GLOSSARY Chloronium ion (Section 6.16): A halonium ion in which th Codon (Section 27 28): Set of three successive nucleotides halogen is chlorine(see halonium ion). mRNA that is unique for a particular amino acid. The 64 Cholesterol (Section 26. 11): The most abundant steroid in codons possible from combinations of A, T, G, and C code mals and the biological precursor to other naturally occur for the 20 amino acids from which proteins are constr ring steroids. including the bile acids. sex hormones and Coenzyme (Section 27. 21): Molecule that acts in combi with an enzyme to bring about a reaction. Chromatography(Section 13. 21): A method for separation and CoenzymeQ (Section 24 14): Naturally occurring group of re- analysis of mixtures based on the different rates at which lated quinones involved in the chemistry of cellular respira- different compounds are removed from a stationary phas tion. also known as ubiquinone Combinatorial chemistry (Section 27.18): A method for carry Chromophore (Section 13.20): The structural unit of a mole ing out a large number of reactions on a small scale in the cule principally responsible for absorption of radiation of a solid phase so as to generate a"library"of related com- particular frequency; a term usually applied to ultraviolet- pounds for further study, such as biological testing. visible spectroscopy. Combustion(Section 2.15): Burning of a substance in the pres- Chymotrypsin(Section 27. 10): A digestive enzyme that cat- ence of oxygen. All hydrocarbons yield carbon dioxide and alyzes the hydrolysis of proteins. Chymotrypsin selectively when they undergo combustion. catalyzes the cleavage of the peptide bond between the car ommon nomenclature (Section 2. 8): Names given to com- pounds on some basis other than a comprehensive system- tic set of rules cis-(Section 3.12): Stereochemical prefix indicating that two Concerted reaction (Section 4.7): Reaction that occurs in a sin- substituents are on the same side of a ring or double bond gle elementary step (Contrast with the prefix trans-) Condensation polymer(Section 20.16): Polymer in which the Claisen condensation (Section 21.1): Reaction in which a bonds that connect the monomers are formed by condensa- B-keto ester is formed by condensation of two moles of tion reactions. Typical condensation polymers include poly ester in base esters and polyamides. Condensation reaction (Section 15.7): Reaction in which twe molecules combine to give a product accompanied by the RCH, COR RCH-CCHCOR′+R'OH expulsion of some small stable molecule(such as water). An example is acid-catalyzed ether formation Claisen rearrangement (Section 24. 13): Thermal conversion ROR + H20 of an allyl phenyl ether to an o-allyl phenol. The rearrange Condensed structural formula (Section 1.7): A standard way ment proceeds via a cyclohexadienone intermediate of representing structural formulas in which subscripts are used to indicate replicated atoms or groups, as in H2- (CH3)2CHCH2CH Conformational analysis(Section 3. 1 ): Study of the conforma CH CH,CH=CH tions available to a molecule. their relative stabi role they play in defining the properties of the molecule. Claisen-Schmidt condensation (Section 18.10): A mixed al- molecule generated by rotation about single bond ons of Conformations (Section 3. 1): Nonidentical representa dol condensation involving a ketone enolate and an aro- matic aldehyde or ketone. Conformers (Section 3. 1 ): Different conformations of a single Clathrate(Section 2.4): A mixture of two substances in which molecule molecules of the minor component are held by van der onjugate acid(Section 4.6): The species formed from a Bron- Waals forces within a framework of molecules of the major Conjugate addition(Sections 10.10 and 18.12 Addition read oinonen tion in which the reagent adds to the termini of the conju- Clemmensen reduction (Section 12.8): Method for reducing the carbonyl group of aldehydes and ketones to a methylene gated system with migration of the double bond; synony- roup(C=0- CH,) by treatment with zinc amalgam mous with 1, 4 addition. The most common examples [Zn(Hg)] in concentrated hydrochloric acid include conjugate addition to 1, 3-dienes and to a, B-unsatu- Closed-shell electron configuration (Sections 1.1 and 11 rated carbonyl compounds. Stable electron configuration in which all the lowest onjugate base (Section 4.6): The species formed from a Bronsted acid after it has donated orbitals of an atom (in the case of the noble gases), an ion Conjugated diene(Section 10.5): System of the type C NMR (Section 13.14): Nuclear magnetic resonance spec C=C-C=C, in which two pairs of doubly bonded car troscopy in which the environments of individual carbon bons are joined by a single bond. The T electrons are de- localized over the unit of four consecutive sp-hybridized atoms are examined via their mass 13 isotope. Forward Main Menu TOC Study Guide Toc Student OLCMHHE Website
