Chemistry 125: Lecture 53 February 19, 2010 Tuning Polymer Properties. Alkynes, Dienes & Conjugation This For copyright notice see final page of this file

Vulcanization in the Home

Hair before Permanent Wave “Reduce” disulfide cross links with excess basic RSH S S S S S S S S RS - - H SR - H RS-SR - H H H H H H (pK a ~11) + NH 4 HS CO 2 HS CO 2 OH or H SR - H Curl

Permanent Wave H H H H H H BDE kcal/mole HO-OH 52 RS-SR ~ 64 RS-H 87 RO-H 105 S S S S S S S S H H Curl “Oxidize” thiols back to disulfide with HOOH

Synthetic Rubber

Thermoplastic Ionomers Malleable cross links

Julius Nieuwland Cl Neoprene

Natural Rubber vs. Synthetics

Radical Polymerization Poly(styrene) Regiochemistry R           R   head-to-tail random ~ 13 kcal/mole more stable than

Radical Polymerization Poly(propylene) Tacticity CH 3 H H H H H H H H H H H H H H H H H H H H H H H H H H H Isotactic (Radical) (Ziegler-Natta) Syndiotactic Atactic

Radical Copolymerization     CO 2 CH 3 Block CO 2 CH 3 Methyl Methacrylate  Styrene CO 2 CH  [1]2 k relative      CO 2 CH 3 Alternating ? fastest

Anti-Hammond Copolymerization     ~ 20 kcal/mole CO 2 CH 3    not as stable but twice as fast!

Radical Copolymerization CO 2 CH 3      C=O gives unusually low LUMO. Good when SOMO is not low. “Ionic resonance structure stabilizes transition state.” COCH 3 - O  + - O N.B. This special stability applies in TS only,not in the radical product!

Acetylenes Review Sec pp Sections pp

Stepwise Addition of HBr to Alkyne 1-Hexyne + HBr 2-Bromo-1-hexene FeBr 3 15°C with “inhibitor” to trap radicals isolated in 40% yield 100 to 1000x slower than comparable ionic addition to alkene, because vinyl cation is not so great. CH 3 -CH 2 -Cl CH 3 -CH Cl - gas phase 193 kcal/mole CH 2 =CH-Cl CH 2 =CH + + Cl kcal/mole

Stepwise Addition of HBr to Alkyne 1-Hexyne + HBr 2-Bromo-1-hexene FeBr 3 15°C with “inhibitor” to trap radicals isolated in 40% yield But as shown in text, HBr can add again to the bromoalkene (obviously more slowly) to give a second Markovnikov addition If the bromo substituent slows addition to an alkene, why is there Markovnikov orientation?

Stepwise Addition of HBr to Alkyne 1-Hexyne + HBr 2-Bromo-1-hexene FeBr 3 15°C with “inhibitor” to trap radicals isolated in 40% yield The schizophrenic nature of a Br substituent. Br is both electron withdrawing (  ) and electron-donating (  ).

Hydration of Alkyne Markovnikov or anti-Markovnikov Initial enol undergoes acid-catalyzed isomerization. Because C=O is so stable (compare average bond energies) 10.8 to 10.11

Regioselection Stereoselection

First e - First H +

Second e - Second H +

Alkyne Acidity and Isomerization Sec pp

Approximate “pK a ” Values CH 3 -CH 2 CH 2 CH 2 H ~ 52 CH 3 -CH 2 CH=CHH ~ 44 CH 3 -CH 2 C CH ~ 25 ~ 34 H 2 NH = 16 HOH CH 3 -CH=C=CHH CH 3 -C C-CH 2 H ~ 38 sp 3 C _ sp 2 C _ (no  overlap) sp C _ (no  overlap) C _ HOMO -  overlap (better E-match N-H ) (bad E-match O-H ) (best E-match C-H ) * Values are approximate because HA 1 + A 2 - = A HA 2 equilibria for bases stronger that HO - cannot be measured in water. One must “bootstrap” by comparing acid-base pairs in other solvents pK a * : : (allylic) (Acidity of 1-Alkynes Secs p. 129; 12.4 p )

