This is a list of definitions of some terms in organic chemistry:





1.      Electron affinity: The measure of the tendency of an atom to gain an electron and form an anion. Elements on the right side of the periodic table, such as the halogens, have higher electron affinities than elements on the left side.


2.         Electronegativity: The ability of an atom to attract electrons and thereby polarize a bond. As a general rule, electronegativity increases in going across the periodic table from left to right and in going from bottom to top. It is important when explaining inductive effect.



3.   Electrophile: An “electron-lover:’ or substance that accepts an electron pair from a nucleophile in a polar bond-forming reaction. We can therefore say that electrophiles are electron loving species.



4.         Electrophoresis: A technique used for separating charged organic molecules, particularly proteins and amino acids. The mixture to be separated is placed on a buffered gel or paper, and an electric potential is applied across the ends of the apparatus. Negatively charged molecules migrate toward the positive electrode, and positively charged molecules migrate toward the negative electrode.



5.         Elution: The removal of a substance from a chromatography column.


6.   Empirical formula: A formula that gives the relative proportions of elements in a compound in smallest whole numbers.

7.     Enantiomers: Stereoisomers of a chiral substance that have a mirror image relationship. Enantiomers must have opposite configurations at all chiral centers in the molecule. That is they must be chiral.

8.     Endothermic: A term used to describe reactions that absorb energy and therefore have positive enthalpy changes. In reaction energy diagrams, the products of endothermic reactions have higher energy levels than the starting materials. Click here to read more on endothermic reaction.


9.     Entgegen, E: A term used to describe the stereochemistry of a carbon— carbon double bond. The two groups on each carbon are first assigned priorities according to the Cahn—Ingold--Prelog sequence rules, and the two carbons are then compared. If the high-priority groups on each carbon are on opposite sides of the double bond, the bond has E geometry. (See Zusammen.)

10.  Enthalpy change, H°: The heat of reaction. The enthalpy change that occurs during a reaction is a measure of the difference in total bond energy between reactants and products.


11.  Entropy change, S°: The amount of disorder. The entropy change that occurs during a reaction is a measure of the difference in disorder between reactants and products.

12. Enzyme): A biological catalyst. Enzymes are large proteins that catalyze specific biochemical reactions.


13.     Epoxide: A three-membered-ring ether functional group.

14.     Equatorial bond: A bond to cyclohexane that lies along the rough equator of the ring.


15.     Equilibrium constant: A measure of the equilibrium position for a reaction.


16.     Essential oil: The volatile oil that is obtained by steam distillation of a plant extract.


17.   Excited-state configuration: An electronic configuration having a higher energy level than the ground state. Excited states are normally obtained by excitation of an electron from a bonding orbital to an antibonding one, such as occurs during irradiation of a molecule with light of the proper frequency.


18.  Exothermic: A term used to describe reactions that release energy and therefore have negative enthalpy changes. On reaction energy diagrams, the products of exothermic reactions have energy levels lower than those of starting materials. Click here to read more on exothermic reation.




19.     Fat: A solid triacyiglycerol derived from animal sources.


20.     Fibers: Thin threads produced by extruding a molten polymer through small holes in a die.


21.     Fibrous protein: Proteins that consist of polypeptide chains arranged side by side in long threads. These proteins are tough, insoluble in water, and occur in nature in structural materials such as hair, hooves, and fingernails.


22.     Fingerprint region: The complex region of the infrared spectrum from 1500 cm’ to 400 cm’. If two substances have identical absorption patterns in the fingerprint region of the IR, they are almost certainly identical.


23.     Fischer projection: A means of depicting the absolute configuration of chiral molecules on a fiat page. A Fischer projection employs a cross to represent the chiral center; the horizontal arms of the cross represent bonds coming out of the plane of the page, whereas the vertical arms of the cross represent bonds going back into the plane of the page.


24.  Formal charge: The difference in the number of electrons possessed by an atom in a molecule and by the same atom in its elemental state. The formal charge on an atom is given by the formula


25.     Frequency: The number of electromagnetic wave cycles that travel past a fixed point in a given unit of time. Frequencies are usually expressed in units of cycles per second, or hertz.


