Question

9.78  

The two lowest energy conformations of pentane are the anti-anti and the anti-gauche forms, in terms of arrangements around the two central C─C bonds. A recent study analyzed the conformations of 3-heptyne as an “elongated” analogue of pentane, where a carbon-carbon triple bond is “inserted” between C2 and C3 of pentane.[15] Interestingly, the researchers found that in each of the two most stable conformations of 3-heptyne, C1 and C6 are nearly eclipsed (looking down the alkyne group). In one of these conformations, C4 and C7 are anti to each other (looking down the C5─C6 bond); and in the other conformation, C4 and C7 experience a gauche interaction. Draw the following:

(a)  

A wedge-and-dash structure for each of the two lowest energy conformations of pentane.

(b)  

A wedge-and-dash structure of the conformer of 3-heptyne that is analogous to anti-anti pentane.

(c)  

A Newman projection that illustrates the eclipsed nature of the low-energy conformations of 3-heptyne.

(d)  

Newman projections that illustrate the difference between the two lowest energy conformations of 3-heptyne.

Answer 1

Н,сн,с, сH; н. сн, H CH2CH anti-gauche(pentane) a anti- anti (pentane)

н Н CH3 CH2CH3 b anti- anti (heptyne)

CHз 7 Н Н Нас 1 С 2 н Н т

сн, 7 H Hll 6 Н сн, 7 нс 1 Нас 1 d H HT н C4-C7 gauche to each other C4-C7 anti to each other I we ern