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

Alkenes, Alkynes

Required background: Thermodynamics from general chemistry Hybridization Molecular geometry Curved arrow notation Acidity and basicity Essential for: 1. Reactions of elimination 2. Selective reactions 3. Multistep reaction mechanisms 4. Role of petroleum in the economy 5. Stereochemistry of reactions 6. Role of polymers in the economy 7. Chemistry of aromatic compounds

Outline 1. Bonding in Alkenes 2. Nomenclature of Alkenes. Cis-trans-isomerism 3. Physical properties of Alkenes 4. Preparation of Alkenes 5. Catalytic Hydrogenation of Alkenes 6. Carbocations 7. Hydrohalogenation of Alkenes 8. Acid-catalyzed Hydration of Alkenes 9. Halogenation of Alkenes 10. Hydroboration-Oxidation of Alkenes 11. Polymers 12. Bonding in Alkynes 13. Acidity of Alkynes

 (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 normally weaker, 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 sp 3 -hybrid orbitals. When the carbon is bonded by one  -bond, one p-orbital participates in the formation of this  - bond, and the remaining one s-orbital and two p-orbitals are involved in the formation of  - bonds through the sp 2 -hybrid orbitals. The presence of sp 2 -hybridized carbons (hosts of sp 2 -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 (C 2 H 4 ). It is the simplest signaling agent in biology, responsible for ripening apples and other fruits.

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.

Nomenclature of alkenes Same rules as for alkanes, except: 1. Replace “-ane” with “-ene” 2. The principal carbon chain must contain the double bond 3. Numbering of the principal chain: The double bond must have the lowest number 4. In the chemical name, indicate position of the double bond 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

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

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 least number of hydrogens attached Dehydration of alcohols

Dehydrohalogenation Dehydrogenation

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 tertiary alkyl carbocations, based on the  to p interaction. Participation of higher in energy  -orbitals strongly increases stability of carbocations at double bonds due to the  to p interaction. Benzylic > allylic > alkyl > vinylic

The first carbocation (triphenylmethyl cation) was synthesized in 1901 by Noris and Kehrmann. 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 and radicals among the most reactive intermediates in organic chemistry

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 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

This reaction was introduced by Herbert Brown in 1955 and brought 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.

The intermediate R-BH 2 normally reacts further with another molecule of alkene until a trialkylborane R 3 B is formed. It does not change the reaction product, but enhances regioselectivity due to the steric hindrance around the fragment R. The mechanism of hydroboration determines its syn-stereoselectivity:

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 remaining one 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

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

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

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

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