Alkenes and Alkynes Notes

Alkenes and Alkynes

Nomenclature of Alkenes and Cycloalkenes

• Alkenes are named by identifying the longest chain containing the double bond and changing the name of the corresponding parent alkane from -ane to -ene.
• The compound is numbered to give one of the alkene carbons the lowest number.
• The double bond of a cycloalkene must be in position 1 and 2.
• Compounds with double bonds and alcohol hydroxyl groups are called alkenols. The hydroxyl is the group with higher priority and must be given the lowest possible number.
• Two groups which contain double bonds are the vinyl ($CH<em>2=CH$) and the allyl ($CH</em>2=CHCH_2$) groups.
• If two identical groups occur on the same side of the double bond, the compound is cis.
• If they are on opposite sides, the compound is trans.
• Several alkenes have common names which are recognized by IUPAC.

The (E)-(Z) System for Designating Alkene Diastereomers

• The Cahn-Ingold-Prelog convention is used to assign the groups of highest priority on each carbon.
  • If the group of highest priority on one carbon is on the same side as the group of highest priority on the other carbon, the double bond is Z (zusammen).
  • If the highest priority groups are on opposite sides, the alkene is E (entgegen).

Relative Stabilities of Alkenes

• Generally, cis alkenes are less stable than trans alkenes because of steric hindrance.
• Heat of Hydrogenation: The relative stabilities of alkenes can be measured using the exothermic heats of hydrogenation.
  • The same alkane product must be obtained to get accurate results.
• The greater the number of attached alkyl groups, the greater the alkene’s stability.

Synthesis of Alkenes via Elimination Reactions

Dehydrohalogenation

• Reactions by an E2 mechanism are most useful; E1 reactions can be problematic.
• E2 reactions are favored by:
  • Secondary or tertiary alkyl halides
  • Alkoxide bases such as sodium ethoxide or potassium tert-butoxide
• Bulky bases such as potassium tert-butoxide should be used for E2 reactions of primary alkyl halides.

Zaitsev’s Rule

• Formation of the most substituted alkene is favored with a small base.
• Some hydrogen halides can eliminate to give two different alkene products.
• When two different alkene products are possible in an elimination, the most highly substituted (most stable) alkene will be the major product.
  • This is true only if a small base such as ethoxide is used.
• The transition state in this E2 reaction has double bond character.
  • The trisubstituted alkene-like transition state will be most stable and have the lowest $\Delta G^{\ddagger}$.
• Kinetic control of product formation: When one of two products is formed because its free energy of activation is lower and therefore the rate of its formation is higher, this reaction is said to be under kinetic control.

Formation of the Least Substituted Alkene Using a Bulky Base

• Bulky bases such as potassium tert-butoxide have difficulty removing sterically hindered hydrogens and generally only react with more accessible hydrogens (e.g., primary hydrogens).

The Stereochemistry of E2 Reactions: The Orientation of Groups in the Transition State

• All four atoms involved must be in the same plane.
  • Anti coplanar orientation is preferred because all atoms are staggered.
  • In a cyclohexane ring, the eliminating substituents must be diaxial to be anti coplanar.
• Neomenthyl chloride and menthyl chloride give different elimination products because of this requirement.
  • In neomenthyl chloride, the chloride is in the axial position in the most stable conformation. Two axial hydrogens anti to chlorine can eliminate; the Zaitsev product is major.
  • In menthyl chloride, the molecule must first change to a less stable conformer to produce an axial chloride. Elimination is slow and can yield only the least substituted (Hoffman) product from anti elimination.

Acid Catalyzed Dehydration of Alcohols

• Recall that elimination is favored over substitution at higher temperatures.
• Typical acids used in dehydration are sulfuric acid and phosphoric acid.
• The temperature and concentration of acid required to dehydrate depends on the structure of the alcohol.
  • Primary alcohols are most difficult to dehydrate; tertiary are the easiest.
• Rearrangements of the carbon skeleton can occur.

Mechanism for Dehydration of Secondary and Tertiary Alcohols: An E1 Reaction

• Only a catalytic amount of acid is required since it is regenerated in the final step of the reaction.

Carbocation Stability and the Transition State

• Recall the stability of carbocations is: tertiary > secondary > primary.
• The second step of the E1 mechanism in which the carbocation forms is rate determining.
• The transition state for this reaction has carbocation character.
• Tertiary alcohols react the fastest because they have the most stable tertiary carbocation-like transition state in the second step.
• The relative heights of $\Delta G^{\ddagger}$ for the second step of E1 dehydration indicate that primary alcohols have a prohibitively large energy barrier.

