Organic Chemistry Lecture Notes PDF

 first semester by Introduction  Organic structures Organic structures can be written with several formats. Following is an example of propyl alcohol: 5  Geometry Also, the structural formula can inform about...

 first semester by Introduction  Organic structures Organic structures can be written with several formats. Following is an example of propyl alcohol: 5  Geometry Also, the structural formula can inform about the geometry of the organic structures (in three dimensions), for instance, the bonds of the tetrahedral structure of methane can be drawn as 1. a dashed wedge: bond behind plane, 2. solid wedge: bond in front of plane 3. and ordinary line: bond in plane. 6  Other examples include ethylene which represents the geometry of a carbon with double bond that has a trigonal planar geometry. This indicates that all bonds are in one plane, with angle of 120o. In case of acetylene (ethyne), the geometry is linear which indicates that all bonds are in the same plane with angle of 180o. 8  Restricted Rotation & Double Bond Optimum overlap between p orbitals of a π bond occurs when they are parallel. Rotation around the π bond is thus energetically expensive as it breaks the bond (264 kJ/mol). 9  Carbons joined by single bonds (σ bonds) have low energy barrier to rotation (13-26 kJ/mol) and thus rotate freely. 10  stereoisomers Restricted rotation around double bonds introduces a new type of isomerism of cis- trans isomerism. The two compounds are not constitutional isomers because the connectivity is the same. They are classified as stereoisomers and are often called cis-trans isomers. 11  Cis indicates substituents on the same side, while Trans indicates substituents on opposite sides. They are not superposable. 12  Bond Lengths The greater the s-character, the shorter the bond – because – s-orbital is spherical and close to nucleus. The greater the p-character the longer the bond, – because – p-orbital is lobe-shaped and extended away from nucleus. 13  How to Predict Molecular Geometry? Molecular geometry is predicted based on the Valence Shell Electron Pair Repulsion (VSEPR) model: 1. Consider a molecule in which central atom is covalently bonded to two or more groups 2. Consider all electron pairs; bonding pairs and lone pairs. 3. Electron pairs in valence shell repel one another and stay as far apart as possible. Lone pairs are generally more repulsive than bonding pairs. 4. Describe the geometry of the molecule by considering nuclei positions only (disregard lone pairs). 14 Ex Nu of pairs Geometry Angle Structure methane has four pairs tetrahedral of bonding 109.5o. electrons. ammonia (NH3) three bonding trigonal pairs of pyramid 107° electrons and one nonbonding pair water two bonding angular or 104.5°, pairs of bent shape electrons and the two nonbonding electron  There are two groups of hydrocarbons: – Saturated hydrocarbons (paraffins). Alkane – Unsaturated hydrocarbons (olefins) and (acetylenes). Akenes and alkynes Aliphatic hydrocarbons: Depending upon the shape of carbon chain, aliphatic hydrocarbons can also be divided into: – Open chain (acyclic) compounds. Closed chain (cyclic or aliphatic alicyclic) compounds. 17 Saturated Hydrocarbons Alkanes & Cycloalkanes 18  Introduction: - Alkanes have the general formula CnH2n+2 - Cycloalkanes have the general formula CnH2n Sources of alkanes: Petroleum Alkanes may straight chain or branched:  Constitutional isomers: isomers that have the same molecular formula but possess different structures ex: pentane, isopentane, and neopentane are constitutional isomers (they all have C5H12) 20  IUPAC nomenclature of Alkanes IUPAC: International union of pure and applied chemistry. - This union has put rules for naming alkanes, if you know these rules you can name any structure. 1) Straight chain alkanes; – Add the suffix -ane to the end of the number that correspond to the number of carbons in the compound. 21  Application of Lithium Dialkyl Cuprates: The Cory-Posner Whitesides-House synthesis: 41 This is due to +I (+ve inductive effect of the methyl groups attached to the radical).