Organic Chemistry Lecture 5 & 6, 2024-2025, PDF

ORGANIC CHEMISTRY Lecture 5 Module: Chemistry/Biochemistry Academic Year : 2024-2025 www.um6ss.ma Prof. Tarik OUCHBANI 1 1- Organic molecule reactivity: Inductive and Mesomeric e...

ORGANIC CHEMISTRY Lecture 5 Module: Chemistry/Biochemistry Academic Year : 2024-2025 www.um6ss.ma Prof. Tarik OUCHBANI 1 1- Organic molecule reactivity: Inductive and Mesomeric effects Polarization of σ bonds: Cδ+ X δ- (X= I, F, Cl, Br) σ bonds between C and heteroatom Cδ+ O δ- Cδ+ N δ- σ bonds between same atoms C-C or X-X No polarization H Cl Electronegativity decreasing order F ˃O ˃ Cl ˃ N ˃ Br ˃ I ˃ S ˃ C ˃ P≈H ˃ B ˃ Si ˃ Al ˃ Mg ˃ Li ˃ Na 2 Pr. T. Ouchbani 1- Organic molecule reactivity: Inductive and Mesomeric effects Electronic Effects Inductive Effect Effect Mesomere Negative Positive Negative Positive inductive effect inductive inductive inductive (I-) effect (I+) effect (M-) effect (M+) (Attracting (Attracting (Donating (Donating effect) effect) effect) effect) 3 Pr. T. Ouchbani 2- Inductive Effect : § It arises due to electronegativity difference between two atoms forming a sigma bond § It is transmitted through the sigma bonds § The magnitude of inductive effect decreases while moving away from the groups causing it It is a permanent effect § It influences the chemical and physical proprieties of compounds Inductive effect weakens away along the chain and is not significant beyond 3rd carbon atom but can be extended in case of pi bond existence 4 Pr. T. Ouchbani 2- Inductive Effect : 5 Pr. T. Ouchbani 2- Inductive Effect : Cumulative positive Inductive Effect (I+): § Throught the C-H bond is pratically considered as non-polar, there is partial positive charge on hydrogen atom and partial negative charge on carbon atom. § Therefore each hydrogen atoms acts as electron donating group. § This cumulative donation turns the alkyl moiety into an electron donating group 6 Pr. T. Ouchbani 2- Inductive Effect : Cumulative positive Inductive effect Carbon Carbon Carbon Carbon 7 Pr. T. Ouchbani 2- Inductive Effect : Negative inductive effect groups (I-) –F ˃-Cl ˃ – Br ˃ –I Positive inductive effect groups (I+) M= Na, Li, 8 K, Mg Pr. T. Ouchbani 2- Inductive Effect : δ+ δ- δ+ (I-) δ- δ+ (I+) 9 Pr. T. Ouchbani 2- Inductive Effect : δ+ δ- (I+) 10 Pr. T. Ouchbani  3-Applications of Inductive Effect : Stability of Carbocations § The stability of carbocations increases with the increase in number of alkyl groups Due to their(I+) effect. § The alkyl groups release electrons to carbon, bearing positive charge and thus stabilizes the ion. § The order of stability of carbocations is: 11 Pr. T. Ouchbani 3-Applications of Inductive Effect : Stability of carbanions: § the stability of carbanions decreases with increase in the number of alkyl groups since the electron donating alkyl groups destabiliza the carbanions by increasing the electron density. § Thus the order of stability of carbanions is: 12 Pr. T. Ouchbani Applications ofof 3-Applications Inductive Inductive Effect Effect : : Acidic strength of carboxylic acids and phenols: The electron withdrawing groups (-I) decrease the negative charge on the carboxylate ion and thus by stabilizing it. The acidic strength increases when –I groups are present. However the +I groups decrease the acidic strength. E.g; § The acidic strength increases with increase in the number of electron withdrawing Fluorine atoms as shown below. CH3COOH