GLOSSARY G-6 Chloronium ion (Section 6.16): A halonium ion in which the halogen is chlorine (see halonium ion). Cholesterol (Section 26.11): The most abundant steroid in animals and the biological precursor to other naturally occurring steroids, including the bile acids, sex hormones, and corticosteroids. Chromatography (Section 13.21): A method for separation and analysis of mixtures based on the different rates at which different compounds are removed from a stationary phase by a moving phase. Chromophore (Section 13.20): The structural unit of a molecule principally responsible for absorption of radiation of a particular frequency; a term usually applied to ultravioletvisible spectroscopy. Chymotrypsin (Section 27.10): A digestive enzyme that catalyzes the hydrolysis of proteins. Chymotrypsin selectively catalyzes the cleavage of the peptide bond between the carboxyl group of phenylalanine, tyrosine, or tryptophan and some other amino acid. cis- (Section 3.12): Stereochemical prefix indicating that two substituents are on the same side of a ring or double bond. (Contrast with the prefix trans-.) Claisen condensation (Section 21.1): Reaction in which a -keto ester is formed by condensation of two moles of an ester in base: Claisen rearrangement (Section 24.13): Thermal conversion of an allyl phenyl ether to an o-allyl phenol. The rearrangement proceeds via a cyclohexadienone intermediate. Claisen—Schmidt condensation (Section 18.10): A mixed aldol condensation involving a ketone enolate and an aromatic aldehyde or ketone. Clathrate (Section 2.4): A mixture of two substances in which molecules of the minor component are held by van der Waals forces within a framework of molecules of the major component. Clemmensen reduction (Section 12.8): Method for reducing the carbonyl group of aldehydes and ketones to a methylene group (CœO ±£ CH2) by treatment with zinc amalgam [Zn(Hg)] in concentrated hydrochloric acid. Closed-shell electron configuration (Sections 1.1 and 11.6): Stable electron configuration in which all the lowest energy orbitals of an atom (in the case of the noble gases), an ion (e.g., Na), or a molecule (e.g., benzene) are filled. 13C NMR (Section 13.14): Nuclear magnetic resonance spectroscopy in which the environments of individual carbon atoms are examined via their mass 13 isotope. heat O CH2 CH CH2 CH2CH OH CH2 RCH2COR O X RCH2CCHCOR O X O X W R ROH 1. NaOR 2. H Codon (Section 27.28): Set of three successive nucleotides in mRNA that is unique for a particular amino acid. The 64 codons possible from combinations of A, T, G, and C code for the 20 amino acids from which proteins are constructed. Coenzyme (Section 27.21): Molecule that acts in combination with an enzyme to bring about a reaction. Coenzyme Q (Section 24.14): Naturally occurring group of related quinones involved in the chemistry of cellular respiration. Also known as ubiquinone. Combinatorial chemistry (Section 27.18): A method for carrying out a large number of reactions on a small scale in the solid phase so as to generate a “library” of related compounds for further study, such as biological testing. Combustion (Section 2.15): Burning of a substance in the presence of oxygen. All hydrocarbons yield carbon dioxide and water when they undergo combustion. Common nomenclature (Section 2.8): Names given to compounds on some basis other than a comprehensive, systematic set of rules. Concerted reaction (Section 4.7): Reaction that occurs in a single elementary step. Condensation polymer (Section 20.16): Polymer in which the bonds that connect the monomers are formed by condensation reactions. Typical condensation polymers include polyesters and polyamides. Condensation reaction (Section 15.7): Reaction in which two molecules combine to give a product accompanied by the expulsion of some small stable molecule (such as water). An example is acid-catalyzed ether formation: Condensed structural formula (Section 1.7): A standard way of representing structural formulas in which subscripts are used to indicate replicated atoms or groups, as in (CH3)2CHCH2CH3. Conformational analysis (Section 3.1): Study of the conformations available to a molecule, their relative stability, and the role they play in defining the properties of the molecule. Conformations (Section 3.1): Nonidentical representations of a molecule generated by rotation about single bonds. Conformers (Section 3.1): Different conformations of a single molecule. Conjugate acid (Section 4.6): The species formed from a Brønsted base after it has accepted a proton. Conjugate addition (Sections 10.10 and 18.12): Addition reaction in which the reagent adds to the termini of the conjugated system with migration of the double bond; synonymous with 1,4 addition. The most common examples include conjugate addition to 1,3-dienes and to ,-unsaturated carbonyl compounds. Conjugate base (Section 4.6): The species formed from a Brønsted acid after it has donated a proton. Conjugated diene (Section 10.5): System of the type CœC±CœC, in which two pairs of doubly bonded carbons are joined by a single bond. The electrons are delocalized over the unit of four consecutive sp2 -hybridized carbons. 2ROH ROR H2O H2SO4
