Questions on steric and gauche interactions

[rooksai]

Questions about steric strain and gauche interactions?
There are two clarifications I need: 1. In a Newman projection, can steric strain happen in a gauche conformation, where molecular groups don't involve twin methyls? (I know that methyls in a gauche will be a 3.8 kJ steric strain, I'm asking about like CH3-Cl or like F-F, or like F-CH3). 2. In a model of a di-substituted cyclohexane, can gauche interactions involve any other groups besides methyl? (Ex like above)

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[ilovemcat]

Questions about steric strain and gauche interactions?
There are two clarifications I need: 1. In a Newman projection, can steric strain happen in a gauche conformation, where molecular groups don't involve twin methyls? (I know that methyls in a gauche will be a 3.8 kJ steric strain, I'm asking about like CH3-Cl or like F-F, or like F-CH3). 2. In a model of a di-substituted cyclohexane, can gauche interactions involve any other groups besides methyl? (Ex like above)

Yes, for the purposes of the MCAT any non-hydrogen molecule is large enough to cause steric strain, including the examples you gave. This isn't always a bad thing though -- usually it is, but not always. One exception I could think of is if the gauche interaction consisted of two -OH groups (or any two atoms capable of hydrogen bonding). This would actually be a very favorable interaction since the molecule is capable of intramolecular hydrogen bonding (resulting in a very stable structure that dominates over any steric hindrance due to each -OH).

 

[rooksai]

Thanks! And another question, for the exception, is that covered in Organic Chem 1 O_O?

 

[ilovemcat]

Thanks! And another question, for the exception, is that covered in Organic Chem 1 O_O?

It was in my class, that's why I remembered it. I wouldn't have known otherwise. I also seen it in some of the practice materials I used for the MCAT, so it's useful to know.

 

[ilovemcat]

Posting again because I'm superstitious about the triple 6 post count 😀

 

[rooksai]

Haha, i was about to make a reply to that 😛