CHEM 41B-MIdterm 2 Mechanisms

CH. 11-14 Mechanisms

cis (Z)

cis (Z)

the biggest substituents at the same side of double bond

trans (E)

The biggest substituents at opposite sides of double bond

Catalytic hydrogenation

-Pd / Pt as catalysts & produces alkane.

-exothermic

What is heat of hydrogenation

-measure of relative stability of the alkene

-more stable the alkene, the less its heat of hydrogenation.

E1 Elimination (unimolecular)

-The dissociation of the starting material to a carbocation is the rate limiting step.

-The reaction rate depends only on the concentration of starting material.

E2 Elimination (bimolecular)

-The reaction rate depends on the concentration of both the base and the starting alkane.

-The overall reaction proceeds in a single step that includes deprotonation by the base

-creation of double bond

-departure of the leaving group

-Reagents: B-(base) and +BH (conjugate acid)

Hofmann Rule (Bulky bases)

bulky bases remove the most sterically accessible proton. Form less substituted (more kinetically favored alkene)

Saytzev Rule (non-bulky bases)

Non bulky bases remove the most substituted proton. Thus predominantly form the more substituted (more thermodynamically stable alkene).

antiperiplanar

orientation required in order to align all orbitals in desired way and thus allow the elimination to occur in a single step.

Electrophilic Halogenation of alkenes

addition of X2 to form alkyl dihalides

-anti addition

electrophilic addition on the double bond: hydrohalogenation (addition of HX to form alkyl halides)

addition of HX to form alkyl halides

Electrophilic Hydration of alkenes

markovnikov hydration produces an alcohol where the hydroxyl group is installed at most substituted carbon.

-addition of H₂O under acid catalysts(H+ -OSO₃H) to form alcohols

-mostly anti addition

-catalytic in acid

-produces most substituted alcohol & is reversible

Epoxidation

-reaction of alkenes with peroxides to form 3-membered O-containing rings (cyclopropanes)

-The reaction proceeds with syn stereospecificity (trans alkene gives trans epoxide)

- The commonly used peroxide is MCPBA

-The epoxidation proceeds faster with more substituted double bonds.

-The acid catalyzed opening of epoxides in water gives anti diols

Hydrogenation

-addition of H2 to alkenes under Pd catalysis to form alkanes

-proceeds only under Pd/Pt catalysis

-exothermic

-syn addition

-hydrogenation proceeds predominantly from the is hindered face

dihydroxylation

ozonolysis

-reaction of alkenes with ozone (O3) to produce carbonyl groups)

-this is an oxidative cleavage of alkenes to produce carbonyl groups

-two of the three oxygen atoms of ozone are added to the alkene

-Me₂S is used to trap the extra oxygen from ozone

Markovnikov addition (anti addition)

-A,B are added from opposite faces of the double bond

-The A-B bond is broken before the reaction with the alkene

-This addition proceeds in a stepwise manner via an initital 3-membered intermediate (aleken reacts w/ electrophile)

-nucleophile reacts in an SN2 manner (thus from opposite face) @ most substituted (most δ+) carbon center.

anti-Markovnikov additions (syn addition)

-A,B added from same side of double bond

-B-A bond not broken prior to the reaction with the alkene. Reaction proceeds in a concerted manner via a four-membered intermediate.

-predict product by orienting the A-B reactant across the alkene in a manner that complements the relative polarization of all centers.

Hydroboration/oxidation

-addition of BH3 and H202 oxidation to form anti-markovnikov alcohols

-produces an alcohol where the hydroxyl group is at the least substituted carbon (anti-markovnikov hydration)

-addition of water=produces least substituted alcohol

-the regioselectivity of the rxn can be improved using bulky borane (9-BBN)

2-step cyclopropanation

-trapping of bromonium ion by other nucleophiles

-reagents: 1) H2O and Br₂ 2) NaOH

-anti-addition of alkene to cyclo-structures

-markovnikov addition

-2 step cyclopropanation

osmylation

-syn dihydroxylation with OsO4 to form syn diols

-the dihydroxylation with OsO4 prodces after reductive cleavage a syn diol

-the Os(VIII) is reduced to Os(VI)

-H₂S reactant in second step

-the osmylation can be catalytic in osmium by the use of an oxidant (H2O2)

-alkene dihydroxylation can also be done with KMnO4

Preparation of Alkynes

1. from dihaloalkanes by double elimination

2. By addition of an alkynyl group to electrophiles

Markovnikov addition to alkynes (anti additions)

electrophilic addition on the double bond: hydrohalogenation (formation of alkyl halides)

• anti addition of HBr

• Br @ most substituted C

• often double addition (geminal dibromoalkanes)

electrophilic halogenation of alkenes (addition of X2 to form alkyl dihalides (and/or tetrahalides)).

Electrophilic hydration of alkenes: addition of H2O under acid catalysis to form carbonyls (tautomerization)

• The acid-catalyzed hydration of alkynes follows the markovnikov rule

• the hydration is catalyzed by HgSO4 (not always needed but speeds up the process)

• The intermediate enol tautomerizes to the ketone (similar to 1,3-hydrogen shift).

Stepwise reduction of alkynes with Na/liq. NH3 to trans alkenes

• stepwise one electron reduction

• reaction mechanism is NOT required

Hydrogenation (addition of H2 to alkynes under Pd catalysis to form alkenes and alkanes)

• synchronous (syn) addition of H2

• often over reduction to alkanes

• lindlar catalyst: a posioned Pd catalyst that reduces alkynes to alkenes but is very slow and inefficient in reducing alkenes to alkanes.

  • Pd/CaCO₃/quinoline

Hydroboration/Oxidation (alkynes)

• addition of BH3 and H2O2 oxidation to form carbonyls; tautomerization

• the hydroboration is sunchronous and proceeds in one step

• the hydroboration/oxidation introduces the HO-H group from the same face (syn) of the alkyne

• The intermediate enol tautomerizes to the aldehyde (similar to 1,3-hydrogen shift).

reactivity of alkenyl halides

• the double bond of alkenyl halides is electronically rich; thus akynes don’t react with nucleophiles

• under strong basic conditions they eliminate (lose HX) to form alkynes.

• alkyl halides (vinyl halides) can be converted to potent nucleophiles upon (grignard) and can react with electrophiles.