Organic Chemistry Lecture 1 PDF

Fall 2025 INTRODUCTION Dr. Mohammad AbdulWahhab Organic Chemistry-I Lecture (1) 1  What is Organic Chemistry? Organic chemistry started as the chemistry of life, then it became the chemistry of carbon compounds, now it is both. It is the c...

Fall 2025 INTRODUCTION Dr. Mohammad AbdulWahhab Organic Chemistry-I Lecture (1) 1  What is Organic Chemistry? Organic chemistry started as the chemistry of life, then it became the chemistry of carbon compounds, now it is both. It is the chemistry of the compounds of carbon along with other elements such as are found in: - Living things (made of organic chemicals). - Proteins (make up hair). - DNA (controls genetic make-up). - Foods, medicines. Why should I – as a pharmacist – study organic chemistry ?? 3 7 8  Atomic Structure Shell proton + N - + - N neutron electron 9  Atomic Number and Atomic Mass The atomic number (Z ) ‫ العدد الذري‬is the number of protons in the atom's nucleus. The mass number (A) ‫ عدد الكتلة‬is the number of protons plus neutrons. All the atoms of a given element have the same atomic number Isotopes ‫ النظائر‬are atoms of the same element that have different numbers of neutrons and therefore different mass numbers The atomic mass (atomic weight) of an element is the weighted average mass in atomic mass units (amu) of an element’s naturally occurring isotopes atomic weight 10  Shapes of Atomic Orbitals for Electrons Four different kinds of orbitals for electrons based on those derived for a hydrogen atom Denoted s, p, d, and f s and p orbitals most important in organic and biological chemistry s orbitals: spherical, nucleus at center p orbitals: dumbbell-shaped, nucleus at middle d orbitals: elongated dumbbell-shaped, nucleus at center 11  electron configuration and Orbital diagram C6 1S 2 2S 22P 2 ↑↓ ↑↓ ↑ ↑ 1s 2s 2px 2py 2pz N7 1S 2 2S 22P 3 ↑↓ ↑↓ ↑ ↑ ↑ 1s 2s 2px 2py 2pz O8 1S 2 2S 22P 4 ↑↓ ↑↓ ↑↓ ↑ ↑ 1s 2s 2px 2py 2pz Lone Pair: a pair of electrons occupying an orbital in an atom or molecule and not directly involved in bonding. 12  Orbitals Hybridization ‫التهجين‬ Mixing of two or more atomic orbitals to form a new set of hybrid orbitals. Mix at least 2 nonequivalent atomic orbitals (e.g., s and p). Hybrid orbitals have very different shape from original atomic orbitals. Number of hybrid orbitals is equal to number of pure atomic orbitals used in the hybridization process. 13 SP 3 Hybridization sp3 Orbitals and the Structure of Methane, CH4 & Ethane, CH3CH3 Promotion & ↑ ↑ ↑ hybridization ↑ ↑ ↑ ↑↓ 2px 2py 2pz ↑↓ 2sp3 2sp3 2sp3 2s p3 ↑↓ 2s Ground state Atomic 1s sp3 hybrid Atomic Orbitals (AOs) of Orbitals of Carbon 1s Carbon (C) (C) - Carbon has 4 valence electrons (2s2 2p2) - In CH4, all C–H bonds are identical (tetrahedral) - sp3 hybrid orbitals: s orbital and three p orbitals combine to form four equivalent, unsymmetrical, tetrahedral orbitals (sppp = sp3) 14 The Structure of Methane ❑ sp3 Orbitals on C overlap with 1s orbitals on 4 H atoms to form four identical C-H bonds ❑ Each C–H bond has a strength of 436 (438)kJ/mol and length of 109 pm ❑ Bond angle: each H–C–H is 109.5°, the tetrahedral angle 15  Valence-Shell Electron-Pair Repulsion (VSEPR) theory ▪ VSEPR model can be used to predict the bond angles in hybridized compounds. ▪ Outer Shell of valence electrons surrounds an atom. ▪ Negative charged region of space created, and repulsion occurred so various regions of electron density around an atom spread out. ❑ NH3: bond angle is 107.3° not 109.5° why? The unshared pair of electrons on the nitrogen repels adjacent electron pairs more strongly than do bonding pairs of electrons ❑ H2O: bond angle is 104.5° not 109.5° why? The 2 unshared pair of electrons on the oxygen. ❑ H2S: