Organic Chemistry Interactive Notes jim.maxka@nau.edu Chapter 7 Reactions of Alkenes

Part 1: Formation of Alkenes Elimination – based on removal of Beta-hydrogen and the elimination of the halide ion or other leaving group.

C C

R4

R3

H

R1

R2

BrLeaving Group

Beta-Hydrogen What makes a leaving group?

Br

H

O-

CH3 + OH CH3 + Br-

Is methoxide a good enough base to make the reaction go to the right? What is the driving force of this reaction? Another possibility is based on an alcohol. Here we use a catalyst to make the OH into a Leaving Group.

OH

H

O+

P

HO

-

OHOH

OHP

O

OH OH

OH

CH+

H

- H+

Leaving Group

P

O

OH OH

OH

P

O-

OH OH

OH OH

H+

H2O+

Part 2: Reactions of Alkenes Addition of Strong Bronsted-Lowry Acids-Ch 6 from previous chapter. Draw arrows.

CH3

+ H Br

BrCH3

C+

CH3

Br-

• Br goes to the most substituted site. • Rearrangements are possible from the C+ intermediate.

Organic Chemistry Interactive Notes jim.maxka@nau.edu Addition of non-protic Electrophiles These reactions take the general form as below, where E is a Lewis Acid.

RCH2 E Nu

R

CH+

E

fast R

E

Nu

CH3

E

Nu

anti addition E = halogen

RCH2 Br Br

R

CH+

Br

fast R

Br+

Br-

CH3

Br

Br

anti addition When a dihalogen is the electrophile, the reaction has an unusual form. The cation intermediate becomes a bridged halonium ion. We only see anti addition. The Lewis acid nature of Br2.

Cyclic molecules prove the mechanism. Only anti addition explains trans products: Step 1

Step 2

E = halogen, Nu ≠ halogen: E on the least substituted and Nu on the most substituted carbon.

Organic Chemistry Interactive Notes jim.maxka@nau.edu When the alkene is not symmetrical. Here is the Mechanism:

Predict these products. Pay attention to regiochemistry,

Answers: virtual text: http://www.cem.msu.edu/~reusch/VirtualText/addene1.htm#add3 Formation of alcohols Simple acid-catalyzed hydration reactions OH goes to Markovnikov position Rearrangements can occur Simple reaction scheme

CH3

+C

+CH3

O+

H

H H+ OH2

CH3

O+

H

H OH2

CH3

OH

H3O+

What is the role of H+? How many moles of water are consumed?

Organic Chemistry Interactive Notes jim.maxka@nau.edu One example too many. Predict the products of this reaction:

CH3

CH3

CH2

dilute aqueous acid

More sophisticated methods: E Nu Modification Regiochemistry Mercuration-Demurcuration Hg(OAc)+ H2O NaBH4 markovnikov Hydroboration BH3 H:- H2O2/H2O anti-markovnikov Mercuration-Demercuration Control of reaction through a bridged intermediate. What is the mode of the Nu:-- coming into the molecule.

Full Mechanism

Organic Chemistry Interactive Notes jim.maxka@nau.edu Hydroboration BH3 exists as B2H6. To stabilize BH3 for organic synthesis, we make the Lewis acid/Lewis base complex with an ether, usually THF.

O

THF

B

H

H H

borane Oversimplified addition scheme of BH3 – added as E-Nu, where BH3 is E.

More reasonable addition scheme for hydroboration.

http://www.cem.msu.edu/~reusch/VirtualText/addene1.htm#add2 The addition is always syn. Here is the complete mechanism.

BR3 is turned into OH by H2O2 in aqueous base. What is the fate of B?

Organic Chemistry Interactive Notes jim.maxka@nau.edu Summary of 2-step anti and syn mechanisms.

http://www.cem.msu.edu/~reusch/VirtualText/addene1.htm#add2 Putting things together: Write full reactions for the following: Anti-markovnikov syn-addition process to make an alcohol from methylcyclohexene. Mercuration-Demercuration of of methylcyclohexene. Simple hydration of 1-vinyl cyclopentane with 10% H2SO4 Carbene/Carbenoid Additions R2C: is an electron deficient carbon species called carbene. What is the structure of carbene. Carbenes with halogens are stabilized. Draw resonance structures for chlorocarbene.

