Introduction To Organic Chemistry 2024 PDF

By Dr. Zahra Kassem  Objectives 1. To identify different types of chemical bonds. 2. To know how atomic orbitals combine to form molecular orbitals. 3. To know why organic compounds have linear, planar or tetrahedral structure. 4. To know the effect of hybridization on bond...

By Dr. Zahra Kassem  Objectives 1. To identify different types of chemical bonds. 2. To know how atomic orbitals combine to form molecular orbitals. 3. To know why organic compounds have linear, planar or tetrahedral structure. 4. To know the effect of hybridization on bond length and acidity of the compounds. 5. To understand factors affecting electron availability at a carbon atom 3 September 2024 2  PERIODIC TABLE 3 September 2024 3  Types of Chemical Bonds Introduction to Bonding Chemical bonding describes a variety of interactions that hold atoms together in chemical compounds. Chemical bonds are forces that hold atoms together to make compounds or molecules. Chemical bonds include covalent, polar covalent, and ionic bonds. Atoms with relatively similar electronegativities share electrons between them and are connected by covalent bonds. Atoms with large differences in electronegativity transfer electrons to form ions. The ions then are attracted to each other. This attraction is known as an ionic bond. 3 September 2024 4  Definitions Bond: A link or force between neighboring atoms in a molecule or compound. Ionic bond: An attraction between two ions used to create an ionic compound. This attraction usually forms between a metal and a non-metal. Covalent bond: An interaction between two atoms, which involves the sharing of one or more electrons to help each atom satisfy the octet rule. This interaction typically forms between two non-metals. Intramolecular: Refers to interactions within a molecule. Intermolecular forces: Refers to interactions between two or more molecules. 3 September 2024 5  Chemical bonds Chemical bonds are the connections between atoms in a molecule. These bonds include both strong intramolecular interactions, such as covalent and ionic bonds. They are related to weaker intermolecular forces, such as dipole-dipole interactions, the London dispersion forces, and hydrogen bonding. 3 September 2024 6  A. Ionic Bonds For atoms with the largest electronegativity differences (such as metals bonding with nonmetals), the bonding interaction is called ionic, and the valence electrons are typically represented as being transferred from the metal atom to the nonmetal. Once the electrons have been transferred to the non-metal, both the metal and the non-metal are considered to be ions. The two oppositely charged ions attract each other to form an ionic compound. 3 September 2024 7  Forming an Ion Ionic bonds are a class of chemical bonds that result from the exchange of one or more valence electrons from one atom, typically a metal, to another, typically a nonmetal. This electron exchange results in an electrostatic attraction between the two atoms called an ionic bond. An atom that loses one or more valence electrons to become a positively charged ion is known as a cation, while an atom that gains electrons and becomes negatively charged is known as an anion. 3 September 2024 8  A cation is indicated by a positive superscript charge (+ something) to the right of the atom. An anion is indicated by a negative superscript charge (- something) to the right of the atom. For example, if a sodium atom loses one electron, it will have one more proton than electron, giving it an overall +1 charge. The chemical symbol for the sodium ion is Na+1or just Na+ Similarly, if a chlorine atom gains an extra electron, it becomes the chloride ion, Cl-. Both ions form neutral molecule. The ion is more stable than the atom due to the octet rule. 3 September 2024 9  Forming an Ionic Bond Once the oppositely charged ions are formed, they are attracted by their positive and negative charges. Ionic bonds are also formed when there is a large electronegativity difference between two atoms. This difference causes an unequal sharing of electrons such that one atom completely loses one or more electrons and the other atom gains one or more electrons, such as in the creation of an ionic bond between a metal atom (sodium) and a nonmetal (Chlorine). 3 September 2024 10  Types of chemical bonds 1- ionic Bond 11Na -e- 10 Na+ as Ne 17Cl +e- 18 Cl- as Ar Na+ + Cl- NaCl Electrostatic attraction between positive and negative ions 3 September 2024 11 3 September 2024 12  Electronic configuration 3 September 2024 13  B. Covalent Bonds Chemical bonds are the forces of attraction that tie atoms together. Bonds are formed when valence electrons, the electrons in the outermost electronic “shell” of an atom, interact. The nature of the interaction between the atoms depends on their relative electronegativity. Atoms with equal or similar electronegativity form covalent bonds, in which the valence electron density is shared between the two atoms. The electron density resides between the atoms is equal and is attracted to both nuclei. This type of bond forms most frequently between two non- metals. 