Electrophilic Aromatic Substitution – Directing Groups
- “Initial Tails” and “Final Heads”
- 3 Ways To Make OH A Better Leaving Group
- A Simple Formula For 7 Important Aldehyde/Ketone Reactions
- Acetoacetic
- 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
- Amines
- 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
- Conformations
- Conjugate Addition
- Curved Arrow Refresher
- Curved Arrows
- Decarboxylation
- 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
- Enolates
- Epoxides – Basic and Acidic
- Evaluating Resonance Forms
- Figuring Out The Fischer
- Find That Which Is Hidden
- Formal Charge
- Frost Circles
- Gabriel Synthesis
- Grignards
- 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
- Hybridization Shortcut
- Hydroboration
- 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
- Paped
- 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
- Rearrangements
- 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
- Stereochemistry
- Strong Acid Strong Base
- Strong And Weak Oxidants
- Strong and Weak Reductants
- Stronger Donor Wins
- Substitution
- 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
- t-butyl
- Tautomerism
- 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
Today’s post has two big images, but has a really simple message.
The products you get from an electrophilic aromatic substitution are directly related to the stability of the carbocation intermediate.
There are 2 main cases to worry about.
Take a benzene with a group like OCH3 attached to it. Now draw reaction arrows from a Pi bond on the ring towards an electrophile (E) so that it ends up on the ortho, meta, and para substitutents respectively. The most stable carbocations in each case will look like those below.
Make sure you know how to obtain these using curved arrows!
- Note how the carbocations for the “ortho” and “para” cases are the most stable (since every atom has a full octet). This means they’ll be faster to form than the “meta” carbocation, which is less stable. That’s why the major products are ortho and para .
- Note that the para is a little more stable due to reduced steric interactions – (not resonance)
- What I just said for OCH3 also applies for other activating groups like NH2, SR, OH, etc. as well as alkyl groups (which lack lone pairs but are still activating groups).
Now let’s look at CF3. Here it’s the opposite case.
- The carbocations for the “ortho” and “para” cases are the most unstable, since we have a carbocation adjacent to the electron withdrawing CF3. This means that they will be higher in energy (more unstable) than the “meta” carbocation, which is less bad. So the meta product is formed preferentially.
- This is why CF3 is a “meta director” (although I prefer to call groups like CF3 “ortho-para avoiders”.
- This also applies to other meta directors such as NO2, CN, SO3H, ketones, and so on.
I will spare you from putting a third big diagram in here to describe halogens like F, Cl, Br, and I, but they are often a source of confusion. Experiments show us that they are ortho-para directors. So the fact that they can contribute to resonance (like OCH3) is what stabilizes the ortho-para products relative to meta.
The bottom line for today is that groups that can donate electrons will stabilize the intermediate carbocation, favoring ortho-para products. Groups that withdraw electrons will destabilize the intermediate carbocation, favoring meta products. (Halogens are the weird exception: they slow down aromatic substitution, but favor ortho-para products).
Question for you: what do you think happens when you have more than one directing group? Which one “wins” ?