ALKENE AND ALKYNE REACTIONSandSYNTHESIS

Dr. Sheppard

CHEM 2412

Summer 2015

Klein (2nd ed.) sections 11.7, 9.1, 9.3, 11.10, 9.4-9.6, 9.8, 9.7, 14.8, 9.9-9.12, 11.11, 10.4, 10.6-10.8, 10.5, 10.9, 10.3, 10.10, 9.13, 10.11, 12.1-12.6

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

CH2 CH2 CH CH2CH3

ethene (ethylene) propene (propylene)

ExamplesStructure Name

CH3 CH CH CH2

CH2 CH2 CH2

CH3

Alkene substituents

Substituent Name

CH2═ methylene

CH2═CH─ vinyl

CH2═CH─CH2─ 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

C CHCH3

ethyne (acetylene) propyne

CH CH

CH3 C C CH2 CH CH3

CH3

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/

stereochemistryRearrangement

possible? Product

 Oxymercuration-demercuration

Alkene1. Hg(OAc)2, H2O

2. NaBH4 MarkovnikovAnti addition 

 no Alcohol 

           

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

CH2 CH CH3

Br2

h

Allylic carbon

CH2 CH CH2

Br

+ HBr

Allylic Halogenation• Another set of reagents:

• N-bromosuccinimide (NBS), hn• 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

• Another example:

• Which is the major product?

• What is the major product of the following reaction?

CH2

NBS

h

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, H2O, etc.)

• Forms carbocation intermediate

C C + HX C C

H X

C C + H2O C C

H OH

Electrophilic Addition• General mechanism:

• Step 1:

• Step 2:

• Which step is RDS?

C C + E C C

E

C C

E

+ Nu: C C

E Nu

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, sp2 hybridized

HBr

• 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 H2O) 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

Mechanism for Hydrohalogenation

Intermediate Structures• Two possible intermediates:

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

HBrCH3 CH2 CH

CH3

CH CH2

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

• Expected product =

• Actual product =

• What happened?

HBrCH3 CH2 CH

CH3

CH CH2

CH3 CH2 CH

CH3

CH CH3

Br

2-bromo-3-methylpentane

CH3 CH2 C

CH3

CH2 CH3

Br

3-bromo-3-methylpentane

Carbocation Rearrangement• Carbocation intermediates can rearrange to form a more

stable carbocation structure• Hydride shift = H:- moves from C adjacent to carbocation

CH3 CH2 C

CH3

CH CH3

H

CH3 CH2 C

CH3

CH2 CH3

CH3 CH2 C

CH3

CH2 CH3

Br

3-bromo-3-methylpentane

Br

CH3 CH2 CH

CH3

CH CH2

H Br

hydride shif t

Carbocation Rearrangement• Alkyl groups can also shift

• Typically methyl or phenyl

(Major product)

Anti-Markovnikov Addition of HBr• In the presence of peroxides

• H2O2 or R2O2

• Free radical mechanism• Only HBr, not HCl or HI

• If product has a stereocenter, a racemic mixture is produced

HBrCH3 C

CH3

CH CH3 CH3 C

CH3

CH2 CH3

Br

HBrCH3 C

CH3

CH CH3 CH3 CH

CH3

CH CH3peroxides

Br

(Markovnikov)

(anti-Markovnikov)

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 H2SO4 or H3PO4 (or just H3O+)

• Carbocation intermediate, so rearrangement can occur• Example:

CH CH2R CH CH3R

OH

H2O

H+

Mechanism

• Draw the major product of the following reaction.

H2O

H2SO4

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 H3O+

• Electrophile is +HgOAc• Formed by dissociation of AcO-Hg-OAc

• Intermediate is bridged mercurinium ion

CH CH2R CH CH2R

OH

Hg(OAc)2

H2O

HgOAc

NaBH4CH CH2R

OH

H

Oxymercuration-demercuration• Predict the products:

1. Hg(OAc)2, H2O

2. NaBH4

Oxymercuration-demercuration• Oxymercuration mechanism:

• Demercuration mechanism: radical

Oxymercuration-demercuration

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

1. Hg(OAc)2, H2O

2. NaBH4

CH3

1. Hg(OAc)2, H2O

2. NaBH4

CH3

CH3

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 BH3

Hydroboration-oxidation• Mechanism of first step:

• BH2 on the C with more H’s because less steric hindrance• Leads to anti-Markovnikov product

• Second step: H2O2/NaOH replace –BH2 with –OH • Complicated mechanism• Keep same stereochemistry (syn)

CH CH2R CH CH2R

BH2H BH2 H

CH CH2R

BH2H

H2O2

NaOHCH CH2R

OHH

• Draw the major product of the following reactions.

1. BH3 THF

2. H2O2, NaOH

CH3

1. BH3 THF

2. H2O2, NaOH

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

CH3 CH CH CH2

CH3

Halogenation

• Addition of halogens• X2 = Br2 or Cl2 (F2 too reactive, I2 unreactive)

• Solvent = inert, nonaqueous• Stereochemistry = anti addition• Product = a vicinal dihalide

• Two X atoms on adjacent carbons

• Example:

CH CH2R + X2

r.t.

CH2Cl2(CHCl3, CCl4)

CH CH2R

X

X

Mechanism

C C Br Br+

Addition of Halogens

• Draw the major product of the following reaction.

Br2

CCl4

CH3

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 X2, except H2O is the

nucleophile in the second step

Mechanism

Mechanism• Water attacks the carbon with the largest d+

• Results in OH on more substituted carbon

• Example:

C CH2R

R

Br

C CH2R

R

Br

not C CH2R

R

Br

• Draw the major product of the following reaction.

Br2

H2O

CH3

Oxidation and Reduction• What is oxidation?• What is reduction?

• Classify these reactions as oxidation or reduction:

• CH3─CH═CH2 → CH3─CH2─CH3

• CH3─CH2─OH → CH3─CO2H

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