[Rutherford]

Is there a method to determine if these compounds are enantiomers or not? I had a hard time rotating those molecules in my head.

[sjb]

Is there a method to determine if these compounds are enantiomers or not? I had a hard time rotating those molecules in my head.

Make models? Try naming them?

[Rutherford]

I tried but I don't know how to rotate the molecules that have the H atom on the top. They should be on the back because of the lowest priority.

[fledarmus]

Naming them requires that you mentally rotate them so that the hydrogen is in the back.

Make models. Make models. Make more models.

[Rutherford]

I improvised somehow, and only practice is needed now. Solved, A)same compound (R,R); B)enantiomers.

[discodermolide]

I improvised somehow, and only practice is needed now. Solved, A)same compound (R,R); B)enantiomers.

Have you got a models set? You can get them cheap.
Or use cocktail sticks and olives, cherries etc. Then you can break the bonds by eating the atoms!

[fledarmus]

Or gumdrops and toothpicks, or my personal favorite, those mini colored marshmallows 

Do they still have those? Either gumdrops or mini colored marshmallows, I mean... 

[Rutherford]

I improvised somehow, and only practice is needed now. Solved, A)same compound (R,R); B)enantiomers.

Have you got a models set? You can get them cheap.
Or use cocktail sticks and olives, cherries etc. Then you can break the bonds by eating the atoms!

Sticks and plasticine  .

[discodermolide]

I improvised somehow, and only practice is needed now. Solved, A)same compound (R,R); B)enantiomers.

Have you got a models set? You can get them cheap.
Or use cocktail sticks and olives, cherries etc. Then you can break the bonds by eating the atoms!

Sticks and plasticine  .

Prefer cocktail sticks etc.
At least you can eat your mistakes and nobody will ever know.

[Rutherford]

Now a tougher problem: http://orgchem.chem.uconn.edu/courses/243f97-rs-pract-04.html
Why is in (i) the C atom that has an OH group attached R? The H atom is towards me, the OH group is away, if I exchange those two shouldn't I get the S configuration?

[discodermolide]

Now a tougher problem: http://orgchem.chem.uconn.edu/courses/243f97-rs-pract-04.html
Why is in (i) the C atom that has an OH group attached R? The H atom is towards me, the OH group is away, if I exchange those two shouldn't I get the S configuration?

Yes you get the (S) configuration.

[Rutherford]

Okay, then it is a mistake. Thanks.

[Babcock_Hall]

One way to "rotate" a molecule on paper is to hold one bond constant (making it the axis of rotation), and spinning the other three groups as if they were a propeller.  This way you can always put the H in the back.  I like models very much, but I have sometimes found this trick to be handy.

[james_a]

The fastest way is this:

After double checking to see that they have the same connectivity, determine (R) and (S) for each stereocenter. Enantiomers will always have opposite (R)/(S) values.

This is far, far simpler and faster than making models or doing bond rotations.

[Babcock_Hall]

The purpose of building models or of rotating them on paper is to facilitate the R,S assigment process.