Formula sheet: Organic Chemistry basics
Organic Chemistry – Some Basic Principles and Techniques14 min readLesson 1 of 7
1.1 Carbon and its bonds
Organic chemistry is the chemistry of carbon compounds. Carbon is tetravalent and forms four covalent bonds, and it can link to other carbon atoms in chains and rings. The bonds are formed by hybrid orbitals, and the hybridisation decides the geometry: sp three gives a tetrahedral arrangement with bond angles near 109.5 degrees, sp two gives a trigonal planar arrangement with angles near 120 degrees, and sp gives a linear arrangement with angles near 180 degrees.
A single bond is a sigma bond, formed by head-on overlap of orbitals. A double bond has one sigma and one pi bond, and a triple bond has one sigma and two pi bonds. The pi bonds are weaker and more exposed, which is why double and triple bonds undergo addition reactions while single bonds mostly undergo substitution.
1.2 Naming and isomerism
IUPAC names are built from a root for the longest carbon chain, a suffix for the principal functional group and prefixes for substituents. The chain is numbered so that the principal group gets the lowest possible locant, and substituents are listed alphabetically. Functional groups have a fixed priority order for choosing the suffix, and the lower priority groups appear as prefixes.
Isomers are compounds with the same molecular formula but different structures or arrangements. Structural isomerism includes chain, position and functional group isomerism. Stereoisomerism includes geometrical and optical isomerism, which arise from restricted rotation and from chirality.
1.3 Electronic effects
Reactivity in organic molecules is controlled by the distribution of electrons. The inductive effect is the permanent displacement of bond electrons due to electronegativity differences, and it is shown by an arrow along the sigma bond. The resonance effect is the delocalisation of pi electrons over several atoms, and it is shown by several structures called resonance contributors. Hyperconjugation is the interaction of sigma electrons of an adjacent carbon-hydrogen bond with an empty or partly filled p orbital.
These effects stabilise or destabilise ions and radicals. Carbocations, which are electron deficient, are stabilised by electron donating groups and by resonance. Carbanions, which are electron rich, are stabilised by electron withdrawing groups. The stability order decides the product of many reactions, including the Markovnikov rule for addition to alkenes.
Exam
For a nomenclature question, find the longest chain, then the principal functional group, then number to give that group the lowest locant. Substituents come alphabetically, and duplicated substituents get di, tri and tetra without affecting the alphabetical order.