- Email: [email protected]
Some remarks on the nomenclature of refrigerants
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ELSEVIER
Fluid Phase Equilibria 132 (1997) 265-270
Letter to the Editor
Some remarks on the nomenclature of refrigerants Ulrich K. Deiters * Institute of Physical Chemistry, University at Cologne, D-50939 Cologne, Germany Received 18 November 1996; accepted 29 November 1996
Abstract The engineering nomenclature of refrigerants, as defined by ASHRAE (so-called R names), and IUPAC nomenclature are compared. It is shown that the engineering nomenclature can deal with constitutional isomerism only in a limited way, and not at all with stereoisomerism. Recommendations for authors of publications on refrigerants are given. © 1997 Elsevier Science B.V. Keywords: Refrigerants; Nomenclature; Stereoisomerism; Isomerism
1. Refrigerant nomenclature Recently, several publications on thermodynamic properties of fluorochlorohydrocarbons were published in which these substances were exclusively referred to by the 'refrigerant nomenclature', and thus the readers were left with the nontrivial task of deducing the chemical nature of the investigated substances from pieces of a somewhat cryptic number code. According to this nomenclature, as defined by ASHRAE (American Society of Heating, Refrigerating, and Air Conditioning Engineers), refrigerants are characterized by a prefix (usually 'R') and a sequence of digits dchf, where the digits d, c, h and f have the following meaning: d: Number of double bonds; omitted if zero. c: Number of carbon atoms minus 1; omitted if zero. • h: Number of hydrogen atoms plus one. • f : Number of fluorine atoms. If bromine atoms are present, their number is given after a 'B'; cyclic compounds are marked with a 'C'. The remaining substituents are chlorine atoms. A few examples are given in Table 1. 'R' seems to be the most widely used prefix, although some manufacturers of refrigerants are using others. A substance like chlorotrifluoromethane may therefore be referred to as R13, R-13, F-13, or Freon- 13.
* Tel.: +49-221-470 4543; fax: +49-221-470 74900. 0378-3812/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0378-3812(96)03232-3
266
U.K. Deiters / Fluid Phase Equilibria 132 (1997) 265-270
Table 1 Refrigerant nomenclature of halogenated hydrocarbons - - Simple compounds R Nomenclature
Chemical formula
IUPAC name
R 14 R 13 R 13B 1 R 161 RC318 R 1114
CF4 CC1F3 CBrF3 C 2H 5F C 4F8 C 2F4
tetrafluoromethane chlorotrifluoromethane bromotrifluoromethane fluoroethane octafluorocyclobutane tetrafluoroethene
Table 2 Refrigerant nomenclature of halogenated hydrocarbons - - Isomers R Nomenclature
Chemical formula
IUPAC name
R114 R 114a R 143 R 143a
CC1F2CCIF2 CC12FCF3 CHF2CH 2F CFaCH 3
1,2-dichlorotetrafluoroethane 1, l-dichlorotetrafluoroethane 1,1,2-trifluoroethane 1,1,1-trifluoroethane
This nomenclature is intended for small molecules with at m o s t five carbon atoms. A c digit a b o v e 4 has a special meaning, e.g., 5 is reserved for azeotropic mixtures, or 7 for inorganic c o m p o u n d s (e.g., R717 is ammonia). Unfortunately, the nomenclature was m a d e before light hydrocarbons were recognized as substitutes for fluorochlorohydrocarbons. For butane, C4H10, the nomenclature rules lead to an ' o v e r f l o w error' o f the hydrogen count (h = 11 !). Therefore the R name o f butane had to b~ defined as R600, but then it is no longer possible to deduce the chemical structure f r o m this name. While the ' R nomenclature' works well for substituted methanes, it runs into difficulties with substituted ethanes because o f the possibility o f i s o m e r i s m : There is often m o r e than one w a y o f arranging the substituent halogen atoms in the molecule. Table 2 shows some cases. The ' R nomenclature' distinguishes between isomers by attaching lowercase characters a, b . . . . to the n u m b e r code 'in the order of increasing a s y m m e t r y ' [1].This works perhaps if only two kinds of substituents are present (such as H and F in the case of R 1 4 3 / R 1 4 3 a ) l, but in the case o f m o r e
Table 3 Refrigerant nomenclature of halogenated hydrocarbons - - The problem of 'asymmetry' R Nomenclature
Chemical formula
IUPAC name
R 142? R 142? R142b
CH 2 C I C H F 2 CHCIFCH2 F CC1F2CH3
1-chloro-2,2-difluoroethane 1-chloro- 1,2-difluoroethane 1-chloro-l,l-difluoroethane
z The verdict of asymmetry is based on the chemical formula only. A three-dimensional view of R134a would reveal that it has a threefold symmetry axis and three mirror planes, and hence is of higher symmetry than R134.
