The following are definition and  terms associated with organic chemistry. It is therefore important for one to get farmiliarized with them. They include;


1.      Absolute configuration: The actual three-dimensional structure of a chiral molecule. Absolute configurations are specified verbally by the Cahn—Ingold— Prelog R,S convention and are represented on paper by Fischer projections.



2.       Absorption spectrum: A plot of wavelength of incident light versus amount of light absorbed. Organic molecules show absorption spectra in both the infrared and ultraviolet regions of the electromagnetic spectrum. By interpreting these spectra, useful structural information about the sample can be obtained.



3.       Acetal: A functional group consisting of two ether-type oxygen atoms bound to the same carbon, R2C(OR’)2. Acetals are often used as protecting groups for ketones and aldehydes, since they are stable to basic and nucleophilic reagents but can be easily removed by acidic hydrolysis.

4.       Achiral: Having a lack of handedness. A molecule is achiral if it has a plane of symmetry and is thus superimposable on its mirror image.

5.        Activating group: An electron-donating group such as hydroxyl (— OH) or amino (— NH2) that increases the reactivity of an aromatic ring toward electrophilic aromatic substitution. All activating groups are ortho- and para directing.



6.       Activation energy: The difference in energy levels between ground state and transition state. The amount of activation energy required by a reaction determines the rate at which the reaction proceeds. The majority of organic reactions have activation energies of 10—25 kcal/mol.

7.       Acylation: The introduction of an acyl group, — COR, onto a molecule. For example, acylation of an alcohol yields an ester (R’OH —, R’OCOR), acylation of an amine yields an amide (R’NH2 —> R’NHCOR), and acylation of an aromatic ring yields an alkyl aryl ketone (ArH —ArCOR).


8.       Acylium ion: A resonance-stabilized carbocation in which the positive charge is located at a carbonyl-group carbon, R—C+= 0 <-* R— C ó. Acylium ions are strongly electrophilic and are involved as intermediates in Friedel—Crafts acylation reactions.

9.       1,4 Addition: Addition of a reagent to the ends of a conjugated pi system. Conjugated dienes yield 1,4 adducts when treated with electrophiles such as HCl. Conjugated enones yield 1,4 adducts when treated with nucleophiles such as cyanide ion.


10.   Aldaric acid: The dicarboxylic acid resulting from oxidation of an aldose.

11.   Alditol: The polyalcohol resulting from reduction of the carbonyl group of a sugar.


12.   Aldonic acid : The monocarboxylic acid resulting from mild oxidation of an aldose.

13.   Alicyclic: Referring to an aliphatic cyclic hydrocarbon such as acycloalkane or cycloalkene.


14.   Aliphatic: Referring to a nonaromatic hydrocarbon such as a simple alkane, alkene, or alkyne.

15.   Alkaloid: A naturally occurring compound that contains a basic amine functional group. Morphine is an example of an alkaloid.


16.   Alkylation: Introduction of an alkyl group onto a molecule. For example, certain aromatic rings can be alkylated to yield arenes (ArH — ArR), alkoxide anions can be alkylated to yield ethers (R’O — R’OR), and enolate anions can be alkylated to yield alpha-substituted carbonyl compounds [RC=C(R’)O — RRC—COR’].

17.   Allylic: Used to refer to the position next to a double bond. For example, CH2 = CHCH2Br is an allylic bromide, and an allylic radical is a conjugated, resonance-stabilized species in which the unpaired electron is in a p orbital next to a double bond (C=C—C -* C—C=C).


18.   Angle strain: The strain introduced into a molecule when a bond angle is deformed from its ideal value. Angle strain is particularly important in small-ring cycloalkanes, where it results from compression of bond angles to less than their ideal tetrahedral values. For example, cyclopropane has approximately 22 kcal/mol angle strain owing to bond deformations from the 109° tetrahedral angle to 60°.

19.   Anomers: Cyclic stereoisomers of sugars that differ only in their configurations at the hemiacetal (anomeric) carbon.


20.   Antarafacial: A word used to describe the geometry of pericyclic reactions. An antarafacial reaction is one that takes place on opposite faces of the two ends of a pi electron system. (See Suprafacial.)

21.   Anti conformation: The geometric arrangement around a carbon— carbon single bond, in which the two largest substituents are 180° apart as viewed in a Newman projection.


22.   Anti stereochemistry: Referring to opposite sides of a double bond or molecule. An anti addition reaction is one in which the two ends of the double bond are attacked from different sides. For example, addition of Br2 to cyclohexene yields trans-1,2-dibromocyclohexane, the product of anti addition. An anti elimination reaction is one in which the two groups leave from opposite sides of the molecule. (See Syn stereochemistry.)

