rotation of alkane

Acyclic Conformational Analysis

Rotamer Barriers in Saturated Compounds

We have seen that staggered conformers of small hydrocarbons (e.g. ethane & butane) are more stable thaneclipsed conformers and generally predominate at equilibrium. This structural preference is usually attributed tosteric effects, and has been termed torsional strain (or Pitzer strain). In the case of ethane, rotation towards aneclipsed structure brings the electrons in C–H bonds on the different C atoms closer to each other, therebyincreasing repulsion. The distance between closest hydrogen atoms in ethane is roughly 2.5 Å in the staggeredform, compared with 2.3 Å in the eclipsed rotamer. This latter distance is comparable to known instances of sterichindrance, such as the hydrogens in axial-methylcyclohexane and ortho hydrogens in biphenyl. However, since thebond vectors in ethane are diverging by nearly 40º, this comparison may not be compelling.Since the eclipsed conformer of ethane has three eclipsed C–H bonds, it is tempting to attribute 1.0 kcal/mole ofstrain to each, resulting in a total of 3 kcal/mole torsion strain. Indeed, it has been noted that the rotational energybarriers in methyl amine and methanol are lower by 1 kcal/mole for each eclipsed C–H bond that is missing, asshown in group A of the following diagram. Although the non-bonding electron pair(s) in these compounds are

shown in spatially directed sp3 orbitals, the exact location is a subject of debate and they do not appear to offerany significant steric hindrance. It might be argued that the C–X bond length is shortened and the C–X–H bondangle narrowed in the alcohol and amine, but molecular structure calculations indicate the eclipsed H---H distanceremains the same.

The second group of structures (B) shows the influence of one or more methyl substituents on one of the ethanecarbons. Each methyl increases the rotational barrier by about 0.5 kcal/mole, although the effect ofthe third substituent (neopentane) is somewhat greater. Because a methyl group is substantiallylarger than a hydrogen, one might have expected a greater barrier to rotation; however, the C–Cbond is more than 40% longer than the C–H bond, reducing the overall steric crowding. Thediagram on the right illustrates this feature, which is amplified by the divergent vectors of the C–H

and C–Y bonds. Consequently, the relative distance r2 is greater than r1. Since it takes less energy to bend acovalent bond than to stretch or shorten it, some of the crowding strain in propane and isobutane might be relievedby opening the C-C-C bond angle. However, this would not be effective in neopentane (the last structure in B),hence this barrier is slightly higher than expected.

Supplemental Topics http://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/rotconf1.htm

1 of 10 06/06/2012 4:31 PM

The influence of bond length on steric hindrance is dramatically demonstrated by the group of compounds labeledD. The covalent radii of substituent atoms increases in the order H < F < Cl < Br < I, with F being less than twice aslarge as H, Cl being three times as large, and Br and I being roughly 3.5 and 4 times the size of hydrogen. The C–Xbond lengths also increase in the same order, with C–F being 25% longer than C–H, C–Cl being 62% longer, andthe Br and I bonds longer still. The consequences of these opposing factors is apparent in the barriers. Additionalchlorine substituents increase the rotational barrier, with the 1,1,1-trichloro compound having a greater thanexpected effect, similar to the neopentane case.Stretching the C–C bond would also reduce the steric hindrance to rotation. Although this not normally possible, anapproximation is found in the two C–Si compounds listed in group F. Because the C–Si bond is 21% longer thanC–C and Si–H is over 35% longer than C–H the barrier in CH3–SiH3 is nearly half of that in ethane. Remarkably,the barrier in tetramethylsilane is even lower. Measurements on eclipsed conformers of these compounds indicatethe H:Y distance (noted in the above diagram) is 2.74 Å when Y=H and 2.94 when Y=CH3.

The barriers in dimethyl ether and trimethyl amine are unexpectedly high, as shown in group C. Each methyl groupseems to add 1.2 to 1.6 kcal/mole to the barrier. For example, the barrier in dimethyl amine is 3.6 kcal/mole, whichagrees well with the methyl component in dimethyl ether (1.6 in both cases). However the second methyl in trimethylamine then adds only 0.8 kcal/mole. The shorter bond lengths and smaller bond angles in these compounds maycontribute to these variations, which are poorly understood.

