Pharmaceutical Organic Chemistry 3 (Spectroscopy) Lecture Notes PDF

Pharmaceutical Organic Chemistry 3 (Spectroscopy) By Dr. Aya Almatary  Chemical Bonding The octet rule: Atoms tend to attain electron configuration of the nearest noble gases. (Why?) Because the electron configuration of noble gase...

Pharmaceutical Organic Chemistry 3 (Spectroscopy) By Dr. Aya Almatary  Chemical Bonding The octet rule: Atoms tend to attain electron configuration of the nearest noble gases. (Why?) Because the electron configuration of noble gases is very stable, because (e-) have filled up their orbitals, having very high ionization energy, thus it’s extremely difficult to break an e- away) How this occurs? By: 1) Losing electrons 2) Gaining electrons 3) Sharing electrons 2  Types of bonds Ionic Bonds Covalent Bonds 3  Ionic Bonds It is formed by the transfer of one or more electrons from one atom to another to create ions. It is an attractive force between oppositely charged ions and the products are called salts. Atoms involved widely differ in electronegativity. (Electronegativity measures the ability of an atom to attract electrons 4  Covalent Bonds When 2 or more atoms of the same or similar electronegativities react, a complete transfer of electrons does not occur. Covalent bond is a chemical bond formed by overlap of 2 atomic orbitals (AOs), one from each atom. Interaction of the 2 AOs actually creates new molecular orbitals (MOs) which involves both atoms. 5 Covalent Bonds 6  Valence The number of atoms which are typically bonded to a given atom is termed the valence of that atom. Non-bonding electrons Valence electrons not used in bonding are called nonbonding electrons, or lone-pair electrons. e.g. Nitrogen atom in ammonia (NH3), shares six valence electrons in three covalent bonds and remaining two valence electrons are nonbonding (lone pair) 7  Hybridization theory ❑ It was promoted by chemist Linus Pauling in order to explain structure of molecules like methane (CH4). ❑ The valence bond theory would predict that C forms 2 covalent bonds. However, CH2, methylene is a very reactive molecule, can’t exist outside of a molecular system. Findings that lead to Hybridization theory: Modern x-ray diffraction methods proved that all C–H bonds are identical (tetrahedral). Carbon should have 4 orbitals with the correct symmetry to bond equally to the 4 hydrogen atoms. Any H can be removed from methane (CH4) carbon with equal ease. 8  Orbital Hybridization Definition: Mixing of 2 or more non equivalent atomic orbitals (e.g. s and p) to form a new set of hybrid orbitals Shape: Hybrid orbitals have totally different shape from original atomic orbitals. Number of hybrid orbitals: is equal to number of pure atomic orbitals used in hybridization process. Types of hybridization: 1- Sp3 (All bonds of the atom are single) 2- Sp2 3- sp 9  1- sp3 Hybridization It occurs when one s-orbital and 3 p orbitals combine to form 4 equivalent, unsymmetrical, tetrahedral orbitals (sppp = sp3) 10  1- sp3 Hybridization Example 1 In methane (CH4), sp3 Orbitals on C-atom overlap with 1s orbitals on 4 H- atoms to form 4 identical C-H bonds (σ bond), having tetrahedral shape, with H–C–H angle of 109.5° 11 1- sp3 Hybridization Methane 12  Valance-Shell Electron-Pair Repulsion (VSEPR) Model ❑ Valence electrons may form single, double, or triple bonds, or they may be unshared. ❑ Negatively charged region of space is created, repulsion occurs so various regions of electron density around an atom spread out, so that each is far away from the other as possible. 13 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°. Distortion is greater, why? The 2 unshared pair of electrons on the oxygen repels adjacent electron pairs more strongly than do bonding pairs of electrons. H2S: bond angle is 92° not 109.5°. Why? The 2 unshared pair of electrons on the sulfur repels adjacent electron pairs more strongly than do bonding pairs of electrons. But bond angle distortion is greater than H2O, why? Difference in electronegativity between O & S atoms (O > S) makes displacement of electrons towards O in O-H bond, leaving slight +ve charge on H atoms to repel one another, keeping the bond angle maximum. 14  sp3 Hybridization Ethane In ethane (C2H6), two C-atoms bond to each other by σ- overlap of an sp3orbital from each. -3 sp3 orbitals on each C-atom overlap with 1s orbitals of H-atoms to form six C–H bonds. 15  sp2 Hybridization It occurs when s-orbital combines with two 2 p orbitals, giving 3 orbitals (spp = sp2). This results in a double bond. sp2 orbitals are in a plane with 120°angles, trigonal planar. Remaining p orbital is perpendicular to the plane. 16 In ethylene, sp2 Hybridization 4 sp2 orbitals of the two carbons form 4 σ bonds with 4 H-atoms. Two sp2-hybridized orbitals overlap to form a sigma (σ) bond. P-orbitals overlap side-to-side to form 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. Electrons in the σ bond are centered between nuclei. Electrons in the π bond occupy regions on either side of a line between nuclei. C=C double bond in ethylene shorter (C=C 134 pm) and stronger than single bond in ethane(C–C 154 pm). 17  What is double bond? The combination of the single bond, formed from the sp2 orbitals and the π– bond, are represented as a carbon-carbon double bond. 18  sp Hybridization It occurs when 1 s-orbital combines with a single 2 p orbital, giving 2 sp hybridized orbitals, the remaining 2 p orbitals remain unchanged. sp orbitals are linear, 180° apart on x-axis. 2 p orbitals are perpendicular on the y-axis and the z-axis. 19  sp Hybridization In acetylene, (CH≡CH) 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 6 electrons forms triple bond (C≡C). 2 sp orbitals form σ-bonds with 2 H-atoms. 20 Differences between the different kinds of hybridization 21  Differences between sigma (σ) and pi (π) bond A covalent bond is a chemical bond formed by overlap of 2 atomic orbitals (AOs), one from each atom. Interaction of the 2 AOs actually creates new molecular orbitals (MOs) which involves both atoms. 22  Practice Assign the following for this compound 1- Sigma bonds 2- Pi bonds 3- Atoms containing non-bonding electrons 4-sp3 hybridized atoms 5-sp2 hybridized atoms 6- sp hybridized atoms 7- head to head overlaps 8- Side to side overlaps 23 THANKS! Any questions? You can find me at ✘ [email protected] ✘ Office hours: Saturday, 12-2 p.m. At p323 24