23.   Antibonding orbital: A molecular orbital that is higher in energy than the atomic orbitals from which it is formed.


24.   Anticodon: A sequence of three bases on tRNA that reads the codons on mRNA and brings the correct amino acids into position for protein synthesis.

25.   Aromaticity: The special characteristics of cyclic conjugated pi electron systems that result from their electronic structures. These characteristics include unusual stability, the presence of a ring current in the ‘H NMR spectrum, and a tendency to undergo substitution reactions rather than addition reactions on treatment with electrophiles. Aromatic molecules must be planar, cyclic, conjugated species that have (4n + 2) pi electrons.


26.   Asymmetric center: See Chiral center.

27.   Atactic polymers: Chain-growth polymers that have a random stereochemical arrangement of substituents on the polymer backbone. These polymers result from high-temperature radical-initiated polymerization of alkene monomers.


28.   Aufbau principle: A guide for determining the ground-state electronic configuration of elements by filling the lowest-energy orbitals first.

29.   Axial bond: A bond to chair cyclohexane that lies along the ring axis perpendicular to the rough plane of the ring. (See Equatorial bond.) Axial bonds
30.   Base peak: The most intense peak in a mass spectrum.

31.   Bent bonds: The bonds in small rings such as cyclopropane that bend away from the internuclear line and overlap at a slight angle, rather than head-on. Bent bonds are highly strained and highly reactive.


32.   Benzylic: Referring to the position next to an aromatic ring. For example, a benzylic cation is a resonance-stabilized, conjugated carbocation having its positive charge located on a carbon atom next to the benzene ring in a pi orbital that overlaps the aromatic pi system.


33.   Benzyne: An unstable intermediate having a triple bond in a benzene ring. Benzynes are implicated as intermediates in certain nucleophilic aromatic substitution reactions of aryl halides with strong bases.


34.   Betaine: A neutral dipolar molecule that has nonadjacent positive and negative charges. For example, the initial adducts of Wittig reagents with carbonyl compounds are betaines:


35.   Bimolecular reaction: A reaction that occurs between two reagents.




36.   Block copolymer: A polymer consisting of alternating homopolymer blocks. Block copolymers are usually prepared by initiating chain-growth polymerization of one monomer, followed by addition of an excess of a second monomer.


37.   Boat cyclohexane: A three-dimensional conformation of cyclohexane that bears a slight resemblance to a boat. Boat cyclohexane has no angle strain, but has a large number of eclipsing interactions that make it less stable than chair cyclohexane:


38.   Bond angle: The angle formed between two adjacent bonds.


39.    Bond-dissociation energy: The amount of energy needed to homolytically break a bond to produce two radical fragments.


40.   Bond length: The equilibrium distance between the nuclei of two atoms that are bonded to each other.


41.   Bond strength: See Bond-dissociation energy.


42.   Bonding orbital: A molecular orbital that is lower in energy than the atomic orbitals from which it is formed.


43.   Bromohydrin: The 1,2-disubstituted bromoalcohol that is obtained by addition of HOBr to an alkene.


44.   Brønsted acid: A substance that donates a hydrogen ion (proton) to a base.


45.   Carbanion: A carbon anion, or a substance that contains a trivalent, negatively charged carbon atom. Carbanions are sp3 hybridized and have eight electrons in the outer shell of the negatively charged carbon.


46.   Carbene: A neutral substance that contains a divalent carbon atom having only six electrons in its outer shell (R2C:).


47.   Carbinolamine: A molecule that contains the R2C(OH)NH2 functional group. Carbinolamines are produced as unstable intermediates during the nucleophilic addition of amines to carbonyl groups.


48.   Carbocation: A carbon cation, or a substance that contains a trivalent, positively charged carbon atom having six electrons in its outer shell (R3C+ Carbocations are planar and sp2 hybridized).

49.   Carbocycle: A cyclic molecule that has only carbon atoms in the ring. (See Heterocycle.)


50.   Carbohydrate: A polyhydroxy aldehyde or polyhydroxy ketone. The name derives from the fact that glucose, the most abundant carbohydrate, has the formula C6H1206 and was originally thought to be a “hydrate of carbon.” Carbohydrates can be either simple sugars such as glucose or complex sugars such as cellulose. Simple sugars are those that cannot be hydrolyzed to yield smaller molecules, whereas complex sugars are those that can be hydrolyzed to yield simpler sugars.


51.   Chain-growth polymer: A polymer produced by a chain-reaction procedure in which an initiator adds to a carbon—carbon double bond to yield a reactive intermediate. The chain is then built as more monomers add successively to the reactive end of the growing chain.


52.   Chain reaction: A reaction that, once initiated, sustains itself in an endlessly repeating cycle of propagation steps. The radical chlorination of alkanes is an example of a chain reaction that is initiated by irradiation with light and then continues in a series of propagation steps: eg chlorination reaction.


