Chapter 14: Introduction to Organic Chemistry

Introduction to Organic Chemistry introduces the vast field of organic chemistry, focusing on compounds built primarily upon carbon atoms, which form the structural backbone of molecules of life like DNA and proteins. It establishes the importance of the hydrocarbon class, defined as compounds containing only carbon and hydrogen, such as alkanes, which notably lack a functional group. Students learn essential methods for representing organic molecules, progressing from the empirical formula (simplest atom ratio) to the molecular formula (actual atom count), the structural formula (showing bonding arrangement per carbon atom), the displayed formula (showing all bonds), and the skeletal formula (a simplified representation). The concept of homologous series is central, where compounds share a common chemical behavior determined by a specific functional group, such as the C-C double bond in alkenes or the COOH group in carboxylic acids. Naming conventions follow the IUPAC system, using stems like 'meth-' or 'eth-' to denote carbon chain length and numbering chains to indicate the lowest possible position of alkyl side-chains or functional groups. A detailed explanation of bonding explores how carbon atoms achieve stability. Single bonds are sigma (σ) bonds, associated with sp3 hybridisation and a tetrahedral shape with bond angles near 109.5 degrees, as seen in alkanes. Double bonds, like in ethene, consist of one σ bond and one pi (π) bond, resulting from sp2 hybridisation and a planar geometry with 120-degree angles. Triple bonds feature one σ bond and two π bonds, arising from sp hybridisation and producing a linear molecule. The chapter then distinguishes two main types of isomerism: structural isomerism, where molecules share the same molecular formula but possess different structural arrangements, categorized into chain, position, and functional group isomerism. Stereoisomerism involves molecules with identical bonding but different spatial arrangements. This includes geometrical (cis/trans) isomerism, which occurs when rotation is restricted around double bonds or in cyclic structures, and optical isomerism, which requires a chiral centre (a carbon atom bonded to four distinct groups), creating non-superimposable mirror image molecules called enantiomers. Finally, fundamental reaction types and mechanisms are defined. Covalent bonds can break via homolytic fission, yielding highly reactive free radicals (characterized by initiation, propagation, and termination steps), or via heterolytic fission, forming charged ions. Charged species involved in mechanisms include carbocations (electron-deficient electrophiles), with tertiary carbocations being the most stable due to the positive inductive effect of surrounding alkyl groups. Conversely, electron-rich species are termed nucleophiles. Organic reactions are classified into addition (two reactants forming one product), elimination (removing a small molecule), substitution (replacement), hydrolysis (breakdown by water), condensation, oxidation (loss of hydrogen or gain of oxygen), and reduction (gain of hydrogen or loss of oxygen).