A Mechanism for Dehydration of Primary Alcohols: An E2 Reaction

• Primary alcohols cannot undergo E1 dehydration because of the instability of the carbocation-like transition state in the 2nd step.
• In the E2 dehydration, the first step is again protonation of the hydroxyl to yield the good leaving group water.

Carbocation Stability and the Occurrence of Molecular Rearrangements

• Rearrangements of carbocations occur if a more stable carbocation can be obtained.
• In the third step, the less stable 2° carbocation rearranges by shift of a methyl group with its electrons (a methanide); this is called a 1,2 shift.
• The removal of a proton to form the alkene occurs to give the Zaitzev (most substituted) product as the major one.
• A hydride shift (migration of a hydrogen with its electrons) can also occur to yield the most stable carbocation.
• Carbocation rearrangements can lead to formation of different ring sizes.

Chemical Reactions of Alkenes - Addition

• i) H2
• ii) HX
• iii) Hydration
  • a) H2O, H+
  • b) oxymercuration-demercuration
  • c) hydroboration
• iv) H2SO4
• v) X2 in CCl4 & H2O

Chemical Reactions of Alkenes - Oxidation

• i) Syn 1,2-dihydroxylation
• ii) Cleavage with KMnO4, OH-, heat
• iii) Cleavage with O3

Carbenes

• Methylene & dihalocarbenes

Hydrogenation

• Hydrogen adds to alkenes in the presence of metal catalysts.
  • Heterogeneous catalysts: finely divided insoluble platinum, palladium or nickel catalysts.
  • Homogeneous catalysts: Catalyst (typically rhodium or ruthenium based) is soluble in the reaction medium.
    • Wilkinson’s catalyst is $Rh[(C<em>6H</em>5)<em>3P]</em>3Cl$.
• This process is called a reduction or hydrogenation.
  • An unsaturated compound becomes a saturated (with hydrogen) compound.
• The catalyst provides a new reaction pathway with lower $\Delta G^{\ddagger}$ values.
• In heterogeneous catalysis, the hydrogen and alkene adsorb to the catalyst surface and then a step-wise formation of C-H bonds occurs.
  • Both hydrogens add to the same face of the alkene (a syn addition).
  • Addition to opposite faces of the double bond is called anti addition.

Addition of HX: Markovnikov’s Rule

• Addition of HBr to propene occurs to give 2-bromopropane as the major product.
• Markovnikov’s Rule (Original): addition of HX to an alkene proceeds so that the hydrogen atom adds to the carbon that already has the most hydrogen atoms.
• The reaction has a highly endergonic first step (rate determining) and a highly exergonic second step.

Theoretical Explanation of Markovnikov’s Rule

• The product with the more stable carbocation intermediate predominates.
• The most stable carbocation is formed fastest because it has a lower $\Delta G^{\ddagger}$.
• The transition state for the rate determining step (first step) resembles a carbocation and is stabilized by factors which stabilize carbocations.
• Addition of HBr to 2-methylpropene gives only tert-butyl bromide.
• Modern Statement of Markovnikov’s Rule: In the ionic addition of an unsymmetrical reagent to a double bond, the positive portion of the adding reagent attaches itself to a carbon atom of the double bond so as to yield the more stable carbocation as an intermediate.
• Regioselective Reaction: When a reaction that can potentially yield two or more constitutional isomers actually produces only one or a predominance of one isomer.

Hydration

Acid-Catalyzed Hydration

• The reaction of alkenes with dilute aqueous acid leads to Markovnikov addition of water.
• The mechanism is the reverse of that for dehydration of an alcohol.
  • The first step in which a carbocation is formed is rate determining.
• The hydration of alkenes and the dehydration of alcohols are simply reverse reactions of one other.
  • The reaction is governed by the position of all the equilibria.
  • Hydration is favored by addition of a small amount of acid and a large amount of water.
  • Dehydration is favored by concentrated acid with very little water present (removal of water produced also helps favor dehydration).
• Carbocation rearrangements can occur.

Oxymercuration-Demercuration: Markovnikov Addition

• The procedure gives high yields of alcohols and avoids rearrangements.
• The reaction shows Markovnikov selectivity.
• The mechanism involves formation of a bridged mercurinium ion.

Hydroboration: Anti-Markovnikov Syn Hydration

• The reaction leads to syn and anti-Markovnikov addition of water to alkenes.
• The elements of hydrogen and boron are added across the double bond.
  • In practice, a borane complex with the solvent tetrahydrofuran (THF) is often used.

