Common Terminologies in Organic Chemistry

Acid

an agent able to produce positively charged hydrogen ions (H+). [Since the hydrogen ion is a bare proton, it usually exists in a solvated form (such as H3O+).] (proton donor)

Achiral

not chiral. A compound (or object) that is super imposible on its mirror image.

Activation energy

the minimum energy which reacting species must possess in order to be able to form an 'activated complex', or 'transition state', before proceeding to the products.

Addition reactions

reactions in which an unsaturated system is saturated or part saturated by the addition of a molecule across the multiple bond.[E.g. the addition of bromine to ethene to form 1,2-dibromoethane]

Allyl group

a group containing 3 carbon atoms and a double bond [C1=C2-C3, where C3 is called the allyliuc position or allylic carbon atom].

Allylic rearrangement

the migration of a double bond in a 3-carbon system from carbon atoms one and two to carbon atoms two and three, e.g. C1=C2-C3-X X-C1-C2=C3

Anomers

the specific term used to describe carbohydrate stereoisomers differing only in configuration at the hemi-acetal carbon atom.

Aromatic

an aromatic molecule or ion possesses aromaticity. For benzene, the most common aromatic system (n = 1, therefore 6 pi electrons), the aromaticity confers the characteristic reactivity of electrophilic substitution.

Asymmetry

a term applied an object or molecule that does not possess symmetry.

Atomic orbital

the energy levels of electrons in an atom which may be described in terms of the four quantum numbers.

Avogadro's constant

the number of particles (atoms or molecules) in one mole of any pure substance. [6.022 x 1023]

Base

a base is a substance that can combine with a proton. (proton acceptor)

Bond energy

the energy required to break a particular bond by a homolytic process.

Bonding Molecular Orbital

when atomic orbitals with the same phase sign interact

Buffer solution

a solution of definite pH made up in such a way that the pH alters only gradually with the addition of an acid or a base. Solution that can resist sudden changes in pH.

Catalyst

a substance that when added to a reaction mixture, speeds up the rate of attainment of equilibrium in the system without itself undergoing a permanent chemical change.

Catenation

the formation of chains or rings by atoms of the same element bonded to each other.

Chirality

a term which may be applied to any asymmetric object or molecule. The property of non-identity of an object with its mirror image.

Compound

a term used generally to indicate a definite combination of elements into a more complex structure (a molecule) but it is also applied to systems with non-stoichiometric proportions of elements.

Configuration

the order and relative spatial arrangement of the atoms in a molecule. Absolute configuration is when the relative 3 dimensional arrangement in space of atoms in a chiral molecule have been correlated with an absolute standard.

Configurational isomers

a series of compounds which have the same constitution and bonding of atoms but which differ in their atomic spatial arrangement. [E.g. glucose and mannose are configurational isomers. Also called stereoisomers.

Conformation

the spatial arrangement of a molecule in space at any particular moment in time.

Conjugation

a sequence of alternating double (or triple) and single bonds. [E.g. C=C-C=C and C=C-C=O. Conjugation can also be relayed by the participation of lone pairs of electrons or vacant orbitals.]

Co-ordinate bond

The linkage of two atoms by a pair of electrons both electrons being provided by one of the atoms (the donor).

Covalency number

refers to the number of unpaired electrons in the orbital of an atom; determines the number of covalent bonds

Covalent bond

the linkage of two atoms by the m of two electrons.

Dextrorotatory

the phenomenon in which plane polarised light is turned in a clockwise direction.

Diastereomers (or diastereoisomers)

stereoisomeric structures which are not enantiomers (mirror images) of one another. [Often applied to systems which differ only in the configuration at one carbon atom, e.g. meso- and d- or l-tartaric acids are diastereoisomeric.]

Dissociation

The process whereby a molecule is split into simpler fragments which may be smaller molecules, atoms, free radicals or ions.

Dissociation constant

the measure of the extent of dissociation, measured by the dissociation constant K. For the process: AB = A + B K = ([A][B])/[AB]

Electronegativity

the tendency for atoms in a molecule to attract electrons.

Electronic configuration

the particular order in which electrons are arranged in an atom or molecule.

Electrophile

an atom, molecule or ion able to accept an electron pair. (electron poor specie/s) [E.g. H3O+, BF3, AlCl3, Br2, Cl2, I2]

Electrophilic substitution

an overall reaction in which an electrophile binds to a substrate with the expulsion of another electrophile.

Electrovalent (ionic) bond

bonding by electrostatic attraction. Formed by of electron/s from one atom to another

Element

a substance which cannot be further subdivided by chemical methods.

Enantiomers

a pair of isomers which are related as mirror images of one another. [E.g. isomers differing only in the configuration about the chiral atoms.]

Endothermic

a reaction in which heat is absorbed.

Enthalpy

a thermodynamic state function, generally measured in kilojoules per mole. In chemical reactions the enthalpy change (deltaH) is related to changes in the free energy (deltaG) and entropy (deltaS) by the equation: deltaG = deltaH - T.deltaS

Entropy

a thermodynamic quantity which is a measure of the degree of disorder within any system. [The greater the degree of order the higher the entropy; for an increase in entropy S is positive. Entropy has the units of joules per degree K per mole.]

