Chapter 4-1. Alkenes: Reactions and Synthesis Based on McMurry’s Organic Chemistry, 6th edition

Alkene Reactions Addition Substitution Diels-Alder Cleavage

Addition Reaction The characteristic reaction of alkenes is addition—the  bond is broken and two new  bonds are formed.

Diverse Reactions of Alkenes Alkenes react with many electrophiles to give useful products by addition reactions (often through special reagents) alcohols (add H-OH) alkanes (add H-H) halohydrins (add HO-X) dihalides (add X-X) halides (add H-X) diols (add HO-OH)

Types of Additions

Carbocation formation Radical formation Addition Reactions Syn addition Anti addition Carbocation formation Radical formation

Syn & Anti Addition to Alkenes Because the carbon atoms of a double bond are both trigonal planar, the elements of X and Y can be added to them from the same side or from opposite sides.

syn-Addition versus anti-Adition anti addition 18

Syn Addition Reactions Hydrogenation Hydroxylation Hydroboration

Anti Addition Reactions Hydroxylation Halogenation Halohydrin formation

Hydroxylation Syn addition Anti addition

Halogenation Halohydrin formation Anti addition Alkene Addition Reactions via Halonium Ions

Carbocation Formation Hydration Alcohol formation HX Addition Alkyl halide formation

Electrophilic Addition The most common reaction of alkenes is electrophilic addition. E+ Nu-- loosely held  electrons E + Carbocation intermediate E+ electrophile nucleophile E+ Nu Nu--

Electrophilic Addition of HX to Alkenes General reaction mechanism: electrophilic addition Attack of electrophile (such as HBr) on  bond of alkene produces carbocation and bromide ion Carbocation is itself an electrophile, reacting with nucleophilic bromide ion

Electrophilic Addition for Syntheses The reaction is successful with HCl and with HI as well as HBr. Note that HI is generated from KI and phosphoric acid

Regioselectivity of Hydrogen Halide Addition: Markovnikov's Rule 7

Orientation of Electrophilic Addition: 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 regiospecific Markovnikov observed in the 19th century that in the addition of HX to alkene, the H attaches to the carbon with the most H’s and X attaches to the other end (to the one with the most alkyl substituents) This is Markovnikov’s rule

Markovnikov's Rule When an unsymmetrically substituted alkene reacts with a hydrogen halide, the hydrogen adds to the carbon that has the greater number of hydrogen substituents, and the halogen adds to the carbon that has the fewer hydrogen substituents. 8

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

Mechanistic Basis for Markovnikov's Rule: acetic acid HBr CH2 CH3CH2CH Br CH3CH2CHCH3 9

Mechanistic Basis for Markovnikov's Rule: Example 1 CH3CH2CH—CH3 + Br – + HBr CH2 CH3CH2CH Br CH3CH2CHCH3 9

Mechanistic Basis for Markovnikov's Rule: Example 1 + CH3CH2CH2—CH2 primary carbocation is less stable: not formed CH3CH2CH—CH3 + Br – + HBr CH2 CH3CH2CH Br CH3CH2CHCH3 9

Mechanistic Basis for Markovnikov's Rule Protonation of double bond occurs in direction that gives more stable of two possible carbocations. 12

Markovnikov’s Rule

Mechanistic Basis for Markovnikov's Rule: Example 3 Cl CH3 0°C HCl 14

Mechanistic Basis for Markovnikov's Rule: Example 3 + CH3 Cl – HCl CH3 H Cl CH3 0°C HCl 14

Mechanistic Basis for Markovnikov's Rule: Example 3 secondary carbocation is less stable: not formed H H H + CH3 + CH3 Cl – H HCl CH3 H Cl CH3 0°C HCl 14

Practice Problem:

Solution:

Problem: Major products?

Problem: Which alkene?

Stability of Carbocations and Markovnikov’s Rule More stable carbocation forms faster Tertiary cations and associated transition states are more stable than primary cations

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 Therefore stability of carbocations: 3º > 2º > 1º > +CH3

Carbocation Stability stability due to hyperconjugation

Carbocation Rearrangements in Hydrogen Halide Addition to Alkenes 15

Carbocation Rearrangements supports carbocation formation. 2° carbocation rearranges to more stable 3° carbocation by a hydride shift. Alkyl groups can also rearrange by an alkyl shift.

Example Account for the formation of 2-chloro-3-methylbutane and

Carbocation Rearrangements

Carbocation Rearrangements

Free Radical Addition of HBr (not HCl or HI) Peroxides 0% 100% Anti-Markovnikov Product Mechanism: Free radical chain reaction Initiation R-O-O-R 2 R-O. R-O. + HBr R-O-H + Br. warm

. . Propagation a. b. What is the alternative reaction in step (a)? 30 radical –stable Br. . What is the alternative reaction in step (a)? Answer: It is possible to form a 20 radical. However, since it is less stable, it does not form!

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

The addition of H2O In the presence of a strong acid (e.g. H2SO4) and water, the addition reaction also takes place.

Another example: isobutene

Polymerization In the presence of only a catalytic amount of H+ and a high concentration of the olefin, polymerization takes place.

Problem: Which starting Alkenes?