bond angle is 92° not 109.5° why? The 2 unshared pair of electrons on the sulfur. bond angle is not like in H2O why? Difference in electronegativity between O & S atoms (O > S) 16 SP 2 Hybridization sp2 Orbitals and the Structure of Ethylene, CH2=CH2 Promotion & ↑ ↑ ↑ hybridization ↑ ↑ ↑ 2p ↑↓ 2px 2py 2pz ↑↓ 2sp2 2sp2 2sp2 ↑↓ 2s Ground state Atomic 1s sp2 hybrid Atomic Orbitals (AOs) of Carbon Orbitals of Carbon (C) 1s (C) sp2 hybrid orbitals: 2s orbital combines with two 2p orbitals, giving 3 orbitals (spp = sp2). This results in a double bond. sp2 orbitals are in a plane with120°angles, trigonal planar Remaining p orbital is perpendicular to the plane 17 Bonds From sp2 Hybrid Orbitals ❑ Two sp2-hybridized orbitals overlap to form a sigma (σ) bond ❑ p orbitals overlap side-to-side to formation a pi (π) bond ❑ sp2–sp2 σ bond and 2p–2p π-bond result in sharing four electrons and formation of C=C double bond ❑ H atoms form σ bonds with four sp2 orbitals ❑ H–C–H and H–C–C bond angles of about 120° ❑ C=C double bond in ethylene shorter and stronger than C-C single bond in ethane ❑ Ethylene C=C bond length 134 pm (C–C 154 pm) 18 SP Hybridization sp Orbitals and the Structure of Acetylene, CHΞCH Promotion & ↑ ↑ ↑ ↑ hybridization ↑ ↑ 2p 2p ↑↓ 2px 2py 2pz ↑↓ 2sp 2sp ↑↓ 2s Ground state Atomic 1s sp hybrid Atomic Orbitals (AOs) of Carbon Orbitals of Carbon (C) 1s (C) ❑ C-C a triple bond sharing six electrons ❑ Carbon 2s orbital hybridizes with a single p orbital giving two sp hybrids - two p orbitals remain unchanged ❑ sp orbitals are linear, 180° apart on x-axis ❑ Two p orbitals are perpendicular on the y-axis and the z-axis 19 Orbitals of Acetylene ❑ Two sp hybrid orbitals from each C form sp–sp σ bond ❑ pz orbitals from each C form a pz–pz π bond by sideways overlap and py orbitals overlap similarly ❑ Sharing of six electrons forms C≡C ❑ Two sp orbitals form σ bonds with hydrogens 20  Comparison between the different kinds of hybridization No. of Bond S Type remaining p No.of bonds Geometry Example angle character orbitals 109.5 (4)sigma bonds (σ) Tetrahedral CH4 sp3 107.3 0 (3)sigma bonds (σ) 25% (1\4) Trigonal pyramidal NH3 104.5 (2)sigma bonds (σ) Bent H2O (3)sigma bonds (σ) C2H4 sp2 120 1 + 33% (1\3) Trigonal planar H2C=O (1)pi bond (π( R2C=O (3)sigma bonds (σ) + HCΞCH (2)pi bonds (π) sp 180 2 (2)sigma bonds (σ) 50% (1\2) linear O=C=O + (CO2) (2)pi bonds (π) HCΞN 21  Rule for determining hybridization of any atom ❑ The rule says that for any atom: No of hybridized orbitals( steric number) = No of attached atoms + No of LPs ❑ But take care of the exception: If an atom has 1 or more LPs and attached to a SP2 atom then its hybridization is SP2 also. 22  Differences between sigma (σ) and pi (π) MOs σ-Bond π-Bond Formed by side to side (lateral) overlap Formed by head-to-head overlap of Aos of AOs Has free rotation Has NO free rotation Only one bond can exist between 2 One or two bonds can exist between 2 atoms atoms Lower energy Higher energy Single bond: 1 σ-bond Double bond: 1 σ-bond and 1 π-bond Triple bond: 1 σ-bond and 2 π-bonds 23  From the following numbered structure (A), write or draw the desired item(s) beside the corresponding number in your answer paper: Bond-line structures are drawn in a zigzag format (⸜⸝⸜⸝⸜), where each corner or endpoint represents a carbon atom. 1) An example of sp2 carbon: C atom No… 2) An example of sp3 carbon: C atom No: …… 3) An example of sp carbon: C atom No……………………….................. 4) The shortest chemical bond lies between atom No… 5) The hybridization of N-1 is …………& The electronic configuration of nitrogen atom in ground state is drawn as ……………………… 6) The angle between C8-C9-C10 is about:… …………….. 7) The angle between C3-C4-C5 is about:…… 25