preparation of alkenes: crude oil & elimination reactions

Preparation of Alkenes:Crude Oil & Elimination

Reactions

Alkenes: Crude Oil Sources & Products

-Elimination Reactions / Alkenes

X Y

dehydrogenation of alkanes: (enzymatic and limited

industrial use)X = Y = H

dehydration of alcohols:X = H; Y = OH

dehydrohalogenation of alkyl halides:X = H; Y = Br, etc.

C CC C + X Y

-Elimination Reactions Overview

Because of steric strain, cis isomers are generally less stable than trans.

The difference in stability can be quantified by comparing the heats of combustion.

Alkene Stability

Alkene Stability

Consider the following stability trend:

What pattern do you see?See SKILLBUILDER 8.3.

Alkene Stability

A. Tetrasubstituted > cis-disubstituted > trans-disubstituted > monosubstitutedB. Trisubstituted > terminal disubstituted > trans-disubstituted > monosubstitutedC. Tetrasubstituted > trans-disubstituted > cis-disubstituted > trisubstituted D. Monosubstituted > cis-disubstituted > trans-disubstituted > trisubstituted 

Question

What is the correct order of stability (most stable to least stable) for alkenes?

Alkene Nomenclature

1) Find the longest continuous chain that includes the double bond.

2) Replace the -ane ending of the unbranched alkane having the same number of carbons with -ene.

3) Number the chain in the direction that gives the lowest number to the doubly bonded carbon.

H2C CHCH2CH3 1-Butene orBut-1-ene

Alkene Nomenclature

4) If a substituent is present, identify its position by number. The double bond takes precedence over alkyl groups and halogens when the chain is numbered.The compound shown above is4-bromo-3-methyl-1-butene.

(or 4-bromo-3-methylbut-1-ene)

H2C CHCHCH2Br

CH3

Alkene Nomenclature

4) If a substituent is present, identify its position by number. Alcohol groups take precedence over the double bond when the chain is numbered.The compound shown above is2-methyl-3-buten-1-ol.

(or 2-methyl-but-3-en-1-ol)

H2C CHCHCH2OH

CH3

Alkene Nomenclature

methylene

vinyl

allyl

CHH2C

H2C CHCH2

H2C

Common Alkenyl Groups

1) Replace the -ane ending of the cycloalkane having the same number of carbons with -ene.

2) Number through the double bond in thedirection that gives the lower number to the first-appearing substituent.

6-Ethyl-1-methylcyclohexeneCH3

CH2CH3

Cycloalkene Nomenclature

1

23

4

5 6

Question

Name the alkene below according to the IUPAC system.

A) 2-bromo-5-heptaneB) 6-bromo-2-trans-hepteneC) 2-bromo-5-cis-hepteneD) 5-bromo-2-heptene

A. (E, 2S)-2-methyl-3-propylhept-5-eneB. (E, 5S)-6-methyl-5-propylhept-2-eneC. (E, 5S)-5-isopropyloct-2-eneD. (E, 2R)- 2-methyl-3-propylhept-5-eneE. (E, 5R)- 5-isopropyloct-2-ene

What is the correct IUPAC name for the following structure?

Question

Dehydration of Alcohols

H2SO4

160°CCH3CH2OH H2C CH2 + H2O

(82%)

C OH

CH3

CH3

H3CH2SO4

heatCH2

H3C

C

H3C

+ H2O

Dehydration of Alcohols

(79-87%)

H2SO4

140°C+ H2OOH

Question

The dehydration of 2-methyl-2-propanol cannot be accomplished by using which of thefollowing reagents?A) H2SO4

B) H3PO4

C) HClD) K2SO4

R

R'

R"

OHC

R

R'

H

OHC

R

H

H

OHC

Relative Reactivity

tertiary:most reactive

primary:least reactive

Question

Of the isomeric alcohols having the molecular formula C5H12O, which one will undergo

acid-catalyzed dehydration most readily?A) 2-pentanolB) 2-methyl-1-butanolC) 2-methyl-2-butanolD) 3-methyl-2-butanol

Regioselectivity in Alcohol Dehydration:The Zaitsev Rule

OH

10 % 90 %

HOH2SO4

80°C+

Regioselectivity

A reaction that can proceed in more than one direction, but in which one direction predominates, is said to be regioselective.

Regioselectivity

A reaction that can proceed in more than one way to produce different products involving different carbon atoms, where one predominates. It is said to be regioselective.

84 %

H3PO4

heat+

16 %

CH3

OH

CH3 CH3

When elimination can occur in more than one direction, the principal alkene is the one formed by loss of H from the carbon having thefewest hydrogens.

R OH

CH3

C C

H

R CH2R

three protons on this carbon

The Zaitsev Rule

When elimination can occur in more than one direction, the principal alkene is the one formed by loss of H from the carbon having thefewest hydrogens.

R OH

CH3

C C

H

R CH2R

two protons on this carbon

The Zaitsev Rule

When elimination can occur in more than one direction, the principal alkene is the one formed by loss of H from the carbon having thefewest hydrogens.

R OH

CH3

C C

H

R CH2R

R

R

CH2R

CH3

C C

only one proton on this carbon

The Zaitsev Rule

Question

What is the major product in the reaction of 2-methyl-2-butanol with H2SO4 at 80°C?

A) B)

C) D)

Question

What is the major product of the dehydration of 2-methylcyclohexanol?A) 1-methylcyclohexeneB) 3-methylcyclohexeneC) 4-methylcyclohexeneD) cyclohexene

Stereoselectivity in Alcohol Dehydration

A stereoselective reaction is one in which a single starting material can yield two or more stereoisomeric products, but one is produced in greater amounts than any other.