26.     Functional group: An atom or group of atoms that is part of a larger molecule and has a characteristic chemical reactivity. Functional groups display the same chemistry in all molecules of which they are a part. Or we can say that, functional groups arew atoms, bonds or group of atoms that determines the chemical and biological properties of a compound.


27.     Gated-decoupled mode: A mode of 13C NMR spectrometer operation in which all one-carbon resonances are of equal intensity. Operating in this mode allows one to integrate the spectrum to find out how many of each kind of carbon atom is present.


28.     Gauche conformation: The conformation of butane in which the two methyl groups lie 60° apart as viewed in a Newman projection. This conformation has 0.9 kcal/mol steric strain:


29.    Gibbs free-energy change, G°: The total amount of free-energy change, both enthalpy and entropy, that occurs during a reaction. The standard Gibbs free-energy change for a reaction is given by the formula G° = H° — TAS°.


30.  Globular protein: Proteins that are coiled into compact, nearly spherical shapes. These proteins, which are generally water-soluble and mobile within the cell, are the structural class to which enzymes belong.


31.     Glycol: A 1,2-diol such as ethylene glycol, HOCH2CH2OH.


32.     Glycoside: A cyclic acetal formed by reaction of a sugar with another alcohol.


33.  Graft copolymer: A copolymer that consists of homopolymer chains grafted onto a different homopolymer backbone. Graft copolymers are prepared by X-ray irradiation of a homopolymer to generate radical sites along the chain, followed by addition of a second monomer.



34.     Ground state: The most stable, lowest-energy electronic configuration of a molecule.


35.     Halohydrin: A 1,2-disubstituted haloalcohol such as is obtained on addition of HOBr to an alkene.


36.  Haloniuin ion: A species containing a positively charged, divalent halogen. Three-membered-ring bromonium ions are implicated as intermediates in the electrophilic addition of bromine to alkenes.




37.  Hammond postulate: A postulate stating that we can get a picture of what a given transition state looks like by looking at the structure of the nearest stable species. Exothermic reactions have transition states that resemble starting material, whereas endothermic reactions have transition states that resemble products.


38.   Haworth projection: A means of viewing stereochemistry in cyclic hemiacetal forms of sugars. Haworth projections are drawn so that the ring is flat and is viewed from an oblique angle with the hemiacetal oxygen at the upper right.


39.  Heat of combustion: The amount of heat released when a compound is burned in a calorimeter.


40.   Heat of hydrogenation: The amount of heat released when a carbon— carbon double bond is hydrogenated. Comparison of heats of hydrogenation for different alkenes allows one to determine the stability of the different double bonds.


41.  Heterocycle: A cyclic molecule whose ring contains more than one kind of atom. For example, pyridine is a heterocycle that contains five carbon atoms and one nitrogen atom in its ring.


42.   Heterogemc bond formation: What occurs when one partner donates both electrons in forming a new bond. Polar reactions always involve heterogenic bond formation:
                                       A + B:- —k A:B

43.   Heterolytic bond breakage: The kind of bond breaking that occurs in polar reactions when one fragment leaves with both of the bonding electrons, as in the equation
                                       A:B — A + B:



44.  HOMO: An acronym for highest occupied molecular orbital. The symmetries of the HOMO and LUMO are important in pericyclic reactions. (See LUMO.)


45.     Homogenic bond formation: What occurs in radical reactions when each partner donates one electron to the new bond:
                                              A• + B- — A:B


46.   Homolytic bond breakage: The kind of bond breaking that occurs in radical reactions when each fragment leaves with one bonding electron according to the equation.
                                               A:B —* A• + B

47.  Homopolymer: A polymer made by chain-growth polymerization of a single monomer unit.


48.   Huckel’s rule: A rule stating that monocyclic conjugated molecules having (4n + 2) pi electrons (n = an integer) show the unusual stability associated with aromaticity.