U.K. Deiters / Fluid Phase Equilibria 132 (1997) 265-270
267
complicated molecules this rule is rather unclear. Table 3 illustrates this problem for R142 (chlorodifluoroethane), which has three isomers: The third isomer is clearly the most asymmetric and therefore earns the label 'b'. But which of the other isomers is the more asymmetric? The first one has a higher dipole moment, the second one a more asymmetric mass distribution. To increase the confusion: The third isomer has been referred to in literature as R142b [2] and as R142 [3]. Why methylpropane (R600a) with its threefold symmetry axis is regarded as more asymmetric than butane (R600), is really hard to understand. There seems to be no unambiguous way to deduce the chemical constitution from 'R nomenclature' and vice versa. Hence, it is necessary that authors use a more precise nomenclature system in their publications.
2. IUPAC nomenclature The IUPAC nomenclature of organic compounds, as laid down in the 'Green Book'[4], is a systematic means of expressing total chemical structure by words 2. There is not enough space for this letter to explore the numerous possibilities and ramifications of this nomenclature, but it may be worthwhile to demonstrate its usage for some simple compounds like alkanes and refrigerants. The first nomenclature rules are as follows: (1) Chains of carbon atoms are identified with the following prefix: C 1: meth-
C 2: eth-
C 3: prop-
C 4: b u t -
Names of longer chains are derived from Greek numerals: C 5: pent-
C 6: h e x -
C 7: hept- etc.
(2) Names of normal alkanes are formed from the chain length prefix and the ending -ane: CH 4: C2H6: C 6H 14:
methane ethane hexane
Note that all substituents which have not been specified are assumed to be hydrogen. (3) Cycloalkanes receive the prefix c y c l o - . (4) Presence of a double bond is indicated by the ending -ene: CH 2 = CH 2:
ethene
In longer chains, the position of a double bond between carbon atoms i and i + 1 is specified by giving i in the name: CH 2 = C H - C H 2 - C H 3: CH 3-CH = C H - C H 3:
1-butene or but- 1-ene 2-butene or but-2-ene
2 Thus IUPAC nomenclature can be regarded as a modem counterpart of the Entish language [5].
U.K. Deiters / Fluid Phase Equilibria 132 (1997) 265-270
268
This is especially important when describing molecules with more than one double bond: CH 2 = C H - C H = CH 2: CH 2 = C = C H - C H 3:
1,3-butadiene 1,2-butadiene
The number of double bonds is indicated by a Greek numeral prefix to the -ene ending. (5) In the case of double bonds, it may be necessary to distinguish between cis- and trans-isomers (another feature that the refrigerant nomenclature cannot do): c/s-2-butene
trans-2-butene
CH3 CH3 \ / CH-CH
CH3 \ CH-CH \ CH3
(6) Substituents to a carbon chain are indicated by putting their name before the chain name. If necessary, their position is specified by giving the carbon atom numbers to which they are attached: CC14"
CHBr2CH 3:
tetrachloromethane 1,1-dibromoethane
If there is more than one way to number the carbon atoms, the numbering scheme is preferred which gives the lowest substituent numbers. Tables 1-3 contain further examples. (7) Names of sidechains are formed from a chain length prefix and the ending -yl. (8) For a branched alkane, the name is derived from longest carbon chain; the other parts of the molecule are regarded as substituents: CH 3-CH(CH3)-CH3" CH 3-CH(CH 3)-CH 2-CH 2-CH 3:
methylpropane (isobutane) 2-methylpentane
This nomenclature evidently can describe molecular constitution in fine detail.
