Organic chemistry - Chapter 2: Electronic & steric effects - Nam Phan

Nội dung text: Organic chemistry - Chapter 2: Electronic & steric effects - Nam Phan

1. Chapter 2: ELECTRONIC & STERIC EFFECTS Effects Steric Electronic Inductive Hyperconjugation Conjugation / Mesomeric 2
2. The more electronegative the X, the stronger the –I effect 4
3. Through a period in a periodic table -I Through a group in a periodic table -I 6
4. CONJUGATION / MESOMERIC EFFECTS (C / M) Electron delocalization in a conjugated system: Alternating single & multiple bonds 8
5. CCH CH CH CHR -C +C COR -C +C R CH CH CH CH O R -C +C 10
6. -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
7. 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 HCH CH C atoms: almost the same H -C 14
8. 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
9. 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
10. Steric hindrance 20
11. Electron- donating groups Electron- withdrawing groups 24
12. If –I groups are introduced on phenol rings, the effect will depend on the distance: + The closer the –I group is to the 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
13. OH OH OH OCH3 > > OCH3 OCH3 pKa = 9.65 9.98 10.21 OH OH OH Cl > > Cl Cl pKa = 8.48 9.02 9.38 28
14. -I is decreased over long distance +C dominates -I dominates CH3CH2CH CHCOOH < CH2 CHCH2CH2COOH < CH3CH CHCH2COOH pKa = 4.83 pKa = 4.68 pKa = 4.48 +C and -I but +C and -I but -I dominates -I dominates HC C COOH H3C CCCOOH 30 pKa = 1.84 pKa = 2.60
15. Basicity X pKa of ammonium cation of o- m- p- XC6 H4NH 2 CH3 4.39 4.69 5.12 CH3O 4.49 4.20 5.29 C2 H5O 4.47 4.17 5.25 C6 H5 3.78 4.18 4.27 F 3.20 3.59 4.65 Cl 2.61 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 1.74 32 NO2 -0.29 2,50 1,02
16. STABILITY OF CARBOCATIONS +H & +I 34
17. Relative stability of carbocations 38
18. STABILITY OF CARBANIONS 40