What is the most stable conformer for 3-methylpentane, viewed along the #C_2-C_3# bond using Sawhorse projections?

1 Answer
Jan 27, 2015

You can find the general procedure for drawing sawhorse projections at this location.

The condensed structural formula of 3-methylpentane is CH₃CH₂CH(CH₃)CH₂CH₃.

Its bond-line structure is

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Step 1. Convert the bond-line structure to a wedge-dash structure at C-2 and C-3.
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To convert the wedge-dash structure to a sawhorse projection, we must look at the molecule along the C2-C3 bond (from the lower right).

The three bonds nearest us form an inverted "Y".

We draw a sawhorse template with the C2-C3 bond going from lower left to upper right.

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Next we add the atoms.

The groups on C-2 are CH₃, H, and H.
The groups on C-3 are CH₃CH₂, CH₃, and H.

On C-2, we put the bulky CH₃ group at the top of the inverted "Y", with the H atoms on the other two bonds.

On C-3, we put the bulky CH₃CH₂ group at the bottom of the "Y". The CH₃ goes on the left, and the H atom on the right.

The staggered sawhorse structure of 3-methylpentane is then

enter image source here

This is the most stable conformer because it puts the bulky CH₃ group on C-2 anti to the CH₃CH₂ group on C-3.