ALKENE AND ALKYNE REACTIONS Dr. Clower CHEM 2411 Spring 2014 McMurry (8 th ed.) sections 7.7-7.8, 7.10-7.11, 10.3-10.4, 8.2-8.8, 8.10, 8.12, 9.3-9.8, 7.1, 8.1, 9.2, 9.9

Outline • Reactions of alkenes • Reactions of alkynes • Preparation of alkenes and alkynes • Synthesis • No reactions of alkyl halides (originally on syllabus)

Reaction Charts • Help organize reaction details • Organize charts by reaction type, starting material, product • See webpage for template • Example:

Reaction Type

Oxymercuration demercuration

Starting Material

Alkene

Reagent

1. Hg(OAc) 2 , H 2 O 2. NaBH 4

Regiochemistry/ stereochemistry

Markovnikov

Anti

addition

Rearrangement possible?

Product

no Alcohol

Reactions of Alkenes I.

II.

III.

IV.

V.

Allylic halogenation Electrophilic addition Reduction Oxidation Polymerization

I. Allylic Halogenation • Similar to radical halogenation of alkanes • Alkene react with molecular halogen in the presence of heat or light • Alkyl halide is produced • Substitution of –X for –H at the allylic position • Most stable radical intermediate • Stabilized by resonance

Allylic Halogenation • Another set of reagents: • N-bromosuccinimide (NBS),

h

n • Bromination only (no chlorination) • Product is a racemic mixture (if there is a stereocenter)

Radical Stability

• What is the major product of the reaction of 1-octene with NBS (in the presence of light)?

• What is the major product of the reaction of 1-octene with NBS (in the presence of light)?

• Reaction occurs at less sterically hindered carbon and produces the more stable C=C

• What is the major product of the following reaction?

II. Electrophilic Addition • Most common reaction of alkenes • Examples: • • Break p bond of alkene Form new s bonds to each C of double bond • Alkene is nucleophile; reacts with electrophile (HX, H 2 O, etc.) • Forms carbocation intermediate

Electrophilic Addition • General mechanism: • Step 1: • Step 2: • Which step is RDS?

Addition of Hydrogen Halides • HCl, HBr, HI • Example: 2-methylpropene + HBr

• What is the major product of the following reaction?

• Stereochemistry of product = racemic mixture • Carbocation intermediate is planar,

sp

2 hybridized • Regiochemistry of reaction • Which C gets the H? Which C gets the X?

• Reaction is

regiospecific

for one product

Regiochemistry of Electrophilic Addn.

• Markovnikov’s Rule: • • In the addition of HX (or H 2 O) to an alkene, the H will add to the carbon with the greater number of H’s already bonded to it The X (or OH) attaches to the carbon with fewer H’s (the more substituted carbon) • Product = Markovnikov product • Opposite product = anti-Markovnikov or non-Markovnikov • Formed under specific conditions

Markovnikov’s Rule

Markovnikov’s Rule • Why is the Markovnikov product favored?

• Look at reaction intermediate • Carbocation • Markovnikov addition forms the more stable R + • 3º > 2º > 1º • More stable carbocation forms faster, will react to give product

Markovnikov’s Rule

• Draw and name the major product of the following reaction.

• Draw and name the major product of the following reaction.

• Expected product = • Actual product = • What happened?

Carbocation Rearrangement • Carbocation intermediates can rearrange to form a more stable carbocation structure • Hydride shift = H: moves from C adjacent to carbocation

Carbocation Rearrangement • Alkyl groups can also shift • Typically methyl or phenyl

(Major product)

Anti-Markovnikov Addition of HBr • In the presence of peroxides • • H 2 O 2 or R 2 O 2 Free radical mechanism • Only HBr, not HCl or HI

Addition of Halogens • X 2 = Br 2 or Cl 2 (F 2 too reactive, I • Solvent = inert, nonaqueous 2 unreactive) • Stereochemistry =

anti

addition • Two X atoms add from opposite sides of the C=C • Product = a vicinal dihalide • Two X atoms on adjacent carbons

Mechanism

Addition of Halogens

• Draw the major product of the following reaction.

Addition of Halogens in the Presence of Water • Stereochemistry: X and OH add

anti

• Regiochemistry: X adds to the less substituted carbon OH adds to the more substituted carbon • Mechanism the same as addition of X 2 , except H 2 O is the nucleophile in the second step

Mechanism

Mechanism • Water attacks the carbon with the largest d + • Results in OH on more substituted carbon

• Draw the major product of the following reaction.

Hydration • Addition of water • Three methods: A.

B.

Acid-catalyzed hydration Oxymercuration-demercuration C.