49.   Hybrid orbital: An orbital that is mathematically derived from a combination of ground-state (s, p, d) atomic orbitals. Hybrid orbitals, such as the sp3, sp2, and sp hybrids of carbon, are strongly directed and form stronger bonds than ground-state atomic orbitals.


50.     Hydration: Addition of water to a molecule, such as occurs when alkenes are treated with strong sulfuric acid.


51.   Hydroboration: Addition of borane (BH3) or an alkylborane to an alkene. The resultant trialkylborane products are useful synthetic intermediates that can be oxidized to yield alcohols.


52.    Hydrogen bond: A weak (5 kcal/mol) attraction between a hydrogen atom bonded to an electronegative element and an electron lone pair on another atom. Hydrogen bonding plays an important role in determining the secondary structure of proteins and in stabilizing the DNA double helix.

53.   Hydrogenation: Addition of hydrogen to a double or triple bond to yield the saturated product.



54.    Hyperconjugation: A weak stabilizing interaction that results from overlap of a p orbital with a neighboring sigma bond. Hyperconjugation is important in stabilizing carbocations and in stabilizing substituted alkenes.


55.     Inductive effect: The electron-attracting or electron-withdrawing effect that is transmitted through sigma bonds as the result of a nearby dipole. Electronegative elements have an electron-withdrawing inductive effect, whereas electropositive elements have an electron-donating inductive effect.


56. Infrared spectroscopy: A kind of optical spectroscopy that uses infrared energy. JR spectroscopy is particularly useful in organic chemistry for determining the kinds of functional groups present in molecules.


57.  Initiator: A substance with an easily broken bond that is used to initiate radical chain reactions. For example, radical chlorination of alkanes is initiated when light energy breaks the weak chlorine—chlorine bond to form chlorine radicals.
57.

58.     Intermediate: A species that is formed during the course of a multistep reaction but is not the final product. Intermediates are more stable than transition states, but may or may not be stable enough to isolate.


59. Intramolecular, intermolecular: Reactions that occur within the same molecule are intramolecular, whereas reactions that occur between two molecules are intermolecular.

60.  Ion pair: A loose complex between two ions in solution. Ion pairs are implicated as intermediates in SN1 reactions in order to account for the partial retention of stereochemistry that is often observed.



61.  Ionic bond: A bond between two ions due to the electrical attraction of unlike charges. Ionic bonds are formed between strongly electronegative elements (such as the halogens) and strongly electropositive elements (such as the alkali metals).


62.  Ionization energy: The amount of energy required to remove an electron from an atom. Elements on the far right of the periodic table have high ionization energies, and elements on the far left of the periodic table have low ionization energies. Click here to get more insight on ionization energy.


63.  Isoelectric point: The pH at which the number of positive charges and the number of negative charges on a protein or amino acid are exactly balanced.


64.     Isomers: Compounds that have the same molecular formula but different structures.


65.  Isoprene rule: An observation to the effect that terpenoids appear to be made up of isoprene (2-methyl-1,3-butadiene) units connected in a head-to-tail fashion. Monoterpenes have two isoprene units, sesquiterpenes have three isoprene units, diterpenes have four isoprene units, and so on.


66. Isotactic polymer: A chain-growth polymer in which all substituents on the polymer backbone have the same three-dimensional orientation.


67.    Kekulé structure: A representation of molecules in which a line between atoms is used to represent a bond. (See Line-bond structure.)


68.    Kinetic control: Reactions that follow the lowest activation energy pathway are said to be kinetically controlled. The product formed in a kinetically controlled reaction is the one that is formed most rapidly, but is not necessarily the most stable.




69.  Kinetics: Referring to rates of reactions. Kinetics measurements can be extremely important in helping to determine reaction mechanisms.


70.  Leaving group: The group that is replaced in a substitution reaction. The best leaving groups in nucleophilic substitution reactions are those that form the most stable, least basic anions.
71.  Levorotatory: Used to describe an optically active substance that rotates the plane of polarization of plane-polarized light in a left-handed (counterclockwise) direction. (See Dextrorotatory.)


