A few orgo questions...

[192LT192]

---

Members do not see this ad.

1. What's the difference between torsional and steric strain?

2. I always get mixed up with the different types of isomers: conformational, constitutional.. i know there are others. i never know which kind it is. can someone please give an explanation of each? 🙂

thanks!

 

[sixkiller]

1. What's the difference between torsional and steric strain?

2. I always get mixed up with the different types of isomers: conformational, constitutional.. i know there are others. i never know which kind it is. can someone please give an explanation of each? 🙂

thanks!

Steric strain arises from electron cloud repulsions....ie two groups are too close to one another to occupy the same space.

Torsional strain is due to electron pair repulsion between the bonding pairs (between ECLIPSED bonds). Hence torsional strain is also called ECLIPSING STRAIN.

*Keep in mind that eclipsed conformations will have both torsional and steric strain* Draw the conformation of butane with the greatest potential energy and you'll see.

Constitutional isomers are also called structural isomers; same molecular formula different connectivity.

Stereoisomers have the same connectivity but different spatial orientation.

Stereoisomers are either:

1) enantiomers (non super impossable mirror images)

or

2) Diastereomers = (geometric isomers)
a) cis/trans
b) configurational diastereomers (R,S vs S,R)
c) E/Z (alkenes)

* The simple trick with STEREOISOMERS is this...."are the two non-superimpossable mirror images of one another?" If no, then they are geometric isomers or diastereomers.* Of course, this is after you've eliminated conformational and structural isomers.

 

[192LT192]

Constitutional isomers are also called structural isomers; same molecular formula different connectivity.

Stereoisomers have the same connectivity but different spatial orientation.

Stereoisomers are either:

1) enantiomers (non super impossable mirror images)

or

2) Diastereomers = (geometric isomers)
a) cis/trans
b) configurational diastereomers (R,S vs S,R)
c) E/Z (alkenes)

ok thanks a lot! you mentioned constitutional, configurational but what about conformational? i dont know where that fits in. is that just another name for a type youve already mentioned? oh boy these names are getting me all confused. 🙁

 

[UCB05]

conformational isomerism happens around a single bond that's free enough to rotate. If you look directly down the single C2-C3 bond of butane, you can get the eclipsed, gauche, and anti conformational isomers, based on how the C1 and C4 bonds sit.

 

[loveoforganic]

only a part of torsional strain is BE pair repulsion. It's a lot more complicated than that, but it's a good way to think about it