H + (aq) + Equilibrium & Rate kcal/mol CH 3 -CH=C=CH 2 CH 3 -C C-CH 3 CH 3 -CH 2 C CH CH 3 -CH 2 C C CH 3 -CH=C=CH CH 3 -C C-CH 2 pK a  38 K a   G  4/3  38 = 51 pK a  25 K a   G  4/3  25 = %0.03% k   /sec t 1/2 = 0.69/k  sec = yrs  10 4  time since Big Bang [0]

H + (aq) + + HO - favors dissn. by 21 kcal (4/3  16) Equilibrium & Rate kcal/mol H 2 N - favors dissn. by 45 kcal (4/3  34) CH 3 -CH=C=CH 2 CH 3 -C C-CH 3 CH 3 -CH 2 C CH CH 3 -CH 2 C C CH 3 -CH=C=CH CH 3 -C C-CH 2 t 1/2  K %  2 150°C

Trick to obtain terminal acetylene: Equilibrate with RNH _ base (in RNH 2 solvent at room temp) to form terminal anion. “Quench” by adding water which donates H + to terminal anion and to RNH _, leaving OH _, which is too weak to allow equilibration. Or add H +, so even [OH _ ] is very low.

C C Conjugation & Aromaticity (Ch ) Conjugated Pi Systems O C Yoke  Jungere  Jug ó m (to Join)

The Localized Orbital Picture (Pairwise MOs and Isolated AOs) Is Our Intermediate between H-like AOs and Computer MOs When must we think more deeply?

When does conjugation make a difference? Experimental Evidence

Conjugation worth ~5 kcal Conjugation worth <7 kcal

Conjugation worth ~ 4 kcal

Allylic Stabilization: Cation R-Cl  R + + Cl - (gas phase kcal/mol) Cl Anion pK a OH OHO Radical Bond Dissociation Energy (kcal/mol) H H Conjugation worth ~ 13 kcal ! as good as secondary 4/3  6 = 8 kcal

Why is conjugation worth more in allylic systems? Because we can draw reasonable resonance structures? good bad

Conjugation & Aromaticity (Ch ) Simple H ü ckel MOs

: : Sum is same as localized : : Secondary mixing is minor (because of poor E-match) Two Ways to Think about Butadiene  System 4 p-orbitals          How different in overall stability?Very Little! (~3 kcal/mole max) : : Localized  bond picture 4 Delocalized   ::        

Two Ways to Think about Butadiene  System 4 p-orbitals                  : : 4 Delocalized   :: Why ignore this mixing? Despite better E-match, it does not lower energy. (What would be gained on one end would be lost on the other) Orthogonal

But there are substantial differences in HOMO & LUMO energies (Reactivity), and in HOMO-LUMO gap (color) But there are substantial differences in HOMO & LUMO energies (Reactivity), and in HOMO-LUMO gap (Color). Two Ways to Think about Butadiene  System : : How different in overall stability? Very Little! (~3 kcal/mole max) Localized  bond picture 4 Delocalized   :: far UV (167 nm) nearer UV (210 nm)

Is There a Limit to  1 Energy for Long Chains? 8 1/  8 1/8 77/84 1/  4 1/4 33/4 Chain length 2 Normalized AO size 1/  2 Overlap per  bond (AO product) 1/2 Number of  bonds 1 Total overlap stabilization 1/2 N 1/  N 1/N N-1(N-1)/N Yes, the limit is 1, i.e. twice the stabilization of the H 2 C=CH 2  bond. Similarly, the LUMO destabilization limit is twice that of the H 2 C=CH 2   MO.. N.B. Here we are using our own “overlap stabilization” units, which are twice as large as conventional “  ” units.

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