G-7 GLOSSARY Connectivity (Section 1.7): Order in which a molecules atoms are connected. Synonymous with constitution Constitution (Section 1.7): Order of atomic connections that defines a molecule Constitutional isomers(Section 1.8): Isomers that differ in re- the n which the atoms are connected Butane (CH3 CH2CH2CH3)and isobutane [(CH3)3CH] are constitu tional isomers Cyclohexadienyl cation ( Section 12. 2): The key intermediate Copolymer (Section 10. 11): Polymer formed from two or more in electrophilic aromatic substitution reactions. It is repr ferent monome sented by the general structure Coupling constant J (Section 13.7) of the extent to hich two nuclear spins are couple simplest cases, it is equal to the distance betweer t peaks in a split NMR signal. Covalent bond (Section 1.3): Chemical bond between tw atoms that results from their sharing of two electrons petroleo m refining in which high-molecular-weight hydrocarbons are converted where e is derived from the electrophile that attacks the to lower molecular-weight ones by thermal or catalytic ca bon-earbon bond cleavage Critical micelle concentration ( Section 19.5): Concentration bove which substances such as salts of fatty acids aggre- Deactivating substituent (Sections 12.11 and 12.13): A gate to form micelles in aqueous solution. that when present in place of a hydrogen substituent Crown ether(Section 16. 4): A cyclic polyether that, via a particular reaction to occur more slowly. The term most ion-dipole attractive forces, forms stable complexes with often applied to the effect of substituents on the rate metal ions. Such complexes, along with their accompany- Debye unit(D)(Section 1.5): Unit customarily used for mea- ing anion, are soluble in nonpolar solvents C terminus (Section 27.7): The amino acid at the end of a pep- suring dipole moments tide or protein chain that has its carboxyl group intact that 1D=1×10-18 esu.cm is, in which the carboxyl group is not part of a peptide bond. Cumulated diene (Section 10.5): Diene of the type C=C=C, carboxylation(Section 19.17): Reaction of the type in which a single carbon atom participates in double bonds RCO,H - RH CO,, in which carbon dioxide is lost two others from a carboxylic acid. Decarboxylation normally occurs yanohydrin (Section 17.7): Compound of the type readily only when the carboxylic acid is a 1, 3-dicarboxylic Decoupling (Section 13. 17): In NMR spectroscopy, any RCR' process that destroys the coupling of nuclear spins between two nuclei. Two types of decoupling are employed inC NMR spectroscopy. Broadband decoupling removes all the Cyanohydrins are formed by nucleophilic addition of HCN H-C couplings; off-resonance decoupling removes all of H-C couplings except those between directly bonded atoms cycloaddition (Section 10. 12): Addition, such as the Dehydration(Section 5.9): Removal of H and OH from adja Diels-Alder reaction, in which a ring is formed ia a cyclic transition state rm is most commonly employed in ycloalkane(Section 2. 12): An alkane in which a ring of car preparation of alkenes by heating alcohols in the presence of an acid catal bon atoms is present. Cycloalkene(Section 5. 1 ) A cyclic hydrocarbon characterized Dehydrogenation(Section 5.1): Removal of the elements of H2 from adjacent atoms. The term is most commonly en by a double bond between two of the ring carbons. Cycloalkyne (Section 9.4): A cyclic hydrocarbon characterized countered in the industrial preparation of ethylene from by a triple bond between two of the ring carbons. ethane, propene from propane, 1, 3-butadiene from butane, and styrene from ethy benzene. Cyclohexadienyl anion(Section 23. 6): The key intermediate in nucleophilic aromatic substitution by the addition-elimina- Dehydrohalogenation(Section 5. 14): Reaction in which an alkyl halide, on being treated with a base such as sodium tion mechanism. It is represented by the general structure shown, where Y is the nucleophile and X is the leaving ethoxide, is converted to an alkene by loss of a proton fro one carbon and the halogen from the adjacent carbon. Delocalization (Section 1.9): Association of an electron with more than one atom. The simplest example is the shared Forward Main Menu TOC Study Guide Toc Student OLCMHHE Website