Finally the presence of a polar substituent on each ethanecarbon often results in a relatively high barrier, as shown bythe compounds in group E. It is instructive to compare1,2-dichloroethane with butane. From the previousdiscussion, one might anticipate a lower barrier for theformer, as a consequence of the longer C–Cl bond.However, the full barrier for the dichloro compound is roughlytwice as high, and may be attributed to dipole repulsionbetween the polar C–Cl bonds as they approach each other.The energy difference between the anti and gauchestaggered conformers of 1,2-dichloroethane is about 45%higher than the difference in butane, and the lower barrierbetween these conformers is 25% higher. The diagram onthe right, which may be toggled from one compound to theother by clicking, shows these features. Unexpectedly,hexachloroethane has a barrier only about 20% higher than1,2-dichlorooethane. This may be due in part to a 0.03 Åincrease in the C–C bond length (about 2%), caused by theextensive dipole repulsion.The rotational energy diagram on the right reflects enthalpy measurements made in the gas phase. In solution or asthe pure liquid the magnitude of the barriers often decreases, and the energy difference between anti and gaucheconformers changes in favor of the latter. For butane, the anti-gauche difference drops from 0.85 to 0.55 kcal/molin the liquid. In the case of 1,2-dichloroethane solutions, polar solvents lower the energy of the gauche conformer tosuch a degree that it becomes favored at equilibrium.

Recent studies of rotational barriers in substituted ethanes suggest that a significant contribution comes fromsigma bond-antibond interactions which stabilize an anti configuration of W and Z groups in W–C–C–Zconstitutions, relative to their gauche orientation. This stereoelectronic factor, which is similar to hyperconjugation,becomes larger as the electronegativity difference between W and Z increases. The conformational preferences of1,2-difluoroethane and 1-fluoropropane provide instructive examples, as shown on the right.Although 1,2-difluoroethane should suffer dipole repulsion as the fluorine atoms approach each other, the gauche


2 of 10 06/06/2012 4:31 PM

form is more stable than the anti conformer (∆Hº = 0.6 kcal/mol). This surprising fact may be explained by the twohyperconjugative bondinteractions in thegauche form, shownby the resonanceformulas in thegray-shaded box.These act to stabilizethe gauche rotamer,and do not exist in theanti conformer. Theoverall barrier torotation in thiscompound is about 6kcal/mol, significantlylower than the dichloroanalog discussed above.A comparison of 1,2-difluoroethane and 1,2-dichloroethane is informative. The bond dipoles of C–F and C–Cl aresimilar, with the latter being slightly larger because of its longer bond. The hyperconjugative interactions thatstabilize the gauche rotamer are stronger for fluorine than for chlorine, due to the former's greater electronegativityrelative to hydrogen. Taken together, these differences shift the conformational equilibria of 1,2-dihaloethanes frompreferentially anti in the chloro, bromo and iodo compounds to gauche for the difluoride.

The case of 1-fluoropropane demonstrates another factor. The C2–C3 rotational barrier is about 1 kcal/mol lessthan that of butane, and the gauche conformer is slightly favored (∆Hº = 0.09 kcal/mol). A similar behavior isobserved for 1-chloropropane. Hyperconjugative stabilization is possible for all staggered conformations, and isprobably not a determining factor. When the strong C–F bond dipole rotates to approach the CH3–CH2 bond itinduces an opposite dipole in that bond. As drawn on the right above, this may actually result in an attractiveinteraction between the methyl and fluorine substituents (as well as methyl and chlorine in 1-chloropropane).

The concepts described above have been used to explain and predict many other cases of conformationalisomerism. Three such examples are shown below, all measured in the gas phase. The rotational barriers of thetwo 2,3-dichlorobutane diastereomers are significantly different, the meso isomer being ∆Hº = 12.5 kcal/mol, andthe racemic compound being ∆Hº = 9.5 kcal/mol.