Epimerization

a process in which the configuration about one chiral center of a compound, containing more than one chiral atom, is inverted to give the opposite configuration. [The term epimers is often used to describe two related compounds which differ only in the configuration about one chiral atom.]

Equilibrium constant

according to the law of mass action, for any reversible chemical reaction: aA + bB = cC + dD, the equilibrium constant (K)is defined as: K = ([C]c[D]d)/([A]a[B]b)

Excited state

the state of an atom, molecule or group when it has absorbed energy and become excited to a higher energy state as compared to the normal ground state. The excited state may be electronic, vibrational, rotational, etc.

Fischer projection

a convention for drawing carbon chains so that the relative 3-dimensional stereochemistry of the carbon atoms is relatively easy portrayed on a 2-dimensional drawing.

Free energy (deltaG)

a thermodynamic state function; the free energy change (G) in any reaction is related to the enthalpy and entropy: deltaG = deltaH - T.deltaS

Functional group

the atom or group of atoms that defines the structure and at the same time determines the properties of a class or family of organic compounds.

Gauche

a conformational isomer in which the groups are neither eclipsed nor trans to one another.

Ground state

the lowest energy state of an atom, molecule or ion.

Heat of reaction

the amount of heat absorbed or evolved when specified amounts of compounds react under constant pressure. [For exothermic reactions the convention is that the enthalpy change (heat of reaction) is negative.]

Hybridization

the process whereby atomic orbitals of different type but similar energies are combined to form a set of equivalent hybid orbitals which are more stable in directional manner.

Hydrolysis

the addition of the elements of water to a substance, often with the partition of the substance into two parts, such as in the hydrolysis of an ester to an acid and an alcohol.

Ion

an atom or group of atoms that has lost or gained one or more electrons to become a charged species.

Isomers

compounds having the same atomic composition (constitution) but differing in their chemical structure. [They include: structural isomers (chain or positional), tautomeric isomers, and stereoisomers - including geometrical isomers, optical isomers and conformational isomers.]

Levorotatory

the phenomenon that turns plane polarized light in an counterclockwise direction.

Lewis acid

an agent capable of accepoting a pair of electrons to form a coordinate bond.

Lewis base

an agent capable of donating a pair of electrons to form a coordinate bond.

Lone pair

a pair of electrons in a molecule which is not shared by two of the constituent atoms.

Markownikow's rule

in the ionic addition of hydrogen halides to a carbon-carbon double bond the halogen attaches itself to the carbon atom bearing the least number of hydrogen atoms.

Molecular orbitals

the electron orbitals belonging to a group of atoms forming a molecule.

Molecule

the smallest particle of matter that can exist in a free state. In the case of ionic substances, such as sodium chloride, the molecule is considered as a pair of ions, NaCl.

Nucleophile

a substance which donates a pair of electrons in the reaction considered. (electron rich specie/s) [E.g. I-, OH-, SR-, CN-, H2O, R-OH, NH3+]

Nucleophilic substitution

an overall reaction in which a nucleophile reacts with a compound displacing another nucleophile.

Optical activity

the property of certain substances to rotate plane polarized light. [Compounds that possess a chiral carbon atom of all the same 'handedness' will rotate plane polarized light. Isomers that rotate light in equal but opposite directions are sometimes called 'optical isomers', although the better term to use is 'enantiomers'.]

Oxidation

a chemical process in which the proportion of electronegative substituents in a compound is increased, or the charge is made more positive, or the oxidation number is increased.

Racemic mixture, racemate

an equimolar mixture of the two enantiomeric isomers of a compound. [As a consequence of the equal numbers of laevo- and dextro-rotatory molecules present in a racemate, there is no net rotation of plane polarized light.]

Reduction

chemical processes in which the proportion of more electronegative substituents is decreased, or the charge is made more negative, or the oxidation number is lowered.

Reversible process

a process in which the forward reaction can reach an equilibrium with the reverse reaction.

Stability constant

when a complex is formed between a metal ion and an a ligand in solution, the equilibrium may be expressed by a constant which is related to the free energy change for the process: For: M + A = MA : - deltaG = RTlnK

Stereochemistry

the study of the spatial arrangements of atoms in molecules and complexes.

Stereoisomer

another name for configurational isomer.

Steric hindrance

the phenomenon of physical blockage of a particular site within a molecule by the presence of local atoms or groups of atoms. [As a consequence of steric hindrance, a reaction at a particular site will be impeded.]

Structural isomerism

isomers which differ in the order of bonding of the constituent atoms.

Substitution reactions

reactions in which one atom or group of atoms is replaced by another atom or group of atoms. [See electrophilic substitutions and nucleophilic substitutions.]

Tautomerism

a form of structural isomerism where the two structures are interconvertible by means of the migration of a proton.

Transition state

the point of highest energy on energy against reaction coordinate curve. [By definition the transition state is the least stable point (peak) on a reaction path; a reaction path may involve more than one transition states.

Unsaturated

the term given to an organic compound containing multiple bonds.

Vinyl group

the ethenyl group, -CH=CH2.

VSEPR

valence shell electron pair repulsion theory.