Conformations of Propane

Of the three conformations of propane shown which one is the most stable" Which one is the least stable" Why" ... 

Sketch a potential energy diagram for rotation around a carbon-carbon bond in propane. Clearly identify each potential energy maximum and minimum with a structural formula that shows the conformation of propane at that point. Does your diagram more closely resemble that of ethane or of butane Would you expect the activation energy for bond rotation in propane to be more than or less than that of ethane Of butane ... 

The eclipsed conformer of propane has three interactions—two ethane-type hvdroeen-hvdroeen interactions and one additional hvdrogen-methyl interac-tion. Since each eclipsing H<—interaction is the same as that in ethane and thus has an energy "cost" of 4.0 kj/mol, we can assign a value of 14 - (2 X 4.0) = 6.0 kj/mol (1.4 kcal/mol.) to the eclipsing H<— CH3 interaction (Figure 3.8). 

Figure 22.8. Conrotatory ring opening of cyclopropane (7) to what Hoffmann called the (0,0) conformation of propane-1,3-diyl (8). The in-phase combination of Ip-n AOs in the LUMO is destabilized by an antibonding interaction with the re combination of C—H bonding orbitals at C2. A lower energy MO, which is not shown, is stabihzed by h3fperconjugative electron donation that is, the bonding version of this interaction. The out-of-phase combination of 2p-n AOs in the HOMO has a node at C2 hence, it does not mix with the C—H orbitals at this carbon.

In the eclipsed conformation of propane, we now have a larger CH3 close to H atom. This results in increased repulsive force or increased steric strain. The energy difference between the eclipsed and staggered forms of propane is greater than that of ethane. 

All the staggered conformations of propane are equivalent to one another, and all its eclipsed conformations are equivalent to one another. The energy diagram resembles that of ethane in that it is a symmetrical one. 

Consider now three conformations of propanal, 3 and 4, where the carbonyl eclipses the CMe and CH bonds, respectively, and 5, which has the CMe and CO bonds trans to each other. Experiments show that 3 is the preferred configuration.4... 

Draw a plot of energy versus dihedral angle for the conformations of propane about one of the C—C bonds. 

As in the case of ethane, two extreme conformations of propane are possible—in one the C-H and C—C bonds are staggered in the other they are eclipsed. 

Conformation and Conformational Analysis Conformation of Ethane Conformation of Propane Conformation of Butane Eclipsed and Staggered Eorms Ring Strains in Cycloalkanes Principles of Conformation 165... 

Follow this procedure for any C-C bond. With a Newman projection, always consider one C-C bond only and draw the atoms bonded to the carbon atoms, not the carbon atoms in the bond itself. Newman projections for the staggered and eclipsed conformations of propane are drawn in Figure 4.7. 

Newman projections for the staggered and eclipsed conformations of propane... 

In a Newman projection it doesn t matter which C you pick to be in the front or the back. All of the Newman projections shown here represent the staggered conformation of propane. 

The CH3-H eclipsed interaction imposes 1.4 kcal/mol of strain on top of the 2.0 kcal/mol H-H torsional strains in the eclipsed conformation of propane. The 0.4 kcal/mol of additional strain is referred to as steric strain, the repulsion between nonbonded atoms or groups. 

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