Tff 1.0 Kcallmol. edited transition stale. ay ay. AcydicsystemI. Poti. blt groups on adjacent. 1 add to 8. Ent

Transcription

1 Poti AcydicsystemI i ConformationalAnalysis ed diharisiate Ent edited transition stale Torsional strain caused by eclipsing atoms separatedby 3 bonds as eclipsing occurs there is a distortion of the C C and C bonds lengthtangles Butane ay ay gauche CB vicinal repulsion f carbons a anti F rel O En1 0.9 kcalphol Stoic strain steric strain buttressing of groups that destabilizesthemolecule Butane rotation barrier 4.6kcalImoI B 26Keakmol blt groups on adjacent The highest E eclipsing conformation Kcallmol It Tff 1.0 Kcallmol The Group Increment method accountsfor strain using Corrections Gauche 10.8kcalImoI gauche cis alkene tho Kcallmol vs Orthosubs to 6kcalImoI It t one gauche 1 add to 8 e

2 j q Eft Kathol q Clb F Cl s LB 3.7 I ise Ckcallmol But also T C X bondlengthwhich helpsto offset the torsional strain 2,3 Dimethylbutane re Ctb gauche Ctb anti compare f CB vs CE yo T ET 13 Eek1.8 3 x gauche Erd 277 2x gauche Expected seecalesbelow Actual 66 T 33 Geminalrepulsions E of the CBlabsauches making the anti form less stable than ekpeded Germinal repulsion BE gcb A 107 Stoic interaction between methyls on the same C AnAsidei he ratio is a statistical ratio making D E O between the anti e gauche Gauche anti vs 2,3 dimethylbutaneforms tenants identical J p I

3 DG D T O Rfn2 298k DG In 2 Forconformations of the same energy the entropyincreases Rln n 0.490Keat D identicalconformers Mol DG RTInKeg Kegs e BURT Molik ao.cm q.gs o m z g Keg B CAT B A 100 A A t AT A 100 An Aside A 70 So BT Syn pentane asiseiaheathesi.itiesw.ee o the methylgrops not.hwatee TE 3.7 Kcallmol Example Aldol OM Rm Et 4 me t Me 7 syn Me pentane 1 I Et O µ 14 ipr d Me Zimmerman Traxler

4 Cyclopropane Cyclicsystemstl It Stuck in Planoform I I 1 Strain Energy 27,5 Kcallmol DS 7.7 eu forced eclipses Cyclobutane Distortsfrom 2 fi akeaymd additionai7o9s9i7atos7ii Iiu ITF bond frozen cyclopentane Mobileenvelope conformation 6.2 Kcallmol minimizes torsional strain 13.3 eu alwaysmoving still some eclipsing cyclohexane 7 1 still has some gauche interactions Okcallmol 21.2 eu extra jumpdueto chair stability Cycloheptane 0.9kcalImd f P mp increment corrections 19,8 eu more mobility forringstrain CyclohexaseconformationalanalysI unavoidablegauche

5 LI my Adds 2x Gauch interactions 2x 0.9kcalmot me Erels1,8Kcallmol Erel 0 TA value me µ A ie A Value Magnitude ofthe E difference of a substituent goingfrom equatorial to axial i i i ins Transannular Origin of Strain Axial 1,3 Repulsion unfavorable interactions of substituent on nonadjacent C's of ring 1.8 Little 2 I 4.8 Difference Magnitude of A ValueDepends on the ring CB ab 4 2 S L l't Explainwhy C C 1.54 Ao C O 1.43 Ao

6 100 ChairInterconversion x I Frel O Ea 10.5 Y Chair Kcallmol I y's T htap.ch other half chair 1 T off other twist boat a sa Disubstitutedcyclohexane A values are nearly additivefor This breaksdown when groups are trass substituents cis synpentane 83.7 Kcallmol City sitting Latches ByCB g 26, Consider why SS Kcallmol has an additionalgauche

7 OtherEffects ome pdggqation y ome AnomericEffect preference for axial or group Chomeric Carbon at the feet TEI a felon Decalin Bonding trans decal in Cis decalin 4 2 I Cl Cz same OCI y 2.7kcalmot higher Ethan trans C4 1 3x Gauche locked Can't ringflip can undergo ring flip T Bondsinricis is favored until ring becomes very large 712 trans can't be isolated for 48 membered rings linear Cyclooctyne is thesmallest that has beenisolated Bred't Rule Bridgehead olefins are unstable and inaccessible forsmaller rings bridgeheadolefin bicyclo heptane bicyclo nonane

8 Allylicstrain u torsional strain between a allylic and vinyl substituent relevant for planar structures A 2 Strain X Z Z I R 2 t ChemRev A 3 Strain I k t F essentially syn pentane R Generally A 3 is worse than A 2 especially with big R groups us 5 Is 3 NY A'B Strain Good Example PE notes for Test Q in O r vs E morestable O AK strain

9 S trans E A 3 W Ifi A Z WI R CB I of s cis form increases Et SS 45 Is size of R increases c'pr Bu O 100 ExtraiconformationalEffectsonreactivity Ces 3.7 cis O O one Rate I ok I 7 no O trans O OAC 3.7 Rate Evaluate the DE in the RDS r cists higher E because axial group isgettinglarger in the T s Trans Ts trans B Den Ayers faster on

10 Alkeries 8f 8 8 D eat is necessary for A T B c SOKcallmol barrier ethylene bondstrength stabilitytrend ne s TT art s IT justslightly More Stable SubstitutionStabilizes alkenes understanding G C Donationto IJChem.Ed rotation I It stabilizes 2oos 82 BZ thesystem A