Chapter 2: ELECTRONIC & STERIC EFFECTS

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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.

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Nội dung Text: Chapter 2: ELECTRONIC & STERIC EFFECTS

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 1
Chapter 2: ELECTRONIC & STERIC EFFECTS Effects Steric Electronic Inductive Hyperconjugation Conjugation / Mesomeric 2
INDUCTIVE EFFECTS (I) C-C σ bond in butane: almost completely nonpolar C-C σ bond in 1- fluorobutane: polarized δ- δ+ δ''+ δ'+ δ'''+ C1 is more positive than C2 as a result of electron- attracting 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
CH CH CH COOH 1.5 CH3CH2CH(Cl)COOH 139 Strong -I 3 2 CH CH(Cl)CH COOH 8.9 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 CH CH CHR -C +C C O R -C +C R CH CH CH CH O R 10 -C +C
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 O H CH CH CH CH CH CH C H O H CH CH CH CH C H O Mobility of hydrogen H CH CH C atoms: almost the same 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 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 -CH is stronger than H effect of -CH - H H C CH HCl CH CH2 CH3 δ+ δ− H CH3 CH CH2 CH2 CH3 Cl 3 Electron-donating ability of -CH is stronger 17 2 3
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