The Letter "A" in Chemistry

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Absolute configuration - The 3D structure of a chiral compound; designated by R or S

Absorption Band - A peak in a spectrum that occurs as a result of absorption of energy

Accuracy - The agreement of a measured value with the true value.

Acetamidomalonic Ester Synthesis - A method used to synthesize an amino acid that is a variation of the N - phthalimidomalonic ester synthesis

Acetoacetic Ester Synthesis - Synthesis of a methyl ketone, using ethyl acetoacetate as the starting material

Achiral - An optically inactive molecule that has a conformation identical to its mirror image (i.e. has a superimposable mirror image)

Acid - A substance that lowers the pH of a solution by producing H+

Acid - Base Indicator - A substance that changes color when the endpoint of an acid - base reaction has been reached.

Acid - Base Reaction - A reaction in which an acid donates a proton to a base or accepts a share in a base's electrons.

Acid Catalyst - A catalyst that increases the rate of a reaction by donating a proton

Acid dissociation constant (Ka) - The equilibrium constant for the reaction of an acid and water forming a conjugate base and the hydronium ion (H3O+). This value is a measure of the degree to which an acid dissociates in solution.

Actinide Series - The group of 14 elements that follow after actinum in the periodic table.

Activating Substituent - A substituent that increases the reactivity of an aromatic ring. Electron - donating substituents activate aromatic rings toward electrophilic attack. Electron - withdrawing substituents activate aromatic rings toward nucleophilic attack

Activated Complex - The arrangement of atoms found at the top of the potential energy barrier as a reaction proceeds from reactants to products.

Activation Energy - The minimum energy required for a chemical reaction to occur.

Active Site - A pocket or cleft in an enzyme where the substrate is bound.

Acyl - Enzyme Intermediate - An intermediate formed when an amino acid residue of an enzyme
is acetylated

Addition Polymer - A polymer made by adding monomers to the growing end of a chain with no other by – products

Addition Reaction - A reaction in which atoms or groups are added to the reactant

Adiabatic Process - A process that occurs without the transfer of energy as heat

Adsorption - The collection of one substance on the surface of another

Alcohol - An organic compound in which the hydroxyl group ( - OH) is a substituent on a hydrocarbon

Alcoholysis - A reaction with an alcohol

Aldaric Acid - A dicarboxylic acid with an OH group bonded to each carbon.

Aldehyde - An organic compound containing the carbonyl group bonded to at least one hydrogen atom

Alditol - A compound with a hydroxyl group ( - OH) bonded to each carbon.

Aldol Addition - A reaction between two molecules of an aldehyde (or two molecules of a ketone) that connects the (alpha) - carbon of one with the carbonyl carbon of the other.

Aldol Condensation - An aldol addition followed by the elimination of water

Aldonic Acid - A carboxylic acid with an OH group bonded to each carbon.

Aliphatic - A nonaromatic organic compound

Alkali Metal - A Group 1A metal

Alkaline Earth Metal - A Group 2A metal

Alkaloid - A natural byproduct, with one or more nitrogen heteroatoms, found in the leaves, bark, or seeds of plants.

Alkane - A saturated hydrocarbon containing only single bonds with a general formula of CnH2n+2

Alkene - An unsaturated hydrocarbon containing one or more carbon - carbon double bonds with the general formula of CnH2n

Alkene Metathesis - A reaction in which the double bond of an alkene (or the triple bond of an alkyne) and then rejoins the fragments.

Alkylation Reaction - A reaction that adds an alkyl group to a reactant

Alkyl Halide - A compound with a halogen in place of one of the hydrogens of an alkane

Alkyne - An unsaturated hydrocarbon containing a carbon - carbon triple bond with the general formula of CnH2n – 2

Allene - A compound with two adjacent double bonds

Allosteric Activator - A compound that activates an enzyme when it binds to a site on the enzyme (other than the active site)

Allosteric Inhibitor - A compound that inactivates an enzyme when it binds to a site on the enzyme (other than the active site)

Alloy - A substance that contains a mixture of elements and has metallic properties

Allylic Carbon - An sp3 carbon adjacent to a vinylic carbon

Allylic Cation - A species with a positive charge on an allylic carbon

Alpha Particle - A helium nucleus

Alpha Particle Production - A common mode of decay for radioactive nuclides in which the mass number changes

Alternating Copolymer - A copolymer made up of two alternating monomers

Ambident Copolymer - A nucleophile that has two nucleophilic sites

Amine - An organic base derivative of ammonia in which one or more of the hydrogen atoms are replaced by organic substituents.