Rotamer Barriers in Unsaturated Compounds

Alkenes

The rotation of a methyl group (or any alkyl group) relative to a double bond is an important conformational event.Propene provides a simple example, as shown below. In the course of the methyl rotation a C–H bond eclipses

either an sp2C–H bond or the C=CH2 bond, but not simultaneously. The energy of this rotation follows a simple


3 of 10 06/06/2012 4:31 PM

sine-curve similar to the rotational barrier of ethane, with a smaller amplitude (ca. 2 kcal/mol). The conformer inwhich the C=CH2 bond is eclipsed is the lower energy rotamer, possibly reflecting a stabilizing interaction between

σ-C–H bonds of methyl and the π*-antibonding orbital. This conformer will be termed eclipsed, and the higherenergy alternative bisected.

Adding a methyl group to propene, as in 1-butene, doubles the number of eclipsed and bisected conformers.Interestingly, the overall rotational barrier is reduced, and the energy difference between the two eclipsed rotamersis over twenty times greater than the difference between the two bisected forms. This is illustrated in the followingdiagram. The eclipsing shown by magenta colored arrows in conformer B2 is referred to as allylic 1,2-strain (A1,2-strain). If the vinyl hydrogen is replaced by a larger methyl group, as in 2-methyl-1-butene, this strain adds about1.3 kcal/mol to the rotational barrier.A larger substituent, such as tert-butyl in 3,3-dimethyl-1-butene, does not change significantly the stability order ofeclipsed relative to bisected rotamers.

Rotamers of 1-Butene

Double bond stereoisomers, such as (E) and (Z)-2-pentene have very different rotational energy barriers. TheE-isomer, shown below, has a rotational profile very similar to that of 1-butene (above). Since the C-1 methyl isdirected away from the ethyl group, this is not surprising.


4 of 10 06/06/2012 4:31 PM

Rotamers of (E)-2-Pentene

For the Z-isomer, the C-1 methyl is cis to the ethyl group, leading to severe steric crowding in the E1 and B1

conformers, shown again by colored arrows. This destabilizing hindrance is called allylic 1,3-strain (A 1,3-strain).Unexpectedly, A 1,3-strain renders eclipsed conformer E2 slightly higher in energy than bisected conformer B2. The

perpendicular conformation, P, is lower in energy than either E2 or B2.

Rotamers of (Z)-2-Pentene

To examine an energy plot of these rotamers click the following button:

Aldehydes and Ketones

The conformations and rotational energy profile of acetaldehyde, shown below, are very similar to propene. Theactivation energy for rotation is half that of propene, reflecting the absence of a hydrogen on the oxygen atom. Thisis the same kind of change noted earlier for ethane compared with methanol.

By comparison, the conformational energy profile of propanal differs significantly from that of 1-butene. As shownbelow, conformer E1 is more stable than E2 by roughly 0.8 kcal/mol, reflecting the relatively small size of oxygen.Even the larger substituent in 3-methylbutanal prefers this conformation. Furthermore, the activation energy for


5 of 10 06/06/2012 4:31 PM

interconversion of the chiral eclipsed E2 conformers is very small, and it is not possible to relate experimentalbehavior to a preferred conformer. Ketones, of course, will exhibit A-1,2-strain in conformer B2.

Rotamers of Propanal

Although the conformational differences shown here are not large, addition reactions to aldehydes and ketones aresensitive to the configuration of substituents on the α-carbon atom. Because of the importance of such asymmetricinduction in the stereoselective formation of new stereogenic centers, it has been extensively studied. A fulldiscussion of this topic is available in the stereoelectronic effects section of this text.

Esters and Amides

Esters and amides generally have higher rotational barriers and greater conformational preference than doaldehydes and ketones, with the trans or Z form being more stable, as illustrated in the following diagram. Forexample, the CH3O–CHO barrier in methyl formate is 12 to 13 kcal/mol (trans is more stable than cis by 4.75

kcal/mol), and the enthalpic preference for trans increases to ca. 8 kcal/mol in methyl acetate (R1 = R2 = CH3).These planar conformations are favored by a stabilizing n-π conjugation, as shown by the resonance formulas onthe right. This requires the delocalizing n-electron pair on the ether oxygen to occupy a p-orbital-like orientation

ortho to the plane of the carbonyl group. The remaining n-electron pair must then occupy a sp2-orbital, designatedby the cyan colored oval. In the trans-conformer this electron pair overlaps with the anti-bonding C–O σ- bond ofthe carbonyl group, as described elsewhere. Dipole repulsion also contributes to a destabilization of the cis form.

