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Bài giảng Organic chemistry - Chapter 2: Electronic and steric effects - Phan Nam

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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].vn  
1
Chapter 2: ELECTRONIC &  
STERIC EFFECTS  
Effects  
Electronic  
Steric  
Hyperconjugation  
Inductive  
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
Ka.105  
1.5  
CH3CH2CH2COOH  
CH3CH2CH(Cl)COOH 139  
CH3CH(Cl)CH2COOH 8.9  
Strong -I  
weak -I  
ClCH2CH2CH2COOH  
3.0  
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  
C CH  
CH  
CH  
CHR  
-C  
+C  
C O R  
-C +C  
R CH  
CH  
CH  
CH  
O R  
10  
-C  
+C  
O H  
OH  
+C  
-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  
12  
Aromatics or alkenes can be both +C and-C  
Through a period in a periodic table  
+C  
Through a  
group in a  
periodic  
table  
+C  
13  
INDUCTIVE vs CONJUGATION  
EFFECTS  
O
CH CH  
CH  
H
CH  
CH  
CH  
C
H
O
H CH  
CH  
CH  
CH  
CH  
C
H
O
Mobility of hydrogen  
atoms: almost the same  
H CH  
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 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
HCl  
H C CH  
CH  
CH2  
CH2  
CH3  
δ+  
δ−  
H
CH3  
CH  
Cl  
CH2 CH3  
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  
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