326:
209:
483:
464:
317:. The anti conformation is more stable by 0.9 kcal mol. We would expect that butane is roughly 82% anti and 18% gauche at room temperature. However, there are two possible gauche conformations and only one anti conformation. Therefore, entropy makes a contribution of 0.4 kcal in favor of the gauche conformation. We find that the actual conformational distribution of butane is 70% anti and 30% gauche at room temperature.
685:
693:
931:
the allosteric signal will increase. The ratio K2/K1 can be related directly to the strain energy difference between the conformers C1 and C2; if it is small higher concentrations of A will directly bind to C2 and make the effector E inefficient. In addition, the response time of such allosteric switches depends on the strain of the conformer interconversion transitions state.
518:
676:
compounds, including equilibria, redox and solvolysis reactions, which all are characterized by transition between sp2 and sp3 state at the reaction center, correlate with corresponding strain energy differences ïSI (sp2 -sp3). The data reflect mainly the unfavourable vicinal angles in medium rings,
502:
conformation where the two terminal methyl groups are brought into proximity. If the bonds are rotated in the same direction, this doesn't occur. The steric strain between the two terminal methyl groups accounts for the difference in energy between the two similar, yet very different conformations.
571:, it is brought near to an axial gamma hydrogen. The amount of strain is largely dependent on the size of the substituent and can be relieved by forming into the major chair conformation placing the substituent in an equatorial position. The difference in energy between conformations is called the
930:
by the substrate A will lead to binding of A to C2 also in absence of the effector E. Only if the stability of the conformer C2 is significantly smaller, meaning that in absence of an effector E the population of C2 is much smaller than that of C1, the ratio K2/K1 which measures the efficiency of
628:
More complex molecules, such as butane, have more than one possible staggered conformation. The anti conformation of butane is approximately 0.9 kcal mol (3.8 kJ mol) more stable than the gauche conformation. Both of these staggered conformations are much more stable than the
925:
systems there are typically two or more conformers with stability differences due to strain contributions. Positive cooperativity for example results from increased binding of a substrate A to a conformer C2 which is produced by binding of an effector molecule E. If the conformer C2 has a similar
659:
of molecular bonding, the preferred geometry of a molecule is that in which both bonding and non-bonding electrons are as far apart as possible. In molecules, it is quite common for these angles to be somewhat compressed or expanded compared to their optimal value. This strain is referred to as
620:
is approximately 2.9 kcal mol. It was initially believed that the barrier to rotation was due to steric interactions between vicinal hydrogens, but the Van der Waals radius of hydrogen is too small for this to be the case. Recent research has shown that the staggered conformation may be
308:
Enthalpy is typically the more important thermodynamic function for determining a more stable molecular conformation. While there are different types of strain, the strain energy associated with all of them is due to the weakening of bonds within the molecule. Since enthalpy is usually more
344:°) of a compound is described as the enthalpy change when the compound is formed from its separated elements. When the heat of formation for a compound is different from either a prediction or a reference compound, this difference can often be attributed to strain. For example, Î
677:
as illustrated by the severe increase of ketone reduction rates with increasing ïSI (Figure 1). Another example is the solvolysis of bridgehead tosylates with steric energy differences between corresponding bromide derivatives (sp3) and the carbenium ion as sp2- model for the
433:
allow. Specifically, Van der Waals strain is considered a form of strain where the interacting atoms are at least four bonds away from each other. The amount on steric strain in similar molecules is dependent on the size of the interacting groups; bulky
377:
Determining the strain energy within a molecule requires knowledge of the expected internal energy without the strain. There are two ways do this. First, one could compare to a similar compound that lacks strain, such as in the previous
887:
Medium-sized rings (7â13 carbons) experience more strain energy than cyclohexane, due mostly to deviation from ideal vicinal angles, or Pitzer strain. Molecular mechanics calculations indicate that transannular strain, also known as
836:
is considered a benchmark in determining ring strain in cycloalkanes and it is commonly accepted that there is little to no strain energy. In comparison, smaller cycloalkanes are much higher in energy due to increased strain.
188:
845:
experiences similar strain, with bond angles of approximately 88° (it isn't completely planar) and eclipsed hydrogens. The strain energy of cyclopropane and cyclobutane are 27.5 and 26.3 kcal mol, respectively.
