Eclipsing conformation

A stretch-torsion cross term can be used to model the stretching of a bond that occurs in ai eclipsed conformation. Two possible functional forms are ... 

Ethane is the simplest hydrocarbon that can have distinct conformations Two the stag gered conformation and the eclipsed conformation, deserve special mention and are illustrated with molecular models m Figure 3 1... 

In the eclipsed conformation each C—H bond of one carbon is aligned with a C—H bond of the other carbon... 

FIGURE 3 3 Some commonly used draw mgs of the eclipsed conformation of ethane... 

Of the two conformations of ethane the staggered is 12 kJImol (2 9 heal mol) more stable than the eclipsed The staggered conformation is the most stable conformation the eclipsed is the least stable conformation Two main explanations have been offered for the difference in stability between the two conformations One explanation holds that repulsions between bonds on adjacent atoms destabilize the eclipsed conformation The other suggests that better electron delocalization stabilizes the staggered conformation The latter of these two explanations is now believed to be the correct one... 

At any instant almost all of the molecules are in staggered conformations hardly any are in eclipsed conformations... 

At Its most basic level separating the total strain of a structure into its components is a qualita tive exercise For example a computer drawn model of the eclipsed conformation of butane using ideal bond angles and bond distances (Figure 3 8) reveals that two pairs of hydrogens are separated by a distance of only 175 pm a value considerably smaller than the sum of their van der Waals radii (2 X 120 pm = 240 pm) Thus this conformation is destabilized not only by the torsional strain associ ated with its eclipsed bonds but also by van der Waals strain... 

FIGURE 3 8 Ball and spoke and space filling models of methyl-methyl eclipsed conformation of butane... 

Eclipsed conformation of ethane (least stable conformation)... 

The difference m energy between the staggered and eclipsed forms is due almost entirely to the torsional strain m the eclipsed conformation At any instant almost all the molecules of ethane reside m the staggered con formation... 

FIGURE 7 9 Repre sentations of (2/ 3R) dihy droxybutanoic acid (a) The staggered conformation is the most stable but is not properly arranged to show stereochemistry as a Fischer projection (b) Rotation about the C 2-C 3 bond gives the eclipsed conforma tion and projection of the eclipsed conformation onto the page gives (c) a correct Fischer projection... 

One way to demonstrate that meso 2 3 butanediol is achiral is to recognize that its eclipsed conformation has a plane of symmetry that passes through and is perpendicular to the C 2-C 3 bond as illustrated m Eigure 7 11a The anti conformation is achiral as... 

In the same way that a Fis cher formula is a projection of the eclipsed conformation onto the page the line drawn through its center is a projection of the plane of symmetry that is present in the eclipsed conformation of meso 2 3 butanediol... 

The aldotetroses are the four stereoisomers of 2 3 4 trihydroxybutanal Fischer pro jections are constructed by orienting the molecule m an eclipsed conformation with the aldehyde group at the top The four carbon atoms define the mam chain of the Fischer projection and are arranged vertically Horizontal bonds are directed outward vertical bonds back... 

Eclipsed conformation of D erythrose showing C 4 hydroxyl group in position to add to carbonyl group... 

Notice that the eclipsed conformation of d ribose derived directly from the Fischer pro jection does not have its C 4 hydroxyl group properly oriented for furanose ring forma tion We must redraw it m a conformation that permits the five membered cyclic hemi acetal to form This is accomplished by rotation about the C(3)—C(4) bond taking care that the configuration at C 4 is not changed... 

Red circles gauche 60° and 300° Red circles anti 180° Gauche and anti relationships occur only in staggered conformations therefore ignore the eclipsed conformations (0° 120° 240° 360°)... 

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... 

Example Ethane is stable in the staggered (gauche) conformation. The transition state for rotating a methyl group in ethane has the eclipsed conformation. Ageometry optimization starting from an eclipsed conformation yields the transition state. 

Acyclic Compounds. Different conformations of acyclic compounds are best viewed by construction of ball-and-stick molecules or by use of Newman projections (see Fig. 1.2). Both types of representations are shown for ethane. Atoms or groups that are attached at opposite ends of a single bond should be viewed along the bond axis. If two atoms or groups attached at opposite ends of the bond appear one directly behind the other, these atoms or groups are described as eclipsed. That portion of the molecule is described as being in the eclipsed conformation. If not eclipsed, the atoms... 

The calculations of the optimum geometry show a slight lenghthening of the C—H bonds because of the electron release to the tc system. These calculations also reveal a barrier to rotation of the methyl group of about 1.5-2.0 kcal/mol. Interaction between the hydrogens and the n system favors the eclipsed conformation to fliis extent. Let us examine the... 

One of the frmdamental structural facets of organic chemistry, which has been explained most satisfactorily in MO terms, is the existence of a small barrier to rotation about single bonds. In ethane, for example, it is known that the staggered conformation is about 3kcal/mol more stable than the ecl sed conformation so that the eclipsed conformation represents a transition state for transformation of one staggered conformation into another by rotation. 

Ab initio MO calculations using 4-3IG orbitals indicate that the eclipsed conformation of acetaldehyde is more stable than the staggered conformation. 

Plane of symmetry in eclipsed conformation of meso-tartaric acid... 

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