Trans vs Cis and equatorial and axial???

[dray5150]

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If the substituents are cis: they will have one equitorial and one axial
If trans: both equatorial or both axial


Is this a valid way of remembering cis and trans for cyclohexanes?

 

[kia ora]

If the substituents are cis: they will have one equitorial and one axial
If trans: both equatorial or both axial


Is this a valid way of remembering cis and trans for cyclohexanes?

well, it's not very catchy but if you can remember that, then it is valid for you!
if you forget, just do a quick drawing of the chair

 

[loveoforganic]

It's not valid. What about trans-1,3-dimethylcyclohexane? cis-1,3-dimethylcyclohexane?

 

[sv3]

I'm not sure there is a solid connection/rule between the two concepts......

 

[ahs4n]

.

 

[MBHockey]

Why would you waste time trying to memorize this? Just draw it out and you'll see that your "rule" fails a lot of the time.

 

[sv3]

exactly. im not sure how else you should tackle it as I really haven't seen a "rule". Unfortunately drawing it out is the only option i've used.

 

[loveoforganic]

Well they alternate.. If you've got cis-1,4-dimethylcyclohexane - assume position (1) is equa up then say: 2) equa down, 3) equa up, 4) equa down. They're cis, so the methyl subsituent in the 4 position must be axial up

 

[sv3]

Well they alternate.. If you've got cis-1,4-dimethylcyclohexane - assume position (1) is equa up then say: 2) equa down, 3) equa up, 4) equa down. They're cis, so the methyl subsituent in the 4 position must be axial up

That would mean the methyl for position 1 would have to be axial up too no? And if 1 is equa up, how does that work?

Or is it the way you described (methyl on position 1 is equa up and one 4 its axial up) in order to avoid a 1,4 diaxial interaction?

Perhaps I'm totally screwed here......i need to review anyway. physics brainwashed me

 

[loveoforganic]

Sorry, when I posted that I thought it might be unclear. I'll elaborate.

You're given this structure

.

Assume that all the methyl substituents are all cis to one another, all wedged (pointing up).

Carbon 1 is the topmost carbon, and the numbering goes clockwise (e.g. carbon 2 has the second methyl substituent on it). Take the methyl group on carbon 1, and assume it's in equatorial (equatorial up). As you go around the cyclohexane, the next equatorial position would be facing down (because they alternate up->down->up->down). However, since the first one was up and the 2 substituents are cis, we're to assume that that methyl group is in the axial position (up). Carbon 3 is equatorial up, then carbon 4 is equatorial down. For the same reason as carbon 2, we're forced to put the methyl group on carbon 4 in axial position (up).

So we have 1 equatorial and 2 axials - obviously not the least strained conformation. So flip it. You get 1 axial and 2 equatorials, and you end up with the methyl on carbon 1 axial up, on carbon 2 equatorial up, and on carbon 4 equatorial up.

This sounds really complicated writing it out, but it's honestly really easy and fast (definitely faster than having to draw out a chair conformation).

 

[sv3]

hey that was great and helpful. I was having issues mentally picturing the chair conformation (i loathe them) so your explanation and pic were totally clear. Thanks very much. I try to avoid using the chair conformation whenever i can as it slows me down.......

cheers
steve

 

[loveoforganic]

Glad it helps. Chair conformation does suck.

 

[jryck3]

On a lighter note... Remember Cis is related to Z (From E,Z) by yelling in a german accent:

"ZEY ARE ON ZE ZAME ZIDE!!!!!!"

I never forget.. lol

 

[chemtopper]

okay now here is a rule to memorize
1,2 -cis -axial(a),equitorial (e)
1,2 trans is e,e or a,a
1,3 cis is a,a or e,e
1,3 trans is a,e
1,4 -cis is again a,e
1,4 trans is a,a or e,e
It is not difficult to memorize as it goes alternate .
You can understand it by using molecular models or by making structures .

 

[FuturaDocta]

Just pull out your molecule kit and practice for about an hour. It doesn't take long to finally get use to the three dimension stuff. Plus, the kit is probably a good way to take a break from all the studying. Just don't get too carried away. hehe

 

[Bildo]

On a lighter note... Remember Cis is related to Z (From E,Z) by yelling in a german accent:

"ZEY ARE ON ZE ZAME ZIDE!!!!!!"

I never forget.. lol

I remember because they are zusammen (together). But then again I took german.

Chair conformation structures are the easiest way to do it.

Plus what if you have a molecule that doesn't assume chair conformation due to sterics/intramolecular attractions? Doubt that's mcat but still...

 

[Hemichordate]

So let me get this straight:

1) If you have cis- the pattern is alternating between equatorial and axial
2) If you have trans- you always have either all axial or all equatorial

So if you have trans 1-3 dimethylcyclohexane, you get two equatorial methyl groups (one pointing up one point down). Right?

 

[inaccensa]

If the substituents are cis: they will have one equitorial and one axial
If trans: both equatorial or both axial


Is this a valid way of remembering cis and trans for cyclohexanes?

I think the correct way might perhaps be like this
If both substituents are in the same direction ,ie up or down ( irrespective of their axial or equatorial positions) they are cis and if one's up and one's down then they are trans.

All odd positions have axial facing up and all even positions have equatorial facing down.