Organic Chemistry: Key Terms for Reactions & Mechanisms
Terms for Chapter 3: Introduction to Organic Reactions & Mechanisms: Acids & Bases
Substitution Reaction (Sec 3.1, 6.2, 10.3, 13.2): A reaction in which an atom or group in a compound is replaced by another atom or group.
Addition Reaction (Sec 3.1, 8.1, 13.2): A reaction which increases the number of atoms or groups that are attached to a pair of atoms joined by a double or triple bond; all parts of the adding reagent appear in the product, and two molecules become one.
Elimination Reaction (Sec 3.1, 6.15, 7.5): A reaction which results in the loss of two atoms or groups from a substrate, with the formation of a pi bond.
Rearrangement Reaction (Sec 3.1, 7.8): A reaction in which the carbon skeleton of a molecule is rearranged to produce a different isomer of the original molecule.
Reaction Mechanism (Sec 3.1, 3.14, 6.6, 6.10, 7.7): A step-by-step description of the events that take place on a molecular level as reactants are converted to products, including a description of all intermediates and transition states.
Bond Cleavage and Formation
Homolysis (Sec 3.1A, 10.1): The cleavage of a covalent bond so that each fragment departs with one of the electrons of the bond that joined them, which usually produces radicals. Also called homolytic cleavage.
Radical (Sec 3.1A, 10.1, 10.7): An uncharged atom or molecule that contains an unpaired electron. Also called a free radical.
Heterolysis (Sec 3.1A, 3.4): The cleavage of a polar covalent bond so that one fragment departs with both of the electrons of the bond that joined them, which usually produces both positive and negative ions. Also called heterolytic cleavage.
Acids and Bases
Acid (Sec 3.2, 3.3):
- An Arrhenius acid is a compound that dissociates in water and releases hydrogen ions.
- A Brønsted-Lowry acid is a proton donor.
- A Lewis acid is an electron-pair acceptor.
Base (Sec 3.2, 3.3):
- An Arrhenius base is a compound that dissociates in water and releases hydroxide ions.
- A Brønsted-Lowry base is a proton acceptor.
- A Lewis base is an electron-pair donor.
Conjugate Acid (Sec 3.2A, 3.6C, Lab 5.3): A compound or ion that forms from the protonation of a base, which converts B into HB+, or converts B– into HB.
Conjugate Base (Sec 3.2A, 3.6C, Lab 5.3): A compound or ion that forms from the deprotonation of an acid, which converts HA into A–, or converts HA+ into A.
Molecular Orbitals and Reactivity
LUMO (Sec 3.3A, 13.9C): The lowest unoccupied molecular orbital; when a molecule in the ground state is excited, an electron may jump from a HOMO into a LUMO.
HOMO (Sec 3.3A, 13.9C): The highest occupied molecular orbital; when a molecule in the ground state is excited, an electron may jump from a HOMO into a LUMO.
Carbocation (Sec 3.4, 6.11): A chemical species in which a carbon atom bears a formal positive charge; the C+ cation has three sp2 hybrid orbitals, plus one empty p orbital, and it has a trigonal planar molecular geometry; it can act as a Lewis acid and a powerful electrophile. Also called carbonium ion.
Carbanion (Sec 3.4, 3.8, 12.1A, 12.7): A chemical species in which a carbon atom bears a formal negative charge; the :C– anion has four sp3 hybrid orbitals with a lone pair of electrons in one of them, and it has a trigonal pyramidal molecular geometry; it can act as a Lewis base, a very strong Brønsted-Lowry base (pKa = 50), and a powerful nucleophile.
Electrophile (Sec 3.4A, 8.1, Lab 15.1): An “electron-loving” species; a cation or electron deficient molecule (with a δ+ charge, or an empty p orbital), that can accept a pair of electrons to form a new covalent bond; Lewis acids (electron-pair acceptors) are electrophiles.
Nucleophile (Sec 3.4A, 6.3, 6.13B, 8.1, Lab 14): A “nucleus loving” species; an anion or molecule with a lone pair of electrons that donates its electron pair to form a new covalent bond; Lewis bases (electron-pair donors) are nucleophiles.
Curved Arrow Notation (Sec 1.8, 3.5): Shows the direction of electron flow in a reaction; the curved arrow begins with a covalent bond or lone pair of electrons, then points towards the site of electron deficiency; a double-barbed arrow represents two electrons, and a single-barbed arrow represents one electron.
Equilibrium and Thermodynamics
Equilibrium Constant, Keq (Sec 3.6, 6.7): A constant that expresses the position of an equilibrium; for the general chemical equation: aA + bB ↔ cC + dD, the equilibrium constant is: Keq = [C]c[D]d / [A]a[B]b
Acid Dissociation Constant (Acidity Constant), Ka (Sec 3.6): The equilibrium constant for the aqueous reaction: HA + H2O ↔ H3O+ + A– is defined as: Ka = Keq[H2O] = [H3O+][A–] / [HA] and pKa = -log Ka.
