Chapter 4 Reactions of Alkenes and Alkynes Suggested Problems: 21-2,27,33,35-39,41-3,45-8,53

Diverse Reactions of Alkenes Alkenes react with many electrophiles to give useful products by addition (often through special reagents)

4.1 Addition of HX to Alkenes: Markovnikov’s Rule In an unsymmetrical alkene, HX reagents can add in two different ways, but one way may be preferred over the other If one orientation predominates, the reaction is regioselective Markovnikov observed in the 19th century that in the addition of HX to an alkene, the H attaches to the carbon with more H’s and X attaches to the carbon with fewer H’s (to the carbon with more alkyl substituents) This is Markovnikov’s rule

Example of Markovnikov’s Rule Addition of HCl to 2-methylpropene Regiospecific – one product forms where two are possible If both ends have similar substitution, then not regiospecific

Markovnikov’s Rule (restated) More highly substituted carbocation favored as an intermediate rather than less highly substituted one Tertiary cations and associated transition states are more stable than primary cations (3o > 2o >1o)

Markovnikov’s Rule (restated)

4.2 Carbocation Structure and Stability Carbocations are planar and the tricoordinate carbon is surrounded by only 6 electrons in sp2 orbitals the fourth orbital on carbon is a vacant p-orbital the stability of the carbocation (measured by energy needed to form it from R-X) is increased by the presence of alkyl substituents

4.3 Addition of Water to Alkenes Hydration of an alkene is the addition of H-OH to give an alcohol Acid catalysts are used in high temperature industrial processes: ethylene is converted to ethanol

4.3 Addition of Water to Alkenes Hydration of an alkene is the addition of H-OH to give an alcohol Acid catalysts are used in high temperature industrial processes: ethylene is converted to ethanol

4.3 Addition of Water to Alkenes Oxymercuration-demercuration In the laboratory, alkenes are often hydrated by the oxymercuration–demercuration procedure Reaction is initiated by electrophilic addition of Hg2+ ion to the alkene (affords mercurinium ion) Regiochemistry - Markovnikov addition of H2O

Worked Examples What products would you expect from oxymercuration–demercuration of the following alkenes? a) b) Solution: Oxymercuration is equivalent to Markovnikov addition of H2O to an alkene Hydration of Alkenes: Addition of H2O by oxymercuration

Worked Examples b) Hydration of Alkenes: Addition of H2O by oxymercuration

4.3 Addition of Water to Alkenes Hydroboration-Oxidation Hydroboration–oxidation affords alternate regiochemistry – Anti-Markovnikov Hydroboration involves addition of B-H bond of borane, BH3, to an alkene to yield an organoborane intermediate Boron is oxidized to afford an OH in place of BH2

4.3 Addition of Water to Alkenes Hydroboration-Oxidation Hydroboration–oxidation occurs with syn stereochemistry – B, H, and OH add to same face of double bond.

Worked Example What alkene might be used to prepare the following alcohol by hydroboration–oxidation? Solution: The products result from hydroboration/oxidation of a double bond The –OH group is bonded to the less substituted carbon of the double bond in the starting material Hydration of Alkenes: Addition of H2O by hydroboration

4.3 Addition of Water to Alkenes Hydration of carbon-carbon double bonds also occurs in various biological pathways but not by the carbocation mechanism

4.4 Addition of Halogens to Alkenes Bromine and chlorine add to alkenes to give 1,2-dihaldes, an industrially important process F2 is too reactive and I2 does not add Cl2 reacts as Cl+ Cl- Br2 is similar

Addition of Br2 to Cyclopentene Addition is exclusively trans

Mechanism of Bromine Addition Br+ adds to an alkene producing a cyclic ion Bromonium ion, bromine shares charge with carbon Gives trans addition

Bromonium Ion Mechanism Electrophilic addition of bromine to give a cation is followed by cyclization to give a bromonium ion This bromoniun ion is a reactive electrophile and bromide ion is a good nucleophile

4.5 Reduction of Alkenes: Hydrogenation Addition of H-H across C=C Reduction in general is addition of H2 or its equivalent Requires Pt or Pd as powders on carbon and H2 Hydrogen is first adsorbed on catalyst Reaction is heterogeneous (process is not in solution)

Hydrogen Addition - Selectivity Selective for C=C. No reaction with C=O, C=N Polyunsaturated liquid oils become solids If one side is blocked, hydrogen adds to other

Mechanism of Catalytic Hydrogenation Heterogeneous – reaction between phases Addition of H-H is syn

4.6 Oxidation of Alkenes: Epoxidation, Hydroxylation, and Cleavage Epoxidation results in a cyclic ether with an oxygen atom Epoxides undergo acid catalyzed ring-opening . Hydroxylation is the net of the two-step reaction.

Epoxide Ring-Opening Mechanism Protonation of epoxide Nucleophilic addition of water Anti (trans) addition of nucleophile to epoxide

Permangate Hydroxylation of Alkenes Hydroxylation of an alkene can occur in a single step via the reaction with potassium permanganate (KMnO4) Requires a basic solution Note – cis diol formed.

