Organic Chemistry-I Lecture 2 PDF Fall 2025

Fall 2025 INTRODUCTION Dr. Mohammad AbdulWahhab Organic Chemistry-I Lecture (2) 1  Electronegativity Electronegativity (EN) is the affinity for electrons or intrinsic ability of an atom to attract the shared elec...

Fall 2025 INTRODUCTION Dr. Mohammad AbdulWahhab Organic Chemistry-I Lecture (2) 1  Electronegativity Electronegativity (EN) is the affinity for electrons or intrinsic ability of an atom to attract the shared electrons in a covalent bond. ❑ Differences in EN produce bond polarity. ❑ F is most electronegative (EN = 4.0), Cs is least (EN = 0.7) ❑Halogens and other reactive nonmetals on right side of periodic table attract electrons strongly, higher electronegativities F > O > Cl >N ~ Br > I ❑ Covalent bonds can have ionic character ❑ These are polar covalent bonds: - Bonding electrons attracted more strongly by one atom than by the other. - Electron distribution between atoms is not symmetrical 2 The difference in electronegativities determine the type of bond: ❑ Nonpolar Covalent Bonds: atoms with similar EN ( 1.7 C–H bonds, relatively nonpolar while C-O, C-X bonds are polar ❑ Bonding electrons toward electronegative atom: - C acquires partial positive charge, δ+ - Electronegative atom acquires partial negative charge, δ- HCl is polar because it has a positive end and a negative end. (difference in electronegativity) NaCl is ionic because the difference in EN is about 2.3 so form ionic bond (EN for Na = 0.9, Cl = 3.2) 3  Polarity of molecules (Dipole Moments) Dipole moment (μ) is Measure the net polarity of the molecule or it is the vector sum of all the individual bond polarities and lone-pair contributions. 4  Electron displacement factors 1- Inductive 2- Mesomeric 3- Hyperconjugation effect effect The inductive effect decreases rapidly as the distance from the source increases. The inductive effect may be ignored after C2 5  Electron displacement factors 1- Inductive 2- Mesomeric 3- Hyperconjugation effect effect Functional groups can be classified as electron-withdrawing (-I ) or electron-donating (+I ) groups relative to hydrogen. 1) Negative inductive effect (-I): Stability of carbocations in solution obeys the order 3ry > 2ry > 1ry NH3+ > NO2 > CN > SO3H > CHO > CO > COOH > COCl > CONH2 > F > Cl > Br > I > OH > OR > NH2 > C6H5 > H 2) Positive inductive effect (+I): C(CH3)3 > CH(CH3)2 > CH2CH3 > CH3 > H 6  Electron displacement factors 1- Inductive 2- Mesomeric 3- Hyperconjugation effect effect 7  Electron displacement factors 1- Inductive 2- Mesomeric 3- Hyperconjugation effect effect 1- Resonance of -ve charge “Atom to bond and bond to atom” 8  Electron displacement factors 1- Inductive 2- Mesomeric 3- Hyperconjugation effect effect 1- Resonance of -ve charge “Atom to bond and bond to atom” 9  Electron displacement factors 1- Inductive 2- Mesomeric 3- Hyperconjugation effect effect 2- Resonance of +ve charge “bond to bond” 10  Electron displacement factors 1- Inductive 2- Mesomeric 3- Hyperconjugation effect effect 2- Resonance of +ve charge “bond to bond” 11  Electron displacement factors 1- Inductive 2- Mesomeric 3- Hyperconjugation effect effect 12  Electron displacement factors 1- Inductive 2- Mesomeric 3- Hyperconjugation effect effect EDG EWG 13  Electron displacement factors 1- Inductive 2- Mesomeric 3- Hyperconjugation effect effect ❑ This phenomenon is due to the ability of σ electrons of C-H bond to conjugate with π-electrons of C=C bond. (No-Bond resonance) 14  Electron displacement factors 1- Inductive 2- Mesomeric 3- Hyperconjugation effect effect Stability of Carbocations Arrange the following carbocations in an ascending order according to their stability: 8 5 9 2 4 1 Correct order is: (6) < (4) < (5) < (2) < (1) < (3) 15  Intermolecular Forces Noncovalent interactions 1. London Dispersion Forces ❑ Occur between all neighboring molecules and arise because the electron distribution within molecules that are constantly changing. ❑ The weakest of all molecular interactions. ❑ The larger the molecule the greater it’s Dispersion Forces are. 2. Dipole-Dipole Forces ❑ Occur between polar molecules as a result of electrostatic interactions among dipoles. ❑ Forces can be attractive or repulsive depending on orientation of the molecules. 3. Ion-Dipole Forces ❑ The force of attraction between an ion and a polar molecule. ❑ The charge of the ion will attract the oppositely partial charge of the dipole. ❑ Na+ is a cation, so it attracts the slightly negative end of the water (the oxygen). 16 4. Hydrogen Bond Forces ❑ Hydrogen bonds are attractive forces in which a hydrogen covalently bonded to a very electronegative atom. ❑ Usually the electronegative atom is oxygen, nitrogen, or fluorine, which has a partial negative charge. The hydrogen then has the partial positive charge. ❑ Hydrogen bonding is usually stronger than normal dipole forces between molecules ❑Phenomena Explained by hydrogen bond forces: Deviation of Boiling Points: 17  Concepts of Acidity and Basicity The Brønsted-Lowry Definition of Acids and Bases ❑Acid: a substance that can donate a proton (H+) ❑Base: a substance that can accept a proton (H+) Lewis Definition of Acids and Base ❑ Lewis acids are defined as electron-pair acceptors (called electrophile) ❑ Atoms with an empty valence orbital can be Lewis acids as BF3 , AlCl3, FeCl3, SbCl3, ZnCl2, HgCl2 ❑ Lewis bases are defined as electron-pair donors (called nucleophile) 18 Factors Affecting Acid Strength 1. Resonance effect ❑ Resonance in the conjugate bases of carboxylic acids stabilizes the base and makes the acid more acidic. ❑ Phenol is significantly more acidic than alcohol, why? This difference due to resonance model. 2- Inductive effect ❑ Groups that electrons withdrawing (-I) increase the acidity and decrease the basicity, while electron- donating groups (+I) act in the opposite direction. ❑ Electron-withdrawing substituent near the carboxyl group increases the acidity of carboxylic acids. pKa=2.86 pKa=4.05 pKa=4.52 19 3- Hydrogen bonding pKa=4.20 pKa=4.58 ❑ Intramolecular hydrogen bonding can greatly influence acid or base strength pKa=2.98 intermolecular intramolecular 4- Atom size Which compound is more acidic? Which compound is more basic? 20 THANK YOU 21