Question

Which of the following Newman projections that represent the most stable conformation of 2,3-dimethylbutane.

Which of the following Newman projections that represent the most stable conformation of 2,3-dimethylbutane.

Answer 1

Which of the following Newman projections that represent the most stable conformation of 2,3-dimethylbutane.

Recall that these are CONFORMERS, meaning that they can take different positions in order to favor the most stable position.

Analysis will be done on carbns --> C2-C3 respectively

C2 --> front carbon, bonded species --> to H, CH3, CH3

C3 --> back carbon, bonded species --> H, CH3, H

Note that

there are two main types of position; they are either:

0° 60° 60° 120° 120° 60° 60° 60° 60° 120° 0° 0° STAGGERED ECLIPSED

- STAGGERED --> will have 60° separation between all groups, therefore, it is much more stable, the groups are the farthest away possible from all

- ECLIPSED --> Half of the Groups are 0° away from each other, the other half is at 120°.. This requires higher energy since it takes more energy to get together at the 0° configuration.

Also...

keep in mind that bulky groups... Bulky groups are those groups which are "larger" considered to other groups

The least bulky group/species is -H atom... since it is small.

Note that there are also energy considerations depending on the type of configuration:

Bulky - Bulky --> most energy

Bulky - nonbulky --> medium energy requirement

nonbulky - nonbulky --> least energy

Now, knowing this....

Number your species from top to bottom as ;

A,B,C,D,E

ignore all eclipsed configurations, that is, A and E

B,C,D are left

2,3-dimethylbutane. --> total carbons = 2+4 = 6

recall that 2 carbons are not shown explicitly in the newman projeciton, so we must see 6-2 = 4 carbons in the substitents

ignore B since it has only 3

Only C and D are left

note that

D has all carbons on the 50% side of the molecule ( equivalent of Southest side)

therefore, this is loaded to this side, implies high energy

this is not recommended.

The most stable configuration has 50% - 50% in each load

therefore

C is the best configuration