72.   Lewis acid: A substance having a vacant low-energy orbital that can accept an electron pair from a base. All electrophiles are Lewis acids, but transition metal salts such as A1C13 and ZnCl2 are particularly good ones. (See Lewis base.)


73.     Lewis base:  A substance that donates an electron lone pauto an acid All nuc1eopJiles are Lewis bas (See Lewis acid)


74.  Lewis structure: A representation of a molecule showing covalent bonds as a pair of electron dots between atoms.


75.  Line-bond structure: A representation of a molecule showing covalent bonds as lines between atoms. (See Kekulé structure.)


76.   Lipid: A naturally occurring substance isolated from cells and tissues by extraction with nonpolar solvents. Lipids belong to many different structural classes, including fats, terpenes, prostaglandins, and steroids.


77.    Lipophilic: Fat-loving. Long, nonpolar hydrocarbon chains tend to cluster together in polar solvents because of their lipophilic properties.

78.   Lone-pair electrons: Nonbonding electron pairs that occupy valence orbitals. It is the lone-pair electrons that are used by nucleophiles in their reactions with electrophiles.


79.  LUMO: An acronym for lowest unoccupied molecular orbital. The symmetries of the LUMO and HOMO are important in determining the stereochemistry of pericyclic reactions. (See HOMO.)


80.     Nitrogen rule: A rule stating that compounds having an odd number of nitrogens give rise to an odd-numbered molecular ion in the mass spectrum. Conversely, compounds with an even number of nitrogens give rise to even-numbered molecular ions.


81.   Node: The surface of zero electron density between lobes of orbitals. For example, a p orbital has a nodal plane passing through the center of the nucleus, perpendicular to the line of the orbital.


82.     Normal alkane: A straight-chain alkane, as opposed to a branched alkane. Normal alkanes are denoted by the suffix n, as in n-C4H10 (n-butane).


83.     Nuclear magnetic resonance, NMR: A spectroscopic technique that provides information about the carbon—hydrogen framework of a molecule. NMR works by detecting the energy absorption accompanying the transition between nuclear spin states that occurs when a molecule is placed in a strong magnetic field and irradiated with radio-frequency waves. Different nuclei within a molecule are in slightly different magnetic environments and therefore show absorptions at slightly different frequencies.


84.     Nucleophile: A “nucleus-lover,” or species that donates an electron pair to an electrophile in a polar bond-forming reaction. Nucleophiles are also Lewis bases. (See Electrophile.)


85. Nucleoside: A nucleic acid constituent, consisting of a sugar residue bonded to a heterocyclic purine or pyrimidine base.




86.  Nucleotide: A nucleic acid constituent, consisting of a sugar residue bonded both to a heterocyclic purine or pyrimidine base and to a phosphoric acid. Nucleotides are the monomer units from which DNA and RNA are constructed.


87.  Off-resonance mode: A mode of ‘3C NMR spectrometer operation that allows for the observation of spin—spin splitting between carbons and their attached hydrogens. Carbons bonded to one hydrogen show a doublet; carbons attached to two hydrogens show a triplet; and carbons attached to three hydrogens show a quartet in the off-resonance NMR.


88.     Olefin: An alternative name for an alkene.


89.     Optical isomers: See Enantiomers.


90.    Optically active: A substance that rotates the plane of polarization of plane-polarized light. Note that an optically active sample must contain chiral molecules, but all samples with chiral molecules are not optically active. Thus, a racemic sample is optically inactive even though the individual molecules are chiral. (See Chiral.)


91.     Orbital: The volume of space in which an electron is most likely to be found. Orbitals are described mathematically by wave functions, which delineate the behavior of electrons around nuclei.


92.  Ozonide: The product formed by addition of ozone to a carbon—carbon double bond. Ozonides are usually treated with a reducing agent such as zinc in acetic acid to produce carbonyl compounds.


93.  Pauli exclusion principle: A statement of the fact that no more than two electrons can occupy the same orbital, and those two must have spins of opposite sign.


94.     Peptides: Amino acid polymers in which the individual amino acid residues are linked by amide bonds. (See Protein.)