53.   Chair cyclohexane: A three-dimensional conformation of cyclohexane that resembles the rough shape of a chair. The chair form of cyclohexane, which has neither angle strain nor eclipsing strain, represents the lowest-energy conformation of the molecule:


54.   Chemical shift: The position on the NMR chart where a nucleus absorbs. By convention, the chemical shift of tetramethylsilane (TMS) is arbitrarily set at zero, and all other absorptions usually occur downfield (to the left on the chart). Chemical shifts are expressed in delta units, 6, where 1 6 equals 1 ppm of the spectrometer operating frequency. For example, 1 6 on a 60 MHz instrument equals 60 Hz. The chemical shift of a given nucleus is related to the chemical environment of that nucleus in the molecule, thus allowing one to obtain structural information by interpreting the NMR spectrum.


55.   Chiral: Having handedness. Chiral molecules are those that do not have a plane of symmetry and are therefore not superimposable on their mirror image. A chiral molecule thus exists in two forms, one right-handed and one lefthanded. The most common (though not the only) cause of chirality in a molecule is the presence of a carbon atom that is bonded to four different substituents. (See Achiral.)


56.   Chiral center: An atom (usually carbon) that is bonded to four different groups and is therefore chiral. (See Chiral.)




57.   Chlorohydrin: The 1,2-disubstituted chloroalcohol that is obtained by addition of HOC1 to an alkene.


58.   Chromatography: A technique for separating a mixture of compounds into pure components. Chromatography operates on a principle of differential adsorption whereby different compounds adsorb to a stationary support phase and are then carried along at different rates by a mobile phase.


59.   Cis—trans isomers: Special kinds of stereoisomers that differ in their stereochemistry about a double bond or on a ring. Cis—trans isomers are also called geometric isomers.


60.   Codon: A three-base sequence on the mRNA chain that encodes the genetic information necessary to cause specific amino acids to be incorporated into proteins. Codons on mRNA are read by complementary anticodons on tRNA.


61.   Concerted: Referring to a reaction that takes place in a single step without intermediates. For example, the Diels—Alder cycloaddition reaction is a concerted process.


62.   Configuration: The three-dimensional arrangement of atoms bonded to a chiral center relative to the stereochemistry of other chiral centers in the same molecule.


63.   Conformation: The exact three-dimensional shape of a molecule at any given instant, assuming that rotation around single bonds is frozen.

64.   Conformational analysis: A means of assessing the minimum- energy conformation of a substituted cycloalkane by totaling the steric interactions present in the molecule. Conformational analysis is particularly useful in assessing the relative stabilities of different conformations of substituted cyclohexane rings.


65.   Conjugate addition: Addition of a nucleophile to the p-carbon atom of an a,-unsaturated carbonyl compound. (See 1,4-Addition.)


66.   Conjugate base: The anion that results from dissociation of a Brønsted acid.


67.   Conjugation: A series of alternating single and multiple bonds with overlapping p orbitals. For example, 1,3-butadiene is a conjugated diene, 3-buten- 2-one is a conjugated enone, and benzene is a cyclic conjugated triene.


68.   Conrotatory: A term used to indicate the fact that p orbitals must rotate in the same direction during electrocyclic ring opening or ring closure. (See Disrotatory.)

69.   Constitutional isomers: Isomers that have their atoms connected in a different order. For example, butane and 2-methyipropane are constitutional isomers.


70.   Copolyiner: A polymer formed by chain-growth polymerization of a mixture of two or more different monomer units.


71.   Coupling constant: The magnitude (expressed in hertz) of the spin—spin splitting interaction between nuclei whose spins are coupled. Coupling constants are denoted J.


72.   Covalent bond: A bond formed by sharing electrons between two nuclei. (See Ionic bond.)


73.   Cracking: A process used in petroleum refining in which large alkanes are thermally cracked into smaller fragments.


74.   Cycloaddition: A pericyclic reaction in which two reactants add together in a single step to yield a cyclic product. The Diels—Alder reaction between a diene and a dienophile to give a cyclohexene is the best-known example of a cycloaddition.


75.   Deactivating group: An electron-withdrawing substituent that decreases the reactivity of an aromatic ring toward electrophilic aromatic substitution. Most deactivating groups, such as nitro, cyano, and carbonyl are meta-directors, but halogen substituents are ortho- and para-directors.


76.   Decarboxylation: A reaction that involves loss of carbon dioxide from the starting material. /3-Keto acids decarboxylate particularly readily on heating.


77.   Degenerate orbitals: Two or more orbitals that have the same energy level.


78.   Dehydration: A reaction that involves loss of water from the starting material. Most alcohols can be dehydrated to yield alkenes, but aldol condensation products (/3-hydroxy ketones) dehydrate particularly readily.