Amine Inversion - The configuration of an sp3 hybridized nitrogen with a nonbonding pair of electrons that rapidly turns inside out.

(alpha) - Amino Acid - An organic acid in which an amino group and an R group are attached to the carbon atom next to the carboxyl group.

Amorphous Solid - A solid with a disordered arrangement of molecules

Ampere - The unit of electric current equal to one coulomb of charge per second

Amphoteric Substance - A substance that can behave either as an acid or a base

Anabolism - Reactions carried out by living organisms in order to synthesize complex molecules from simple precursor molecules

Anchimeric Assistance - Catalysis in which the catalyst is a part of the molecule undergoing
the reaction.

Angle Strain - The strain in a molecule resulting from bond angles being distorted from their ideal values.

Angular Momentum Quantum Number (l) - The quantum number relating to the shape of an atomic orbital, which can assume any integral value from 0 to (n - 1) for each value of n.

Anion - A negative ion

Anion - Exchange Resin - A positively charged resin used in ion - exchange chromatography

Anionic Polymerization - Chain - growth polymerization in which the initiator is a nucleophile with an anionic propagation site

Anode - The electrode in a galvanic cell at which oxidation occurs.

Annulation Reaction - A ring - forming reaction

Anomeric Carbon - The carbon in a cyclic sugar that is the carbonyl carbon in the open chain form.

Anomers - Two cyclic sugars that differ in configuration only at the carbon that is the carbonyl carbon in the open chain form

Anti Addition - An addition reaction in which two substituents are added at opposite sides of the molecule

Antiaromatic - An unstable cyclic and planar compound with an even number of pairs of pi electrons.

Antibonding Molecular Orbital - An orbital higher in energy than the atomic orbitals of which it is composed. A molecular orbital that results when two atomic orbitals with opposite signs interact. Electrons in an antibonding orbital decrease bond strength

Anticodon - The three bases at the bottom of the middle loop in tRNA

Anti Conformer - The most stable of the staggered conformers

Anti Elimination - An elimination reaction in which the two substituents that are eliminated are removed from opposite sides of the molecule.

Antigene Agent - A polymer designed to bind to DNA at a particular site

Antigens - Compounds that can generate a response from the immune system

Anti - Periplanar - Parallel substituents on opposite sides of the molecule

Antisense Agent - A polymer designed to bind to mRNA at a particular site

Antisense Strand - The strand in DNA that is read during transcription

Antisymmetric Molecular Orbital - A molecular orbital in which one - half is not the mirror of the other half.

Apoenzyme - An enzyme without its cofactor

Aprotic Solvent - A solvent that does not have hydrogen bonded to an oxygen or a nitrogen.

Aqueous Solution - A solution in which water is the solvent

Arene Oxide - An aromatic compound that has had one of its double bonds converted to an epoxide.

Aromatic - A highly stable compound that is cyclic and planar and has an odd number of pairs pi electrons.

Arrhenius Acid - An acid is defined as a compound that produces hydrogen ions in aqueous solution

Arrhenius Base - A base is defined as a compound that produces hydroxide ions in aqueous solution

Asymmetric Center - An atom bonded to four different substituents

Atomic Mass - The sum of neutrons and protons in the nucleus of an atom

Atomic Number - The number of protons in the nucleus of an atom

Atomic Weight - The weighted average mass of the atoms in a naturally occurring element

Atomic Radius - Half the distance between the nuclei in a molecule consisting of identical atoms

Atomic Solid - A solid that contains atoms at the lattice point

AufBau Principle - The principle that states that electrons will always fill the lowest energy orbital first.

Autoionization - The transfer of a proton from one molecule to another of the same substance

Avogadro's Law - Equal volumes of gases at the same temperature and pressure contain the same number of particles

Avogadro's Number - 6.022x10(23) which is exactly equal to number of atoms in exactly 12 grams of pure 12C

Axial Bond - A bond of the chair conformer of cyclohexane that is perpendicular to the plane in which the chair is drawn.

Aziridine - A three - membered - - ring compound in which one atom in the ring is nitrogen.