303:
497:
into a gauche conformation, one of which is 3 kcal mol higher in energy than the other. When the two methyl-substituted bonds are rotated from anti to gauche in opposite directions, the molecule assumes a
489:
There are situations where seemingly identical conformations are not equal in strain energy. Syn-pentane strain is an example of this situation. There are two different ways to put both of the bonds the central in
615:
Torsional strain occurs when atoms separated by three bonds are placed in an eclipsed conformation instead of the more stable staggered conformation. The barrier of rotation between staggered conformations of
660:
angle strain, or Baeyer strain. The simplest examples of angle strain are small cycloalkanes such as cyclopropane and cyclobutane, which are discussed below. Furthermore, there is often eclipsing or
456:
They found that as the size of the alkyl groups on the amine were increased, the equilibrium constant decreased as well. The shift in equilibrium was attributed to steric strain between the
366:
is -25.5 kcal mol. Despite having the same atoms and number of bonds, methylcyclopentane is higher in energy than cyclohexane. This difference in energy can be attributed to the
841:
is analogous to a triangle and thus has bond angles of 60°, much lower than the preferred 109.5° of an sp hybridized carbon. Furthermore, the hydrogens in cyclopropane are eclipsed.
309:
important, entropy can often be ignored. This isn't always the case; if the difference in enthalpy is small, entropy can have a larger effect on the equilibrium. For example,
873:, is noted for being one of the most strained compounds that is isolatable on a large scale; its strain energy is estimated at 63.9 kcal mol (267 kJ mol).
672:
or force field approaches allow to calculate such strain contributions, which then can be correlated e.g. with reaction rates or equilibria. Many reactions of
559:
1,3-diaxial strain is another form of strain similar to syn-pentane. In this case, the strain occurs due to steric interactions between a substituent of a
910:
is commonly the sum of the strain energy in each individual ring. This isn't always the case, as sometimes the fusion of rings induces some extra strain.
55:
stored within it. A strained molecule has an additional amount of internal energy which an unstrained molecule does not. This extra internal energy, or
109:
233:
850:
experiences much less strain, mainly due to torsional strain from eclipsed hydrogens: its preferred conformations interconvert by a process called
545:
methyl group of the olefin. These types of compounds usually take a more linear conformation to avoid the steric strain between the substituents.
892:, does not play an essential role. Transannular reactions however, such as 1,5-shifts in cyclooctane substitution reactions, are well known.
584:
1100:
1033:
1013:
993:
334:
591:
equilibrium for the measurement of axial versus equatorial values of cyclohexanone/cyclohexanol (0.7 kcal mol).
1119:-Butyl- and Neopentyldimethylamines; Interaction of Trimethylboron and Boron Trifluoride with highly hindered bases".
1319:
567:
carbons two bonds away from the substituent in question (hence, 1,3-diaxial interactions). When the substituent is
74:
within that molecule. Without the bonds holding the conformation in place, the strain energy would be released.
370:
of a five-membered ring which is absent in cyclohexane. Experimentally, strain energy is often determined using
383:
382:
example. Unfortunately, it can often be difficult to obtain a suitable compound. An alternative is to use
1206:
386:. As long as suitable group increments are available for the atoms within a compound, a prediction of Î
940:
1051:
1338:; Lampman, G. M.; Ciula, R. P.; Connor, D. S.; Schertler, P.; Lavanish, J. (1965). "Bicyclobutane".
1392:
568:
554:
92:
1289:
1374:
664:
strain in cyclic systems. These and possible transannular interactions were summarized early by
600:
314:
202:
1305:
193:
If there is a decrease in Gibbs free energy from one state to another, this transformation is
1340:
1146:
Eliel, E.L., Wilen, S.H., The
Stereochemistry of Organic Compounds, Wiley-Interscience, 1994.
60:
579:
is a thermodynamic parameter and was originally measured along with other methods using the
564:
477:
430:
426:
421:
100:
88:
1115:; Johannesen, R. B. (1952). "Dissociation of the Addition Compounds of Trimethlboron with
8:
882:
669:
588:
542:
194:
525:
Allylic strain, or A strain is closely associated to syn-pentane strain. An example of
1183:
1074:
825:
In principle, angle strain can occur in acyclic compounds, but the phenomenon is rare.