Hydroboration-oxidation

A. Acid-catalyzed hydration • Regiochemistry = Markovnikov • Acid catalyst typically H 2 SO 4 or H 3 PO 4 (or just H 3 O + ) • Carbocation intermediate, so rearrangement can occur

Mechanism

• Draw the major product of the following reaction.

B. Oxymercuration-demercuration • Step 1: Alkene reacts with mercuric acetate • Step 2: Reduction with sodium borohydride • Regiochemistry =Markovnikov • Stereochemistry =

anti

addition of OH and H • No rearrangements • • Milder conditions than H 3 O + Electrophile is + HgOAc • Formed by dissociation of AcO-Hg-Oac • Intermediate is bridged mercurinium ion (similar to bromonium)

Oxymercuration-demercuration

• Draw the major product for each of the following reactions.

C. Hydroboration-oxidation • Anti-Markovnikov product •

Syn

addition of H and OH (add on same side of C=C) • No rearrangements • THF stabilize highly reactive BH 3

Hydroboration-oxidation • Mechanism of first step: • • BH 2 • on the right because less steric hindrance Leads to anti-Markovnikov product Second step: H • 2 O 2 /NaOH replace Keep same stereochemistry (

syn

) –BH 2 with –OH

• Draw the major product of the following reaction.

• Draw the major product formed when the following alkene undergoes (a) acid-catalyzed hydration, (b) oxymercuration demercuration, and (c) hydroboration-oxidation.

Oxidation and Reduction • What is oxidation?

• What is reduction?

• Classify these reactions as oxidation or reduction: • CH 3 ─CH═CH 2 → CH 3 ─CH 2 ─CH 3 • CH 3 ─CH 2 ─OH → CH 3 ─CO 2 H

III. Reduction • Catalytic hydrogenation • Seen before with heat of hydrogenation (alkene stability) • Catalyst = metal, usually Pd, Pt, or Ni • Reaction takes place on metal surface • Stereochemistry =

syn

(both H’s add to same side of C=C)

Mechanism

Catalytic Hydrogenation • This reduction does not work with C=O, C=N, or benzene except at very high P or T, or with a special catalyst

IV. Oxidation • Three types A.

Epoxidation B.

C.

Hydroxylation Oxidative cleavage

A. Epoxidation • Formation of epoxide • Cyclic ether • Example: • • Reagent is peroxy acid (RCO 3 H) Stereochemistry =

syn

• Another method: treat halohydrin with base:

B. Hydroxylation • Formation of a 1,2-diol/glycol/vicinal diol • Methods: 1.

Opening of epoxide using aqueous acid • Product is

trans

diol • Mechanism:

Hydroxylation 2.

Addition of osmium tetroxide (OsO 4 ) or potassium permanganate (KMnO 4 ) • How do you know these are both oxidizing agents?

• Reaction includes some appropriate work-up • • H 2 O 2 or NaHSO 3 , H 2 O for OsO 4 HO (aq) for KMnO 4 • Stereochemistry =

syn

• Draw the major product of the following reaction.

C. Oxidative Cleavage • Oxidize and alkene and split the C=C • Results in formation of 2 carbonyls • Type of carbonyls depends on alkene structure and the oxidizing agent used • Three types of oxidizing agents 1.

2.

3.

Ozone Potassium permanganate Periodic acid

Oxidative Cleavage 1.

Ozone • Ozonolysis • Reagents: 1. O 3 2. (CH 3 ) 2 S or Zn, H 3 O + • Products = 2 carbonyls (ketones or aldehydes) • Terminal alkenes give CO 2

Oxidative Cleavage 2.

KMnO 4 • Reagents: KMnO 4 • (excess or concentrated) and heat or acid Use heat and excess KMnO 4 to split intermediate glycol • Products = 2 carbonyls (ketones or carboxylic acids) • Aldehydes oxidize to carboxylic acids in KMnO 4 • Terminal alkenes still give CO 2

Oxidative Cleavage 2.

HIO 4 • Specifically used to split glycol

• Draw the major product for each of the following reactions.

V. Polymerization • Polymer = large molecule synthesized by covalently linking single parts (monomers) • Biological polymers: proteins, cellulose, nucleic acids • Organic polymers: plastics • Chain-growth polymers: made from alkene monomers • Radical reaction

Chain-growth Polymerization • Initiation by peroxides: • Propagation: • Termination: R─CH 2 CH 2 • + • CH 2 CH 2 ─R → R─CH 2 CH 2 CH 2 CH 2 ─R

Chain-growth Polymers

• Draw the structure of poly(vinyl chloride).