3. Stereochemistry Unfortunately, organic chemistry has more pitfalls. The second compound in Table 3 or the refrigerant R124 (CHC1FCF3, 1-chloro-l,2,2,2-tetrafluoroethane) both have a carbon atom with four different substituents ( - H , -C1, - F , - C F 3 in the case of R124). Such a carbon atom is a centre of chirality: There are two different forms of R124 as Fig. 1 illustrates. Form a cannot be transformed into form b by any rotation of the molecule as a whole or of its constituent groups around single bonds: a and b are distinct species. In this case, b is a mirror image of a, and a and b are called enantiomers. Because of their symmetry, a and b have the same thermodynamic properties. But their reactivity with or solubility in other chiral material may be not be the same. Since biochemical processes are usually highly stereoselective, the toxicity or the biodegradability may be rather different for enantiomers.
U.K. Deiters / Fluid Phase Equilibria 132 (1997) 265-270
F~.
H
.~CI
Cl~
H
C
I
C
a
b
269
~F
I
Fig. 1. Stereographic projections of the two enantiomeric forms of 1-chloro-l,2,2,2-tetrafluoroethane (R124).
Unless a sample has been prepared from biochemical material or special stereoselective methods of synthesis have been used, it will be composed of equal amounts of enantiomers. Such racemic mixtures should not be confused with pure components: Usually the enantiomers either do not mix in the solid state, or the racemic mixture has a crystal structure of its own. Either way, the melting point of a racemic mixture is usually not the same as that of a pure enantiomer. Because the packing efficiency in the dense liquid may also be different for pure enantiomers and for the racemic mixture, one must expect nonvanishing excess volumina; hence liquid racemic mixtures can depart from ideality. The above discussion might seem far-fetched, as far as refrigerants are concerned. But one should keep in mind that some purification procedures, such as membrane pervaporation or liquid chromatography, might change the ratio of enantiomers, if the membrane or chromatography column contains chiral material like cellulose, dextrane, or gelatine! The configuration of substituents at a centre of chirality can be characterized by the RS nomenclature of Cahn, Ingold, and Prelog. There is no need to explain this nomenclature in this text; instead, the reader is referred to textbooks in structural organic chemistry. Just for completeness, we remark that compound a has an S configuration at its carbon atom 1, and its IUPAC name is therefore (1 S)- 1-chloro- 1,2,2,2-tetrafluoroethane; b has an R configuration. If there are n elements of chirality in a molecule, there are 2" possible molecular configurations. In the case of 2 centres of chirality, we have the combinations (R,R), (R,S), (S,R), and (S,S). The molecule with the configuration (R, R) is an enantiomer of that with (S,S), and the molecule with the configuration (R,S) is an enantiomer of that with (S,R). But (R,R) and (R,S) have no mirror symmetry relation. Generally, molecules which different configurations, which are not enantiomers, are called diastereomers. They must be expected to have different thermodynamic properties. If a molecule contains a double bond or a ring, further distinguishable ways of arranging substituents become possible, from the rather obvious cis/trans isomerism mentioned above to the almost arcane concepts of helicity or plane chirality.