G-7 GLOSSARY Connectivity (Section 1.7): Order in which a molecule’s atoms are connected. Synonymous with constitution. Constitution (Section 1.7): Order of atomic connections that defines a molecule. Constitutional isomers (Section 1.8): Isomers that differ in respect to the order in which the atoms are connected. Butane (CH3CH2CH2CH3) and isobutane [(CH3)3CH] are constitutional isomers. Copolymer (Section 10.11): Polymer formed from two or more different monomers. Coupling constant J (Section 13.7): A measure of the extent to which two nuclear spins are coupled. In the simplest cases, it is equal to the distance between adjacent peaks in a split NMR signal. Covalent bond (Section 1.3): Chemical bond between two atoms that results from their sharing of two electrons. Cracking (Section 2.13): A key step in petroleum refining in which high-molecular-weight hydrocarbons are converted to lower molecular-weight ones by thermal or catalytic carbon—carbon bond cleavage. Critical micelle concentration (Section 19.5): Concentration above which substances such as salts of fatty acids aggregate to form micelles in aqueous solution. Crown ether (Section 16.4): A cyclic polyether that, via ion—dipole attractive forces, forms stable complexes with metal ions. Such complexes, along with their accompanying anion, are soluble in nonpolar solvents. C terminus (Section 27.7): The amino acid at the end of a peptide or protein chain that has its carboxyl group intact that is, in which the carboxyl group is not part of a peptide bond. Cumulated diene (Section 10.5): Diene of the type CœCœC, in which a single carbon atom participates in double bonds with two others. Cyanohydrin (Section 17.7): Compound of the type Cyanohydrins are formed by nucleophilic addition of HCN to the carbonyl group of an aldehyde or a ketone. Cycloaddition (Section 10.12): Addition, such as the Diels—Alder reaction, in which a ring is formed via a cyclic transition state. Cycloalkane (Section 2.12): An alkane in which a ring of carbon atoms is present. Cycloalkene (Section 5.1): A cyclic hydrocarbon characterized by a double bond between two of the ring carbons. Cycloalkyne (Section 9.4): A cyclic hydrocarbon characterized by a triple bond between two of the ring carbons. Cyclohexadienyl anion (Section 23.6): The key intermediate in nucleophilic aromatic substitution by the addition—elimination mechanism. It is represented by the general structure shown, where Y is the nucleophile and X is the leaving group. RCR W W OH CPN Cyclohexadienyl cation (Section 12.2): The key intermediate in electrophilic aromatic substitution reactions. It is represented by the general structure where E is derived from the electrophile that attacks the ring. Deactivating substituent (Sections 12.11 and 12.13): A group that when present in place of a hydrogen substituent causes a particular reaction to occur more slowly. The term is most often applied to the effect of substituents on the rate of electrophilic aromatic substitution. Debye unit (D) (Section 1.5): Unit customarily used for measuring dipole moments: 1 D 1 1018 esucm Decarboxylation (Section 19.17): Reaction of the type RCO2H ±£ RH CO2, in which carbon dioxide is lost from a carboxylic acid. Decarboxylation normally occurs readily only when the carboxylic acid is a 1,3-dicarboxylic acid or a -keto acid. Decoupling (Section 13.17): In NMR spectroscopy, any process that destroys the coupling of nuclear spins between two nuclei. Two types of decoupling are employed in 13C NMR spectroscopy. Broadband decoupling removes all the 1 H—13C couplings; off-resonance decoupling removes all of 1 H—13C couplings except those between directly bonded atoms. Dehydration (Section 5.9): Removal of H and OH from adjacent atoms. The term is most commonly employed in the preparation of alkenes by heating alcohols in the presence of an acid catalyst. Dehydrogenation (Section 5.1): Removal of the elements of H2 from adjacent atoms. The term is most commonly encountered in the industrial preparation of ethylene from ethane, propene from propane, 1,3-butadiene from butane, and styrene from ethylbenzene. Dehydrohalogenation (Section 5.14): Reaction in which an alkyl halide, on being treated with a base such as sodium ethoxide, is converted to an alkene by loss of a proton from one carbon and the halogen from the adjacent carbon. Delocalization (Section 1.9): Association of an electron with more than one atom. The simplest example is the shared E H H H H H H Y X H H H H H
