steric hindrance problem on achiever

[gomawum]

one of question in achiever test 1 in ochem part was

which of conformation of cyclohexane with 3 methyl group (1,2,4) attached are stable.

and answer was the three methyl group all on axial position
but if 1and 2 are both equitorial position, isn't the steric hindrance greater than 1 being equitorial and 2 being axial?

because if 2 is in axial the only steric hindrance is from H from 3 which is better than CH3 from 1 if 2 is in equitorial position..




to me C is less sterically hindered than d but answer was d..
whyyyy..

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[TheWiredNerv]

equatorial positions are always more stable. Think about the structure in 3-d rather than 2-d.

 

[PreDent2009]

The only things that cause steric hinderance in cyclohexanes is 1,3 diaxial repulsion, and gauce pair repulsion. All substituents have 1,3 diaxial repulsion if in axial positions. There are 3 axial positions up, 3 down. In choice C, there is 2X CH3-H 1,3 Diaxial repsulsion because there are 2 other H atoms in the up-axial positions messing with that methly group. In choice D, there is no diaxial repulsion at all. This is why D is the more stable configuration.

 

[Orgodox]

and answer was the three methyl group all on axial position

You mean equatorial right?

It is deceiving when drawn in 2-d if you build a model you will see that they are not as close as they look. Just remember that equatorial is always better than axial (unless you have ability for H bonding, then axial is better because internal H bonding is stabilizing)

 

[CookieDough]

I can see how this question can be tricky, but this may make it a bit easier for you;
1) As stated by all before me, equatorial positions are more stable than axial positions
2) A good way to look at a chair conformation is to envision the substituent groups that are pointing "up" (axial or equatorial) to be a WEDGE in a 3D configuration, and the substituents that are pointing "down" on the chair to be a DASH in 3-D. In this way, you'll see that the methyls that appear very close to one another in the chair conformation are actually about 180* apart in reality.

Hope this helps.

 

[klutzy1987]

In choice D all three of the methyl groups are in the equatorial positiion therefor pointing away from the ring and away from any steric hinderance, however in choice C there is one methyl in equatorial.