Why is head to tail addition favored?

The mechanical strength of a polymer material is dependent on its average molecular weight. The higher it is, the stronger the material will be. Which termination step (combination or disproportionation) favors strong materials?

Formation of Long Branches in Polymers

Polymerization continues from

this point.

b-Cleavage

a,b-unsaturated ketone

b-cleavage reaction

reverse of polymerization : depolymerization (DG = DH – TDS)

Polymerization important at lower temperatures.

Depolymerization important at higher temperatures.

The temperature at which depolymerization becomes more important and the polymer breaks down is known as the ceiling temperature.

Autooxidation

ether diethyl ether tetrahydrofuran

hydroperoxides peroxides

Further reactions occur to various products.

Mechanism

Initiation

1. A radical R’^. is generated somehow.

resonance stabilized, more stable

net stabilization

less stable

Termination

Autooxidation is a slow reaction that results in a change in the structure of a molecule. If this molecule is used in some application, such structural changes generally mean that the molecule will not fulfill its function. Further autooxidation can lead to total degradation. These reactions are partially responsible for the deterioration of food, plastics, paint, dyes, etc.

Ethers: store in dark, preferably cool, store < 1 yr., treat with solution of Fe⁺² salts (FeSO₄)

Antioxidants (a.k.a. inhibitor) react with R’^. faster than R-H, R-O-R, etc.

BHT BHA butylated hydroxyanisole

butylated hydroxytoluene

Vitamin E (a-tocopherol)

What 2 factors lead to this being a very unreactive radical that will inhibit further reaction?

Atherosclerosis

In atherosclerosis, an autooxidation reaction occurs. Molecules (L-H) in low density lipoprotein particles (LDL) undergo reactions with oxygen.

Initiation

Propagation

Vitamin E (a-tocopherol) is believed to act as an inhibitor, but the mechanism is not clear.

V. W. Bowry, K. U. Ingold, Accounts of Chemical Research, 1999, Vol. 32, p. 27

Allylic Halogenation (Bromination)

NBS, N-bromosuccinimide succinimide

Markovnikov Addition

Anti-Markovnikov Addition

So far, 5 different reactions have been covered that add one or more Br atoms to molecules.

(Some free radical initiator present)

Mechanism of Allylic Bromination

Initiation

Propagation

H-Br undergoes a rapid reaction with NBS and will not have a chance to react with any radicals present.

Br-Br is generated at a very low concentration.

The alkene concentration is kept low. Polymerization does not occur.

H-Br reacts with NBS before radicals can. No addition of H-Br occurs.

resonance stabilized, most favorable

vinyl radical, less stable than allylic radical

2° radical, but not resonance stabilized

This pathway is not likely because the bromine concentration is too low. The concentration of all other likely reactants are also low. Most likely pathway will be reverse reaction.

The first step is not reversible because NBS removes H-Br from the reaction mixture. Although the reaction of the radical and Br₂ is slow because of their low concentrations, it is the only likely pathway remaining.

Chapter 11 Conjugated Dienes and the Allyl System:

2p Orbitals in Conjugation

- Large differences in reactivity of C=C in different molecular structures

- How molecules absorb and emit light and why substances have color

- Biochemical process of vision

- Biosynthesis of steroids } terpenes and terpenoids

- Conducting polymers (molecular wires and polymer based LED’s)

Conjugated systems: structures having alternating single and double bonds

e.g. 1,3-butadiene

To some extent, the p orbitals overlap in a sequential manner

unconjugated or nonconjugated structures contain “isolated” double bonds

e.g. 1,4-pentadiene

Other conjugated compounds

1,3,5-hexatriene a,b-unsaturated benzene

carbonyl

allene ketene

Examples of molecules with cumulated double bonds.

TDI, Toluene 2,4-diisocyanate

Conjugated anions, cations, and radicals are more stable than the corresponding nonconjugated species. The charge and electrons are delocalized over more than 1 atom. This is often represented by resonance structures and can be explained through molecular orbital theory.