Download Conjugation and Resonance - Organic Chemistry I - Lecture Slides and more Slides Organic Chemistry in PDF only on Docsity! Resonance: 1. When you can draw more than one classic valence bond structure for a compound that differ only in the arrangement of the electrons, there is resonance. 2. If the structures have approximately the same stability, then resonance is important. 3. If resonance is important, none of the classic structures adequately represent the compound. It is better represented as a hybrid of the classic valence bond structures. 4. The resonance hybrid is more stable than any of the contributing structures ( resonance stabilization energy). docsity.com allylic halogenation of alkenes. CH2=CHCH3 + X2, heat CH2=CHCH2 + HX X 1) X2 2 X• 2) CH2=CHCH3 + •X HX + CH2=CHCH2• allyl free radical 3) CH2=CHCH2• + X2 CH2=CHCH2-X + X• etc. docsity.com proof that the allyl free radical is as proposed: 13CH3CH=CH2 + NBS 13CH2CH=CH2 + 13CH2=CHCH2 Br Br [ 13CH2CH=CH2 13CH2=CHCH2 ] • • docsity.com Dienes: | | | | | | | | | — C = C — C = C — —C = C — C — C = C — | conjugated double bonds isolated double bonds | | — C = C = C — cumulated double bonds docsity.com nomenclature: CH2=CHCH=CH2 CH3CH=CHCH2CH=CHCH3 1,3-butadiene 2,5-heptadiene conjugated isolated 2-methyl-1,3-butadiene (isoprene) conjugated docsity.com Heats of hydrogenation (Kcal/mole) for dienes: 1,4-pentadiene 60.8 isolated 1,5-hexadiene 60.5 isolated 1,3-butadiene 57.1 conjugated 1,3-pentadiene 54.1 conjugated 2-methyl-1,3-pentadiene 53.4 conjugated 2,3-dimethyl-1,3-butadiene 53.9 conjugated 1,2-propadiene (allene) 71.3 cumulated docsity.com Conjugated dienes are more stable (~3/4 Kcal/mole) than predicted. (Isolated dienes are as expected.) Conjugated dienes are the preferred products of eliminations: CH3CH2CHCH2CH=CH2 + KOH(alc) Br CH3CH2CH=CHCH=CH2 only! CH3CH=CHCH2CH=CH2 none! docsity.com isolated dienes: (as expected) 1,5-hexadiene CH2=CHCH2CH2CH=CH2 + H2, Ni CH3CH2CH2CH2CH=CH2 CH2=CHCH2CH2CH=CH2 + 2 H2, Ni CH3CH2CH2CH2CH2CH3 CH2=CHCH2CH2CH=CH2 + Br2 CH2CHCH2CH2CH=CH2 Br Br CH2=CHCH2CH2CH=CH2 + HBr CH3CHCH2CH2CH=CH2 Br CH2=CHCH2CH2CH=CH2 + 2 HBr CH3CHCH2CH2CHCH3 Br Br docsity.com 1,2- plus 1,4-addition of free radicals: perox. CH2=CHCH=CH2 + HBr CH2CHCH=CH2 CH2CH=CHCH2 Br • Br • resonance! allyl free radical: CH3CH--C--CH2 • CH2CHCH=CH2 + CH2CH=CHCH2 Br H Br H 1,2-addition 1,4-addition docsity.com no resonance is possible with isolated double bonds: CH2=CHCH2CH=CH2 + HBr CH2CHCH2CH=CH2 H no resonance possible CH2CHCH2CH=CH2 H Br docsity.com conjugated dienes are unusual in that they: 1) are more stable than predicted 2) are the preferred products of eliminations 3) give 1,2- plus 1,4-addition products docsity.com