4. Conclusions It is important that authors properly identify and characterize the substances with which their publication deals. In particular, the following statements should be regarded: (1) Trading names and application-oriented nomenclature systems, such as the 'R nomenclature' of refrigerants, may be used to render a text more legible. But authors should be aware that the
270
U.K. Deiters / Fluid Phase Equilibria 132 (1997) 265-270
'refrigerant nomenclature' cannot express details of chemical constitution or configuration. It should therefore be used with great caution only. (2) It is mandatory that the publication title and the keyword list contain the IUPAC names. (3) If possible, CAS registry numbers, chemical formulas and R names should also be given in the keyword list or in the abstract. (4) If a substance can have configurational isomers, the authors should explain this in the publication, eventually with the help of three-dimensional chemical structure formulas, and state which stereoisomers were used for the experiments. If random mixtures of stereoisomers were used (racemic mixtures in the case of enantiomers), the randomness should be checked by measuring the optical rotation or other stereospecific effects. Today there are several ways of determining CAS registry numbers and IUPAC names from chemical structure: printed abstract services and collections, like Chemical Abstracts, Beilstein etc. • online electronic data banks, such as CA/CAPLUS (created by Chemical Abstracts), BEILSTEIN (Beilstein), DETHERM (Dechema) 3 local PC-based software, such as the 'electronic journal' ELDATA [6] or the Compounds database, a part of the TRC electronic database set [3].
References [1] W. Gerhartz, Y.S. Yamamoto, B. Elvers, J.F. Rounsaville, G. Schulz (Eds.), Ullmann's Encyclopedia of Industrial Chemistry, AI 1, VCH, Weinheim, 1988, ch. 3, p. 354. [2] D. Baumer, E. Riedel, Gase-Handbuch, 3rd ed., Technical Report 90.1001, Messer Griesheim, 1990. [3] K.N. Marsh, R.C. Wilhoit, S.J. Xu, D. Yin, Compounds 4.3 (TRC Electronic Databases for Chemistry and Engineering). TEES, Thermodynamics Research Center, Texas A&M University, 1994. [4] I. Mills, T. CvitaL K. Homann, N. KaUay, K. Kuchitsu, Quantities, Units and Symbols in Physical Chemistry, 2nd ed., Blackwell, Oxford, 1993. [5] J.R.R. Tolkien, The Lord of the Rings, Part 2: The Two Towers, ch. IV of book III, Allen & Unwin, London, 1966. [6] H.V. Kehiaian, ELDATA: Int. Electronic J. Phys.-Chem. Data. ITODYS, Universit6 Paris VII, since 1995.
3 These three maintained by STN International.
! '~: 1~ ,".
jill
ELSEVIER
Fluid Phase Equilibria 132 (1997) 265-270
Letter to the Editor
Some remarks on the nomenclature of refrigerants Ulrich K. Deiters * Institute of Physical Chemistry, University at Cologne, D-50939 Cologne, Germany Received 18 November 1996; accepted 29 November 1996
Abstract The engineering nomenclature of refrigerants, as defined by ASHRAE (so-called R names), and IUPAC nomenclature are compared. It is shown that the engineering nomenclature can deal with constitutional isomerism only in a limited way, and not at all with stereoisomerism. Recommendations for authors of publications on refrigerants are given. © 1997 Elsevier Science B.V. Keywords: Refrigerants; Nomenclature; Stereoisomerism; Isomerism
1. Refrigerant nomenclature Recently, several publications on thermodynamic properties of fluorochlorohydrocarbons were published in which these substances were exclusively referred to by the 'refrigerant nomenclature', and thus the readers were left with the nontrivial task of deducing the chemical nature of the investigated substances from pieces of a somewhat cryptic number code. According to this nomenclature, as defined by ASHRAE (American Society of Heating, Refrigerating, and Air Conditioning Engineers), refrigerants are characterized by a prefix (usually 'R') and a sequence of digits dchf, where the digits d, c, h and f have the following meaning: d: Number of double bonds; omitted if zero. c: Number of carbon atoms minus 1; omitted if zero. • h: Number of hydrogen atoms plus one. • f : Number of fluorine atoms. If bromine atoms are present, their number is given after a 'B'; cyclic compounds are marked with a 'C'. The remaining substituents are chlorine atoms. A few examples are given in Table 1. 'R' seems to be the most widely used prefix, although some manufacturers of refrigerants are using others. A substance like chlorotrifluoromethane may therefore be referred to as R13, R-13, F-13, or Freon- 13.