The contribution of n-π conjugation to the structure of amides is even greater than observed in esters, as indicatedby their lower C=O stretching frequency. Thus, the barrier to rotation about the NH2–CHO bond in formamide isover 18 kcal/mol, and the CH3NH–CHO bond in N-methylformamide is more than a kcal/mol greater. Such barrierspermit the observation of conformational isomers of amides by room temperature nmr. For simple amides likeN-methylacetamide and the formamides noted above, the trans conformer is more stable than cis by more than 1.5to 2.5 kcal/mol.


6 of 10 06/06/2012 4:31 PM

End of this supplementary topic

Six-Membered Rings

Cyclohexane Derivatives

Conformations of cyclohexane and its simple substituted derivatives have been described elsewhere in this text.The chair conformation is the favored configuration, and bulky substituents prefer to occupy an equatorial location.


7 of 10 06/06/2012 4:31 PM

The left hand structures and table in the following diagram summarize the free energy differences betweenequatorial and axial orientations of some simple groups. These energies are commonly reported as A values. Anaxial methyl group is hindered by two gauche butane interactions, each accounting for ca. 0.9 kcal/mol. Since anaxial ethyl group may rotate so that it appears no larger than a methyl to the remaining axial hydrogens on the sameside of the ring, its A value is the same as methyl. Larger alkyl groups have increased A values, commensurate withincreased crowding with the axial hydrogens. The trimethylsilyl group has a value half that of a tert-butyl group,

reflecting the longer bond length of C–Si. Click the button for a table of common A values.

The heterocyclic compounds on the right side of the diagram illustrate the decreased axial hinderance that resultsfrom the absence of nearby axial hydrogens. From the smaller but significant energy differences shown, it may beconcluded that the steric hindrance of non-bonding electron pairs on oxygen cannot be ignored. Other factors inthese cases are the shorter bond length and tighter C-O-C angle, which may act to increase hindrance, as shown bythe lower right example.

Cyclohexene Derivatives

Inserting a double bond into a cyclohexane ring (exo or endo) introduces distortion that influences structural andchemical behavior. In the case of an exocyclic double bond, an axial hydrogen is removed from one side of the

ring, and the increased bond angle at the sp2-carbon expands some of the remaining axial:axial distances. Theconformational preference for an equatorial methyl substituent in 3-methylcyclohexanone is thereby reduced, asshown on the left of the following diagram. However, the change in bond angle perturbs the strain free cyclohexaneconfiguration to such a degree that double bond addition reactions are exceptionally favored. Thus, the heat ofhydrogenation of methylenecyclohexane is roughly 1 kcal/mol greater than that of methylenecyclopentane ormethylenecycloheptane. Furthermore, the rate and equilibria constants for addition reactions of cyclohexanone aregreater than those for comparable reactions of similar ketones.


8 of 10 06/06/2012 4:31 PM

The endocyclic double bond in cyclohexene produces an even greater change in structure, asillustrated on the right side of the above diagram. The planar configuration of the double bondfavors a pair of half-chair conformations, having a 5.3 kcal/mol equilibrium barrier. A model ofa twist chair is displayed on the right. Twisting of the allylic carbons skews the orientation oftheir axial and equatorial bonds into pseudo axial or equatorial directions. These locations maybe identified in the model by clicking the last button.

Allylic strain exists when an equatorial allylic substituent is hindered by a substituent on thedouble bond. A good example of how this strain may influence the course of a reaction is foundin enamine formation from α-substituted cyclohexanones. The following equations are typical.Most significantly, the enamine double bond is generally formed away from the α-substitution,especially with pyrrolidine. The A 1,3-strain shown in the bracketed formulas is undoubtedlyresponsible for this regioselectivity. Second, when a reference point exists, the α-substituent isfound to be axial in the final product, reflecting A 1,2-strain.

Spacefill

Model Stick

Model Pseudo

Locations On/Off


9 of 10 06/06/2012 4:31 PM


10 of 10 06/06/2012 4:31 PM

rotation of alkane

Documents