Chapter 8: Alkenes & Alkynes

Addition Reactions:

usually exothermic

Binary acids =

HCl, HBr, HI

Addition of H-X to alkenes:

• Binary acids like HCl, HBr, & HI can add across alkenes

• Markovnikov’s rule

Markovnikov’s rule =

the hydrogen adds to the carbon that already has the most hydrogens → “the rich get richer”

Regioselective Reactions:

A reaction that can yield two or more constitutional isomers (generally one is preferred) — will have major and minor products

Exception to Markovnikov’s Rule:

When the reaction of an alkene with HBr is carried out in the presence of peroxide, the Anti-Markovnikov product dominates

Peroxide =

ROOR

Anti-Markovnikov =

poor guy gets rich

3 Types of steps in Radical Reactions:

1. Initiation

2. Propogation

3. Termination

Initiation =

radical created

Propogation =

radical moves

Termination =

radical dies —> sometimes, you don’t see this step

Stereochemistry of Addition to an Alkene:

Carbocation intermediate is susceptible to attack from either side → leads to enantiomers

Hydration of an alkene:

• adding H2O to give an alcohol

• uses a strong catalyst like H3O+ (H3PO4) or cat.H2SO4

• Markovnikov product

Oxymercuration-Demercuration:

• Reaction is initiated by electrophilic addition of Hg2+ ion to the alkene

  • Gives an intermediate mercurinium ion

• Regiochemistry of the reaction corresponds to Markovnikov addition of H2O

• No carbocation

• Forms 3 membered ring intermediate & water is added to it

• Final product will have OH

Adding H2O gives:

alcohols

Adding alcohols gives:

ethers

Formation of an alcohol through Markovnikov addition:

image

What is the Markovnikov site?

the sit where the OH attaches — the more substituted C

Hydroboration:

Process involving addition of a B–H bond of borane, BH3, to an alkene to yield an organoborane intermediate, RBH2

Hydroboration-Oxidation:

• anti-Markovnikov

• Alkene reacts with BH3 in THF solution, rapid addition to the double bond occurs three times and a trialkylborane is formed

• syn addition

• Regioselective

• Non-Markovnikov regiochemistry

What is Deuterium?

a stable, naturally occurring isotope of hydrogen — represented by the symbol ²H or D

Hydroboration-Oxidation: Regioselective

boron and hydrogen add to the alkene from the same face of the double bond with syn stereochemistry

Addition of Halogens to Alkenes:

• Halogens (X2) add across the double bond

• Halogens will add to the alkene via anti addition

• Regioselective & Stereospecific (you’ll only ever get anti addition)

When will enantiomers be present?

when the molecule is chiral

Addition of Halogens to Alkenes: Halogens typically used are

Br & Cl

Halogenation:

• Anti stereochemistry: forms the trans stereoisomer

• The reaction intermediate is bromonium ion, not a carbocation

Stereospecific reaction =

a particular stereoisomeric reactant reacts by a mechanism that gives a specific stereoisomeric form of the product

In halogenation, what is added?

only a halogen

Id the alkene the halogen is being added to is not chiral:

ex.Cl2 added — each Cl does not have to be on opposite sides

Halohydrin Formation =

• both Halogen & H2O added

• Water is the solvent & reacts because it is in high concentration

• The halogen adds to the carbon with the most hydrogens

• anti addition

• only ever seen with Cl and Br

Reaction of alkenes with a halogen and water yields a 1,2-halo alcohol, called a:

halohydrin

Carbene =

R2C: Neutral molecule containing a divalent carbon with only six electrons in its valence shell

• is sp2 like carbocation and has an empty p orbital

• can act as both Nu and Eu

Forming double or triple bonds is done through:

Beta elimination

Ways to make carbenes:

1. Simmons-Smith Cyclopropane Synthesis

2. Alpha (α) elimination

When carbenes are generated, something needs to be lost:

from the carbon containing compound over the arrow

Oxidation of Alkenes: Osmium tetroxide

produces a 1,2-diol (syn addition)

Reduction =

adding H or losing O

Oxidation =

losing H or gaining O

Oxidation of Alkenes: KMnO4 (potassium permanganate)

• Cold conditions produces 1,2-diols (syn addition)

• Hot conditions cleave the double bond

Cold Basic KMnO4:

acts same as OsO4 & makes a syn diol

Hot acidic KMnO4:

cleaves double bonds

Hot acidic KMnO4 # of Hydrogens on Alkene: 0H

Ketone

Hot acidic KMnO4 # of Hydrogens on Alkene: 1H

carboxylic acid

Hot acidic KMnO4 # of Hydrogens on Alkene: 2H

CO2 gas

Oxidation of Alkenes: Ozonolysis

Double bond cleaved and replaced by C=O

Ozonolysis # of H on C of alkene: 0H

ketone

Ozonolysis # of H on C of alkene: 1H

aldehyde

Ozonolysis # of H on C of alkene: 2H

formaldehyde

Periodic (Per-iodic) Acid:

• HIO4 (periodic acid)

• 1,2-diol is converted to C=O

• Cleavage is very molecule specific

• Better definition – cleavage occurs between carbons if both bare an oxygen (-OH or C=O)

Periodic Acid cut between:

2 carbonyls, 2 alcohols, carbonyl next to alcohol

For periodic, you can only laso water from:

OH

For periodic, if you take an Oxygen’s hydrogen, it becomes:

a carbonyl

Epoxides are a:

new functional group

• a subclass of ether

Epoxidation =

formation of a 3-membered ring that contains oxygen

Epoxides can be formed by:

peroxy acids (RCO3H); peroxy acids do syn addition

What is a simplest epoxide?

an oxirane

In the making of epoxides, where does the oxygen come from?

from the one on the OH

Stereochemistry with epoxidations?

• cis alkene = cis

• trans alkene = trans

What are the 2 types of ring openings of epoxides?

Basic & Acidic

Epoxide Ring Opening: Basic

• SN2 attack — stereochemistry flips

• will attack the least hindered site

Epoxide Ring Opening: Acidic

• SN2 attack — stereochemistry flips

• if tertiary site, attack there

• is no tertiary site, attack the least hindered site

Epoxide basic Common Bases:

NaOH, NaOMe, NaOEt

Epoxide acidic Acidic list:

H3O+, cat.H2SO4/MeOH, cat.H2SO4/EtOH, HCl, HBr, HI

Addition of H-X to Alkynes:

Remember: Markovnikov’s rule

Addition of X₂ to Alkynes:

Remember: anti addition

Alkynes: Reaction of HBr & peroxide

Anti-Markovnikov addition of HBr

Alkynes: Oxidative Cleavage

usually get carboxylic acids for this