The Meso Trap
- “Initial Tails” and “Final Heads”
- 3 Ways To Make OH A Better Leaving Group
- A Simple Formula For 7 Important Aldehyde/Ketone Reactions
- Acids (Again!)
- Activating and Deactivating
- Actors In Every Acid Base Reaction
- Addition – Elimination
- Addition Pattern 1 – Carbocations
- Addition pattern 2 – 3 membered rings
- Addition Reactions
- Aldehydes And Ketones – Addition
- Alkene Pattern #3 – The “Concerted” Pathway
- Alkyl Rearrangements
- Alkynes – 3 Patterns
- Alkynes: Deprotonation and SN2
- Aromaticity: Lone Pairs
- Avoid These Resonance Mistakes
- Best Way To Form Amines
- Bulky Bases
- Carbocation Stability
- Carbocation Stability Revisited
- Carboxylic Acids are Acids
- Chair Flips
- Cis and Trans
- Conjugate Addition
- Curved Arrow Refresher
- Curved Arrows
- Determining Aromaticity
- Diels Alder Reaction – 1
- Dipoles: Polar vs. Covalent Bonding
- E2 Reactions
- Electronegativity Is Greed For Electrons
- Electrophilic Aromatic Substitution – Directing Groups
- Elimination Reactions
- Enantiocats and Diastereocats
- Epoxides – Basic and Acidic
- Evaluating Resonance Forms
- Figuring Out The Fischer
- Find That Which Is Hidden
- Formal Charge
- Frost Circles
- Gabriel Synthesis
- Hofmann Elimination
- How Acidity and Basicity Are Related
- How Are These Molecules Related?
- How Stereochemistry matters
- How To Stabilize Negative Charge
- How To Tell Enantiomers From Diastereomers
- Hybridization Shortcut
- Imines and Enamines
- Importance of Stereochemistry
- Intermolecular Forces
- Intro to Resonance
- Ketones on Acid
- Kinetic Thermodynamic
- Making Alcohols Into Good Leaving Groups
- Markovnikov’s rule
- Mechanisms Like Chords
- Mish Mashamine
- More On The E2
- Newman Projections
- Nucleophiles & Electrophiles
- Nucleophilic Aromatic Substitution
- Nucleophilic Aromatic Substitution 2
- Order of Operations!
- Oxidation And Reduction
- Oxidative Cleavage
- Pi Donation
- Pointers on Free Radical Reactions
- Protecting Groups
- Protecting Groups
- Proton Transfer
- Putting it together (1)
- Putting it together (2)
- Putting it together (3)
- Putting the Newman into ACTION
- Reaction Maps
- Recognizing Endo and Exo
- Redraw / Modify
- Robinson Annulation
- Robinson Annulation Mech
- Sigma and Pi Bonding
- SN1 vs SN2
- sn1/sn2 – Putting It Together
- sn1/sn2/e1/e2 – Exceptions
- sn1/sn2/e1/e2 – Nucleophile
- sn1/sn2/e1/e2 – Solvent
- sn1/sn2/e1/e2 – Substrate
- sn1/sn2/e1/e2 – Temperature
- Strong Acid Strong Base
- Strong And Weak Oxidants
- Strong and Weak Reductants
- Stronger Donor Wins
- Sugars (2)
- Synthesis (1) – “What’s Different?”
- Synthesis (2) – What Reactions?
- Synthesis (3) – Figuring Out The Order
- Synthesis Part 1
- Synthesis Study Buddy
- Synthesis: Walkthrough of A Sample Problem
- Synthesis: Working Backwards
- The 4 Actors In Every Acid-Base Reaction
- The Claisen Condensation
- The E1 Reaction
- The Inflection Point
- The Meso Trap
- The Michael Reaction
- The Nucleophile Adds Twice (to the ester)
- The One-Sentence Summary Of Chemistry
- The Second Most Important Carbonyl Mechanism
- The Single Swap Rule
- The SN1 Reaction
- The SN2 Reaction
- The Wittig Reaction
- Three Exam Tips
- Tips On Building Molecular Orbitals
- Top 10 Skills
- Try The Acid-Base Reaction First
- Two Key Reactions of Enolates
- What makes a good leaving group?
- What Makes A Good Nucleophile?
- What to expect in Org 2
- Work Backwards
- Zaitsev’s Rule
I’ve said that enantiomers always have the same names, but opposite R,S designations. This is true.
But there’s one little wrinkle to this that’s important to know.
It is possible for a molecule to have chiral centers, but be achiral overall due to a plane of symmetry.
These types of molecules are called meso compounds.
Why might they be achiral? Because the molecules have mirror planes! This means that the mirror images of these compounds will be superimposable, and thus not enantiomers.
Remember: in chemistry, two molecules that are superimposable mirror images are the same molecule.
You’ll often see questions like this on midterms: two compounds that look like they are enantiomers. Beware!
If you state that these two molecules are enantiomers, you’ve fallen into the Meso Trap. These molecules are actually the same molecule, drawn in two different ways. It’s a meso compound.
Sometimes it’s immediately apparent that a molecule has a mirror plane, like in the example above. But other times, the molecules might be drawn in ways that make it harder to see.
In these cases, there are two ways to spot meso compounds: the easy way, and the hard way.
The hard way is trying to do the bond rotations to make the molecule look like it has a mirror plane. This takes skill – and time, which might be lacking if you’re in an exam situation.
The easier way is to learn to do R/S naming quickly. Then, make note of the following:
- Since the stereocenters of meso compounds are always mirror images of each other, their R/S designations are always opposite, like (R,S) or (S,R). Meso compounds are never (R,R) or (S,S).
- If you encounter a molecule with opposite R/S configurations, double-check the structure. Are the atoms that comprise each stereocenter the same? Does the molecule appear to have a line of symmetry, where the “left half” is equal to the “right half” ? If so, you’re looking at a meso compound.
This will help you avoid falling into the trap. Here are some more examples of meso compounds.
Thanks for reading! James
P.S. Further reading: The Meso Trap
P.P.S. This is the one exception I was talking about in the last post. Meso compounds have opposite R,S designations,