Question: Are these molecules enantiomers, diastereomers, or the same?

If you immediately recognized this as a molecule with an internal plane of symmetry (and thus an achiral molecule, incapable of having an enantiomer), congratulations. If not, you just fell into The Meso Trap.

The Meso Trap is a common feature on exams and tests to make sure that you understand the concept of chirality and that you are paying attention.

Just to make things clear, a meso compound is a molecule that has chiral centers  but also has an internal plane of symmetry. This renders the molecule achiral: it does not have an enantiomer, and it does not rotate plane polarized light .

It is similar to a phenomenon found in certain two-headed, two-tailed cats.

The Meso Trap usually comes up in conjunction with the testing of some other important skills, notably:

1) identifying types of isomers

2) manipulating various types of chemical diagrams

3) determining (R)/(S) nomenclature for given stereocenters

4) identifying the stereochemistry of various types of reactions

These are all fundamental skills. We are not talking about trivial stuff here.

It can come up in glaringly obvious ways (like in the above example) or in a number of other ways, as in these examples, where the meso compound is not nearly as obvious, and drawn in different projections.

The Meso Trap can also sneak up on you in other ways, like when you’re being asked for the products of a reaction. In the following example, you might be tempted to draw in not only the product of the reaction but also its enantiomer. In this case, however, the product is a meso compound… meaning that there is only one product.

How To Avoid Falling into The Meso Trap

Identifying a meso compound means being able to identify a plane of symmetry in a molecule. It’s important to realize that this can cut either through a bond or through an atom, as in the following examples.

What if the plane of symmetry isn’t obvious? That’s when you really have to demonstrate that you have a few important skills – like the ability to recognize molecules drawn in different projections, how to do bond rotations, and how to recognize (R)/(S) designations.

There are really two basic strategies for identifying a meso compound if it isn’t drawn in an obvious configuration.

The path of greatest resistance, but greatest reward is to learn to master how to perform  bond rotations on line-wedge diagrams, Newman projections, Fischer projections, as well as to be able to convert between the three types of drawings as necessary. In the beginning stages, using a model kit to check your work is invaluable. With these skills you can take any structure given to you and be able to rotate the bonds in a way to test whether the two molecules are mirror images.

A slightly easier method to determine whether a given compound is meso is to take advantage of a simple principle: meso compounds have an internal mirror plane. That is to say that not only must 1) every carbon on each side of a mirror plane have the same substituents, but in addition 2) every (R) stereocenter on one side of the mirror plane must be balanced by an (S) stereocenter on the opposite side. So if you can quickly determine (R)/(S), you can also quickly determine whether a given compound is meso without having to do bond rotations. If the stereocenters aren’t opposite, it can’t be meso.  Depending on the complexity of the molecule, this can be a much faster way to do things.

The key ingredient in avoiding falling into the Meso Trap is paranoia. When asked if molecules are diastereomers/enantiomers/the same, ask yourself – “is this a symmetric molecule?”. Every time. After doing an addition reaction to a double bond, ask yourself: “is this a symmetric molecule?”. This comes up a lot more often than you’d expect – especially on tests.

It also comes up later in NMR, where recognizing a plane of symmetry tells you how many distinct proton or carbon signals you will see.\

Answers: 1) the same (meso) 2) a), b), c) are all examples of the same (meso) compounds, drawn differently. 3) a gives cis-dimethylcyclohexane (a meso compound), b) gives (R,S)-2-3-dibromobutane, also a meso compound.

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{ 20 comments… read them below or add one }

alan January 14, 2011 at 11:35 pm

hey! thanks for your words of encouragement last month. my blog is kind of just my little secret space where i pout and complain and pour all my frustrations into; stuff i don’t reveal to people in real life cause it would probably make people think i’m self-centered (especially fussing over a B+).

i guess you could say i have a love-hate relationship with organic chemistry. it’s quite a tough class, but i really do thoroughly enjoy learning the material–at least in comparison with general chemistry. my two apartmentmates (both extremely brilliant curve setters) and i are taking the honors sequence starting this quarter. hopefully i won’t make the same mistake again by doing well in the beginning and underestimating the final exam.

