[vivekfan]

In a molecule with restricted rotation

Can cis be if two substituents of the same kind are on the left side and two different identical substituents are on the right side? (so identical substituents are arranged up and down instead of right and left on the upper half and lower half respectively? Also, in cycloalkanes does cis necessarily mean that both are axial or both are equatorial? And does trans necessarily mean one is equatorial and axial?

Also, I know that conformations exist because of free rotation about single bonds. But since cycloalkanes have restricted rotation (and thus have cis/trans forms), why can it have conformations?

Also, does it matter which way you number the cycloalkane or the chair conformation as long as you number the substituents with the lowest number?

[vivekfan]

What does this mean?

[Squirmy]

Can cis be if two substituents of the same kind are on the left side and two different identical substituents are on the right side? (so identical substituents are arranged up and down instead of right and left on the upper half and lower half respectively?

No, it can't. As soon as you have one carbon of the double bond with two of the same substituents, you don't have cis/trans isomerism.

Also, in cycloalkanes does cis necessarily mean that both are axial or both are equatorial? And does trans necessarily mean one is equatorial and axial?

Cis and trans on a cyclohexane refer to the substituents being on the same side or opposite side of the ring, respectively. When you go to a chair conformation, half the axial positions are up, half are down. Likewise, half the equatorial positions are up, half are down. A cis structure would have both substituents up or both substituents down, trans would be one up/one down. Whether they're both equatorial or axial or one equatorial/one axial depends on the relative positions of the substitutents.

Also, I know that conformations exist because of free rotation about single bonds. But since cycloalkanes have restricted rotation (and thus have cis/trans forms), why can it have conformations?

The bonds in a cyclohexane can't rotate a full 360 degrees, but there is still some rotation around the single bonds.

Also, does it matter which way you number the cycloalkane or the chair conformation as long as you number the substituents with the lowest number?

It matters for nomenclature, but not really for drawing the chair conformation. If you're given a flat structure and asked to draw chairs for it, you can number it however you like, as long as the numbering in the flat structure is consistent with the numbering in the chair.

[sjb]

In a molecule with restricted rotation

Can cis be if two substituents of the same kind are on the left side and two different identical substituents are on the right side? (so identical substituents are arranged up and down instead of right and left on the upper half and lower half respectively?

Assuming your double bond is horizontal, like an equals sign, then this will not give isomers, as indicated.

[vivekfan]

Can cis be if two substituents of the same kind are on the left side and two different identical substituents are on the right side? (so identical substituents are arranged up and down instead of right and left on the upper half and lower half respectively?

No, it can't. As soon as you have one carbon of the double bond with two of the same substituents, you don't have cis/trans isomerism.

Also, in cycloalkanes does cis necessarily mean that both are axial or both are equatorial? And does trans necessarily mean one is equatorial and axial?

Cis and trans on a cyclohexane refer to the substituents being on the same side or opposite side of the ring, respectively. When you go to a chair conformation, half the axial positions are up, half are down. Likewise, half the equatorial positions are up, half are down. A cis structure would have both substituents up or both substituents down, trans would be one up/one down. Whether they're both equatorial or axial or one equatorial/one axial depends on the relative positions of the substitutents.

Also, I know that conformations exist because of free rotation about single bonds. But since cycloalkanes have restricted rotation (and thus have cis/trans forms), why can it have conformations?

The bonds in a cyclohexane can't rotate a full 360 degrees, but there is still some rotation around the single bonds.

Also, does it matter which way you number the cycloalkane or the chair conformation as long as you number the substituents with the lowest number?

It matters for nomenclature, but not really for drawing the chair conformation. If you're given a flat structure and asked to draw chairs for it, you can number it however you like, as long as the numbering in the flat structure is consistent with the numbering in the chair.

So is the cis/trans different for cycloalkanes and double bonds, since there is no rotation in double bonds but some rotation in the cyclic molecule? Because I thought cis/trans could only exist if there was no rotation? Please explain...

[Squirmy]

You are correct about the differences between cis/trans in alkenes vs. cyclic structures. However, to understand the use of the same descriptors, it helps to look at the similarities.

1. For both, you would have to break bonds in order to go from one isomer to the other. Even though there is some bond rotation in the cyclic structures, these rotations will never convert between cis and trans isomers.

2. In both cases, the substituents can either be on the same side (cis) or on opposite sides (trans) of a defined plane.

For the alkenes, this plane is a cross-section through the double bond that bisects the substituents on each carbon, for the cycloalkanes, the plane is defined by the atoms of the ring (even if they aren't truly planar as in a chair conformation, they can be drawn as if they were flat).

I've shown simple examples below where the plane in both cases is shown in light blue. I'm wishing I'd picked a different color considering the page background...let me know if it's not clear.

[vivekfan]

You are correct about the differences between cis/trans in alkenes vs. cyclic structures. However, to understand the use of the same descriptors, it helps to look at the similarities.

1. For both, you would have to break bonds in order to go from one isomer to the other. Even though there is some bond rotation in the cyclic structures, these rotations will never convert between cis and trans isomers.

2. In both cases, the substituents can either be on the same side (cis) or on opposite sides (trans) of a defined plane.

For the alkenes, this plane is a cross-section through the double bond that bisects the substituents on each carbon, for the cycloalkanes, the plane is defined by the atoms of the ring (even if they aren't truly planar as in a chair conformation, they can be drawn as if they were flat).

I've shown simple examples below where the plane in both cases is shown in light blue. I'm wishing I'd picked a different color considering the page background...let me know if it's not clear.

So am i understanding correctly if I say that both cyclic molecules and molecules can have cis/trans isomers because they are restricted in rotation. But out of the two only cyclic can have conformations because there is some rotation? 

[Squirmy]

Alkenes can still have conformations...just not by rotating around the C=C bond.

Check out http://www.cem.msu.edu/~reusch/VirtualText/rotconf1.htm and scroll down to "Rotamer Barriers in Unsaturated Compounds". It's something usually saved for graduate students, so don't sweat the details too much.