Base Dissociation Constant, Kb (Sec 3.6): The equilibrium constant for the aqueous reaction: B + H2O ↔ HB+ + OH– is defined as: Kb = Keq[H2O] = [HB+][OH–] / [B] and pKb = -log Kb; it is often converted into the pKa of its conjugate acid by the formula: pKa = 14.00 – pKb.
Acid Strength (Sec 3.6): The strength of an acid is related to its acid dissociation constant, Ka; the stronger the acid, the larger its Ka, and the smaller its pKa.
Base Strength (Sec 3.6): The strength of a base is inversely related to its conjugate acid dissociation constant, Ka; the stronger the base, the weaker its conjugate acid, thus the smaller its Ka, and the larger its pKa.
Conjugate Acid-Base Pair (Sec 3.6C, Lab 5.3): Two compounds that differ only in the presence or absence of a hydrogen ion; the strength of a conjugate acid is inversely related to the strength of its conjugate base; stronger acids have weaker conjugate bases, and vice versa.
Electronic and Structural Effects
Inductive Effect (Sec 3.8B, 3.11B, 15.10, 15.11): Polarization of a molecule, due to the effect of a group that is either electron-donating or electron-withdrawing, which is transmitted through space and through the bonds of the molecule, but the effect gets weaker with increasing distance from the group.
Energy (Sec 3.9): The capacity to move matter and to do work.
Kinetic Energy, KE (Sec 3.9): Energy that results from the motion of an object, defined as: KE = ½ mv2, where m is the mass of the object, and v is its velocity.
Potential Energy, PE (Sec 3.9): Stored energy due to attractive or repulsive forces between objects.
Enthalpy, H (Sec 3.10): The heat content of a system, defined as: H = E + PV, where E is the internal energy of the system, P is the pressure, and V is the volume.
Enthalpy Change, ΔH (Sec 3.10): The heat of reaction, which measures the amount of heat released or absorbed by a system.
Exothermic Reaction (Sec 3.10, 6.7): A reaction that releases heat, and has a negative ΔH. (Origin: exo = external; therm = heat)
Endothermic Reaction (Sec 3.10, 6.7): A reaction that absorbs heat, and has a positive ΔH. (Origin: endo = within; therm = heat)
Entropy, S (Sec 3.10): (1) A measure of the degree of disorder of a system. (2) The part of the energy of a system which is not available to perform useful work.
Entropy Change, ΔS (Sec 3.10): The change in the relative order of a system; a reaction that increases the disorder of a system has a positive ΔS; total entropy increases in all spontaneous natural processes.
Gibbs Free Energy Change, ΔG (Sec 3.10, 6.7, 10.5A): A measure of the amount of free energy of a system (energy available to do work); defined as: ΔG = ΔH – TΔS, where ΔH is the enthalpy change, T is the absolute temperature, and ΔS is the entropy change. The relation between the standard Gibbs free energy change, ΔG°, and the equilibrium constant, Keq, is defined as: ΔG° = ─RTln(Keq), where R is the gas constant (8.314 J/mol ∙ K), and ln is the natural logarithm function.
Exergonic Reaction (Sec 6.7): A reaction that releases energy, and has a negative ΔG. (Origin: ex = external; ergon = work)
Endergonic Reaction (Sec 6.7): A reaction that absorbs energy, and has a positive ΔG. (Origin: end = within; ergon = work)
Delocalization Effect (Sec 3.11A): An effect by which resonance causes the dispersal of electron density or electrical charge into two or more atoms, which always stabilizes a system.
Substituent Effect (Sec 3.11D, 15.10): An effect on the rate of reaction (or on the equilibrium constant) caused by the replacement of a hydrogen atom by another atom or group; effects include those caused by the size of the atom or group, called steric effects, and those effects caused by the ability of the group to withdraw or donate electron density, called electronic effects (inductive effects or resonance effects).
Solvents and Reactivity
Protic Solvent (Sec 3.12, 6.13C): A solvent whose molecules have a hydrogen atom bound to an oxygen or nitrogen atom, and thus can form hydrogen bonds to solute molecules that contain oxygen or nitrogen atoms; it solvates anions and cations, and is an excellent solvent for SN1 reactions because it stabilizes carbocations; common protic solvents include water, alcohols (such as ethanol), and carboxylic acids (such as acetic acid).
Alkyloxonium Ion (Sec 3.13, 11.5): A protonated alcohol, ROH2+, with a positive charge on the oxygen atom, which is the conjugate acid of an alcohol, ROH.
Alkoxide Ion (Sec 3.15, 11.6): A deprotonated alcohol, RO–, with a negative charge on the oxygen atom, which is the conjugate base of an alcohol, ROH.
Leveling Effect of a Solvent (Sec 3.15): An effect that restricts the use of certain solvents with strong acids and bases; no acid stronger than the conjugate acid of the solvent, and no base stronger than the conjugate base of the solvent, can exist to an appreciable extent in that solvent.