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Kenny November 14, 2011 at 11:57 pm

OMG! I love you!! :D THIS ARTCILE have save me from an extra 2 hours of confusion!! YAY :D

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chris raabe February 25, 2012 at 9:41 am

James, I noticed there are no answers posted to these problems on this “meso trap” post. Also there is no explanation to the diagrams where you rotate 120 degrees and in some cases 60 degrees. When can we do this and what are the rules? Thanks! Chris Raabe, UAA

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james February 26, 2012 at 4:49 pm

Posted answers, thanks for mentioning this. Regarding bond rotations: If you interchange any three groups on a carbon, you’ve done a bond rotation. Rotating a group 60 degrees is a little trickier, I don’t have a good shortcut for learning how to do this.

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stefani johnson April 11, 2014 at 11:40 am

for 2,3-dibromo butane i’m not seeing R

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lisa January 28, 2013 at 8:19 am

OMG!!!! I just love it when some one knows their stuff AND can teach it. It is sooooo rare.

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Ivan K September 14, 2013 at 12:50 am

I found this site by accident but what a fortuitous mistake.

Thank you for this level of detail. I have an upcoming exam and likely would have fallen for this.

Do you happen to have any more sections on your webpage regarding tricks that professors like to throw into their exams? Your site is unique in that it might as well have been made by a professor (your credentials are more impressive than most professors). Your insight is appreciated tremendously.

With kind regards

-Ivan

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Jessica.T October 3, 2013 at 9:42 pm

Thank’s a million for all of this great information. I found your site while looking for tips on orgo and I have to say this is a goldmine! I am sure you get this all the time but thank you so much for all of your help. I had just being looking at this one stereochemistry problem for the past 10 minutes and this saved me so much more time.

Great stuff!

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Ptachia Bar-On October 20, 2013 at 12:16 pm

Just one word: Excellent

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laila December 3, 2013 at 8:18 am

thank you so much you saved my life !! thanks but i still dont know how to determine theR and S for newman projection ?

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Miranda Hurrle December 30, 2013 at 3:45 pm

You sir, deserve an award! (And my ochem professor deserves to be fired and exiled from campus)

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ochem_student April 13, 2014 at 4:46 pm

very helpful!! quick question though, just to clarify. If a molecule is chiral, is it automatically not meso? It seems like that is true from the first few sentences, but I just want to verify.

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writer_who_needs2know August 8, 2014 at 12:15 pm

You ARE a Professor and clearly an excellent one no matter what condition academe is in. Kudos for continuing to teach and doing so accessibly vs. blooming in the desert of some community college–great for the few that go there but not for the rest of us!

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Rajni September 18, 2014 at 7:22 am

Indeed a great work……. Thanks a lot for the material and that too presented in an easy way …. You are a genius.

I have a query… How can a Newman projection be easily converted to Flying Wedge formula.

Thanks

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Orgo Girl October 7, 2014 at 9:50 pm

This is what I thought!

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Sankalp Sinha October 12, 2014 at 1:25 am

Great, that finally cleared my doubts. You explained it a gazillion times better than our chem teacher. Thanks a lot.

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Ally October 27, 2014 at 7:22 pm

2)A the fisher projection does not seem to be meso to me. from what I see, the two are mirror images, plus have opposing chiral centers (R,S) (S,R). I also can’t seem to superimpose them. And, they both have been singly substituted at each stereo-center. I also cannot find a plane of symmetry.
How is 2A a meso compound?

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James October 30, 2014 at 5:32 pm

In order to see the mirror plane you’d need to do some bond rotations. Further down the page I show how to do a rotation on the left-hand molecule so that you can clearly see the mirror plane. You can do the same thing (except opposite direction) to the molecule on the right, and you’ll also see that it has a mirror plane.

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jenny November 7, 2014 at 12:04 am

Thanks for the info! I am a bit confused though because i thought number 2b was an enantiomer due to the r,s configuration being opposite in both. can you explain?

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James November 11, 2014 at 5:37 pm

in 2b, if you rotate each Newman so that the OH groups line up you’ll see that the front carbon completely superimposes on the back – the molecules have mirror planes. They are actually the same molecule, just rotated 180 degrees.

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