95.  Pericyclic reaction: A reaction that occurs by a concerted reorganization of bonding electrons in a cyclic transition state.


96. Periplanar: A conformation in which bonds to neighboring atoms have a parallel arrangement. In an eclipsed conformation, the neighboring bonds are syn periplanar; in a staggered conformation, the bonds are anti periplanar: Anti periplanar Syn periplanar


97.     Peroxide: A molecule containing an oxygen—oxygen bond functional group, R—O—O—R’ or R—O—O—H. The “peroxides” present as explosive impurities in ether solvents are usually of the latter type. Since the oxygen—oxygen bond is weak and easily broken, peroxides are often used to initiate radical chain reactions.


98.     Phase-transfer catalysts: Agents that cause the transfer of ionic reagents between phases, thus catalyzing reactions. Tetraalkylammonium salts, R4N X, are often used to transport inorganic anions from the aqueous phase to the organic phase where the desired reaction then occurs. For example, permanganate ion is solubilized in benzene in the presence of tetraalkylammonium ions.


99.   Phospholipid: Lipids that contain a phosphate residue. For example, phosphoglycerides contain a glycerol backbone linked to two fatty acids and a phosphoric acid.


100. Pi bond: The covalent bond formed by sideways overlap of atomic orbitals. For example, carbon—carbon double bonds contain a pi bond formed by sideways overlap of two p orbitals.

101. Plane of symmetry: An imaginary plane that bisects a molecule such that one half of the molecule is the mirror image of the other half. Molecules containing a plane of symmetry are achiral.


102. Plane-polarized light: Ordinary light that has its electric vectors in a single plane rather than in random planes. The plane of polarization is rotated when the light is passed through a solution of a chiral substance.

103. Polar reaction: A reaction in which bonds are made when a nucleophile donates two electrons to an electrophile and bonds are broken when one fragment leaves with both electrons from the bond. Polar reactions are the most common class of reactions. (See Heterogenic bond formation; Heterolytic bond breakage.)


104. Polarity: The unsymmetrical distribution of electrons in molecules that results when one atom attracts electrons more strongly than another.

105. Polarizability: The measure of the change in the electron distribution in a molecule in response to changing electric interactions with solvents or ionic reagents.


106. Polymer: A large molecule made up of repeating smaller units. For example, polyethylene is a synthetic polymer made from repeating ethylene units, and DNA is a biopolymer made of repeating deoxyribonucleotide units.

107. Paraffins: An alternative name for alkanes.

108. Primary, secondary, tertiary, quaternary: Terms used to describe the substitution pattern at a specific site. A primary site has one organic substituent attached to it, a secondary site has two organic substituents, a tertiary site has three, and a quaternary site has four:


109. Primary structure: The amino acid sequence in a protein. (See Secondary structure; Tertiary structure.)

110. Principle of maximum overlap: The strongest bonds are formed when overlap between orbitals is greatest.
111. Propagation step: The step or series of steps in a radical chain reaction that carry on the chain. The propagation steps must yield both product and a reactive intermediate to carry on the chain.


112. Prostaglandin: A member of the class of lipids with the general carbon skeleton Prostaglandins are present in nearly all body tissues and fluids, where they serve a large number of important hormonal functions.


113. Protecting group: A group that is introduced to protect a sensitive functional group from reaction elsewhere in the molecule. After serving its protective function, the group is then removed. For example, ketones and aldehydes are often protected as acetals by reaction with ethylene glycol, and alcohols are often protected as tetrahydropyranyl ethers.

114. Protein: A large peptide, containing fifty or more amino acid residues. Proteins serve both as structural materials (hair, horns, fingernails) and as enzymes that control an organism’s chemistry. (See Peptides.)





115. Protic solvent: A solvent such as water or alcohol that can serve as a proton donor. Protic solvents are particularly good at stabilizing anions by hydrogen bonding, thereby lowering their reactivity. (See Dipolar aprotic solvent.)


