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Most stable conformation, BR vs Methyl

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somuchwater

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This is referring to TBR , Orgo Section IV, Passage III, Q 16. If you don't have it i'll break it down.

We have 2 bromobutane. What is the most stable conformation C2 and C3? The book says it is when the methyl and ethyl are anti. I figured it'd be bromo and ethyl anti to each other because isn't the bromo more bulky (mass = 80 g) vs methyl?
 

bestinthewest

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the user above nailed it. dont succumb to the pitfall that this question should be answered from a quantitative perspective. its all about the steric hindrance created by those bulky hydrogens. Bromine is heavier but thats irrelevant since it doesn't take up space the same way branching H's do. Draw the newman projection and dont abbreviate the atoms. Draw everything and youll see the hydrogens come awfully close to touching one another.
 
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