GLOSSARY G-8 electron pair(covalent)bond Delocalization is important in Dipole-tipole attraction(Section 2.14): A force of attraction onjugated T electron systems, where an electron may be between oppositely polarized atoms. ssociated with several carbon atoms Dipole/induced-dipole attraction(Section 4.5): A force of at- Deoxy sugar (Section 25.10): A carbohydrate in which one of traction that results when a species with a permanent dipole EPT (Section 13. 18): Abbreviation for distortionless enhance- induces a complementary dipole in a second species Dipole moment(Section 1.5): Product of the attractive force ment of polarization transfer. DEPT is an NMR technique between two opposite charges and the distance between that reveals the number of hydrogens directly attached to a them. Dipole moment has the symbol u and is measured in carbon responsible for a particular signal Debye units( D). Detergents(Section 195): Substances that clean by micellar Disaccharide (Sections 25.1 and 25. 14): A carbohydrate that action. Although the term usually refers to a synthetic de- yields two monosaccharide units(which may be the same tergent, soaps are also detergents or different) on hydrolysis Diastereomers( Section 7. 10): Stereoisomers that are not enan- Dispersion force (Section 2. 14): See induced-dipole/induced tiomers stereoisomers that are not mirror images of one dipole attrac another substituted alkene ( Section 5.6): Alkene of the type Diastereotopic(Section 13.6): Describing two atoms or groups H, or rCh R may be the same a molecule that are attached to the same atom but are in or different, they may be any length, and they may be stereochemically different environments that are not mirror branched or unbranched. The significant point is that there mages of each other. The two protons shown in bold in are two carbons directly bonded to the carbons of the dou CH2=CHCl, for example, are diastereotopic. One is cis to ble bone Disulfide bridge(Section 27.7): An S-S bond between the 1, 3-Diaxial repulsion(Section 3. 8): Repulsive forces between sulfur atoms of two cysteine residues in a peptide or protein axial substituents on the same side of a cyclohexane ring. DNA (deoxyribonucleic acid)(Section 27. 26): A polynu- Diazonium ion (Sections 22.16-22.17): lon of the type cleotide of 2 -deoxyribose present in the nuclei of cells that R-NEN: Aryl diazonium ions are formed by treatment serves to store and replicate genetic information. Genes are of primary aromatic amines with nitrous acid. They are ex DNA tremely useful in the preparation of aryl halides, phenols, Double bond (Section 1. 4): Bond formed by the sharing of Diazotization(Section 22.17): The reaction by which a pri Double dehydrohalogenation(Section 9.7): Reaction in mary arylamine is converted to the corresponding diazo- which a geminal dihalide or vicinal dihalide, on being nium ion by nitrosation. treated with a very strong base such as sodium amide, is Dieckmann reaction(Section 21.2 ): An intramolecular version converted to an alkyne by loss of two protons and the twe of the claisen condensation halogen substituents. Dielectric constant(Section 8. 12): A measure of the ability of a Double helix (Section 27. 27) The form in which DNA nor- material to disperse the force of attraction between oppositely mally occurs in living systems. Two complementary strands charged particles. The symbol for dielectric constant ise of DNA are associated with each other by hydrogen bonds Diels-Alder reaction (Section 10.12): Conjugate addition of an between their base pairs, and each DNA strand adopts a alkene to a conjugated diene to give a cyclohexene deriva helical shape tive. Diels-Alder reactions are extremely useful in synthesis Section 13.4): The low-field region of an NMR Dienophile (Section 10.12): The alkene that adds to the diene n. a signal that is downfield with respect to another a Diels-Alder re left on the spectrum. B-Diketone (Section 18.5): Compound of the tyr Eclipsed conformation ( Section 3.1): Conformation in which bonds on adjacent atoms are aligned with one another. For example, the C-H bonds indicated in the structure sho Dimer (Section 6.21): Molecule formed by the combination of two identical molecules Dipeptide (Section 27.7): A compound in which two a-an acids are linked by an amide bond between the amino group of one and the carboxyl group of the other Edman degradation(Section 27. 13): Method for determining the N-terminal amino acid of a peptide or protein. It in- RCHC— NHCHCO volves treating the material with phenyl isothiocyanate NH R (CHSN=C=S), cleaving with acid, and then identifying the phenylthiohydantoin(PTH derivative) produced Forward Main Menu TOC Study Guide Toc Student OLCMHHE Website
