Ch. 5 & Lecture OChem

Oxymercuration-Reduction

A method for converting an alkene to an alcohol. The alkene is treated with mercury (II) acetate followed by reduction with sodium borohydride -the overall result is the net addition of the elements of water (H and OH) to an alkene double bond in a highly regioselective manner: **the ¬OH group is added to the more branched carbon of the double bond.

Oxymercuration of Alkenes

In oxymercuration, alkenes react with mercuric acetate, Hg(OAc)2, in aqueous solution to give addition products in which an ¬HgOAc (acetoxymer- curi) group and an ¬OH (hydroxy) group derived from water have added to the double bond

Reaction of a free radical with a carbon-carbon bond is another common process encountered in free-radical chemistry.

The pi bond reacts, rather than a sigma bond, because carbon-carbon pi bonds are weaker than carbon-carbon sigma bonds.

steric effect

When a chemical phenomenon (such as a reaction) is affected by van der Waals repulsions

bromohydrin

a compound containing both an -OH and a -Br group

atom abstraction

a free radical removes an atom from another molecule, and a new free radical is formed

vicinal dihalide

two halogen atoms are bonded to adjacent carbons

termination step in a free-radical reaction

two radicals react to give nonradical products

calculate ΔH° of the reaction

ΔH° = BDE (bonds broken) - BDE (bonds formed)

monomers

A chemical subunit that serves as a building block of a polymer.

free radical chain reaction

A free-radical chain reaction involves free-radical intermediates and consists of the following three fundamental reaction steps: -initiation steps, -propagation steps, and -termination steps. **Antioxidant agents stop radicals by donating an electron

bromonium ion

A positively charged, bridged intermediate formed during the addition reaction that occurs when an alkene is treated with molecular bromine (Br2). A bromonium ion is a species that contains a bromine bonded to two carbon atoms; the bromine has an octet of electrons and a positive charge. Formation of the bromonium ion occurs in a single mechanistic step involving three curved arrows. *Analogous cyclic ions form in chlorine and iodine addition.

Regioselectivity of Bromonium ion

In this structure, about 90% of the positive charge resides on the tertiary carbon, and the bond between this carbon and the bromine is so long and weak that this species is essentially a carbocation containing a weak carbon-bromine interaction

stability of free radicals

tertiary > secondary > primary

step 2: oxymercuration

the mercurinium ion reacts with the solvent water

free radical polymerization

the process of forming an addition polymer by chain-growth polymerization involving a free radical at the end of the growing chain

halohydrin

-Addition of X2 -Reagents: Br2 or Cl2 in H2O -Bromonium or chloronium ion intercepted by H2O -Markovnikov addition of H2O -Anti addition stereochemical preference -most common type of halohydrin, the two groups occupy adjacent(i.e. vicinal) positions

mercuric acetate

Hg(OAc)2

polymers

chains of monomers

Ozonolysis: ozonide treated with water, hydrogen peroxide (H2O2)

ldehydes are converted into carboxylic acids, but ketones are unaffected

In electrophilic addition reactions, when the two groups that add are different, the carbon of the double bond with fewer alkyl substituents becomes bonded to the _______________ (more or less) electronegative group, and the carbon of the double bond with more alkyl substituents becomes bonded to the _______________ (more or less) electronegative group.

less, more

Hydroboration-Oxidation

A method for converting an alkene to an alcohol in 2 steps (1. BH3, THF, 2. H2O2, NaOH) -the reaction adds H and OH to a double bond so that the ¬OH group adds to the carbon with fewer alkyl substituents **anti-markovnikov

Ozonolysis step 1a: initial cycloaddition

Addition reaction of the alkene bond. -The central oxygen of ozone is a positively charged electronegative atom and therefore strongly attracts electrons. -The curved-arrow notation shows that this oxygen can accept an electron pair when the other oxygen of the O=O bond accepts electrons from the alkene

electrophilic addition reaction

An addition reaction is an electrophilic addition when it begins with the donation of an electron pair from a bond to an electrophilic atom.

free radical

Any species with at least one unpaired electron

Why don't free radicals simply recombine before they propagate any chains?

Free-radical intermediates are present in very low concentration, but the other reactants are present in much higher concentration, so free-radicals are less likely to collide with each other

halohydration regioselectivity

In halohydrin formation, the reaction is highly regioselective only if one of the alkene carbons has two alkyl branches.

oxymercuration regioselectivity

In oxymercuration, the reaction of water occurs almost exclusively at the carbon with more alkyl substituents, even if that carbon has only one alkyl substituent (compare to halohydrin formation, where the reaction is highly regioselective only if one of the alkene carbons has two alkyl branches)

Space-filling models of the alternative transition states for the addi- tion of a bromine atom to 2-methylpropene.

In part (a), the bromine is adding to the carbon of the double bond that has the two methyl substituents. This transition state contains van der Waals repulsions between the bromine and four of the six methyl hydrogens, which are shown in pink. (Three of these are shown; one is hidden from view.) In part (b), the bromine is adding to the CH2 carbon of the double bond, and the van der Waals repulsions shown in part (a) are absent. The transition state in part (b) has lower energy and therefore leads to the observed product.

