Organic A

Chapter 8Alkenes (I)

By Prof. Dr.Adel M. Awadallah

Islamic University of Gaza

Alkenes and AlkynesHydrocarbons (contain only carbon and hydrogen)

a) Saturated: (Contain only single bonds)

Alkanes (CnH2N + 2 )

Cycloalkanes (CnH2N )

b) Unsaturated: contain

Alkenes: double bonds (,,,CnH2N)

Alkynes: triple bonds ((CnH2N - 2)

Aromatic: benzene like compounds

Facts about double and triple bonds

HH

bond angle 109.5 120 o 180 o

bond length 154 pm 134 pm 121 pm

rotation possible restricted restricted

geometry tetrahedral triagonal planer linear

Hypridization sp3 sp2 sp

Bond Length in Benzene 139 pm (plannar, sp2 hypridized)

A pi bond is one in which the electrons in the p orbitals are held above and below the plane of the molecule.The sigma bond is stronger than the pi bond.A double bond is formed from a sigma bond and a pi bond, and so it is stronger than a single bond.

Physical Properties

• Physical properties:

• non-polar or weakly polar

• no hydrogen bonding

• relatively low mp/bp ~ alkanes

• water insoluble

• Importance:

• common group in biological molecules

• starting material for synthesis of many plastics

The Chemistry of Vision

The more substituted alkene will form

• Saytzeff orientation:• In dehydrohalogenation the preferred product is the alkene that has

the greater number of alkyl groups attached to the doubly bonded carbon atoms

• (the more substituted alkene will form)

• Ease of formation of alkenes:• R2C=CR2 > R2C=CHR > R2C=CH2, RCH=CHR > RCH=CH2 >

CH2=CH2

• Stability of alkenes:

• R2C=CR2 > R2C=CHR > R2C=CH2, RCH=CHR > RCH=CH2 > CH2=CH2

• CH3CH2CHCH3 + KOH(alc) CH3CH2CH=CH2 RCH=CH2

• Br 1-butene 19%• sec-butyl bromide +• CH3CH=CHCH3 RCH=CHR• 2-butene 81%

Mechanisms of EliminationE2 with concentrated base 3>2>1second order rate = K[RX][B]

Mechanisms of EliminationE1 with dilute or weak base 3>2

first order rate = K[RX]

• Order of reactivity in E2: 3o > 2o > 1o

• CH3CH2-X CH2=CH2 3 adj. H’s

• CH3CHCH3 CH3CH=CH2 6 adj. H’s & more stable

• X alkene

• CH3 CH3• CH3CCH3 CH=CCH3 9 adj. H’s & most stable• X alkene

Evidence for the E2 mechanism1) second order2) No Rearrangement3) Show a large hydrogen isotope effect

Primary hydrogen isotope effect:

A bond to hydrogen (protium) is broken faster than a bond to deuterium (D) KH / KD = 5 - 8

This means that the breaking of hydrogen is in the rate determining step

Evidence for the E2 mechanismThe Absence of Hydrogen Exchange

The carbanion mechanism (E1cB elimination unimolecular of the conjugate base)

Run the reaction until about half the substrate had been converted into alkene. Unconsumed 2-phenylethyl bromide was recovered. It contained no deuterium. So, the reaction was not acompanied by hydrogen exchange. This rules out the carbanion mechanism

Evidence for the E2 mechanismThe Element Effect (is the breaking of the C-X bond in the rate determining step????)

Strength of the bond

R-F > R-Cl > R-Br > RI

Reactivity toward SN2, SN1, E2 and E1

R-I > R-Br > R-Cl > R-F

So, R-X bond breaking is in the rate determining step

E1 Mechanism

•Elimination, unimolecular E1

•a) RX: 3o > 2o > 1o •b) rearragement possible

•c) may yield mixtures •d) Saytzeff orientation

•e) element effect•f) no isotope effect

•g) rate = k [RW]

The E1 reaction: Orientation

Elimination vs. substitution

Substitution is generally the main reaction, but, E1 Elimination occurs more with 3 > 2 >1

CH3CH3

Br

CH3

CH3CH3

Br

H

CH3CH3

OH

CH3

CH3CH3

OH

H

CH3 CH2

CH3

CH3 CH2

H

+

+

EtOH / H2O

EtOH / H2O

80 oC

80 oC

19%

5%

2. dehydration of alcohols:a) ROH: 3o > 2o > 1o

b) acid is a catalystc) rearrangements are possible d) mixtures are possible e) Saytzefff) mechanism is E1

80 oC80 oC

Mechanism of Dehydration (E1)

Dehydration (Rearrangement)

E1 Mechanism, Rearrangement

Synthesis of 1-butene from 1-butanol:

• CH3CH2CH2CH2-OH + HBr CH3CH2CH2CH2-Br• SN2• E2 KOH(alc)•• CH3CH2CH=CH2

• only!

• To avoid the rearrangement in the dehydration of the alcohol the alcohol is first converted into an alkyl halide.