Conformation, staggered

Therefore, if you are confronted with a particular sawhorse or Newman formula and you have to decide whether it represents a meso compound, the best procedure is to make a ball-and-stick model of the conformation and then rotate around the bonds to see if it can be brought into a conformation (staggered or eclipsed) that has a plane of symmetry (such as 27) or is identical with its mirror image. 

Know the meaning of conformation, staggered, eclipsed, dash-wedge projection, Newman projection, sawhorse projection, rotational isomers, rotamers. 

If the coordinate system of propane, with all partial conformations staggered (point group C2v), has been oriented with the x axis perpendicular to the plane containing the three carbon atoms and to the 2(2) axis, and if the numbering of the orbitals refers to the valence shell only, then the band assignments proposed by different authors compare as in diagram 67. Note that Kimura and his coworkers proposed two assignments ... 

The conformation about the C—C bond of ethane is determined by the three equivalent hydrogens as substituents on each of the carbon atoms. Thus, there are three equal energy maxima and three equal energy minima but only two different possible conformations staggered and eclipsed. 

The origin of a torsional barrier can be studied best in simple cases like ethane. Here, rotation about the central carbon-carbon bond results in three staggered and three eclipsed stationary points on the potential energy surface, at least when symmetry considerations are not taken into account. Quantum mechanically, the barrier of rotation is explained by anti-bonding interactions between the hydrogens attached to different carbon atoms. These interactions are small when the conformation of ethane is staggered, and reach a maximum value when the molecule approaches an eclipsed geometry. 

In the staggered conformation each C—H bond of one carbon bisects an H—C—H angle of the other carbon... 

FIGURE 3 2 Some commonly used draw mgs of the staggered conformation of ethane... 

Eclipsed bonds are characterized by a torsion angle of 0° When the torsion angle is approximately 60° we say that the spatial relationship is gauche, and when it is 180° we say that it is anti Staggered conformations have only gauche or anti relationships between bonds on adjacent atoms... 

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... 

Conformations m which the torsion angles between adjacent bonds are other than 60° are said to have torsional strain Eclipsed bonds produce the most torsional strain staggered bonds none Because three pairs of eclipsed bonds are responsible for 12 kJ/mol (2 9 kcal/mol) of torsional strain m ethane it is reasonable to assign an energy cost of 4 kJ/mol (1 kcal/mol) to each pair In this chapter we 11 learn of additional sources of strain m molecules which together with torsional strain comprise steric strain... 

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

Rotation around carbon-carbon bonds is one of the fastest processes m chemistry Among the ways that we can describe the rate of a process is by its half life which is the length of time it takes for one half of the molecules to react It takes less than 10 s for half of the molecules m a sample of ethane to go from one staggered conformation to another at 25°C... 

Figure 3 7 illustrates the potential energy relationships among the various confer matrons of butane The staggered conformations are more stable than the eclipsed At any instant almost all the molecules exist m staggered conformations and more are present m the anti conformation than m the gauche The point of maximum potential... 

Staggered arrangement of bonds in chair conformation of cyclohexane... 

It IS no accident that sections of our chair cyclohexane drawings resemble saw horse projections of staggered conformations of alkanes The same spatial relationships seen m alkanes carry over to substituents on a six membered ring In the structure... 

Section 3 1 The most stable conformation of ethane is the staggered conformation It IS approximately 12 kJ/mol (3 kcal/mol) more stable than the eclipsed, which IS the least stable conformation... 

Staggered conformation of ethane (most 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... 

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