Permangate Oxidation of Alkenes Potassium permanganate (KMnO4) can cleave the double bond and produce carboxylic acids and carbon dioxide if H’s are present on C=C If hydrogens are not present on the double bond, carbonyl containing products are produced. Requires an acid solution.

Permangate Oxidation of Alkenes The type of product produced varies with the number of hydrogens on the double bond.

4.7 Addition of Radicals to Alkenes: Polymers A polymer is a very large molecule consisting of repeating units of simpler molecules, formed by polymerization Alkenes react with radical catalysts to undergo radical polymerization Ethylene is polymerized to polyethylene, for example Examples on this slide result from condensation rxns

Free Radical Polymerization: Initiation Initiation - a few radicals are generated by the reaction of a molecule that readily forms radicals from a nonradical molecule A bond is broken homolytically

Polymerization: Propagation Radical from initiation adds to alkene to generate alkene derived radical This radical adds to another alkene, and so on many times Note: BzO represents a benzoyloxy moiety

Polymerization: Termination Chain propagation ends when two radical chains combine Not controlled specifically but affected by reactivity and concentration

Other Polymers Other alkenes give other common polymers

4.8 Conjugated Dienes Compounds can have more than one double or triple bond If they are separated by only one single bond they are conjugated and their orbitals interact The conjugated diene 1,3-butadiene has properties that are very different from those of the nonconjugated diene, 1,4-pentadiene.

Orbital Interaction Orbital view of buta-1,3-diene Each of the four carbon atoms has a p orbital Allowing for an electronic interaction across the C2-C3 single bond

Addition of HX and X2 Conjugated diene reactions are similar to that of alkenes But, distinct differences in the addition of HX and X2

Carbocations from Conjugated Dienes Addition of H+ leads to delocalized secondary allylic carbocation

4.9 Stability of Allylic Carbocations: Resonance Some molecules have structures that cannot be shown with a single representation In these cases we draw structures that contribute to the final structure but which differ in the position of the  bond(s) or lone pair(s) Such a structure is delocalized and is represented by resonance forms The resonance forms are connected by a double-headed arrow

Allylic Carbocations Three electrons are delocalized over three carbons Spin density surface shows single electron is dispersed

Allylic Carbocations and Resonance A structure with resonance forms does not alternate between the forms Instead, it is a hybrid of the resonance forms, so the structure is called a resonance hybrid

Products of Addition to Delocalized Carbocation Nucleophile can add to either cationic site The transition states for the two possible products are not equal in energy

4.10 Drawing and Interpreting Resonance Forms Resonance is an extremely useful explanatory concept for a variety of chemical phenomena In the acetate ion, both C—O bonds are identical Acetate is a resonance hybrid of two structures with both oxygen atoms sharing the p electrons and the negative charge equally.

Resonance Hybrids Another example, benzene (C6H6) has two resonance forms with alternating double and single bonds In the resonance hybrid, the actual structure, all its C-C bonds are equivalent, midway between double and single

Rules for Resonance Forms Individual resonance forms are imaginary - the real structure is a hybrid (only by knowing the contributors can you visualize the actual structure) Resonance forms differ only in the placement of their  or nonbonding electrons Different resonance forms of a substance do not have to be equivalent Resonance forms must be valid Lewis structures: the octet rule generally applies The resonance hybrid is more stable than any individual resonance form would be

4.11 Alkynes and Their Reactions Reduction accomplished by addition of H2 over a metal catalyst Is a two-step reaction using an alkene intermediate Reduction of alkynes

4.11 Alkynes and Their Reactions Addition of H2 using chemically deactivated palladium on calcium carbonate as a catalyst (the Lindlar catalyst) produces a cis alkene The two hydrogens add syn (on the same side of the triple bond)

Addition of HX to Alkynes Involves Vinylic Carbocations Addition of H-X to alkyne should produce a vinylic carbocation intermediate Secondary vinyl carbocations form less readily than primary alkyl carbocations Primary vinyl carbocations probably do not form at all Nonetheless, H-Br can add to an alkyne to give a vinyl bromide if the Br does not end up on a primary carbon Markovnikov’s Rule applies

Addition of Bromine and Chlorine Initial addition gives trans intermediate Product with excess reagent is tetrahalide

Addition of H2O Addition of H-OH as in alkenes Mercury (II) catalyzes Markovnikov oriented addition Hydroboration-oxidation gives the anti-Markovnikov product

Formation of Acetylide Anions Terminal alkynes are weak Brønsted acids (alkenes and alkanes are much less acidic (pKa ~ 25. See Table 9.1 for comparisons)) Reaction of strong anhydrous bases with a terminal alkyne produces an acetylide ion (NaNH2 is base of choice) The sp-hydbridization at carbon holds negative charge relatively close to the positive nucleus

Formation of Acetylide Anions Acetylide ions can react as nucleophiles as well as bases Reaction with a primary alkyl halide produces a hydrocarbon that contains carbons from both partners, providing a general route to larger alkynes

Let’s Work a Problem Prepare n-octane from 1-pentyne.

Let’s Work a Problem Prepare n-octane from 1-pentyne. The best strategy to approach this problem is to use acetylide coupling: