Chapter 2: ELECTRONIC & STERIC EFFECTS pps

1 ORGANIC CHEMISTRY Dr Nam T. S. Phan Faculty of Chemical Engineering HCMC University of Technology Office: room 211, B2 Building Phone: 8647256 ext. 5681 Email: ptsnam@hcmut.edu.vn 2 Chapter 2: ELECTRONIC & STERIC EFFECTS Conjugation / Mesomeric Steric Electronic Inductive Hyperconjugation Effects 3 INDUCTIVE EFFECTS (I) C-C σ bond in butane: almost completely nonpolar δ- δ+ δ'+ δ''+ δ'''+ C-C σ bond in 1- fluorobutane: polarized C1 is more positive than C2 as a result of electron- attracting ability of F 4 The more electronegative the X, the stronger the –I effect 5 The more electropositive the Z, the stronger the +I effect 6 -I -I Through a period in a periodic table Through a group in a periodic table 7 K a .10 5 CH 3 CH 2 CH 2 COOH 1.5 CH 3 CH 2 CH(Cl)COOH 139 CH 3 CH(Cl)CH 2 COOH 8.9 ClCH 2 CH 2 CH 2 COOH 3.0 Strong -I weak -I 8 CONJUGATION / MESOMERIC EFFECTS (C / M) Electron delocalization in a conjugated system: Alternating single & multiple bonds 9 O is more electronegative than C Electrons move through π-bond network towards C=O The conjugated system is polarized C=O has negative conjugation / mesomeric effect (- C / -M) on the conjugated system 10 +C -C C CH CH CH CHR +C-C C O R CH CH CH CH R O R +C -C [...]... INDUCTIVE vs CONJUGATION EFFECTS H CH CH H CH CH Mobility of hydrogen atoms: almost the same CH CH CH CH H CH CH O C H CH O C H CH O C H 14 -C INDUCTIVE vs CONJUGATION EFFECTS • C effects are generally stronger than I effects • C effects can be effective over much longer distances than I effects – provided that conjugation is present • I effects are determined by distance, C effects are determined by... EFFECTS • A steric effect is an effect on relative rates caused by space-filling properties of those parts of a molecule attached at / near the reacting site • Steric hindrance: the spatial arrangement of the atoms / groups at / near the reacting site hinders / retards a reaction • Generally, very large & bulky groups can hinder the formation of the required transition state 18 Steric hindrance 19 Steric. .. EFFECTS (H) Electron density from Cα-H flows into the vacant p orbital (in carbocation / C=C / C≡ C) because orbitals can partially overlap Hyperconjugation effects 16 (H) •H effect of -CH3 is stronger than H effect of -CH2•H effect is generally stronger than I effect H H C CH δ+ H CH δ− CH3 CH CH2 CH2 CH3 CH2 HCl CH3 Cl Electron-donating ability of -CH3 is stronger than that of -CH2CH3 17 STERIC EFFECTS. .. 3.34 3.98 Br 2.60 3.51 3.91 I 2.60 3,61 3,78 CH3OCO 2,23 3.64 2.38 CF3 - 3.50 2.60 CN - 2.76 32 1.74 33 STABILITY OF CARBOCATIONS +H & +I 34 Allylic & benzylic carbocations +C +C Allylic & benzylic carbocations are generally stable due to the electron delocalization (+C effects) 35 36 ... bulky groups can hinder the formation of the required transition state 18 Steric hindrance 19 Steric hindrance 20 ACIDITY & BASICITY 21 22 Electrondonating groups Electronwithdrawing groups 23 Electrondonating groups Electronwithdrawing groups 24 If –C groups are introduced at ortho- & para position on phenol rings: + The anion (-O-) can be further stabilized by delocalization through the conjugated system... negative charge (-O-), the greater the stabilizing effect is + The closer the –I group is to the –OH, the O-H bond is more polarized Acidity of phenols is generally increased Note: there might be ortho -effects 26 OH OH OH NO2 > > NO2 = 7.23 8.4 OH pKa NO2 7.15 OH OH > CH3 > CH3 pKa = 10.08 CH3 10.14 10.28 27 OH OH OH OCH3 > > OCH3 pKa OCH3 9.98 = 9.65 10.21 OH OH OH Cl > > Cl pKa = 8.48 Cl 9.02 9.38 . 5681 Email: ptsnam@hcmut.edu.vn 2 Chapter 2: ELECTRONIC & STERIC EFFECTS Conjugation / Mesomeric Steric Electronic Inductive Hyperconjugation Effects 3 INDUCTIVE EFFECTS (I) C-C σ bond in butane:. 3 INDUCTIVE EFFECTS (I) C-C σ bond in butane: almost completely nonpolar δ- δ+ δ'+ δ''+ δ'''+ C-C σ bond in 1- fluorobutane: polarized C1 is more positive than C2 as. CH CH C CH CH CH CH CH C CH H O H H O H O INDUCTIVE vs CONJUGATION EFFECTS Mobility of hydrogen atoms: almost the same -C 15 INDUCTIVE vs CONJUGATION EFFECTS • C effects are generally stronger than I effects • C effects can be effective