88:
965:
17:
222:
199:
899:
943:
919:
187:
345:
337:
139:
because substituents in the axial position are relatively close to two other axial substituents. This makes it very crowded when bulky substituents are oriented in the axial position. These types of steric interactions are commonly known as 1,3 diaxial interactions. These types of interactions are not present with substituents at the equatorial position.
1005:. Bromine, iodine, and chlorine all have similar A-values even though their atomic radii differ. A-values then, predict the apparent size of a substituent, and the relative apparent sizes determine the differences in steric effects between compounds. Thus, A-values are useful tools in determining compound reactivity in chemical reactions.
138:
Steric effects play a major role in the assignment of configurations in cyclohexanes. One can use steric hindrances to determine the propensity of a substituent to reside in the axial or equatorial plane. It is known that axial bonds are more hindered than the corresponding equatorial bonds. This is
914:
It was observed that the cis compound underwent oxidation at a much faster rate than the trans compound. The proposition was that the large hydroxyl group in the axial position was disfavored and formed the carbonyl more readily to relieve this strain. The trans compound had rates identical to
352:
Due to the larger number of possible conformations of ethyl cyclohexane, the A value is reduced from what would be predicted based purely on enthalpic terms. Due to these favorable entropic conditions, the steric relevance of an ethyl group is similar to that of a methyl substituent.
83:
of cyclohexane rings. The most stable conformation will be the one which has the substituent or substituents equatorial. When multiple substituents are taken into consideration, the conformation where the substituent with the largest A-value is equatorial is favored.
955:
A-Values are measured using a mono-substituted cyclohexane ring, and are an indication of only the sterics a particular substituent imparts on the molecule. This leads to a problem when there are possible stabilizing electronic factors in a different system. The
1001:-butyl group. The longer bond allows for less interactions with neighboring substituents, which effectively makes the trimethylsilyl group less sterically hindering, thus, lowering its A-value. This can also be seen when comparing the
1279:
Huang, C.-Y.; Cabell, L.A.; Anslyn, E.V. (1994). "Molecular
Recognition of Cyclitols by Neutral Polyaza-Hydrogen-Bonding Receptors: The Strength and Influence of Intramolecular Hydrogen Bonds between Vicinal Alcohols".
960:
substituent shown below is axial in the ground state, despite a positive A-value. From this observation, it is clear that there are other possible electronic interactions that stabilize the axial conformation.
192:
The dashed lines indicate 6-atom interactions found in this conformation of ethyl cyclohexane, which amounts to approximately 2.7 kcal/mol (11 kJ/mol) in the enthalpic term of Free Energy.
204:
The dashed lines here signify the 7 atom interactions, which contribute approximately 8 kcal/mol (33 kJ/mol) to the enthalpic term making this conformation unrealistically high in energy.
968:
Equilibrium representation of a chair flip of a carboxylic acid; the axial position is preferred due to favorable electronic factors, despite a steric bias favoring the equatorial position.
324:
71:(ΔG) between the higher energy conformation (axial substitution) and the lower energy conformation (equatorial substitution) is the A-value for that particular substituent.
178:
between conformations. Each 6-atom interaction is worth 0.9 kcal/mol (3.8 kJ/mol) and each 7-atom interaction is worth 4 kcal/mol (17 kJ/mol).
28:
above. This means it costs 1.74 kcal/mol (7.3 kJ/mol) of energy to have a methyl group in the axial position compared to the equatorial position.
939:
Using the A-values of the hydroxyl and isopropyl subunit, the energetic value of a favorable intramolecular hydrogen bond can be calculated.
285:
also plays a role in a substituent's preference for the equatorial position. The entropic component is determined by the following formula:
946:
Possible chair conformations and the favorable hydrogen bond available in the conformation where both hydroxyl substituents are equatorial
1282:
1262:
1156:
1120:
1083:
1069:
1320:
103:
The utility of A-values can be generalized for use outside of cyclohexane conformations. A-values can help predict the
330:
1107:]. Topics in Current Chemistry Fortschritte der Chemischen Forschung. Vol. 45. Springer-Verlag. p. 139.
107:
of a substituent. In general, the larger a substituent's A-value, the larger the steric effect of that substituent. A
1190:
269:
251:
247:
159:, defined as the torsional strain arising from 1,2 interactions between groups attached to contiguous carbons,
130:
than methyl. This difference in steric effects can be used to help predict reactivity in chemical reactions.