* Tel.: +49-221-470 4543; fax: +49-221-470 74900. 0378-3812/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0378-3812(96)03232-3
266
U.K. Deiters / Fluid Phase Equilibria 132 (1997) 265-270
Table 1 Refrigerant nomenclature of halogenated hydrocarbons - - Simple compounds R Nomenclature
Chemical formula
IUPAC name
R 14 R 13 R 13B 1 R 161 RC318 R 1114
CF4 CC1F3 CBrF3 C 2H 5F C 4F8 C 2F4
tetrafluoromethane chlorotrifluoromethane bromotrifluoromethane fluoroethane octafluorocyclobutane tetrafluoroethene
Table 2 Refrigerant nomenclature of halogenated hydrocarbons - - Isomers R Nomenclature
Chemical formula
IUPAC name
R114 R 114a R 143 R 143a
CC1F2CCIF2 CC12FCF3 CHF2CH 2F CFaCH 3
1,2-dichlorotetrafluoroethane 1, l-dichlorotetrafluoroethane 1,1,2-trifluoroethane 1,1,1-trifluoroethane
This nomenclature is intended for small molecules with at m o s t five carbon atoms. A c digit a b o v e 4 has a special meaning, e.g., 5 is reserved for azeotropic mixtures, or 7 for inorganic c o m p o u n d s (e.g., R717 is ammonia). Unfortunately, the nomenclature was m a d e before light hydrocarbons were recognized as substitutes for fluorochlorohydrocarbons. For butane, C4H10, the nomenclature rules lead to an ' o v e r f l o w error' o f the hydrogen count (h = 11 !). Therefore the R name o f butane had to b~ defined as R600, but then it is no longer possible to deduce the chemical structure f r o m this name. While the ' R nomenclature' works well for substituted methanes, it runs into difficulties with substituted ethanes because o f the possibility o f i s o m e r i s m : There is often m o r e than one w a y o f arranging the substituent halogen atoms in the molecule. Table 2 shows some cases. The ' R nomenclature' distinguishes between isomers by attaching lowercase characters a, b . . . . to the n u m b e r code 'in the order of increasing a s y m m e t r y ' [1].This works perhaps if only two kinds of substituents are present (such as H and F in the case of R 1 4 3 / R 1 4 3 a ) l, but in the case o f m o r e
Table 3 Refrigerant nomenclature of halogenated hydrocarbons - - The problem of 'asymmetry' R Nomenclature
Chemical formula
IUPAC name
R 142? R 142? R142b
CH 2 C I C H F 2 CHCIFCH2 F CC1F2CH3
1-chloro-2,2-difluoroethane 1-chloro- 1,2-difluoroethane 1-chloro-l,l-difluoroethane
z The verdict of asymmetry is based on the chemical formula only. A three-dimensional view of R134a would reveal that it has a threefold symmetry axis and three mirror planes, and hence is of higher symmetry than R134.
U.K. Deiters / Fluid Phase Equilibria 132 (1997) 265-270
267
complicated molecules this rule is rather unclear. Table 3 illustrates this problem for R142 (chlorodifluoroethane), which has three isomers: The third isomer is clearly the most asymmetric and therefore earns the label 'b'. But which of the other isomers is the more asymmetric? The first one has a higher dipole moment, the second one a more asymmetric mass distribution. To increase the confusion: The third isomer has been referred to in literature as R142b [2] and as R142 [3]. Why methylpropane (R600a) with its threefold symmetry axis is regarded as more asymmetric than butane (R600), is really hard to understand. There seems to be no unambiguous way to deduce the chemical constitution from 'R nomenclature' and vice versa. Hence, it is necessary that authors use a more precise nomenclature system in their publications.
2. IUPAC nomenclature The IUPAC nomenclature of organic compounds, as laid down in the 'Green Book'[4], is a systematic means of expressing total chemical structure by words 2. There is not enough space for this letter to explore the numerous possibilities and ramifications of this nomenclature, but it may be worthwhile to demonstrate its usage for some simple compounds like alkanes and refrigerants. The first nomenclature rules are as follows: (1) Chains of carbon atoms are identified with the following prefix: C 1: meth-
C 2: eth-
C 3: prop-
C 4: b u t -
Names of longer chains are derived from Greek numerals: C 5: pent-
C 6: h e x -
C 7: hept- etc.
