How To Tell Enantiomers From Diastereomers
- “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
Bottom line for today: you can tell if molecules are enantiomers or diastereomers by looking at their (R,S) designations.
Enantiomers are non-superimposable mirror images of each other. Pardon me while I push the Caps Lock button:
ENANTIOMERS ALWAYS HAVE OPPOSITE R,S DESIGNATIONS.
By “opposite” I mean they have the same names, but their R’s and S’s are reversed.
The enantiomer of (S)-2-chlorobutane? It’s (R)-2-chlorobutane.
The enantiomer of (R)-2-butanol? It’s (S)-2-butanol.
- Let’s say you’ve got a molecule with two stereocenters and the configuration is (R,R). The enantiomer will be the molecule with the exact same name except it’s (S,S). For instance, the enantiomer of (2R,3R)2-bromo-3-iodobutane is (2S,3S)2-bromo-3-iodobutane.
- If you’ve got a chiral molecule with two stereocenters and the configuration is (R,S), the enantiomer will have the (S,R) configuration.
- Taking it even further, if we had a molecule that was (R,R,S,R)-(followed by long chemical name), what would its enantiomer be? It would be (S,S,R,S). Get the idea?
This is why learning to figure out R/S designations is such a key skill! You can figure out whether two molecules are enantiomers (or not) simply by examining their names and their (R,S) designations!
Now, what if we have a molecule with the exact same name, except their (R,S) designations are not opposite, but not identical either?
Like (R,R) and (R,S)…. or (S,S) and (S,R) ?
Or: (R,R,R) and (R,S,R)
Or: (S,S,R,S) and (S,S,S,S)
These molecules will be stereoisomers…. but not enantiomers. So they’re diastereomers! Caps lock time agein…
DIASTEREOMERS WILL ALWAYS HAVE NON-IDENTICAL (BUT NON-OPPOSITE) R,S DESIGNATIONS. Like (2S,3R)-2,3-butanediol and (2R,3R)-2,3-butanediol.
OK! Quiz time.
1. Maybe you’ve heard of pseudoephedrine (Sudafed)? It’s (S,S)-2-methylamino-1-phenylpropan-1-ol. Guess what its enantiomer is? (Answer below)
2. As you might have guessed, there’s also a molecule called ephedrine. It’s (R,S)-2-(methylamino)-1-phenylpropan-1-ol. Guess what its enantiomer is?
3. Now guess how ephedrine and pseudoephedrine are related to each other?
Tomorrow: let’s sum up the week.
Thanks for reading! James
- The enantiomer would be (R,R)-2-methylamino-1-phenylpropan-1-ol
- The enantiomer would be (S,R)-2-methylamino-1-phenylpropan-1-ol
- Pseudoephedrine (S,S) and ephedrine (R,S) are diastereomers of each other.
P.P.S. You might ask if there are any exceptions to this. Yes and no. Enantiomers always have opposite R,S designations. BUT (and this is important) not all molecules with opposite R,S designations are enantiomers! As we’ll see later, it’s possible for a molecule to have chiral centers but be an achiral molecule due to a plane of symmetry. These are called meso compounds, and we’ll talk about them next week.