79.   Dehydrohalogenation: A reaction that involves loss of fiX from the starting material. Alkyl halides undergo dehydrohalogenation to yield alkenes on treatment with strong base.


80.   Delocalization: A spreading out of electron density over a conjugated pi electron system. For example, allylic cations and allylic anions are delocalized because their charges are spread out by resonance stabilization over the entire pi electron system.




81.   Denaturation: The physical changes that occur in proteins when secondary and tertiary structures are disrupted. Denaturation is usually brought about by heat treatment or by a change in pH and is accompanied by a loss of biological activity.


82.   Deshielding: An effect observed in NMR that causes a nucleus to absorb downfield (to the left) of TMS standard. Deshielding is caused by a withdrawal of electron density from the nucleus and is responsible for the observed chemical shifts of vinylic and aromatic protons.


83.   Deuteriuin isotope effect: A tool for use in mechanistic investigations to establish whether or not a C — H bond is broken in the rate-limiting step of a reaction. Since carbon—deuterium bonds are stronger and less easily broken than carbon—protium bonds, one can measure the reaction rates of both protiumand deuterium-substituted substrates and see if they are the same. If they are different, a deuterium isotope effect is present, indicating that C—H bond breakage is rate-limiting.


84.   Dextrorotatory: A word used to describe an optically active substance that rotates the plane of polarization of plane-polarized light in a right-handed (clockwise) direction. The direction of rotation is not related to the absolute configuration of the molecule. (See Levorotatory.)


85.   Diastereomer: A term that indicates the relationship between non- mirror-image stereoisomers. Diastereomers are stereoisomers that have the same configuration at one or more chiral centers, but differ at other chiral centers.


86.   Diazotization: The conversion of a primary amine, RNH2, into a diazonium salt, RN, by treatment with nitrous acid. Aryl diazonium salts are stable, but alkyl diazonium salts are extremely reactive and are rarely isolable.


87.   Dielectric constant: A measure of the ability of a solvent to act as an insulator of electric charge. Solvents that have high dielectric constants are highly polar and are particularly valuable in 5N1 reactions because of their ability to stabilize the developing positive charge of the intermediate carbocation.




88.   Dienophile: A compound containing a double bond that can take part in the Diels—Alder cycloaddition reaction. The most reactive dienophiles are those that have electron-withdrawing groups such as nitro, cyano, or carbonyl on the double bond.


89.   Bipolar aprotic solvent: Dipolar solvent that cannot function as a hydrogen ion donor. Dipolar aprotic solvents such as dimethyl sulfoxide (DMSO), hexamethylphosphoramide (HMPA), and dimethylformamide (DMF) are particularly useful in 5N2 reactions because of their ability to solvate cations.


90.   Dipole moment, z: A measure of the polarity of a molecule. A dipole moment arises when the centers of gravity of positive and negative charges within a molecule do not coincide.


91.   Disrotatory: A term used to indicate the fact that p orbitals rotate in opposite directions during electrocyclic ring opening or ring closing. (See Conrotatory.)

92.   d,l form: A shorthand way of indicating the racemic modification of a compound. (See Racemic mixture.)




93.   DNA: Deoxyribonucleic acid, the biopolymer consisting of deoxyribonucleotide units linked together through phosphate—sugar bonds. DNA, which is found in the nucleus of cells, contains an organism’s genetic information.


94.   Doublet: A two-line NMR absorption caused by spin—spin splitting when the spin of the nucleus under observation couples with the spin of a neighboring magnetic nucleus.


95.   Downfield: Refers to the left-hand portion of the NMR chart. (See Deshielding.)


96.   Eclipsed conformation: The geometric arrangement around a carbon—carbon single bond in which the bonds to substituents on one carbon are parallel to the bonds to substituents on the neighboring carbon as viewed in a Newman projection. For example, the eclipsed conformation of ethane has the C — H bonds on one carbon lined up with the C H bonds on the neighboring carbon:

97.   Eclipsing strain: The strain energy in a molecule caused by electron repulsions between eclipsed bonds. Eclipsing strain is also called torsional strain.

98.   Elastomers: Amorphous polymers that have the ability to stretch out and then return to their previous shape. These polymers have irregular shapes that prevent crystallite formation and have little cross-linking between chains.


 99.   Electrocyclic reaction: A unimolecular pericyclic reaction in which a ring is formed or broken by a concerted reorganization of electrons through a cyclic transition state. For example, the cyclization of 1,3,5-hexatriene to yield 1,3- cyclohexadiene is an electrocyclic reaction.

100.Electromagnetic spectrum: The range of electromagnetic energy, including infrared, ultraviolet, and visible radiation.

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