ALKENE AND ALKYNE REACTIONS and SYNTHESIS Dr. Sheppard CHEM 2412 Summer 2015 Klein (2 nd ed.) sections 11.7, 9.1, 9.3, 11.10, , 9.8, 9.7, 14.8, , 11.11, 10.4, , 10.5, 10.9, 10.3, 10.10, 9.13, 10.11,

Outline Nomenclature review Preparation and reactions of alkenes (Ch. 9 and 11) Preparation and reactions of alkynes (Ch. 10) Synthesis (9.13, 10.11, Ch. 12)

Alkene Nomenclature Similar to alkanes; change infix from “-an-” to “-en-” For larger alkenes: 1. Parent is longest C chain containing both carbons of C=C 2. Number chain so C=C has lowest possible number If the double bond is equidistant from both ends, start numbering at end nearest the first substituent Show location of C=C by first number Alkenes with >1 C=C use “-adiene”, “-atriene”, etc. in place of “-ene” and show location of all double bonds 3. Name and number substituents and write the full name 4. Determine stereochemistry (cis, trans, E, Z) if applicable

Examples StructureName

Alkene substituents SubstituentName CH 2 ═methylene CH 2 ═CH─vinyl CH 2 ═CH─CH 2 ─allyl

Alkyne Nomenclature Similar to alkenes; change infix from “-en-” to “-yn-” For larger alkynes: 1. Parent is longest C chain containing both carbons of C≡C 2. Number chain so C≡C has lowest possible number Same rules as alkenes 3. Name and number substituents and write the full name Example

Preparation of Alkenes: Review Alkyl halide eliminates HX to form an alkene Reagent = base Alcohol eliminates water to form an alkene Reagent = acid

Reactions of Alkenes I. Allylic halogenation II. Electrophilic addition III. Reduction IV. Oxidation V. Polymerization

Reaction Charts Help organize reaction details Organize charts by reaction type, starting material, product See webpage for template Example: Reaction Type Starting Material Reagent Regiochemistry/ stereochemistry Rearrangement possible? Product Oxymercuration- demercuration Alkene 1. Hg(OAc) 2, H 2 O 2. NaBH 4 Markovnikov Anti addition noAlcohol

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 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)?

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

Another example: Which is the major product?

What is the major product of the following reaction?

II. Electrophilic Addition Most common reaction of alkenes Examples: Break  bond of alkene Form new  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?

Hydrohalogenation Addition of hydrogen halides HCl, HBr, HI Example: cis-3-hexane + 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 regioselective

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

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

Mechanism for Hydrohalogenation

Intermediate Structures Two possible intermediates:

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 If product has a stereocenter, a racemic mixture is produced

Mechanism

Stability of Intermediate In both Markovnikov and anti-Markovnikov reactions, the most stable intermediate is formed

Hydration Addition of water Three methods: A. Acid-catalyzed hydration B. Oxymercuration-demercuration C. Hydroboration-oxidation

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

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 Anti addition of OH and H (add on opposite sides of C=C) No rearrangements Milder conditions than H 3 O + Electrophile is + HgOAc Formed by dissociation of AcO-Hg-OAc Intermediate is bridged mercurinium ion

Oxymercuration-demercuration Predict the products:

Oxymercuration-demercuration Oxymercuration mechanism: Demercuration mechanism: radical

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 C with more H’s because less steric hindrance Leads to anti-Markovnikov product Second step: H 2 O 2 /NaOH replace –BH 2 with –OH Complicated mechanism Keep same stereochemistry (syn)

Draw the major product of the following reactions.

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

Halogenation Addition of halogens X 2 = Br 2 or Cl 2 (F 2 too reactive, I 2 unreactive) Solvent = inert, nonaqueous Stereochemistry = anti addition Product = a vicinal dihalide Two X atoms on adjacent carbons Example:

Mechanism

Addition of Halogens

Draw the major product of the following reaction.

Halohydrin formation 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

Water attacks the carbon with the largest  + Results in OH on more substituted carbon Example:

Draw the major product of the following reaction.

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 No enantiomer in this example because the product is achiral