GLOSSARY G-8 electron pair (covalent) bond. Delocalization is important in conjugated electron systems, where an electron may be associated with several carbon atoms. Deoxy sugar (Section 25.10): A carbohydrate in which one of the hydroxyl groups has been replaced by a hydrogen. DEPT (Section 13.18): Abbreviation for distortionless enhancement of polarization transfer. DEPT is an NMR technique that reveals the number of hydrogens directly attached to a carbon responsible for a particular signal. Detergents (Section 19.5): Substances that clean by micellar action. Although the term usually refers to a synthetic detergent, soaps are also detergents. Diastereomers (Section 7.10): Stereoisomers that are not enantiomers stereoisomers that are not mirror images of one another. Diastereotopic (Section 13.6): Describing two atoms or groups in a molecule that are attached to the same atom but are in stereochemically different environments that are not mirror images of each other. The two protons shown in bold in CH2œCHCl, for example, are diastereotopic. One is cis to chlorine, the other is trans. 1,3-Diaxial repulsion (Section 3.8): Repulsive forces between axial substituents on the same side of a cyclohexane ring. Diazonium ion (Sections 22.16—22.17): Ion of the type . Aryl diazonium ions are formed by treatment of primary aromatic amines with nitrous acid. They are extremely useful in the preparation of aryl halides, phenols, and aryl cyanides. Diazotization (Section 22.17): The reaction by which a primary arylamine is converted to the corresponding diazonium ion by nitrosation. Dieckmann reaction (Section 21.2): An intramolecular version of the Claisen condensation. Dielectric constant (Section 8.12): A measure of the ability of a material to disperse the force of attraction between oppositely charged particles. The symbol for dielectric constant is . Diels—Alder reaction (Section 10.12): Conjugate addition of an alkene to a conjugated diene to give a cyclohexene derivative. Diels—Alder reactions are extremely useful in synthesis. Dienophile (Section 10.12): The alkene that adds to the diene in a Diels—Alder reaction. -Diketone (Section 18.5): Compound of the type also referred to as a 1,3-diketone. Dimer (Section 6.21): Molecule formed by the combination of two identical molecules. Dipeptide (Section 27.7): A compound in which two -amino acids are linked by an amide bond between the amino group of one and the carboxyl group of the other: RCHC±NHCHCO2 W NH3 W R O X R O O R R±NPN Dipole—dipole attraction (Section 2.14): A force of attraction between oppositely polarized atoms. Dipole/induced-dipole attraction (Section 4.5): A force of attraction that results when a species with a permanent dipole induces a complementary dipole in a second species. Dipole moment (Section 1.5): Product of the attractive force between two opposite charges and the distance between them. Dipole moment has the symbol and is measured in Debye units (D). Disaccharide (Sections 25.1 and 25.14): A carbohydrate that yields two monosaccharide units (which may be the same or different) on hydrolysis. Dispersion force (Section 2.14): See induced-dipole/induceddipole attraction. Disubstituted alkene (Section 5.6): Alkene of the type R2CœCH2 or RCHœCHR. The groups R may be the same or different, they may be any length, and they may be branched or unbranched. The significant point is that there are two carbons directly bonded to the carbons of the double bond. Disulfide bridge (Section 27.7): An S±S bond between the sulfur atoms of two cysteine residues in a peptide or protein. DNA (deoxyribonucleic acid) (Section 27.26): A polynucleotide of 2-deoxyribose present in the nuclei of cells that serves to store and replicate genetic information. Genes are DNA. Double bond (Section 1.4): Bond formed by the sharing of four electrons between two atoms. Double dehydrohalogenation (Section 9.7): Reaction in which a geminal dihalide or vicinal dihalide, on being treated with a very strong base such as sodium amide, is converted to an alkyne by loss of two protons and the two halogen substituents. Double helix (Section 27.27) The form in which DNA normally occurs in living systems. Two complementary strands of DNA are associated with each other by hydrogen bonds between their base pairs, and each DNA strand adopts a helical shape. Downfield (Section 13.4): The low-field region of an NMR spectrum. A signal that is downfield with respect to another lies to its left on the spectrum. Eclipsed conformation (Section 3.1): Conformation in which bonds on adjacent atoms are aligned with one another. For example, the C±H bonds indicated in the structure shown are eclipsed. Edman degradation (Section 27.13): Method for determining the N-terminal amino acid of a peptide or protein. It involves treating the material with phenyl isothiocyanate (C6H5NœCœS), cleaving with acid, and then identifying the phenylthiohydantoin (PTH derivative) produced. H H