizing effect . This overlap results in some electron density beingshared with the carbonium ion, somewhat helping the electron deficiency. This phenomenon is called Hyperconjugation and is a very important concept which will reoccur throughout the course. Themore nearby alkyl groups the greater the effect. Thus the order of stability of carbocations is:

Tertiary > Secondary > Primary > MethylThus in the case of addition of HBr to 1-hexene, the major product is 2-bromohexane (as

predicted by Markovnikov's Rule) because addition of the electrophile to the double bond at thenumber one carbon gives the more stable seco ndary carbocation at the number 2 carbon.

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CH3

H3C

H3CH

H3C

H3CH

H

H3CH

H

H

H

H

Relative Stability of Carbocations (Carbonium ions).

Tertiary 3˚

Primary 1˚

MethylSecondary 2˚

> > >

Carbocation stability is caused by hyperconjugation

filled

sp3

orbitals

emptyp orbital

Hyperconjugation between the empty p orbital andadjacent sigma bonds formed from sp3 orbitalshelps to stabilize the carbocation.

c. Carbonium Ion Rearrangements

In some cases, an initial carbonium ion intermediate will migrate to a different portion of themolecule by shifting a nearby alkyl group or hydride. This is done so as to produce a more stable carbocation . This will give, after trapping by the nucleophile, a rearranged product. The drivingforce for all these rearrangements is to form a more stable intermediate carbocation!(i.e. primary to secondary, secondary to tertiary, etc.)

H3C

CC

CH2

H3CCH3

H

H Cl

H3C

CCH

CH3

H3CCH3

H3C

CCH

CH3

H3CCH3

Cl

H3C

CH3C

C H

CH3

CH3 CH3

CH3C C H

CH3

CH3

Cl

Note direction ofcurved arrows.

STEP 1:Electrophillic Additionfollows MarkovnikovRule

Intermediate is asecondary cation

Cl-

STEP 2:Rearrangementmethyl groupshift......forms tertiarycarbocation

2-Chloro-3,3-dimethylbutane17% of product mixture

Cl-

2-chloro-2,3-dimethylbutane83% of product mixture

Rearrangement of Carbonium Ions

104 . Hammond Postulate

Certain reactions have their product distributions determined by the rates of a pair of competitive pathways . Since these reactions do not involve product equilibration, it is the relative height of the transition states leading to the two products that determines the ratio of the twoproducts. Pathways with lower barriers will occur faster, hence more of the reactantwill cycle over to this product. These reactions are said to occur under kinetic control.

+ HBr

Br

Br

ENERGY

REACTION COORDINATE

TS 1

TS 2

The transition state leading to the secondary carbonium ion (TS 1) will have some ofthe structure of this species. This transition state will be lower in energy than the transition state leading to the primary carbonium ion (TS 2), which has some of the structure of the less stable primary carbonium ion. Thus the rate at which 2-bromo-propane will be formed is higher than that for 1-bromopropane; 2-bromopropane will be the predominate product.

Hammond's Postulate Applied to Alkene Addition