A propagation step in a radical mechanism

Is characterized by the presence of the same number of radicals on either side of the reaction -radicals react with non-radical starting materials to give other radicals; -starting materials are consumed and products are formed. -Propagation steps occur repeatedly. -propagation steps continue in a chainlike fashion until the reactants are consumed -when the propagation steps are considered together, there is no net formation or destruction of any of the radical species involved. -> if a radical is formed, it must be consumed in a subsequent propagation step and another radical must be formed to take its place

Conversion of alkenes to alcohols

Oxymercuration-reduction and hydroboration-oxidation bring about the overall addition of H and OH to a double bond. The two sequences are complementary because they occur with opposite regiospecificities. (Notice the position of the ¬OH group in the product.) (Although the hydration of alkenes is used industrially for the preparation of particular alcohols, it is rarely used for the laboratory preparation of alcohols)

Hydroboration-oxidation is an effective way to synthesize certain alcohols from alkenes. It is particularly useful to prepare alcohols of the general structure:

R2CH¬CH2¬OH or R¬CH2¬CH2¬OH *Because carbocations are not involved in either the hydroboration or the oxidation reaction, the alcohol products are not contaminated by constitutional isomers arising from rearrangements

HBr addition to alkene in free radical reaction

Reaction of the bromine atom at the alkene carbon with fewer alkyl substituents to give the more alkyl-substituted free radical is faster because the reaction with the transition state of lower energy is the faster reaction,

tetrahydrofuran (THF)

THF dissolves both water and many water-insoluble organic compounds. -In oxymercuration it dissolvse both the alkene and the aqueous mercuric acetate solution. -Recall that alkenes are not soluble in water alone. -Water is required as both a reactant and as a solvent for the mercuric acetate.

Ozonolysis step 1b: formation of the ozonide

The addition of ozone to alkenes occurs at low temperature and breaks the carbon-carbon bond to give an unstable addition product. The spontaneous conversion of this addition product into an ozonide breaks the second carbon-carbon bond.

Ozonolysis of alkenes

The reaction of an alkene with ozone to yield products of double-bond cleavage is called ozonolysis. 1) alkene reacts with ozone to form an addition product called an ozonide. 2) the ozonide is treated with oxidizing or reducing agents to form products.

The peroxide effect

The reversal of orientation of HBr addition to alkenes in the presence of peroxides. A free-radical mechanism is responsible for the peroxide effect. -The regioselectivity of HI or HCl addition to alkenes is not affected by the presence of peroxides.

Writing organic reactions: by convention, organic chemists abbreviate reactions by showing only the organic starting materials and the major organic product(s). The other reactants and conditions are written over the arrow.

This "shorthand" way of writing organic reactions is frequently used because it saves space and time. When equations are written this way, by-products are not given and, in many cases, the equation is not balanced.

Example of a solvent acting as a nucleophile

When an alkene is treated with bromine in a solvent containing a large excess of water, a water molecule rather than bromide ion reacts with the bromonium ion, because water is present in much higher concen- tration than bromide ion.

homolytic cleavage

When the bond breaks, each atom gets one electron -initiated by light or heat -produces two free radical halogens

heterolytic cleavage

When the bond breaks, the most electronegative atom gets both electrons

free-radical initiator

a molecule that undergoes homolysis with particular ease -The initiator is the source of free radicals. -Peroxides are frequently used as free-radical initiators (but not H2O2)

addition polymer

a polymer in which no atoms of the monomer unit are lost as a result of the polymerization reaction

thermal cracking

a process that breaks large hydrocarbon molecules into smaller ones by heating them to a high temperature

polymerization reaction

a reaction in which monomer units are bonded together to form a polymer

concerted mechanism

a reaction that occurs in a single step without intermediates

recall: percentage yield

actual yield/theoretical yield x 100

The recombination reactions of free radicals are in general highly _________ (exothermic or endothermic)

exothermic; that is, they have very favorable, or negative, ΔH° values -there is no ΔG°‡ for radical recombination

step 1: oxymercuration

formation of a cyclic ion called a mercurinium ion

radical recombination reaction

in which two radicals come together to form a covalent bond -radical recombination is the reverse of a homolysis -each recombination reaction takes two free radicals "out of circulation," -> it terminates two propagation reactions and breaks two free-radical chains

unpaired electron on the carbon with

more alkyl substituents (forms a more stable free radical)

Polyethylene

polymer formed from a chain containing many ethylene units; often used in plastic bags and plastic bottles -subscript n means that a typical polyethylene molecule contains a very large number of repeating units

step 3: oxymercuration

protonation of the acetate ion

azoisobutyronitrile (AIBN)

readily forms free radi- cals because the very stable molecule dinitrogen is liberated as a result of homolytic cleavage

bond dissociation energy

standard enthalpy ΔH° of the reaction -measures the intrinsic strength of a chemical bond