Mechanism of Hydroboration

• Boron hydride adds successively to three molecules of alkene.
• Boron becomes attached to the least substituted carbon of the double bond.
  • The bulky boron group can approach the least sterically hindered carbon more easily.
  • This orientation also allows a $\delta+$ charge in the transition state to reside at the most substituted carbon.
  • This orientation leads to anti-Markovnikov product.
• The boron and hydride add with syn stereochemistry.

Oxidation and Hydrolysis of Alkylboranes

• Oxidation and hydrolysis to the alcohol takes place with retention of stereochemistry at the carbon bonded to boron.
• Hydroboration of methylcyclopentene gives the anti-Markovnikov product with syn addition of the elements of water.

Summary of Alkene Hydration Methods

• Acid-catalyzed hydrolysis: Markovnikov addition
• Oxymercuration: Markovnikov addition
• Hydroboration-Oxidation: anti-Markovnikov and syn addition

Addition of Sulfuric Acid to Alkenes

• Addition of concentrated sulfuric acid to alkenes leads to alkyl hydrogen sulfates which are soluble in the acid. The addition follows Markovnikov’s rule.
• The sulfate can be hydrolyzed by heating with water. The net result is Markovnikov addition of water to an alkene.

Addition of Bromine and Chlorine to Alkenes

• Addition produces vicinal dihalides.
• This reaction is used as a test for alkenes because the red color of the bromine reagent disappears when an alkene (or alkyne) is present.
  • Alkanes do not react with bromine in the dark.
• A bromonium ion intermediate results instead of the carbocation seen in other addition reactions.

Stereochemistry of the addition of Halogens to Alkenes

• The net result is anti addition because of SN2 attack on the bromonium ion intermediate.
• When cyclopentene reacts, the product is a racemic mixture of trans-1,2-dibromocyclopentane enantiomers.

Stereospecific Reactions

• A reaction is stereospecific if a particular stereoisomeric form of the starting material reacts in such a way that it gives a specific stereoisomeric form of the product.
• Example: cis- and trans-2-butene give stereoisomeric products when halogenated. Halogenation of double bonds is stereospecific.
• In unsymmetrical alkenes, the bromonium ion will have some of its $\delta+$ charge density on the most substituted of the two carbons.
  • The most substituted carbon can best accommodate $\delta+$ charge.
  • The water nucleophile will tend to react at the carbon with the most $\delta+$ charge.

Chemical Reaction of Alkenes-Oxidations

Syn 1,2-Dihydroxylation

• Either OsO4 or KMnO4 will give 1,2 diols (glycols).

Mechanism for Syn Hydroxylation of Alkenes

• Cyclic intermediates result from reaction of the oxidized metals.
• The initial syn addition of the oxygens is preserved when the oxygen-metal bonds are cleaved and the products are syn diols.

Oxidation of Alkenes

Cleavage with KMnO4, OH-, heat

• Reaction of an alkene with hot KMnO4 results in cleavage of the double bond and formation of highly oxidized carbons.
  • Unsubstituted carbons become CO2, monosubstituted carbons become carboxylates, and disubstituted carbons become ketones.
• This can be used as a chemical test for alkenes in which the purple color of the KMnO4 disappears and forms brown MnO2 residue if alkene (or alkyne) is present.

Cleavage with O3-Ozonolysis

• Cleavage of alkenes with ozone and workup with zinc in acetic acid leads to less highly oxidized carbons than products from cleavage with hot KMnO4.
  • Unsubstituted carbons are oxidized to formaldehyde, monosubstituted carbons are oxidized to aldehydes, and disubstituted carbons are oxidized to ketones.
• Ozone adds across the double bond to form the initial ozonide which rearranges to a highly unstable ozonide.
• The ozonides react with zinc and acetic acid to effect the cleavage.

Divalent Carbon Compounds: Carbenes

• Carbenes have divalent but neutral carbons with a lone pair of electrons. Carbenes are highly reactive.

Structure and Reaction of Methylene

• Methylene can be made by heat or light initiated decomposition of diazomethane.
  • Loss of a molecule of the stable gas nitrogen drives this reaction.
• Methylene reacts with alkenes to form cyclopropanes.

Reactions of Other Carbenes: Dihalocarbenes

• Carbenes add to double bonds in a stereospecific manner.
• Dihalocarbenes are formed by $\alpha$-elimination of compounds such as chloroform.

Carbenoids: The Simmons-Smith Cyclopropane Synthesis

• A carbene-like species is formed which then reacts with alkenes.