Gauche or Anti?!

[PooyaH]

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Which one is more stable? I have two different sources in front of me that say the exact opposite things.

Thank you,

P

 

[flin5845]

Which one is more stable? I have two different sources in front of me that say the exact opposite things.

Thank you,

P

you want the biggest groups the furthest apart....This would be Anti.

 

[PooyaH]

you want the biggest groups the furthest apart....This would be Anti.

That's my understanding too. But there's a problem in Destroyer Odyssey that has the Newman structure of a 1,2 ethanediol and says that normally it would be the anti but in this case it is the gauche that is the most stable! Maybe it's just this particular example in which I don't know the reason why.

 

[flin5845]

I think it's due to the abilities of each OH group to hydrogen bond with each other

 

[yappy]

Correct - in that question the gauche was more stable because INTRAmolecular H bonding occuring between the alcohol groups.

I hope there are not questions like that on the DAT because it seems like such a judgement call. I have not read any text that even teaches to look for intramolecular h bonding when evaluating stability of ecliped, anti, or gauche.

I mean really... how can we qualitatively know that.

Steric hinderence + H bonding = less energy than zero steric hinderence? lol.

I think it's due to the abilities of each OH group to hydrogen bond with each other

 

[PooyaH]

I think it's due to the abilities of each OH group to hydrogen bond with each other

Uhuh, yeah in that case it makes sense. Thanks.

 

[Moka11]

Why is a hydroxyl group smaller than a methyl group for Newman projection when an O is larger than a C?

 

[Slumber]

Why is a hydroxyl group smaller than a methyl group for Newman projection when an O is larger than a C?

I think it's because methyl has 4 atoms whereas a hydroxyl group only has 2.

 

[Moka11]

I think it's because methyl has 4 atoms whereas a hydroxyl group only has 2.

Thanks. So whats the rule of thumb? Amount of bonds?

Because if not O-H still has a larger MW (17) than CH3 (15)

 

[KyoPhan]

Why is a hydroxyl group smaller than a methyl group for Newman projection when an O is larger than a C?

O isn't larger than C. Going to the right of peroidic table means the atom gets smaller.

Just did random google search to try to find a rule in finding steric strain. Couldn't really find a rule. But this is from www.people.vcu.edu/~s2cmring/Chapter 3.doc

These are for a ring, but the rule is pretty much the same; you want the largest group furthest apart.
"If the ring is substituted, you want to get the largest substituent in the equatorial position. This minimizes steric strain. The largest substituent is not defined by molecular weight but by physical size
Ex. CH3 is bigger than Br, even though it weighs less"

I remember chad mentioning in his video that CH3 is bigger than Br/Cl/OH.

Also here's his response to Br vs CH3 in a ring.
"The methyl group it turns out does indeed have the priority here and should get to be equatorial before the Br. There's no intuitive way to know this; it's just something we determined empirically and now you have to remember it."

 

[Moka11]

Ok thanks a lot. Guess Ill just commit that to memory.

 

[joemario]

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