Conformational v Configurational isomers

[sc4s2cg]

I don't know what it is, but apparently I've forgotten the super basic stuff from 4 years ago.

Can someone explain to me why enantiomers can't be considered conformational isomers? I understand the definitions (conformational isomers can be turned into one another by rotating around a sigma bond without breaking the bond; configurational isomers need to break a bond to make them look alike), but when I look at examples like:

I mean, you can definitely rotate the top half of #2 to the right and get it to look exactly like number one. So why are they configurational instead of conformational isomers?

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[chemist16]

The very definition of an enantiomer is that you can't rotate it to get the other one. Like how you can't rotate your right hand around to get your left hand. Can you clarify what you mean by #2 here?

 

[chemist16]

Also, part of the problem is that the way the molecules are drawn is not representative of how they actually are. The bond angles should be closer to 109 degrees, and I suspect that if you visualize that, you'll understand why you can't rotate the molecules in the parentheses to get the first molecule. Try drawing the Newman projections and rotating that.

 

[sc4s2cg]

Ah, sorry about the confusion.

I was saying if I took the circled portion below, and flip it, it will look just like the compound on the left.

 

[chemist16]

Ah. Then that's because of the flattening out of the molecule from how it's drawn. Actually, the methyl is coming out of the plane of the screen and the hydrogen is going behind it. It's really hard to explain in words but try drawing the Newman projection to visualize it. If you have a stereocenter and switch the positions of any two groups, you get the enantiomer. That's the simple rule. If you have to visualize it, then you should draw it using dashes and wedges rather than the Fischer projection, since a 2D representative of a 3D molecule tends to distort things.

 

[chemtopper]

Ah, sorry about the confusion.

I was saying if I took the circled portion below, and flip it, it will look just like the compound on the left.

You can rotate atoms clockwise or anticlockwise ..

 

[tommyngu]

It might be easier to convert your Fischer projection to a sawhorse projection. You'll easily be able to see if you rotate the molecule between C2 and C3 to get the other conformational isomers of the given molecule that they won't be the same molecule.

 

[RH8448]

It makes sense if you actually make a model, use Newman projections, or really understand ficher projections. The best way to interpret fisher projection is to imagine the type of hug you give someone like your grandma. You give them a hug with your butt out and head not too close. The ficher projection has its arms pointing towards you like it wants to hug, however, the top and bottom groups are pointing away so that the hug is not too intimate.

 

[PharmHoppe]

For conformational isomers, you can easily rotate them to superimpose onto one another. Using wedge and dash drawing as others have echoed, you will see that it is impossible to rotate that circled carbon to superimpose it onto the far left structure.