Anti-Markovnikov addition of H2O Anti-Markovnikov addition of water is achieved via a hydroboration reaction, i.e. addition of BH3 to the double bond.

hydroboration-oxidation Anti-Markovnikov orientation.  100% yields.  no rearrangements 

Addition of Water to Alkenes: Hydroboration Herbert Brown (HB) invented hydroboration (HB) Borane (BH3) is a Lewis acid Borane adds to an alkene to give an organoborane

The mechanism (I) The nucleophilic alkene attacks the electron poor boron atom

Hydroboration-Oxidation Forms an Alcohol from an Alkene Addition of H-BH2 (from BH3-THF complex) to three alkenes gives a trialkylborane Oxidation with alkaline hydrogen peroxide in water produces the alcohol derived from the alkene

Orientation in Hydration via Hydroboration Regiochemistry is opposite to Markovnikov orientation OH is added to carbon with most H’s H and OH add with syn stereochemistry, to the same face of the alkene (opposite of anti addition)

Hydroboration, Electronic Effects give Non-Markovnikov More stable carbocation is consistent with steric preferences

BH3 can add to more olefins

Mechanism of oxidation (III) Syn Stereoselective: OH has same positional orientation as the B in the trialkylborane

The overall result

Predict the Product => syn addition

Addition of Halogens to Alkenes Bromine and chlorine add to alkenes to give 1,2-dihalides, an industrially important process F2 is too reactive and I2 too unreactive Cl2 reacts as “Cl+ Cl-”; Br2 is similar

Addition of Halogens to Alkenes Bromine and chlorine add to alkenes to give 1,2-dihalides, an industrially important process F2 is too reactive and I2 too unreactive Cl2 reacts as “Cl+ Cl-”; Br2 is similar

Addition of Br2 to Cyclopentene Addition is exclusively trans (stereospecific)

Mechanism of Bromine Addition Br+ adds to an alkene producing a cyclic cation: a bromonium ion, in which bromine shares charge with carbon

Mechanism of Bromine Addition Since the Br blocks one face, one must get anti (trans) addition

For a cyclohexene, anti addition corresponds to trans diaxial addition

TESTS FOR C=C DOUBLE BONDS

PERMANGANATE TEST FOR A C=C DOUBLE BOND Baeyer Test POSITIVE TEST KMnO4 reacts implies the presence of a double bond KMnO4 brown sludge MnO2 (+) purple NEGATIVE TEST (-) KMnO4 is not decolorized compound with a suspected double bond implies that a double bond is not present

BROMINE TEST FOR A C=C DOUBLE BOND POSITIVE TEST Br2 reacts implies the presence of a double bond Br2 / CCl4 (+) purple colorless NEGATIVE TEST (-) Br2 is not decolorized (does not react) compound with a suspected double bond implies that a double bond is not present

Halohydrin Formation If a halogen is added in the presence of water, a halohydrin is formed Water is the nucleophile, instead of halide. Product is Markovnikov and anti.

Regioselectivity in Halohydrin Fromation Nucleophilic attack occurs at the more substituted carbon end of the bridged halonium ion because that carbon is better able to accommodate the partial positive charge in the transition state.

Regiospecificity The most highly substituted carbon has the most positive charge, so nucleophile attacks there.

Predict the Product Predict the product when the given alkene reacts with chlorine in water.

Reduction of Alkenes: Catalytic Hydrogenation Addition of H-H across C=C Reduction in general is addition of H2 or its equivalent

Catalytic Hydrogenation Alkene + H2  Alkane Catalyst required, usually Pt, Pd, or Ni. Finely divided metal, heterogeneous Syn addition

Syn Addition

Mechanism of catalytic hydrogenation. B X Y H 8

Mechanism of catalytic hydrogenation. B Y C C A X H H H H 9

Mechanism of catalytic hydrogenation. B X Y H H H H 9

Mechanism of catalytic hydrogenation. B Y A X H H H C C H 9

Mechanism of catalytic hydrogenation. B X Y H H 9

Mechanism of catalytic hydrogenation. B X Y H H 9

Selectivity in Hydrogen Addition 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

Solid Fats from Liquid Oils

fats & oils: triglycerides CH2—O—CCH2CH2CH2CH2CH2CH2CH2CH2CH3 | O CH—O—CCH2CH2CH2CH2CH2CH2CH2CH2CH3 CH2—O—CCH2CH2CH2CH2CH2CH2CH3 “saturated” fat

Stereoselectivity A reaction in which a single starting material can give two or more stereoisomeric products but yields one of them in greater amounts than the other (or even to the exclusion of the other) is said to be stereoselective. 20

Example of stereoselective reaction H3C CH3 H Example of stereoselective reaction H2, cat CH3 H3C H CH3 H3C H Both products correspond to syn addition of H2. 21

Example of stereoselective reaction H3C CH3 H Example of stereoselective reaction H2, cat CH3 H3C H But only this one is formed. 21

Example of stereoselective reaction H3C CH3 H Example of stereoselective reaction H2, cat Top face of double bond blocked by this methyl group CH3 H3C H 21

Example of stereoselective reaction H3C CH3 H Example of stereoselective reaction H2, cat CH3 H3C H H2 adds to bottom face of double bond. 21

Six-membered ring formation (4+2) Diels-Alder Reaction Six-membered ring formation (4+2) Diene + Dienophile

Chapter 4-1, Questions 25, 26, 33, 36, 37, 40