371:
363:
36:
32:
1353:
1335:
1315:
1301:
1201:
1175:
1121:
1096:
1029:
1009:
989:
907:
901:
858:
580:
379:
325:
96:
44:
1078:
625:. Rotation away from the staggered conformation interrupts this stabilizing force.
1349:
1269:
1250:
1239:
1214:
1187:
1167:
1158:
1129:
1112:
1066:
678:
622:
450:
183:{\displaystyle K_{\rm {eq}}=\exp \left(-{\frac {\Delta {G^{\circ }}}{RT}}\right)\,}
67:
66:. Much like a compressed spring must be held in place to prevent release of its
63:
48:
40:
16:
When a molecule is deformed from its lowest-energy conformation by applied stress
1311:
851:
629:
eclipsed conformations. Instead of a hyperconjugative effect, such as that in
512:
446:
298:{\displaystyle \Delta {G^{\circ }}=\Delta {H^{\circ }}-T\Delta {S^{\circ }}\,.}
1070:
1386:
889:
862:
638:
71:
70:, a molecule can be held in an energetically unfavorable conformation by the
56:
1304:(1968). "Small Ring Bicycloalkanes". In Hart, H.; Karabatsos, G. J. (eds.).
1218:
1179:
847:
838:
656:
608:
is the resistance to bond twisting. In cyclic molecules, it is also called
499:
439:
563:
ring ('α') and gauche interactions between the alpha substituent and both
482:
429:, or steric strain, occurs when atoms are forced to get closer than their
927:
842:
833:
702:
650:
560:
534:
367:
352:
224:
208:
1133:
922:
665:
205:
will spontaneously convert to the lower energy molecular conformation.
1171:
673:
541:
to rotate such that the terminal methyl group is brought near to the
530:
223:
are related to Gibbs free energy through the equation (at a constant
20:
445:
The effects of steric strain in the reaction of trialkylamines and
216:
24:
1156:
Weinhold, F. (2001). "Chemistry: A New Twist on
Molecular Shape".
576:
572:
491:
407:°, this difference in energy can be attributed to strain energy.
220:
661:
634:
630:
617:
538:
310:
198:
52:
1290:
https://onlinelibrary.wiley.com/doi/epdf/10.1002/poc.610080802
1375:
https://pubs.rsc.org/en/content/articlepdf/2016/ob/c6ob01303a
1111:
684:
526:
463:
457:
435:
692:
99:
of the two conformations. From this energy difference, the
517:
1334:
212:
Examples of the anti and gauche conformations of butane.
575:
and is well known for many different substituents. The
1204:(1986). "The Concept of Strain in Organic Chemistry".
236:
112:
1242:DOI: 10.1021/ja01145a072 ; H.C. Brown, G. Ham
926:stability as another equilibrating conformer C1 a
521:
Allylic methyl and ethyl groups are close together.
442:and often experience greater steric interactions.
374:which is typically an easy experiment to perform.
297:
182:
1105:
1384:
1270:https://pubs.acs.org/doi/pdf/10.1021/ja00349a031
1251:https://pubs.acs.org/doi/pdf/10.1021/ja01593a024
1240:https://pubs.acs.org/doi/pdf/10.1021/ja01145a072
916:
641:and angle strain caused by these interactions.
320:
1231:H. C. Brown , R.S. Fletcher, R.B. Johannes
1008:, 3rd ed., Blackie Academic & Pro., 1993,
467:Reaction of trialkylamines and trimethylboron.
1045:
1043:
1041:
460:of the amine and the methyl groups on boron.
329:Images of cyclohexane and methylcyclopentane.
103:for the two conformations can be determined.
