Alkenes, Alkynes. Required background: Thermodynamics from general chemistry Hybridization Molecular geometry Curved arrow notation Acidity and basicity

Alkenes, Alkynes

Required background:Thermodynamics from general chemistryHybridizationMolecular geometryCurved arrow notationAcidity and basicity Essential for:1. Reactions of elimination2. Selective reactions3. Multistep reaction mechanisms4. Role of petroleum in the economy5. Stereochemistry of reactions6. Role of polymers in the economy7. Chemistry of aromatic compounds

Outline1. Bonding in Alkenes2. Nomenclature of Alkenes. Cis-trans-isomerism3. Physical properties of Alkenes4. Preparation of Alkenes5. Catalytic Hydrogenation of Alkenes6. Carbocations7. Hydrohalogenation of Alkenes8. Acid-catalyzed Hydration of Alkenes9. Halogenation of Alkenes10. Hydroboration-Oxidation of Alkenes11. Polymers12. Bonding in Alkynes13. Acidity of Alkynes

C

HH

H

H

-bond

sp3 s

A B

-bond

pp

(sigma)-bonds are symmetrical with respect to rotation around the bond.Rotation of fragments around the bond does not break the bond.

(pi)-bonds are non-symmetrical with respect to rotation around the bond.Rotation of fragments around the bond breaks the bond. The -bonds are normallyweaker, than the -bonds, due to a weaker orbital overlap.

Together,- and - bonds form a double bond.Now we need to choose a hybridization to describe systems, containing a double bond.

The valence shell of the atom of carbon has one s-orbital and three p-orbitals.When the carbon is not bonded by any -bonds, all s- and p-orbitals are involved in the formation of -bonds, through the sp3-hybrid orbitals.

When the carbon is bonded by one-bond, one p-orbital participates in the formation of this - bond, and the remainingone s-orbital and two p-orbitals are involved in the formation of -bonds through the sp2-hybrid orbitals.

The presence of sp2-hybridized carbons (hosts of sp2-orbitals) is characteristic for alkenes.

The presence of a weaker -bond in alkenes accounts for their higher chemical reactivity, comparing with alkanes.

The simplest alkene is ethylene (C2H4). It is the simplest signaling agent in biology, responsible for ripening apples and other fruits.

Propane

C3H8

CnH2n+2

Propene

C3H6

CnH2n

Homologous formula for alkenes

(but not only for them)

CH3CH2

CH3 CH3

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Due to the planar geometry of the double bond, two substituents can be located at either the same side of the double bond, or at the opposite sides of the double bond.

Note: cis-trans-isomerism is impossible if at least one carbon at the double bond has two identical substituents.

H

R

H

R

R

H

H

R

cis-isomer trans-isomer

R

R

H

R

R

R

H

R

identical



Nomenclature of alkenes

Same rules as for alkanes, except:

1. Replace “-ane” with “-ene”2. The principal carbon chain must contain the double bond3. Numbering of the principal chain:The double bond must have the lowest number4. In the chemical name, indicate position of the double bond

CH2 CH3

CH37

1

2

3 4

5

6

2-propyl-1-heptene

5. If the compound contains more, than one double bond, replace “-ene” with “-diene”, “-triene” etc.6. Indicate stereochemistry (cis- or trans-)


Step 1. For each double bond, assign relative priorities of attached fragments

Step 2. Find highest priority fragments at each carbon. If they are “cis-”, the isomer is “Z”. If they are “trans”, the isomer is “E”.

E-, Z- Nomenclature

CH3

Cl

Br

H

Cl

CH3

Br

H

E-isomer Z-isomer



Dipole moments are a little higher, than for alkanes due to polarization of -bonds

Dipole moments for cis-isomers are normally higher, than for trans-isomers

CH2CH3 CH3

CH3

= 0.46D = 0.085D

H

Cl

Cl

H

Cl

H

Cl

H = 0 > 0

More substituted double bonds are more stable, than less substituted double bonds.

Trans-isomers are more stable, than cis-isomers.

To compare relative stability of isomeric alkenes with higher accuracy, heats of hydrogenation can be used instead of heats of combustion.