(2) Names of normal alkanes are formed from the chain length prefix and the ending -ane: CH 4: C2H6: C 6H 14:
methane ethane hexane
Note that all substituents which have not been specified are assumed to be hydrogen. (3) Cycloalkanes receive the prefix c y c l o - . (4) Presence of a double bond is indicated by the ending -ene: CH 2 = CH 2:
ethene
In longer chains, the position of a double bond between carbon atoms i and i + 1 is specified by giving i in the name: CH 2 = C H - C H 2 - C H 3: CH 3-CH = C H - C H 3:
1-butene or but- 1-ene 2-butene or but-2-ene
2 Thus IUPAC nomenclature can be regarded as a modem counterpart of the Entish language [5].
U.K. Deiters / Fluid Phase Equilibria 132 (1997) 265-270
268
This is especially important when describing molecules with more than one double bond: CH 2 = C H - C H = CH 2: CH 2 = C = C H - C H 3:
1,3-butadiene 1,2-butadiene
The number of double bonds is indicated by a Greek numeral prefix to the -ene ending. (5) In the case of double bonds, it may be necessary to distinguish between cis- and trans-isomers (another feature that the refrigerant nomenclature cannot do): c/s-2-butene
trans-2-butene
CH3 CH3 \ / CH-CH
CH3 \ CH-CH \ CH3
(6) Substituents to a carbon chain are indicated by putting their name before the chain name. If necessary, their position is specified by giving the carbon atom numbers to which they are attached: CC14"
CHBr2CH 3:
tetrachloromethane 1,1-dibromoethane
If there is more than one way to number the carbon atoms, the numbering scheme is preferred which gives the lowest substituent numbers. Tables 1-3 contain further examples. (7) Names of sidechains are formed from a chain length prefix and the ending -yl. (8) For a branched alkane, the name is derived from longest carbon chain; the other parts of the molecule are regarded as substituents: CH 3-CH(CH3)-CH3" CH 3-CH(CH 3)-CH 2-CH 2-CH 3:
methylpropane (isobutane) 2-methylpentane
This nomenclature evidently can describe molecular constitution in fine detail.
3. Stereochemistry Unfortunately, organic chemistry has more pitfalls. The second compound in Table 3 or the refrigerant R124 (CHC1FCF3, 1-chloro-l,2,2,2-tetrafluoroethane) both have a carbon atom with four different substituents ( - H , -C1, - F , - C F 3 in the case of R124). Such a carbon atom is a centre of chirality: There are two different forms of R124 as Fig. 1 illustrates. Form a cannot be transformed into form b by any rotation of the molecule as a whole or of its constituent groups around single bonds: a and b are distinct species. In this case, b is a mirror image of a, and a and b are called enantiomers. Because of their symmetry, a and b have the same thermodynamic properties. But their reactivity with or solubility in other chiral material may be not be the same. Since biochemical processes are usually highly stereoselective, the toxicity or the biodegradability may be rather different for enantiomers.
U.K. Deiters / Fluid Phase Equilibria 132 (1997) 265-270
F~.
H
.~CI
Cl~
H
C
I
C
a
b
269
~F
I
Fig. 1. Stereographic projections of the two enantiomeric forms of 1-chloro-l,2,2,2-tetrafluoroethane (R124).