1155:
1049:
637:is due to both steric interactions between
1261:H.-J. Schneider , G. Schmidt , F. Thomas
1200:
1038:
857:Ring strain can be considerably higher in
1052:"Conformational analysis of cycloalkanes"
980:
978:
976:
291:
179:
43:in comparison to a strain-free reference
974:
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1300:
876:
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1018:
953:
548:
471:
393:° can be made. If the experimental Î
1085:
998:
201:. A highly strained, higher energy
895:
594:
529:strain can be seen in the compound
355:is -29.9 kcal mol while Î
197:and the lower energy state is more
95:is determined by the difference in
13:
988:, University Science Books, 2006,
668:as internal strain, or I-Strain.
585:MeerweinâPonndorfâVerley reduction
410:
276:
255:
237:
148:
122:
119:
51:of a molecule consists of all the
14:
1404:
1280:P. MĂŒller, J. Mareda , D. Milin
986:Modern Physical Organic Chemistry
506:
82:
313:has two possible conformations,
1360:
1328:
1307:Advances in Alicyclic Chemistry
1294:
1274:
1255:
1225:
1006:Principles of Organic Synthesis
906:The amount of strain energy in
583:equation and, for example, the
449:were studied by Nobel laureate
1140:
1095:, 4th ed., Brooks/Cole, 2005,
1028:, 5th ed., McGraw-Hill, 2002,
828:
721:Strain energy (kcal mol)
713:Strain energy (kcal mol)
644:
400:° differs from the predicted Î
1:
1354:10.1016/S0040-4020(01)98361-9
946:
438:take up much more space than
384:Benson group increment theory
1050:Dragojlovic, Veljko (2015).
917:Strain in allosteric systems
321:Determining molecular strain
7:
1207:Angew. Chem. Int. Ed. Engl.
1091:Brown, Foote, and Iverson,
934:
10:
1409:
941:Strain (materials science)
899:
880:
648:
598:
552:
510:
475:
419:
335:standard heat of formation
77:
1071:10.1007/s40828-015-0014-0
715:
533:. It's possible for the
555:Cyclohexane conformation
633:, the strain energy in
623:hyperconjugative effect
93:molecular conformations
1219:10.1002/anie.198603121
984:Anslyn and Dougherty,
701:Strain of some common
697:
689:
601:Alkane stereochemistry
522:
486:
468:
330:
299:
213:
203:molecular conformation
184:
59:, can be likened to a
695:
687:
621:more stable due to a
599:Further information:
520:
485:
466:
328:
300:
211:
185:
1314:. pp. 185â254.
478:Pentane interference
427:Van der Waals strain
422:Van der Waals strain
416:Van der Waals strain
234:
110:
101:equilibrium constant
1134:10.1021/ja01097a005
883:Transannular strain
877:Transannular strain
706:
670:Molecular mechanics
589:Oppenauer oxidation
537:substituent of the
431:Van der Waals radii
372:heats of combustion
1368:Org. Biomol. Chem.
1282:J. Phys.Org. Chem.
1026:Physical Chemistry
1004:Coxon and Norman,
700:
698:
690:
549:1,3-diaxial strain
523:
487:
472:Syn-pentane strain
469:
364:methylcyclopentane
331:
295:
214:
180:
33:chemical structure
1366:H.-J. Schneider.
1348:(10): 2749â2769.
1263:J. Am. Chem. Soc.
1244:J. Am. Chem. Soc.
1233:J. Am. Chem. Soc.
1166:(6837): 539â541.
1122:J. Am. Chem. Soc.
1101:978-0-534-46773-9
1093:Organic Chemistry
1034:978-0-07-253495-5
1014:978-0-7514-0126-4
994:978-1-891389-31-3
902:Bicyclic molecule
823:
822:
655:According to the
581:Gibbs free energy
500:cyclopentane-like
436:tert-butyl groups
380:methylcyclohexane
172:
97:Gibbs free energy
39:which raises its
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908:bicyclic systems
896:Bicyclic systems
861:. For example,
859:bicyclic systems
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679:transition state
606:Torsional strain
595:Torsional strain
451:Herbert C. Brown
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1393:Stereochemistry
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1310:. Vol. 2.
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49:internal energy
41:internal energy
35:undergoes some
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681:. (Figure 2)
649:Main article:
646:
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596:
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553:Main article:
550:
547:
513:Allylic strain
511:Main article:
508:
507:Allylic strain
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476:Main article:
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447:trimethylboron
420:Main article:
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1302:Wiberg, K. B.