(25%) (75%)

+

OH

H2SO4

heat

Stereoselectivity

Question

The major product of the dehydration of 1-phenyl-2-propanol is

A) B)

C) D)

(0%) (100%)

+

OH

dehydrogenase

Stereospecificity

A stereospecific reaction is one in which a single starting material yields only a single stereoisomer although other steroeisomers are possible. Enzymes do this commonly, but there are not many processes invented by man which do.

The E1 & E2 Mechanismsof Alcohol Dehydration

Dehydration of alcohols and the reaction of alcohols with hydrogen halides share thefollowing common features:

1) Both reactions are promoted by acids

2) The relative rates/ reactivity decreases in theorder tertiary > secondary > primary

These similarities promote the idea that carbocations areintermediates in the acid-catalyzed dehydration ofalcohols, just as they are in the reaction of alcoholswith hydrogen halides.

A Connecting Point...

first two steps of mechanism are identical tothose for the reaction of tert-butyl alcohol withhydrogen halides

Dehydration of tert-Butyl Alcohol

C OH

CH3

CH3

H3CH2SO4

heatCH2

H3C

C

H3C

+ H2O

first two steps of mechanism are identical tothose for the reaction of tert-butyl alcohol withhydrogen halides

Dehydration of tert-Butyl Alcohol

C OH

CH3

CH3

H3CH2SO4

heatCH2

H3C

C

H3C

+ H2O

Step 1: Proton transfer to tert-butyl alcohol

(CH3)3C O

H

..: H O+ ..

H

H

+

H

O: +(CH3)3C

H+

fast, bimolecular

tert-Butyloxonium ion

O

H

:

H

:

Mechanism

Step 2: Dissociation of tert-butyloxonium ionto carbocation

(CH3)3C O

H

:

H+

+

slow, unimolecular

(CH3)3C O

H

:

H

:

tert-Butyl cation

+

Mechanism

Because rate-determiningstep is unimolecular, thisis called the E1 mechanism.

Step 3: Deprotonation of tert-butyl cation

+ O

H

:

H

:

CH2+

H3C

C

H3C

H

fast, bimolecular

CH2

H3C

C

H3C

+ O

H

:

H

H+

Mechanism

Carbocation intermediates can:

1) react with nucleophiles

and/or

2) lose a -proton to form an alkene

Carbocations

avoids carbocation because primary carbocations

are not thermodynamically stable

oxonium ion loses water and a proton in abimolecular step

H2SO4

160°CCH3CH2OH H2C CH2 + H2O

Dehydration of Primary Alcohols

Step 1: Proton transfer from acid to ethanol

H

..: H O+OCH3CH2 ..

H

H

H

O: +

H+

fast, bimolecular

Ethyloxonium ion

CH3CH2 O

H

:

H

:

Mechanism

Step 2: Oxonium ion loses both a proton and a water molecule in the same step.

+

H

O:

H+

CH2CH2HO

H

:

H

:

slow, bimolecular

+ O

H

:

H

:O

H

H

: H+

H2C CH2+

Mechanism

Step 2: Oxonium ion loses both a proton and a water molecule in the same step.

+

H

O:

H+

CH2CH2HO

H

:

H

:

slow, bimolecular

+ O

H

:

H

:O

H

H

: H+

H2C CH2+

Mechanism

Because rate-determiningstep is bimolecular, thisis called the E2 mechanism.

The alkene product may not have the same carbon skeleton as the starting alcohol.

Rearrangements in Alcohol Dehydration

OH

H3PO4, heat

3% 64% 33%

+ +

Example

Example

3%

CH3

CHCH3CH3

CH3

+C

Carbocation can lose a proton as shown;

or it can undergo a 1,2- methyl migration.

CH3 group migrates with its pair of electrons to adjacent positively charged carbon.

Rearrangement Involves Alkyl Group Migration

3%

CH3

CHCH3

CH3

+CH3

97%

CH3

CHCH3CH3

CH3

+C C

tertiary carbocation; more stable

Rearrangement Involves Alkyl Group Migration

3%

CH3

CHCH3

CH3

+CH3

97%

CH3

CHCH3CH3

CH3

+C C

Rearrangement Involves Alkyl Group Migration

Example

Example

CH3CH2CH2CH2OH

H3PO4, heat

12%

+

mixture of cis (32%)and trans-2-butene (56%)

CH2CH3CH2CH CHCH3CH3CH

Another Rearrangement

Oxonium ion can losewater and a proton(from C-2) to give1-butene;

doesn't give a carbocation directlybecause primarycarbocations are toounstable.

CH3CH2CH2CH2 O

H

H

+:

CH2CH3CH2CH

Rearrangement Involves Hydride Shift

Hydrogen migrates with its pair of electrons from C-2 to C-1 as water is lost;

carbocation formed by hydride shift is secondary.

CH3CH2CH2CH2 O

H

H

+:

CH2CH3CH2CH

CH3CH2CHCH3+

Rearrangement Involves Hydride Shift

CH3CH2CH2CH2 O

H

H

+:

CH2CH3CH2CH

CH3CH2CHCH3+

mixture of cisand trans-2-butene

CHCH3CH3CH

Rearrangement Involves Hydride Shift

H

H

CH3CH2CHCH2 O

H

+:

+CH3CH2CHCH2 +

H

O

H

H

::

Hydride Shift

• react with nucleophiles

• lose a proton from the -carbon to form an alkene

• rearrange (less stable to more stable)

Carbocations Can...

Question

What is the major product in the reaction of 2-methyl-1-butanol with H2SO4 at 80°C?

A) B)

C) D)

H2SO4/Heat

A. B.

C. D.

OH

What is the major product of the following reaction?

Question