The characteristic reaction of alkenes is addition to the double bond. + A—B C C A C C B Reactions of Alkenes

Reactions involving carbocations: 1. 1.Substitutions via Sn Eliminations via the E Additions to alkenes and alkynes. Reminder: Reactions of Alkenes CH 3 + 2° carbocation CH 3 + 3° carbocation 1,2-alkyl shift for stability

Terms: Driven by weaker pi bond to 2, new, stronger sigma bonds. Overall Transformation: C=C to a new functional group. Regioselectivity: Which side a specific group is added to. Stereoselectivity: Whether the addition is cis or trans. Nucleophile: pi bond has high electron density; reacts with electrophiles. Electrophile: An electron poor species, which reacts with electron rich species. Electrophilic Addition: An electrophile adds to a nucleophile, where two systems combine to form one product. In actuality, the nucleophile attacks the electrophile Alkene Addition Terms

syn additionanti addition syn Addition versus anti Addition

General Format: Depending on timing, two different mechanisms are possible: Two Step: Full Carbocation Mechanism 1: Reaction of the C=C with E+  Carbocation 2: Nucleophilic attack. Electrophilic Addition :Nu - Nu E + E Nu  +  - E +

One Step: Partial Carbocation Mechanism Simultaneous formation of the two sigma bonds. Tips: 1. 1.Find the electrophile The alkene attacks the electrophile - Regioselectivity. 3. Figure out if carbocations are formed. - 2 step reaction is not stereoselective. Electrophilic Addition ENu ENu

One Step: Partial Carbocation Mechanism Simultaneous formation of the two sigma bonds. Tips: 4.Determine if the new sigma bonds are from the same species - Syn addition. 5.Determine if the two new sigma bonds are from different species - Anti addition. Electrophilic Addition ENu ENu

CH 3 CH 2 CH 2 CH 3 HH CH 3 CH 2 CH 2 CHCH 2 CH 3 Br (76%) CHCl 3, -30°C C C HBr Example

Electrophilic addition of hydrogen halides to alkenes proceeds by rate-determining formation of a carbocation intermediate. Mechanism

Electrons flow from the  system of the alkene (electron rich) toward the positively polarized proton of the hydrogen halide. Mechanism

Free Radical Mechanism: Free Radical Mechanism Br Br H Br Br H

Which mechanism is used for the following reactions? Carbocation Radical Learning Check

Electrophilic Addition: Hydrogenation: Hydrohalogenation: Hydration: Hydroboration: Alkene Reactions

Halogenation: Halohydration: Hydrohalogenation: Epoxidation: Ozonolysis: Alkene Reactions

Reaction Type: Electrophilic Addition (Reduction). Used for heats of hydrogenation stability. Overall Reaction: H 2 + Alkene  Alkane Requires: Metal catalyst (Pt, Pd, Ni, Rh). Regioselectivity: None. Stereospecificity: Only syn addition products. Hydrogenation

HH C C A B X Y HH Mechanism of Catalytic Hydrogenation

H HHH C C A B X Y Absorption of H by catalyst

H HHH C C A B X Y Mechanism of Catalytic Hydrogenation Addition of alkene to catalyst.

H HHH C C A B X Y Mechanism of Catalytic Hydrogenation

HH H C C A B X Y H Addition of H to alkene.

HH H C C A B X Y H Mechanism of Catalytic Hydrogenation Alkane leaves catalyst with H in syn position.

CO 2 CH 3 (100%) H 2, Pt Example of syn-Addition CO 2 CH 3 H H

Example 1: Example 2: Learning Check H 2 or

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. Markovnikov's Rule

Occurs due to the formation of the most stable carbocation: Markovnikov rules extend to other additions which don’t involve halides: Markovnikov Example Major product

Br CH 3 CH 2 CHCH 3 CH 2 CH 3 CH 2 CH acetic acid HBr Mechanistic Basis for Markovnikov's Rule: Example 1

Br CH 3 CH 2 CHCH 3 CH 2 CH 3 CH 2 CH HBr CH 3 CH 2 CH—CH 3 + Br – + CH 3 CH 2 CH 2 —CH 2 + primary carbocation is less stable: not formed Mechanistic Basis for Markovnikov's Rule: Example 1

Mechanistic Basis for Markovnikov's Rule: Example 2 H CH 3 Cl HCl 0°C

HCl H CH 3 Cl H H CH 3 + secondary carbocation is less stable: not formed CH 3 H + H Cl –

Reactions that are Anti Markovnikov add the hydrogen to the side with the least number of hydrogens. The ‘group’ is attached to the less substituted carbon. Reaction does not go through a carbocation intermediate. Anti Markovnikov

Reaction of Alkenes with HBr (no radical formation). Reaction Type: Overall Reaction: HX + Alkenes  Alkyl Halide. Hydrogen Halide Reactivity Order: HI > HBr > HCl > HF (same as acidity order) Regioselectivity: Markovnikov Rule. “For addition of hydrogen halides to alkenes, the H atom adds to the C with the most H atoms already present” meaning it goes through the more stable carbocation intermediate.” Hydrohalogenation Electrophilic Addition; Reduction (H) and Oxidation (X)

Reaction of Alkenes with HBr (no radical formation). Stereoselectivity: None, during mechanism, a planar carbocation is formed. Requirements: HBr, No light (hv), no peroxides, no radicals created. Hydrohalogenation

Mechanism: Hydrohalogenation H H H CH 3 H + H C CH 3 CH 3 + Br - H C CH 3 CH 3 Br Protonate alkene. Can be viewed as an acid base reaction (Lewis). Pi electrons act as a Lewis Base. Bromide ion is nucleophile. Carbocation is electrophile. Forms most stable carbocation. Nucleophilic attack. Forms alkyl bromide.