291:
989:-butyl group actually occupies less space. This difference can be attributed to the longer length of the
174:
When comparing relative stability, 6- and 7-atom interactions can be used to approximate differences in
1028:
895:-butyl group used locks the conformation of each molecule, placing it equatorial (cis compound shown).
162:
243:
232:
80:
52:
44:
977:
A-values do not predict the physical size of a molecule, only the steric effect. For example, the
990:
236:
1148:
994:
1140:
25:
8:
64:
1315:
1047:
1186:
1152:
1141:
1116:
1079:
1051:
891:
of oxidation in trans and cis substituted rings using a chromium catalyst. The large
143:
68:
1291:
1108:
1037:
1024:"Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)"
887:
One of the original experiments performed by
Winston and Holness was measuring the
142:
There are generally considered three principle contributions to the conformational
112:
1251:
957:
35:
are numerical values used in the determination of the most stable orientation of
1023:
982:
1309:
934:
888:
156:
150:
127:
104:
1078:] (second ed.). New Jersey: John Wiley and Sons, Inc. p. 167.
1042:
108:
87:
21:
1112:
964:
60:
56:
48:
1295:
1278:
1240:(fourth ed.). New York: John Wiley & Sons, Inc. p. 40.
1225:(first ed.). New York: John Wiley & Sons, Inc. p. 199.
221:
16:
175:
95:-butyl substituent; therefore the most stable conformation has the
40:
1206:
Eliel, E.L.; Allinger, N.L.; Angyal, S.J.; G.A., Morrison (1965).
1002:
282:
51:. A-values are derived from energy measurements of the different
153:, defined as the strain arising from deformation of bond angles.
91:
A methyl substituent has a significantly smaller A-value than a
898:
198:
942:
918:
36:
186:
935:
Approximating intramolecular force strength using A-values
1236:
Romers, C.; Altona, C.; Buys, H. R.; Havinga, E. (1969).
981:-butyl group (A-value=4.9) has a larger A-value than the
344:
336:
1235:
1205:
348:
Possible equatorial conformations of ethyl cyclohexane.
294:
1261:. University of British Columbia. 21 November 2012.
1147:. Sausalito, CA: University Science Books. pp.
362:A-values (in kcal/mol) of some common substituents
340:Possible axial conformations of ethyl cyclohexane.
318:
1134:
1132:
915:those found in the monosubstituted cyclohexanol.
1307:
1180:
165:, which are similar to 1,3 diaxial interactions.
1181:Eliel, E.L.; Wilen, S.H.; Mander, L.N. (1994).
1138:
1139:Anslyn, Eric V.; Dougherty, Dennis A. (2006).
1129:
1063:
1061:
133:
118:has an A-value of ~5. Because the A-value of
55:of a monosubstituted cyclohexane chemical.
1199:
1272:
1229:
250:. Unsourced material may be challenged and
1058:
1015:
47:), as well as a general representation of
1174:
1041:
1021:
270:Learn how and when to remove this message
1283:Journal of the American Chemical Society
1098:
963:
941:
917:
897:
882:
343:
335:
86:
15:
1171:Note: measured under diverse conditions
1092:
1067:
972:
169:
1308:
1220:
211:
1268:from the original on 22 January 2021.
1214:
319:{\displaystyle \Delta S=R\ln \sigma }
1210:. New York: Interscience Publishers.
1183:Stereochemistry of Organic Compounds
356:
248:adding citations to reliable sources
215:
13:
329:Where σ is equal to the number of
295:
99:-butyl in the equatorial position.
14:
1332:
1143:Modern Physical Organic Chemistry
333:available for each conformation.
902:Possible chair conformations of
220:
197:
185:
67:to the axial. The difference in
1244:
877:
1259:Advanced Organic Chemistry 330
1165:
950:
1:
1008:
985:group (A-value=2.5), yet the
111:has an A-value of 1.74 while
63:ring prefer to reside in the
24:is 1.74 as derived from the
7:
1105:Topics in Current Chemistry
1099:Anderson, J. Edgar (1974).
1068:Hoffman, Robert V. (2004).
10:
1337:
1321:Physical organic chemistry
1029:Pure and Applied Chemistry
380:
372:
163:Van der Waals interactions
134:Free energy considerations
79:A-values help predict the
74:
1238:Topics in Stereochemistry
1223:Topics in Stereochemistry
53:cyclohexane conformations
1208:Conformational Analysis
1043:10.1351/pac199466051077
45:conformational analysis
1221:Hirsch, J. A. (1967).