CCl

ClCCl

Cl

Organic Chemistry Interactive Notes jim.maxka@nau.edu Carbene-like species can be created by the Metal reduction of haloalkanes to form carbenoids that can add to alkenes to make cyclopropane rings. Such a reaction would look like CHCl3 + KOH :CCl2 + (H-Cl—KOH) acid + base titration :CCl2 Predict the reaction of CHCl3 + propene in strong base (KOH) Another form is the Simmons-Smith reaction. This reaction makes :CH2. The driving force for this reaction is the removal of I by a metal. CH2I2 + Zn ZnI2 + :CH2

CH2I2 + Zn + propene Practice: Write a reaction that would make 1,1-dichloro-2-ethylcyclopropane Bicyclo[4.1.0]heptane (cyclohexane fused with cyclopropane) Transition State of Carbene Reactions and the Hammond Postulate

H2C CH2

C:ClCl

DonorAcceptor

H2C CH2

C:Cl

Cl

approach of carbene transistion state

CH2H2C

ClCl

************************************************************************************************************** Reduction and Oxidation Reduction is adding bonds to H. Oxidation is adding bonds to O. Oxidation #’s in organic chemistry O2- is -2; H+ is +1. Cl- is -1., etc. All else is derived. What about H2O? Consider metals first. What is the oxidation state of Mn in MnO4

-? In MnO2? FeCl2 FeCl3 Fe2O3? What is the most reduced form of C? oxidized form of C? What is the oxidation state on C in ethane? In ethanol CH3CH2OH? What is the process of nitrogen fixation? N2 NH3 NO2

Organic Chemistry Interactive Notes jim.maxka@nau.edu Reduction of alkenes with H2 – catalytic hydrogentation. This reaction occurs at the surface of a metal – usually Pt or Pd. Always syn.

http://www.cem.msu.edu/~reusch/VirtualText/addene2.htm#add3c Complete these reduction reactions

CH3 CH3H2

Pt

CH3 CH3

CH3 CH3

H2

Pt

CH3

CH3

H2

Pd

Oxidation reactions Syn Hydroxylation

• MnO4 cold in basic solution • OsO4 followed by mild reductive elimination of the metal

http://www.cem.msu.edu/~reusch/VirtualText/addene2.htm#add3c Be sure that you see why hydration (formation of alcohol) is not oxidation. H2C=CRH + H2O (dilute sulfuric acid) CH3CRH-OH

Organic Chemistry Interactive Notes jim.maxka@nau.edu Ozonolysis Addition of ozone cleaves the double bond and makes some unstable intermediates. The break up of these intermediates is usually controlled by either further oxidation leads to ketones or carboxylic acids. or reduction leads to ketones or aldehydes.

Bill Reusch, Virtual Textbook Some examples of reductive workup. Zn or Me2S:

http://www.cem.msu.edu/~reusch/VirtualText/addene2.htm#add3c Oxidative workup cleaves all C-H bonds but not C-C bonds. So, any C-H bond becomes a carboxylic acid and any C-C bond becomes a ketone.

H

CH3

1. O3

2. KMnO4 or H2O2

Because KMnO4 is such a strong oxidizing agent, it will accomplish the above two steps at once if reacted hot in acidic solution.

CH2

CH3

CH3

KMnO4

Hot

Organic Chemistry Interactive Notes jim.maxka@nau.edu Retracing Steps—What alkenes produced these ozonolysis products:

http://www.cem.msu.edu/~reusch/VirtualText/addene2.htm#add3c Oxidative Diol Cleavage 1,2 – alcohols can be selectively cleaved with strong oxoacids:

http://www.cem.msu.edu/~reusch/VirtualText/addene2.htm#add3c The products are always ketones or aldehydes – never a carboxylic acid.

Organic Chemistry Interactive Notes jim.maxka@nau.edu Polymers

Organic Chemistry Interactive Notes jim.maxka@nau.edu Answers to practice: Electrophilic Additions of alkenes with Lewis Acids

Bill Reusch, Virtual Textbook Alkene retracing from ozonolysis products:

Bill Reusch, Virtual Textbook.