Q 3-3-52 E - Organic Chemistry (Mcmurry)

Question

Increased substitution around a bond leads to increased strain. Take the four substituted butanes listed below, for example. For each compound, sight along the C2–C3 bond and draw Newman projections of the most stable and least stable conformations. Use the data in Table 3-5 to assign strain-energy values to each conformation. Which of the eight conformations is most strained? Which is least strained? 

(a) 2-Methylbutane 

(b) 2,2-Dimethylbutane 

(c) 2,3-Dimethylbutane 

(d) 2,2,3-Trimethylbutane

Step-by-Step Solution

Answer

a) 

 

       Most stable

        less stable

b) 

           Most stable

           Less stable

c)

            Most stable

           Less stable

d) 

           Most stable

          Less stable

Newman projections

It shows the conformation of the molecules from front to back in which the front atom is represented by dot and the back atom is represented by the circle.

Newman projections of 2-methyl butane

The two projections are the anti and gauche the anti is more stable than the gauche conformation because in anti the methyl substituent is present anti(opposite) to each other with no steric hindrance and in gauche conformation the methyl substituent is present on the same position that the steric hindrance is very high makes the conformation less stable as shown below:

             Most stable

                Less stable 

Newman projections of 2,2-dimethyl butane

The two projections are the anti and gauche the anti is more stable than the gauche conformation because in anti the methyl substituent is present anti(opposite) to each other with no steric hindrance and in gauche conformation the methyl substituent is present in the same position that the steric hindrance is very high makes the conformation less stable as shown below:

           Most stable

                  Less stable

Newman projections of 2,3-dimethyl butane

The two projections are the anti and gauche the anti is more stable than the gauche conformation because in anti the methyl substituent is present anti(opposite) to each other with no steric hindrance and in gauche conformation the methyl substituent is present on the same position that the steric hindrance is very high makes the conformation less stable as shown below:

            Most stable

             Less stable

Newman projections of 2,2,3-trimethyl butane

The two projections are the anti and gauche the anti is more stable than the gauche conformation because in anti the methyl substituent is present anti(opposite) to each other with no steric hindrance and in gauche conformation the methyl substituent is present on the same position and next to each other that the steric hindrance is very high makes the conformation less stable as shown below:

               Most stable

           less stable