116. Proton noise-decoupled mode: The most common manner of ‘3C NMR spectrometer operation, in which all nonequivalent carbon atoms in the sample show a single, unsplit resonance. Operating the spectrometer in this mode allows one to count the number of chemically different carbon atoms present in the sample molecule.
117. Quartet: A set of four peaks in the NMR, caused by spin—spin splitting of a signal by three adjacent nuclear spins.

118. Quaternary: See Primary.


119. Quaternary structure: The highest level of protein structure, involving a specific aggregation of individual proteins into a larger cluster.


120. R,S convention: A method for defining the absolute configuration around chiral centers. The Cahn—Ingold--Prelog sequence rules are used to assign


121. salts, which could be separated by fractional crystallization. Regeneration of the free acids would then yield the two pure enantiomeric acids.


122. Resonance effect: The effect by which substituents donate or withdraw electron density through orbital overlap with neighboring pi bonds. For example, an oxygen or nitrogen substituent donates electron density to an aromatic ring by overlap of the 0 or N orbital with the aromatic ring p orbitals. A carbonyl substituent, however, withdraws electron density from an aromatic ring by p orbital overlap. These effects are particularly important in determining whether a given group is meta-directing or ortho- and para-directing in electrophilic aromatic- substitution reactions.


123. Resonance hybrid: A molecule, such as benzene, that cannot be represented adequately by a single Kekulé structure but must instead be considered as an average of two or more resonance structures. The resonance structures themselves differ only in the positions of their electrons, not of their nuclei.


124. Ring current: The circulation of pi electrons induced in aromatic rings by an external magnetic field. This effect accounts for the pronounced downfield shift of aromatic ring protons in the ‘H NMR.


125. Ring-flip: The molecular motion that converts one chair conformation of cyclohexane into another chair conformation. The effect of a ring-flip is to convert an axial substituent into an equatorial substituent.


126. RNA: Ribonucleic acid, the biopolymer found in cells that serves to transcribe the genetic information found in DNA and uses that information to direct the synthesis of proteins.


127. Saccharide: A sugar.


128. Saponification: An old term for the base-induced hydrolysis of an ester to yield a carboxylic acid salt.

129. Saturated: A saturated molecule is one that has only single bonds and thus cannot undergo addition reactions. Alkanes, for example, are saturated, but alkenes are unsaturated.
130. Sawhorse structure: A stereochemical manner of representation that portrays a molecule using a stick drawing and gives a perspective view of the conformation around single bonds. Sawhorse structure


131. Second-order reaction: A reaction whose rate-limiting step is bimolecular and whose kinetics are therefore dependent on the concentration of two reagents.


132. Secondary: See Primary.


133. Secondary structure: The level of protein substructure that involves organization of chain sections into ordered arrangements such as β-pleated sheets or α-helices.


134. Sequence rules: A series of rules devised by Cahn, Ingold, and Prelog for assigning relative priorities to substituent groups on a double-bond carbon atom or on a chiral center. Once priorities have been established, E,Z double-bond geometry and R,S configurational assignments can be made. (See Entgegen; R,S convention; Zusammen.)


135. Shielding: An effect observed in NMR that causes a nucleus to absorb toward the right (upfield) side of the chart. Shielding is caused by donation of electron density to the nucleus. (See Deshielding.)

136. Sigma bond: A covalent bond formed by head-on overlap of atomic orbitals.


137. Sigmatropic reaction: A pericyclic reaction that involves the migration of a group from one end of a pi electron system to the other. For example, the [1,5] sigmatropic rearrangement of a hydrogen atom in cyclopentadiene is such a reaction.
137.

138. Skew conformation: Any conformation about a single bond that is intermediate between staggered and eclipsed. (See Staggered conformation; Eclipsed conformation.).



139. Soap: The mixture of long-chain fatty acid salts obtained on base hydrolysis of animal fat.


140. Solid-phase synthesis: A technique of synthesis whereby the starting material is covalently bound to a solid polymer bead and reactions are carried out on the bound substrate. After the desired transformations have been effected, the product is cleaved from the polymer and is isolated. This technique is particularly useful in peptide synthesis (Merrifield method).




