Tài liệu Bài giảng organic chemistry chapter2

ORGANIC CHEMISTRY Dr Nam T. S. Phan Faculty of Chemical Engineering HCMC University of Technology Office: room 211, B2 Building Phone: 38647256 ext. 5681 Email: [email protected] 1 Chapter 2: ELECTRONIC & STERIC EFFECTS Effects Electronic Steric Inductive Hyperconjugation Conjugation / Mesomeric 2 INDUCTIVE EFFECTS (I) C-C σ bond in butane: almost completely nonpolar C-C σ bond in 1fluorobutane: polarized δ- δ''+ δ'''+ δ+ δ'+ C1 is more positive than C2 as a result of electronattracting ability of F 3 The more electronegative the X, the stronger the –I effect 4 The more electropositive the Z, the stronger the +I effect 5 Through a period in a periodic table -I Through a group in a periodic table -I 6 CH3CH2CH2COOH CH3CH2CH(Cl)COOH CH3CH(Cl)CH2COOH ClCH2CH2CH2COOH Ka.105 1.5 139 8.9 3.0 Strong -I weak -I 7 CONJUGATION / MESOMERIC EFFECTS (C / M) Electron delocalization in a conjugated system: Alternating single & multiple bonds 8 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 9 C CH -C CH CH CHR +C C -C R CH CH CH CH -C O R +C O R +C 10 OH +C O H -C -C +C N NH2 +C -C -C +C 11 -C groups generally contain an electronegative atom (s) or / and a π-bond (s): CHO, C(O)R, COOH, COOR, NO2, CN, aromatics, alkenes +C groups generally contain a lone pair of electrons or a π-bond (s): Cl, Br, OH, OR, SH, SR, NH2, NHR, NR2, aromatics, alkenes Aromatics or alkenes can be both +C and-C 12 Through a period in a periodic table +C Through a group in a periodic table +C 13 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 -C 14 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 relative positions 15 HYPERCONJUGATION 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 (H) 16 •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 • 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 hindrance 20