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.

 

 

 

 

 

 

                                                  

 

 

Somewhat more stable than corresponding nonconjugated compounds.

 

                                              

Heats of Hydrogenation

 

 

DH° kcal/mole

-57.1

-60.8

-60.5

 

                                                                       

Hydrogenation of 1,3-butadiene is ~ 4 kcal/mole less exothermic.  This means that it is ~ 4 kcal/mole more stable. 

 

appears to have a small amount of p bond character

                                 ¯

                        ¬  looks like normal double bond

 

On average    C-C : 1.54 A

                        C=C : 1.33 A

                                   

 

 

            shorter than single bond but longer than a double bond, indicates some          but not complete p bond character

 

nearly free rotation

E barrier = 2.9 kcal/mole

no bond rotation at RT

E barrier » 66 kcal/mole

 

 

                                               

           

no bond rotation

E barrier similar to other p bonds

rotation occurs

E barrier » 4-5 kcal/mole indicates some p character

 

            2 conformations possible, both overall planar

 

s-cis

s-cis-1,3-butadiene

s-tran

s-trans-1,3-butadie

 

           

Which conformation appears to be more stable?

 

 

 

Butadiene

 

 

Would forms A, C, and D contribute significantly?

 

Molecular Orbitals of 1,3-Butadiene

 

       s MO’s are much lower in E than p MO’s

                        s* MO’s are much higher in E than p* MO’s

 

                        p MO will be HOMO

                        p* MO will be LUMO

 

p orbital, 1 node (region of 0 electron density separating regions of high         electron density

 

 

     p MO, 1 node, 1 bonding interaction (lobes match)

 

 

     p* MO, 2 nodes, 1 new node relative to lowest E MO, 1 antibonding                 interaction (lobes opposite)

 

To determine the relative energies of the MO’s, the following must be compared:

 

a. the # of new nodes relative to the lowest E MO

(lower # new  nodes, lower E)

 

b.  the net # of bonding/antibonding interactions

(more bonding or less antibonding, lower E)

 

 

 

Ethylene         p MO lower in E than  p* MO

 

 

            0 new nodes                          1 new node

            1 bonding int.                         0 bonding int.

            0 antibonding int.                   1 antibonding int.

 

1,3-Butadiene

                       

 

Both (pe1 + pe2) and (pe1 - pe2) are bonding MO’s and are energetically stabilizing.

 

In (pe1 + pe2) there is a bonding interaction between C2 and C3.

In (pe1 - pe2) there is an antibonding interaction between C2 and C3.

 

They should cancel out except that since (pe1 + pe2) is lower in energy, there will be a small net bonding interaction giving C2-C3 some p bond character.

 

 

photon E = hn = hc/l where E = energy

                                                            h = Planck’s constant

                                                            n = frequency (1/s)

                                                            l = wavelength (nm)

If E photon = DE B-C, then there is a probability that the photon will be absorbed by the molecule resulting in an electron being promoted from B to C*. This is called an electronic transition.

 

Commonly Observed Transitions in Organic Molecules

 

            s - s*

            p - p*

            n - s*

            n - p*

 

n - s* and n - p* occur when a molecule has nonbonding or lone pair electrons, e.g. allyl carbanion

 

 

e.g.      n - p*