141. sp orbital: A hybrid orbital mathematically derived from the combination of an s and a p atomic orbital. The two sp orbitals that result from hybridization are oriented at an angle of 180° to each other.


142. sp2 orbital: A hybrid orbital mathematically derived by combination of an s atomic orbital with two p atomic orbitals. The three sp2 hybrid orbitals that result lie in a plane at angles of 120° to each other.


143. sp3 orbital: A hybrid orbital mathematically derived by combination of an s atomic orbital with three p atomic orbitals. The four sp3 hybrid orbitals that result are directed toward the corners of a tetrahedron at angles of 109° to each other.


144. Specific rotation,: The specific rotation of a chiral compound is a physical constant.


145. Spin—spin splitting: The splitting of an NMR signal into a multiplet caused by an interaction between nearby magnetic nuclei whose spins are coupled. The magnitude of spin—spin splitting is given by the coupling constant, J.


146. Staggered conformation: The three-dimensional arrangement of atoms around a carbon—carbon single bond in which the bonds on one carbon exactly bisect the bond angles on the second carbon as viewed end-on. (See Eclipsed conformation.) Staggered conformation


147. Stationary phase: The solid support used in chromatography. The molecules to be chromatographically separated adsorb to the stationary phase and are moved along by the mobile phase. Silica gel (hydrated SiO2) and alumina (A12O3) are often used as stationary phases in column chromatography of organic mixtures. (See Mobile phase.)


148. Step-growth polymer: A polymer produced by a series of polar reactions between two difunctional monomers. The polymer normally has the two monomer units in alternating order and usually has other atoms in addition to carbon in the polymer backbone. Nylon, a polyamide produced by reaction between a diacid and a diamine, is an example of such a polymer.


149. Stereochemistry: The branch of chemistry concerned with the three-dimensional arrangement of atoms in molecules.


150. Stereoisomers: Isomers that have their atoms connected in the same order but in different three-dimensional arrangements. The term stereoisomer includes both enantiomers and diastereomers, but does not include constitutional isomers.


151. Stereospecific: A term indicating that only a single stereoisomer is reduced in a given reaction, rather than a mixture.




152. Steric strain: The _goups are o close together and try to occupy the sajmpce. Steric strain is Tesponsible both for the greater stability of trans versus cis alkenes, and for the greater stability of equatorially substituted versus axially substituted cyclohexanes.


153. Steroid: A lipid whose structure is based on the tetracyclic carbon skeleton. Steroids occur in both plants and animals and have a variety of important hormonal functions.


154. Suprafacial: A word used to describe the geometry of pericyclic reactions. Suprafacial reactions take place on the same side of the two ends of a pi electron system. (See Antarafacial.)


155. Symmetry-allowed, symmetry-disallowed: A symmetry-allowed reaction is a pericyclic process that has a favorable orbital symmetry for reaction through a concerted pathway. A symmetry-disallowed reaction is one that does not have favorable orbital symmetry for reaction through a concerted pathway.


156. Syn stereochemistry: A syn addition reaction is one in which the two ends of the double bond are attacked from the same side. For example, 0504 induced hydroxylation of cyclohexene yields cis-1,2-cyclohexanediol, the product of syn addition. A syn elimination is one in which the two groups leave from the same side of the molecule. (See Anti stereochemistry.)


157. Syndiotactic polymer: A chain-growth polymer in which the substituents on the polymer backbone have a regular alternating stereochemistry.


158. Tautomers: Isomers that are rapidly interconverted. For example, enols and ketones are tautomers, since they are rapidly interconverted on treatment with either acid or base catalysts.


159. Terpenes: Lipids that are formally derived by head-to-tail polymerization of isoprene units. (See Isoprene rule.)


160. Tertiary: See Primary.


161. Tertiary structure: The level of protein structure that involves the manner in which the entire protein chain is folded into a specific three-dimensional arrangement.

162. Thermodynamic control: Equilibrium reactions that yield the lowest-energy, most stable product are said to be thermodynamically controlled. Although most stable, the product of a thermodynamically controlled reaction is not necessarily formed fastest. (See Kinetic control.)