969:
947:
931:
930:-butyl-cyclohexan-1-ol
922:Chromium oxidation of
911:
910:-butyl-cyclohexan-1-ol
349:
341:
320:
100:
29:
1113:10.1007/3-540-06471-0
967:
945:
921:
901:
883:Predicting reactivity
347:
339:
321:
90:
19:
1076:An Intermediate Text
973:Other considerations
292:
244:improve this section
170:Enthalpic components
126:-butyl has a larger
26:chemical equilibrium
1296:10.1021/ja00086a011
1252:"Table of A-Values"
1185:. New York: Wiley.
993:as compared to the
991:carbon–silicon bond
363:
212:Entropic components
65:equatorial position
1022:Muller, P (1994).
995:carbon–carbon bond
970:
948:
932:
912:
361:
350:
342:
316:
122:-butyl is higher,
101:
30:
20:The A-value for a
1158:978-1-891389-31-3
1122:978-3-540-06471-8
1101:Dynamic Chemistry
1085:978-0-471-45024-5
1071:Organic Chemistry
875:
874:
357:Table of A-values
280:
279:
272:
69:Gibbs free energy
1328:
1300:
1299:
1290:(7): 2778–2792.
1276:
1270:
1269:
1267:
1256:
1248:
1242:
1241:
1233:
1227:
1226:
1218:
1212:
1211:
1203:
1197:
1196:
1178:
1172:
1169:
1163:
1162:
1146:
1136:
1127:
1126:
1096:
1090:
1089:
1065:
1056:
1055:
1045:
1036:(5): 1077–1184.
1019:
364:
360:
325:
323:
322:
317:
275:
268:
264:
261:
255:
224:
216:
201:
189:
1336:
1335:
1331:
1330:
1329:
1327:
1326:
1325:
1306:
1305:
1304:
1303:
1277:
1273:
1265:
1254:
1250:
1249:
1245:
1234:
1230:
1219:
1215:
1204:
1200:
1193:
1179:
1175:
1170:
1166:
1159:
1137:
1130:
1123:
1097:
1093:
1086:
1066:
1059:
1020:
1016:
1011:
975:
958:carboxylic acid
953:
937:
885:
880:
868:
864:
854:
826:
802:
791:
768:
753:
742:
738:
723:
719:
708:
699:
690:
679:
664:
653:
643:
619:
596:
592:
569:
545:
528:
524:
501:
492:
488:
471:
462:
458:
435:
431:
414:
410:
400:
359:
293:
290:
289:
276:
265:
259:
256:
241:
225:
214:
209:
208:
207:
206:
205:
202:
194:
193:
190:
172:
136:
77:
12:
11:
5:
1334:
1324:
1323:
1318:
1302:
1301:
1271:
1243:
1228:
1213:
1198:
1191:
1173:
1164:
1157:
1128:
1121:
1091:
1084:
1057:
1013:
1012:
1010:
1007:
983:trimethylsilyl
974:
971:
952:
949:
936:
933:
884:
881:
879:
876:
873:
872:
869:
866:
862:
859:
856:
852:
849:
847:
844:
843:
840:
837:
834:
831:
828:
824:
820:
819:
816:
813:
810:
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804:
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796:
795:
792:
789:
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772:
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766:
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751:
747:
746:
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740:
736:
733:
730:
727:
724:
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135:
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76:
73:
9:
6:
4:
3:
2:
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1317:
1314:
1313:
1311:
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1194:
1192:81-224-0570-3
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1014:
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1004:
1000:
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992:
988:
984:
980:
966:
962:
959:
944:
940:
929:
925:
920:
916:
909:
905:
900:
896:
894:
890:
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860:
857:
850:
848:
846:
845:
841:
838:
835:
832:
829:
822:
821:
817:
814:
811:
808:
805:
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764:
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749:
748:
744:
734:
731:
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701:
695:
692:
686:
685:
681:
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672:
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660:
659:
655:
649:
646:
639:
636:
633:
632:
628:
625:
622:
615:
612:
609:
608:
604:
601:
598:
588:
585:
582:
581:
577:
574:
571:
565:
562:
559:
558:
554:
551:
548:
541:
538:
535:
534:
530:
520:
517:
514:
511:
508:
507:
503:
497:
494:
484:
481:
478:
477:
473:
467:
464:
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451:
448:
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427:
424:
421:
420:
416:
406:
403:
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382:
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334:
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313:
310:
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301:
298:
288:
287:
286:
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274:
271:
263:
253:
249:
245:
239:
238:
234:
229:This section
227:
223:
218:
217:
200:
188:
179:
177:
164:
161:
158:
157:Pitzer strain
155:
152:
151:Baeyer strain
149:
148:
147:
145:
140:
131:
129:
128:steric effect
125:
121:
117:
115:
110:
106:
105:steric effect
98:
94:
89:
85:
82:
72:
70:
66:
62:
58:
54:
50:
46:
42:
38:
34:
27:
23:
18:
1287:
1281:
1274:
1258:
1246:
1237:
1231:
1222:
1216:
1207:
1201:
1182:
1176:
1167:
1142:
1104:
1100:
1094:
1075:
1070:
1033:
1027:
1017:
998:
986:
978:
976:
954:
938:
927:
923:
913:
907:
903:
892:
886:
878:Applications
351:
328:
281:
266:
257:
242:Please help
230:
173:
141:
137:
123:
119:
116:-butyl group
113:
109:methyl group
102:
96:
92:
81:conformation
78:
57:Substituents
32:
31:
22:methyl group
951:Limitations
383:Substituent
375:Substituent
367:Substituent
331:microstates
144:free energy
61:cyclohexane
49:steric bulk
1310:Categories
1009:References
1316:Isomerism
1052:195819485
314:σ
311:
296:Δ
231:does not
1263:Archived
1003:halogens
386:A-value
378:A-value
370:A-value
260:May 2019
176:enthalpy
41:molecule
33:A-values
997:of the
283:Entropy
252:removed
237:sources
75:Utility
1189:
1155:
1151:–105.
1119:
1082:
1050:
634:N=C=NR
407:OSi(CH
1266:(PDF)
1255:(PDF)
1103:[
1074:[
1048:S2CID
861:Si(CH
750:CH=CH
656:0.59
605:0.27
578:0.68
575:O-TFA
504:0.56
495:>4
444:0.87
428:CH(CH
417:0.74
394:0.006
59:on a
39:in a
37:atoms
1187:ISBN
1153:ISBN
1117:ISBN
1080:ISBN
999:tert
987:tert
979:tert
928:tert
908:tert
893:tert
889:rate
871:2.5
842:0.3
839:HgCl
833:SOPh
830:1.75
815:HgBr
794:1.1
779:0.41
771:1.9
756:1.35
745:2.1
735:N(CH
726:1.75
705:NHCH
702:1.17
696:COCH
682:1.6
673:1.25
670:COCl
647:0.96
629:0.5
626:O-Ts
613:0.28
602:OCHO
599:1.27
586:0.51
572:1.92
563:0.21
555:0.6
552:O-Ac
549:1.35
539:0.17
531:0.9
512:0.43
485:C(CH
482:0.38
474:0.6
465:2.15
452:0.43
438:2.15
425:0.15
404:1.79
235:any
233:cite
124:tert
120:tert
114:tert
97:tert
93:tert
1292:doi
1288:116
1149:104
1109:doi
1038:doi
926:-4-
924:cis
906:-4-
904:cis
858:2.5
836:1.9
827:OTs
818:~0
812:1.3
785:0.8
782:SPh
776:CCH
762:0.7
759:SMe
732:0.9
693:2.1
667:1.7
650:ONO
623:1.2
610:NCS
583:NCO
521:OCH
498:OCD
468:OCH
455:c-C
246:by
1312::
1286:.
1257:.
1131:^
1115:.
1060:^
1046:.
1034:66
1032:.
1026:.
855:Ph
851:SO
823:CH
803:Bu
799:CH
788:NO
765:NH
729:SH
720:CH
716:CH
711:1
687:CF
676:NH
661:CH
644:Pr
640:CO
620:Et
616:CO
593:CH
589:CO
566:CO
560:NC
542:CO
536:CN
525:CH
515:Ph
479:Br
461:11
449:Cl
441:OH
401:Br
397:CH
308:ln
146::
1298:.
1294::
1195:.
1161:.
1125:.
1111::
1088:.
1054:.
1040::
867:3
865:)
863:3
853:2
825:2
809:S
806:2
801:2
790:2
767:3
752:2
741:2
739:)
737:3
722:3
718:2
707:3
698:3
689:3
678:2
663:3
652:2
642:2
637:1
618:2
595:3
591:2
568:2
546:H
544:2
527:3
523:2
518:3
509:I
500:3
491:3
489:)
487:3
470:3
459:H
457:6
434:2
432:)
430:3
422:F
413:3
411:)
409:3
399:2
391:D
305:R
302:=
299:S
273:)
267:(
262:)
258:(
254:.
240:.
43:(
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