3 September 2024 14  When there is a greater electronegativity difference than between covalently bonded atoms, the pair of atoms usually forms a polar covalent bond. The electrons are still shared between the atoms, but the electrons are not equally attracted to both elements. As a result, the electrons tend to be found near one particular atom most of the time. Again, polar covalent bonds tend to occur between non-metals. 3 September 2024 15  Forming Covalent Bonds Covalent bonds are a class of chemical bonds where valence electrons are shared between two atoms, typically two nonmetals. The formation of a covalent bond allows the nonmetals to obey the octet rule and thus become more stable. For example: A fluorine atom has seven valence electrons. If it shares one electron with a carbon atom (which has four valence electrons), the fluorine will have a full octet (its seven electrons plus the one it is sharing with carbon). 3 September 2024 16  Carbon will then have four valence electrons (its four and the one its sharing with fluorine). Covalently sharing two electrons is also known as a “single bond.” Carbon will have to form four single bonds with four different fluorine atoms to fill its octet. The result is CF4 or carbon tetrafluoride. 3 September 2024 17  Covalent bonding requires a specific orientation between atoms in order to achieve the overlap between bonding orbitals. Covalent bonding interactions include sigma-bonding (σ) and pi-bonding (π). Sigma bonds are the strongest type of covalent interaction and are formed via the overlap of atomic orbitals along the orbital axis. The overlapped orbitals allow the shared electrons to move freely between atoms. Pi bonds are a weaker type of covalent interactions and result from the overlap of two lobes of the interacting atomic orbitals above and below the orbital axis. 3 September 2024 18  Covalent bonds can be single, double, and triple bonds. Single bonds occur when two electrons are shared and are composed of one sigma bond between the two atoms. Double bonds occur when four electrons are shared between the two atoms and consist of one sigma bond and one pi bond. Triple bonds occur when six electrons are shared between the two atoms and consist of one sigma bond and two pi bonds. 3 September 2024 19  2- Covalent Bond Sharing of electrons between 2 atoms arise through overlap of atomic orbitals to form either σ or π bonds: s spherical p dumbbell-shape 3 September 2024 20 Types of overlap: 1) s and s overlap (form σ bond) 2) s and p overlap (overlap with only one lobe) (form σ bond) 3 September 2024 3) p and p overlap a) head on overlap b) Sidewise overlap A π bond is not formed unless a σ bond is first formed 3 September 2024 22 3 September 2024 23  3- Coordination Bond ( Semi-ionic Bond) A coordinate bond (also called ‘’dative covalent bond or dipolar bond’’) is a covalent bond (shared pair of electrons) in which both electrons come from the same atom. 3 September 2024 24  4- Hydrogen Bond Types of Hydrogen Bonds a- Intermolecular hydrogen bond Hydrogen Bond is due to electrostatic attraction between δ+ and δ- Association hydrogen bond leads to increase in boiling point. The increase in boiling point happens because the molecules are getting larger with more electrons, and so van der Waals dispersion forces become greater. 3 September 2024 25 b- Intramolecular hydrogen bond 3 September 2024 26  valence 3 September 2024 27  Valency It is number of unpaired electrons present in the outermost shells. 3 September 2024 28  Hybridization Is the idea that atomic orbitals fuse to form newly hybridized orbitals, which in turn, influences molecular geometry and bonding properties. 3 September 2024 29 3 September 2024 30  Organic Chemistry is the study of carbon compounds Methane CH4 Overlap of four sp3 orbitals of c atom with four 1s orbitals of four H atoms. The bond angles in methane equal to 109.5°. 3 September 2024 Methane CH4 As seen in methane (CH4), carbon can form 4 bonds. The rationale behind this phenomenon is hybridization. Supporting evidence shows that 1 s and 3 p orbitals are being combined to form hybrid orbitals, allowing polyatomic molecule to have 25% s character and 75% p character. Thus, we call methane a sp3-hybridized molecule. 3 September 2024 32 3 September 2024 33 3 September 2024 34 3 September 2024 35  Ethylene The 2p orbital here is considered low enough energy to be classified within the same energy level as the sp2 orbitals. The figure below portrays the correct way to distribute electrons. 3 September 2024 36 Ethylene Notice how that lone electron in the 2p orbital is separate from the electrons in the sp2 orbitals. This is what influence ethylene's shape. The lone electron from each carbon will remain in its respective p orbital and form a pi bond with the other p orbital electron. Thus, ethylene is a planar molecule, with orbitals spaced 120 degree angles apart. 