163. Thermoplastic: A polymer that is hard at room temperature but becomes soft and pliable when heated. Thermoplastics are used for the manufacture of a variety of molded objects.


164. Thermosetting resin: A polymer that is highly cross-linked and sets into a hard, insoluble mass when heated. Bakelite is the best-known example of such a polymer.


165. Torsional strain: The strain in a molecule caused by electron repulsion between eclipsed bonds. Torsional strain is also called eclipsing strain.


166. Transcription: The process by which the genetic information encoded in DNA is read and used to synthesize RNA in the nucleus of the cell. A small portion of double-stranded DNA uncoils, and complementary ribonucleotides line up in the correct sequence for RNA synthesis.


167. Transition state: An imaginary activated complex between reagents, representing the highest-energy point on a reaction curve. Transition states are unstable complexes that cannot be isolated.
168. Translation: The process by which the genetic information transcribed from DNA onto mRNA is read by tRNA and used to direct protein synthesis.

169. Tree diagram: A diagram used in NMR to help sort out the complicated splitting patterns that can arise from multiple couplings.


170. Triacylglycerol: Lipids such as animal fat and vegetable oil consisting chemically of triesters of glycerol with long-chain fatty acids. Triplet: A symmetrical three-line splitting pattern observed in the 1H NMR when a proton has two equivalent neighbor protons or in the ‘3C NMR when a carbon is bonded to two hydrogens.


171. Ultraviolet (UV) spectroscopy: An optical spectroscopy employing ultraviolet irradiation. UV spectroscopy provides structural information about the extent of pi electron conjugation in organic molecules.

172. Unimolecular reaction: A reaction that occurs by spontaneous transformation of the starting material without the intervention of other reagents. For example, the dissociation of a tertiary alkyl halide in the SN reaction is a unimolecular process.





173. Unsaturated: An unsaturated molecule is one that has multiple bonds and can undergo addition reactions. Alkenes and alkynes, for example, are unsaturated. (See Saturated.)

174. Upfield: Used to refer to the right-hand portion of the NMR chart. (See Shielding.)


175. Van der Waals forces: The attractive forces between molecules that are caused by dipole—dipole interactions. Van der Waals forces are one of the primary forces responsible for holding molecules together in the liquid state.

176. Vicinal: A term used to refer to a 1,2-disubstitution pattern. For example, 1,2-dibromoethane is a vicinal dibromide.


177. Vinylic: A term that refers to a substituent at a double-bond carbon atom. For example, chloroethylene is a vinylic chloride, and enols are vinylic alcohols.

178. Vulcanization: A process for hardening rubber by heating in the presence of elemental sulfur. The sulfur functions by forming cross-links between polymer chains.


179. Wave function: The mathematical expression that defines the behavior of an electron. The square of the wave function is the probability function that defines the shapes of orbitals.


180. Wave number: The wave number is the reciprocal of the wavelength in centimeters. Thus, wave numbers are expressed in cm’.


181. Wavelength: The length of a wave from peak to peak. The wavelength of electromagnetic radiation is inversely proportional to frequency and inversely proportional to energy. (See Frequency.)





182. Ylide: A neutral dipolar molecule in which the positive and negative charges are adjacent. For example, the phosphoranes used in Wittig reactions are ylides.


183. Zaitsev’s rule: A rule stating that E2 elimination reactions normally yield the more highly substituted alkene as major product.



184. Zusammen, Z: A term used to describe the stereochemistry of a carbon—carbon double bond. The two groups on each carbon are assigned priorities according to the Cahn—Ingold—Prelog sequence rules, and the two carbons are compared. If the high-priority groups on each carbon are on the same side of the double bond, the bond has Z geometry. (See Entgegen; Sequence rules.)

185. Zwitterion: A neutral dipolar molecule in which the positive and negative charges are not adjacent. For example, amino acids exist as zwitterions, H3N—CHR—COO. (Zwitterions are also called betaines.)



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