Definitions & pronunciations — Organic Chem Basics

Commonly used terms that may be new to you. Select Say it to hear each term pronounced.

Term	Definition	Pronounce
Alkane	Compounds that contain only C and H atoms and only single (sigma) bonds; no pi bonds present; also includes cycles.
Alkene	Compounds that contain at least one pi bond as part of a carbon-carbon double bond; also includes cycles.
Alkyne	Compounds that contain pi bonds in a carbon-carbon triple bond; also cyclic compounds if the cycle is big enough.
Atomic orbital	An orbital that an atom presents into space; may be hybridized or unhybridized, e.g. 2p or sp3.
Bond-line structure	A short-hand way of drawing Organic molecules that uses lines for bonds and assumes the presence of C and H atoms.
Carbocation	An intermediate structure in which C has lost a pair of electrons from its octet; resultant 6-electron species is electron-poor.
Concerted	A mechanistic event in which all bonds are formed and broken at the same time (in concert) without going through intermediate(s).
Constitutional isomer	Compounds that contain the same number and type of atoms but those atoms are attached to each other in different ways.
Covalent bond	A bond formed between atoms of similar electronegativities in which the bond electrons are shared between the atoms.
Delocalization	The spreading of electron density through a pi system usually resulting in extra stability for a collection of conjugated atoms.
Double bond	Two pairs of electrons shared between two atoms; one as sigma (single) bond, second as pi bond. Sigma + pi = double bond.
Electronegativity	The attraction of an atom or group for electrons; related to atomic nuclear charge and atom size.
Electronic	Used in conjunction with molecular “effects” such as resonance and hyperconjugation; stabilizing or destabilizing.
Electron poor	Atom or region in a molecule missing electron density, e.g. from being attached to a highly electronegative atom or group.
Electron rich	Atom or region in a molecule with excess electron density, e.g. having a lone pair or being an electronegative atom or group.
Electrophile	An electron-poor atom or a species such as a carbocation that may attract and accept electron density.
Enthalpy	Related to heat and energy within a system; relative bond strengths, stability of molecular shapes, etc.
Entropy	Ability of a system (molecule, bond) to dissipate energy through degrees of freedom; e.g. open-chain molecules vs. cycles.
Formal charge	A book-keeping device to designate an atom sharing more electrons or carrying more than it would in simple molecules.
Gibbs free energy	The free energy (ΔG) in a system as calculated from the relevant enthalpy (H) and entropy (S) factors.
Hofmann selectivity	The preference for a less-substituted alkene to be formed when options are possible; often due to steric factors.
Hybrid orbital	Mixed orbital models used to explain the shapes of atoms within molecules; limited to sp3, sp2, sp for C, N, O.
Hyperconjugation	Donation of electron density from adjacent sigma bonds to stabilize species like carbocations and radicals.
Induction	Electron donation or withdrawal of electron density through single bonds creating dipoles within molecules.
Inversion	A switch in the stereochemical identity of a chiral center, for example during a bimolecular substitution reaction.
Ionic bond	A bond formed when electrons are swapped between atoms of quite different electronegativity values.
Irreversible	A step in a reaction pathway in which the products are much more stable than the reacting species.
Kinetics	The measurement and use of rates of reaction to determine mechanism by working out rate-determining steps.
Lone pair	A pair of valence electrons on an atom that do not need to be shared with another atom in a covalent bond.
Loss of leaving group	Separation of an atom or group that takes a pair of valence electrons away from carbon; usually electronegative.
Mechanism	The description of how bonds are formed and broken on the way from starting materials to products.
Molecular orbital	Orbitals used to hold atoms together in molecules; either bonding (constructive) or antibonding (deconstructive).
Nucleophile	An electron-rich species capable of donating a pair of electrons to an electrophilic (electron-poor) species.
Octet rule	The goal for atoms at the top of the Periodic Table to achieve the valence electron configuration of a Noble gas.
Pi bond	A bond above and below a sigma bond formed from overlapping p orbitals; weaker than a sigma bond and easier to break.
Polar covalent bond	A covalent bond between atoms of differing electronegativity where the electron density shifts towards the more electronegative atom.
Proton transfer	When a proton is transferred in a single step from an acid to a base to produce a conjugate base and a conjugate acid.
Radical	A species that has lost an electron from its octet; while neutral, radicals tend to be very reactive with other molecules.
Rearrangement	When an atom or group migrates within a molecule to leave a more stable situation e.g. a secondary carbocation becoming tertiary.
Recitation	A tutorial class in which concepts from lecture are reiterated and related problems are discussed in a smaller group.
Regiochemistry	When there are options for the placement of a functional group within a molecule, for example when a pi bond is introduced.
Regioselectivity	The preference for chemistry at one place in a molecule over another, for example when a pi bond is introduced.
Resonance	The delocalization of electron density across a pi system; resonance structures help describe a resonance hybrid structure.
Retention	When the stereochemistry of a chiral center remains the same throughout a reaction, for example during a migration step.
Reversible	When reactants and products in a step are close enough in energy to interconvert as long as the reverse step is viable.
Sigma bond	A single bond formed between two atoms along the inter-nuclear axis; typically stronger than a pi bond.
Stepwise	When a mechanistic pathway involves discrete steps and intermediates; bonds are formed and broken sequentially.
Steric	Related to size/accessibility; can lead to competition in paths, for example when substitution and elimination are possible.
Thermodynamics	The study of energetics in a system; relative bond strengths, stability of species and consequences in equilibria.
Triple bond	When two atoms are attached through a sigma and two pi bonds to give a linear, electron rich, triple bond.
Valence	The outermost electron shell that an atom uses to bond with other elements; in Organic, typically mono- to tetravalent.
Zaitsev selectivity	The preference for the more highly substituted alkene to be formed when there are regioisomers possible from a reaction.