Unless a sample has been prepared from biochemical material or special stereoselective methods of synthesis have been used, it will be composed of equal amounts of enantiomers. Such racemic mixtures should not be confused with pure components: Usually the enantiomers either do not mix in the solid state, or the racemic mixture has a crystal structure of its own. Either way, the melting point of a racemic mixture is usually not the same as that of a pure enantiomer. Because the packing efficiency in the dense liquid may also be different for pure enantiomers and for the racemic mixture, one must expect nonvanishing excess volumina; hence liquid racemic mixtures can depart from ideality. The above discussion might seem far-fetched, as far as refrigerants are concerned. But one should keep in mind that some purification procedures, such as membrane pervaporation or liquid chromatography, might change the ratio of enantiomers, if the membrane or chromatography column contains chiral material like cellulose, dextrane, or gelatine! The configuration of substituents at a centre of chirality can be characterized by the RS nomenclature of Cahn, Ingold, and Prelog. There is no need to explain this nomenclature in this text; instead, the reader is referred to textbooks in structural organic chemistry. Just for completeness, we remark that compound a has an S configuration at its carbon atom 1, and its IUPAC name is therefore (1 S)- 1-chloro- 1,2,2,2-tetrafluoroethane; b has an R configuration. If there are n elements of chirality in a molecule, there are 2" possible molecular configurations. In the case of 2 centres of chirality, we have the combinations (R,R), (R,S), (S,R), and (S,S). The molecule with the configuration (R, R) is an enantiomer of that with (S,S), and the molecule with the configuration (R,S) is an enantiomer of that with (S,R). But (R,R) and (R,S) have no mirror symmetry relation. Generally, molecules which different configurations, which are not enantiomers, are called diastereomers. They must be expected to have different thermodynamic properties. If a molecule contains a double bond or a ring, further distinguishable ways of arranging substituents become possible, from the rather obvious cis/trans isomerism mentioned above to the almost arcane concepts of helicity or plane chirality.
4. Conclusions It is important that authors properly identify and characterize the substances with which their publication deals. In particular, the following statements should be regarded: (1) Trading names and application-oriented nomenclature systems, such as the 'R nomenclature' of refrigerants, may be used to render a text more legible. But authors should be aware that the
270
U.K. Deiters / Fluid Phase Equilibria 132 (1997) 265-270
'refrigerant nomenclature' cannot express details of chemical constitution or configuration. It should therefore be used with great caution only. (2) It is mandatory that the publication title and the keyword list contain the IUPAC names. (3) If possible, CAS registry numbers, chemical formulas and R names should also be given in the keyword list or in the abstract. (4) If a substance can have configurational isomers, the authors should explain this in the publication, eventually with the help of three-dimensional chemical structure formulas, and state which stereoisomers were used for the experiments. If random mixtures of stereoisomers were used (racemic mixtures in the case of enantiomers), the randomness should be checked by measuring the optical rotation or other stereospecific effects. Today there are several ways of determining CAS registry numbers and IUPAC names from chemical structure: printed abstract services and collections, like Chemical Abstracts, Beilstein etc. • online electronic data banks, such as CA/CAPLUS (created by Chemical Abstracts), BEILSTEIN (Beilstein), DETHERM (Dechema) 3 local PC-based software, such as the 'electronic journal' ELDATA [6] or the Compounds database, a part of the TRC electronic database set [3].
References [1] W. Gerhartz, Y.S. Yamamoto, B. Elvers, J.F. Rounsaville, G. Schulz (Eds.), Ullmann's Encyclopedia of Industrial Chemistry, AI 1, VCH, Weinheim, 1988, ch. 3, p. 354. [2] D. Baumer, E. Riedel, Gase-Handbuch, 3rd ed., Technical Report 90.1001, Messer Griesheim, 1990. [3] K.N. Marsh, R.C. Wilhoit, S.J. Xu, D. Yin, Compounds 4.3 (TRC Electronic Databases for Chemistry and Engineering). TEES, Thermodynamics Research Center, Texas A&M University, 1994. [4] I. Mills, T. CvitaL K. Homann, N. KaUay, K. Kuchitsu, Quantities, Units and Symbols in Physical Chemistry, 2nd ed., Blackwell, Oxford, 1993. [5] J.R.R. Tolkien, The Lord of the Rings, Part 2: The Two Towers, ch. IV of book III, Allen & Unwin, London, 1966. [6] H.V. Kehiaian, ELDATA: Int. Electronic J. Phys.-Chem. Data. ITODYS, Universit6 Paris VII, since 1995.
3 These three maintained by STN International.