3 September 2024 37 3 September 2024 38 3 September 2024 39 Acetylene 1 s orbital is being mixed with 1 p orbital. 3 September 2024 40 Acetylene the lone electrons in the 2p orbitals are not part of the sp hybride orbitals. Instead, each electron is in its respective p orbital, and will bond with its respective p orbital of the other carbon. This in itself will create a sigma bond and two pi bonds, leading to the formation of a linear molecule! 3 September 2024 41  Effect of hybridization on bond length and acidity Electrons in S orbitals are strongly attached to nucleus 3 September 2024 42  Effect of hybridization on bond length and acidity Sp3 C Sp2c Sp C Bond type C-C C=C C ≡C Example CH3-CH3 CH2=CH2 CH ≡ CH % S Character 25 33.3 50 Bond length 1.526 1.335 1.206 A O Acidity Non acidic Less acidic Most acidic 3 September 2024 43  Bond Length and Bond Energy Bond Energy : Is the amount of energy required to separate the bond pairs of atoms to undefined distance : σ > π bonds. Triple>double>single Bond Length: It is the average distance between centers of two adjacent atoms which are bonded together ,bond length in single bond is much longer than the double bond. 3 September 2024 44  Electronegativity is the ability of an atom to attract bonded electrons closer to itself According to electronegativity: F > O > N, Cl > Br > C,S > H 4.0 3.5 3.0 2.82 2.5 2.25 3 September 2024 45  Bonds, Stability, and Compounds Covalent interactions are directional and depend on orbital overlap, while ionic interactions have no particular directionality. Each of these interactions allows the atoms involved to gain eight electrons in their valence shell, satisfying the octet rule and making the atoms more stable. These atomic properties help describe the macroscopic properties of compounds. For example, smaller covalent compounds that are held together by weaker bonds are frequently soft and malleable. On the other hand, longer-range covalent interactions can be quite strong, making their compounds very durable. Ionic compounds, though composed of strong bonding interactions, tend to form brittle crystalline lattices. 3 September 2024 46  Ionic Compounds v. Molecular Compounds Unlike an ionic bond, a covalent bond is stronger between two atoms with similar electronegativity. For atoms with equal electronegativity, the bond between them will be a non- polar covalent interaction. In non-polar covalent bonds, the electrons are equally shared between the two atoms. For atoms with differing electronegativity, the bond will be a polar covalent interaction, where the electrons will not be shared equally. 3 September 2024 47  Ionic solids are generally characterized by high melting and boiling points along with brittle, crystalline structures. Covalent compounds, on the other hand, have lower melting and boiling points. Unlike ionic compounds, they are often not soluble in water and do not conduct electricity when solubilized. 3 September 2024 48  Factors Affecting Electron Availability at an Atom 1 - Inductive effect 2 - Resonance 3 - Hyperconjugation 3 September 2024 49  1- Inductive effect - I( out of Carbon ) +I ( to carbon) electron withdrawing gp electron donating gp e.g. NO₂,CN,SO₃H,CHO,COR e.g. Alkyl groups F , Cl , Br , I 3 September 2024 50  2- Resonance 3 September 2024 51  ↑ number of canonical structures →↑ stability 3 September 2024 52  Inductive effect Resonance Occur only in saturated Occur in unsaturated and systems conjugated systems It is the tendency of an Delocalization of π es or atom or group of atoms to lone pair in a planar attract or release electrons system (parallel p orbitals) Transmission effect fades Continue as long as by increasing distance conjugation extends 3 September 2024 53  Breaking of a covalent bond A) homolytic fission B) heterolytic fission 3 September 2024 54  Breaking of a covalent bond Homolytic bond fission, is the breaking of a bond that results in one electron going to each atom in the bond. Homolytic cleavage is the source behind free radicals--atoms with one electron in an orbital and no charge (actual atoms--not ions). Heterolytic cleavage, which is more common, refers to a bond breaking and one atom getting both electrons while the other gets none. 3 September 2024 55  List of References Essential Books (Text Books): - Textbook of Organic Medicinal and Pharmaceutical Chemistry, Wilson and Gisvold's, 12th Ed., International Edition, Philadelphia : Lippincott Williams & Wilkins, 2011. - Textbook of Organic Medicinal and Pharmaceutical Chemistry, Wilson and Gisvold's, 11th Ed., Philadelphia : Lippincott Williams & Wilkins, 2004. - Organic Chemistry ,T. W. Graham Solomons, Craig B. Fryhle, 9th Ed., N.Y.: John Wiley & Sons, Inc., 2006. - Organic Chemistry, T. W. Graham Solomons, Craig B. Fryhle, 8th Ed., N.Y.: John Wiley & Sons, Inc., 2004. - March's Advanced Organic Chemistry,: Reactions, Mechanisms, And Structure, Michael B. Smith and Jerry March, 6th Ed., New Jersey : John Wiley and Sons, Inc., 2007. - Organic Chemistry, John McMurry, 7th Ed., Australia : Thomson Brooks/Cole, 2008. 3 September 2024 56