Organic chem question

[drsoni]

When we draw chair structures and Newman projections, do we look at the size of the atoms in the molecule or the number of atoms in the molecule to decide their position? For instance, is ch3-ch3 staggered better than Br-Ch3 staggered, or vice versa?

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[averageasian]

well both come into consideration to calculate A values. what youre saying about size/number of atoms is not always true. for instance, a nitrile is two atoms but actually has a lower A-value than a halide such as chlorine. this is because the CN bond is linear (little steric strain on ring) and cl is a little bigger than carbon. Having the more electronegative, larger cl in the equatorial position is better for everyones sake.

For newman projections, you want the largest groups with the highest A values furthest away from the next biggest group, period

 

[DAT Destroyer]

When we draw chair structures and Newman projections, do we look at the size of the atoms in the molecule or the number of atoms in the molecule to decide their position? For instance, is ch3-ch3 staggered better than Br-Ch3 staggered, or vice versa?

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The Newman conformer that is more stable is clearly the anti in most cases. If gauche, the methyl group occupies more space than a Br or I......thus these groups MUST be further apart. Quantum Mechanic calculations clearly show it is more stable and lower energy to place the methyl groups further apart than a Methyl - Bromine when in the staggered conformation.

Hope this helps.

Dr. Romano

 

[drsoni]

The Newman conformer that is more stable is clearly the anti in most cases. If gauche, the methyl group occupies more space than a Br or I......thus these groups MUST be further apart. Quantum Mechanic calculations clearly show it is more stable and lower energy to place the methyl groups further apart than a Methyl - Bromine when in the staggered conformation.

Hope this helps.

Dr. Romano

Thank you! That makes sense! However, if we compare this to cis-trans isomers, why does the halogen get priority over methyl group in cis-trans group?

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