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921:In synthetic
914:
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890:Prelog strain
884:
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863:bicyclobutane
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206:
204:
200:
196:
175:
168:
165:
157:
153:
142:
138:
134:
131:
128:
114:
106:
105:
104:
102:
98:
94:
90:
75:
73:
69:
65:
62:
58:
57:strain energy
54:
50:
46:
42:
38:
34:
30:
26:
22:
1370:
1367:
1362:
1345:
1339:
1330:
1306:
1296:
1288:, 8, 507.
1285:
1281:
1276:
1265:
1262:
1257:
1249:, 78 , 2735
1246:
1243:
1235:
1232:
1227:
1210:
1205:
1196:
1163:
1157:
1151:
1142:
1125:
1120:
1116:
1113:Brown, H. C.
1107:
1092:
1087:
1062:
1058:
1025:
1020:
1005:
1000:
985:
920:
912:
905:
886:
856:
848:Cyclopentane
839:Cyclopropane
832:
824:
657:VSEPR theory
654:
627:
614:
609:
605:
604:
558:
524:
492:
488:
458:alkyl groups
453:
444:
425:
404:
397:
390:
376:
359:
348:
341:
332:
307:
215:
192:
86:
28:
27:experiences
18:
1373:,14, 7994.
1341:Tetrahedron
928:fit induced
843:Cyclobutane
834:Cyclohexane
829:Small rings
705:ring-sizes
703:cycloalkane
651:Ring strain
645:Ring strain
561:cyclohexane
368:ring strain
353:cyclohexane
225:temperature
195:spontaneous
89:equilibrium
1202:Wiberg, K.
947:References
923:allosteric
718:Ring size
710:Ring size
696:Figure 2 B
688:Figure 1 B
666:H.C. Brown
61:compressed
1238:73, 212.
1128:: 16â20.
1059:Chemtexts
674:alicyclic
565:methylene
531:2-pentene
286:∘
277:Δ
271:−
265:∘
256:Δ
247:∘
238:Δ
158:∘
149:Δ
143:−
135:
31:when its
21:chemistry
1387:Category
1180:11385553
1079:94348487
1024:Levine,
935:See also
495:-pentane
311:n-butane
217:Enthalpy
45:compound
25:molecule
1188:9812878
577:A value
573:A value
543:vicinal
527:allylic
221:entropy
91:of two
78:Summary
47:. The
1318:
1186:
1178:
1159:Nature
1099:
1077:
1032:
1012:
992:
662:Pitzer
635:butane
631:ethane
618:ethane
539:olefin
454:et al.
362:° for
351:° for
199:stable
64:spring
53:energy
37:stress
29:strain
1236:1951,
1184:S2CID
1075:S2CID
1065:(3).
1055:(PDF)
813:12.6
749:11.3
743:26.3
735:12.4
729:27.5
569:axial
535:ethyl
72:bonds
1371:2016
1316:ISBN
1286:1995
1266:1983
1247:1956
1176:PMID
1097:ISBN
1030:ISBN
1010:ISBN
990:ISBN
819:2.0
805:1.9
799:9.7
791:1.9
785:6.2
777:5.2
771:0.1
763:4.1
757:6.2
333:The
219:and
87:The
23:, a
1350:doi
1284:,
1215:doi
1168:doi
1164:411
1130:doi
1067:doi
865:, C
816:16
802:15
788:14
774:13
760:12
746:11
732:10
227:):
132:exp
19:In
1389::
1346:21
1344:.
1211:25
1182:.
1174:.
1162:.
1126:75
1073:.
1061:.
1057:.
1040:^
955:^
854:.
810:9
796:8
782:7
768:6
754:5
740:4
726:3
612:.
337:(Î
1356:.
1352::
1324:.
1221:.
1217::
1190:.
1170::
1136:.
1132::
1117:n
1081:.
1069::
1063:1
871:6
869:H
867:4
587:/
493:n
405:H
402:f
398:H
395:f
391:H
388:f
360:H
357:f
349:H
346:f
342:H
339:f
293:.
282:S
274:T
261:H
253:=
243:G
176:)
169:T
166:R
154:G
139:(
129:=
123:q
120:e
115:K
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