tbr cbt 1 Q 45 leaving groups

[Whiteshoes]

Trans-platin is less reactive than cis-platin, because its chlorine ligands are less labile. How does this difference in lability explain the difference in reactivity?


A. The trans effect makes Cl a better leaving group when it is trans to an amine rather than trans to Cl.

B. The trans effect makes Cl a worse leaving group when it is trans to an amine rather than trans to Cl.

A is the best answer. Steric hindrance is greater in the cis species, but the chlorine ligands are better leaving groups in the cis species, meaning choices C and D are eliminated. The trans effect of nitrogen ligands is that their donation of a lone pair weakens the bond trans to the nitrogen, making it a better leaving group. This makes choice A the best answer. It could also have been determined by eliminating choice B, because Cl is a better, not a worse, leaving group. The best answer is A.


If trans makes it less reactive as stated in the question then shouldn't it make Cl a worse leaving group. If Cl becomes a better leaving than it would become more reactive. (can't have a rxn if leaving group doesn't leave) Can some please clarify. Thanks

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[Dirty Harry]

Oh goodness, I thought I was the only one. I know I am reviving an old thread that is almost two years old, but I thought B was the answer as well because A makes absolutely no sense for trans-platin to be less reactive. No one clarified your answer, but I am in agreement with you that B is right.

 

[enervate]

Does the prompt give you the structures of either molecule?

 

[Dirty Harry]

The exam only gives you the cis-platin image.

 

[enervate]

Maybe it would be easier if we broke down the choices:

A. Chlorine is a better leaving group when it is trans to an amine.
B. Chlorine is a better leaving group when it is trans to the other chlorine.

From the Wikipedia article on coordination complexes: "When two ligands are mutually adjacent they are said to be cis, when opposite each other, trans."

That can be inferred from the prompt and structure but it certainly isn't something I've encountered in my experience as an undergraduate. We can see that chlorine atoms are opposite (and therefore trans) to the amine groups in cis-platin but it's counterintuitive to infer that they are trans in a molecule named cis-platin!

At any rate, if we ignore the names and acknowledge that we are concerned with the reactivity of the cis-platin structure, we can see that the chlorine atoms are across from the amine groups. To me, that would unequivocally suggest trans rather than cis.

Those names are really red herrings! It's a good thing you wouldn't be encountering something like this on the MCAT for the first time.

 

[Dirty Harry]

Hmmm, wow, so this is a confusing language answer choice. Thank you, enervate. In that sense, the molecules are trans in a molecule called cis-platin. Counterproductive, indeed. Like you said, if the MCAT had something like this, I be totally banging my head and now that I seen this special cis/trans ligand trickery, I might feel a little better.🙂