Transcript Major product
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).