Zaitsev’s Rule: Hydrogen comes off the carbon with the leastnumber of hydrogens attached

Dehydration of alcohols

CH3

CH3

OH

CH2

CH3

+

CH3H

CH3 H

CH3CH3

H H

+ Major products

Minor product

H+

CH3

CH3

O+

H H

CH3

CH+

CH3

H

Dehydrohalogenation

Dehydrogenation

CH2 CH2

Br

H

CH3O -

CH2 CH2 + + CH3OHBr -

CH2 CH2 + H2CH3 CH3

Catalyst, heat

Catalyst = Pt or Ni or other catalysts


R1

R2

R3

R4

+ X - Y

R3

R4

Y

X

R2

R1

Addition to double bonds in general

CH2 CH2 + H2 CH3 CH3

Catalyst = Pt or Ni or other catalysts


Stability of alkyl substituted carbocations: tertiary > secondary > primary

Delocalization of positive charge increases stability of carbocations.So far, we compared stabilities of primary, secondary and tertiaryalkyl carbocations, based on the to p interaction.

Participation of higher in energy -orbitals strongly increases stabilityof carbocations at double bonds due to the to p interaction.

Benzylic > allylic > alkyl > vinylic

The first carbocation (triphenylmethyl cation) was synthesizedin 1901 by Noris and Kehrmann.

C+

Existence of carbocations was proved by George Olah (NMR, X-Ray)and brought him the Nobel Prize in 1994.

Carbocations are considered alongside with carboanions, carbenes andradicals among the most reactive intermediates in organic chemistry


CH2

R

CH3

R

Cl

CH2 CH2

R

Cl

a

bWhich path is preferred?HCl

Path a

Path b

CH2

R

CH2

R

H+

H+

CH CH3

R

CH3

R

Cl

CH2 CH2

R R

Cl

+

+

Cl-

Cl-

secondary

primary

Markovnikov’s rule (1896)

The halogen of a hydrogen halide attaches to the carbon of the alkene bearing the fewer number of hydrogens and greater number of carbons

Examples of hydrohalogenation


For the reaction of hydration the Markovnikov’s rule works the best

CH3

CH3

CH2 + H2OH+

CH3

CH3

CH3

OH

+

CH3

CH3

OH

Exclusive product

CH2 CH2 + H2OH3PO4

300 oCCH3 OH

600,000,000 lb of ethanol is produced annually in the US by this reaction.

Hydration is the reaction of dehydration, going backwards.


There are several mechanisms of halogenation of alkenes.We will consider electrophilic halogenation.

Consequence of the bromonium cation formation: anti-stereoselectivity of addition

Br+

H

H

Br

Br

+ Br Br- Br -

Step 1

Step 2

Electrophilic attack

Bromonium cation

Nucleophilic addition (anti-)

Br+

H

H

Br -Trans-product


This reaction was introduced by Herbert Brown in 1955 andbrought him the Nobel Prize in 1979. Hydroboration, followed by oxidation is used when we need to perform hydration of a double bond against the Markovnikov’s rule.

CH2

R

CH3

R

OH

CH2 CH2

R

OH

a

bWhich path is preferred?

1. BH3

2. H2O2, OH-

Path a

Path b

CH2

R

CH2

R

CH CH2

R

CH3

R

BH2

CH CH2

R R

BH2

+

+

More stabletransition state

BH2 H

BH2H

H2B H

BH2H

CH3

R

OH

R

OH

H2O2, OH-

H2O2, OH-

The intermediate R-BH2 normally reacts further with another molecule of alkene until a trialkylborane R3B is formed. It does not change the reaction product, but enhancesregioselectivity due to the steric hindrance around the fragment R.

The mechanism of hydroboration determines its syn-stereoselectivity:

1. BH3

2. H2O2, OH-CH3

CH3

CH3

CH3

H

OH


Monomer

R1

R2

R3

R4

R1

R2

R3

R4n

n

PolymerExtent of polymerization

Typically n = 3,000 - 40,000


In alkynes, one- and two - bonds form a triple bond.Now we need to choose a hybridization to describe systems, containing a triple bond.

The valence shell of the atom of carbon has one s-orbital and three p-orbitals.

When the carbon is bonded by two-bonds, each of two p-orbitals participates in the formation of this - bond, and the remainingone s-orbital and one p-orbital are equally involved in the formation of -bonds, giving rise to the sp-hybridization state.

The presence of sp-hybridized carbons is characteristic for alkynes.

- and -bonds in alkynes

-bondsp

s

C C HH

p

CH CH

-bond -bond


The relatively high acidity of alkynes significantly affects their chemical properties.

C C HR1

NaNH2C C NaR1

C CR1 Na+-

+

C C HR1

RMgBrC C MgBrR1 + RH

C C HR1

CuCl, NH3C C CuR1

A colored precipitate,explosive when dry

Same reaction takes place, if Cu is replaced with Ag

An acetylenide (a strong nucleophile)

Documents

Alkenes, Alkynes. Required background: Thermodynamics from general chemistry Hybridization Molecular geometry Curved arrow notation Acidity and basicity