DAT Gen Chem--Gauche Interactions

[Dr.BabyKarott]

Hi all,

I have a question that I cannot find the clear answer to anywhere. For gauche interactions on Newman projections, how do we decide which gauche interaction is more stable? I came across a problem in destroyer that asks you to select the most stable Newman projection, and one structure had a gauche between CH3 and Br, and another had a gauche interaction between Br and Br. The Br-Br was most stable... My question is, how do we know that? If we have gauche interactions, we want them to be between the LEAST bulky groups? Or is my thinking wrong? Br is less bulky than CH3, that's why I think that.

Thank you in advance! (Sorry if this thread already exists somewhere--couldn't find it!)

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[8_man]

That's my guess as well.

Here you go
https://www.google.com/amp/s/forums...ulkiness-of-br-vs-ch3.1029944/?amp=1403728759

 

[socal6]

You're right, Br is less bulky than CH3. Even though it weighs more than CH3, Br does not have the 3 hydrogens bonded to it so it's not as sterically hindered. You want to minimize interactions between groups, so that's why you would choose gauche Br-Br over gauche Br-CH3. Hope this helps!

 

[schmoob]

Moving to DAT.

 

[DAT Destroyer]

Hi all,

I have a question that I cannot find the clear answer to anywhere. For gauche interactions on Newman projections, how do we decide which gauche interaction is more stable? I came across a problem in destroyer that asks you to select the most stable Newman projection, and one structure had a gauche between CH3 and Br, and another had a gauche interaction between Br and Br. The Br-Br was most stable... My question is, how do we know that? If we have gauche interactions, we want them to be between the LEAST bulky groups? Or is my thinking wrong? Br is less bulky than CH3, that's why I think that.

Thank you in advance! (Sorry if this thread already exists somewhere--couldn't find it!)

When deciding which gauche is best, the main criteria is steric ! A bromine is a single atom.....a methyl group is 4 !!! Consequently, the methyl groups would display a greater destabilization if they were close. In Advanced Organic Chemistry, we often use what is called " A " values, which guide us in deciding what groups need to be placed further apart. Let us do a quick problem that I show my students and nail 99% of them . OK...are you ready ? Which needs to be placed equatorially preferentially, a Br or CN ? Think about it......Br is one atom CN is two atoms......BUT........the CN has a triple bond, hence the overall steric bulk is diminished greatly !!! Thus....Br is placed equatorially in preference of the small CN !!!!!!

Hope this helps, keep up the great work.

Dr. Romano

 

[SmileItsLife]

When deciding which gauche is best, the main criteria is steric ! A bromine is a single atom.....a methyl group is 4 !!! Consequently, the methyl groups would display a greater destabilization if they were close. In Advanced Organic Chemistry, we often use what is called " A " values, which guide us in deciding what groups need to be placed further apart. Let us do a quick problem that I show my students and nail 99% of them . OK...are you ready ? Which needs to be placed equatorially preferentially, a Br or CN ? Think about it......Br is one atom CN is two atoms......BUT........the CN has a triple bond, hence the overall steric bulk is diminished greatly !!! Thus....Br is placed equatorially in preference of the small CN !!!!!!

Hope this helps, keep up the great work.

Dr. Romano

How sad that I understood how to do OP's question, but got stuck on CN v. Br... I need to study more.