Addition Pattern 1 – Carbocations
- “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
OK, now that I’ve hopefully sensitized you to the importance of regiochemistry and stereochemistry, let’s dig into some of the key patterns you’ll see in addition reactions to alkenes.
There are 3 main patterns you’ll see in the addition reactions of alkenes. It’s a slight oversimplification, but not too much of one. If you can get these 3 different patterns straight, you’ll be in good shape.
The first pattern – what I’ll talk about today – is what you could call theCarbocation Pathway.
A perfect example is the reaction of an alkene with a hydrogen halide such as HBr.
In the first step, the alkene acts as a nucleophile, attacking the electrophilic hydrogen of H-Br. This leads to formation of a carbocation.
Important! Think back to the SN1 – what factors were important for carbocation stability? – remember that carbocation stability increases as we go from primary to secondary to tertiary?
The “big barrier” for this reaction is also carbocation stability – so we’re going to preferentially add the hydrogen to the alkene so as to provide the most stable carbocation.
Once we form the carbocation, a nucleophile (in this case, bromide) can come in and attack it. The product will have the bromine attached to the most substituted carbon of the alkene.
In other words, it follows Markovnikoff’s rule!
What about stereochemistry?
Let’s remember the SN1 again. Remember how in the SN1, the carbocation was flat, so the nucleophile could attack from either side? Same thing applies here!
This means that the orientation of the hydrogen and the bromide could be syn or anti – they have no strong preference. In other words, it’s random.
One last thing. We’re forming a carbocation. So be on the lookout for the possibility of rearrangements. If the carbocation is formed adjacent to a carbon that could rearrange to give a more substituted carbocation, it will happen.
Putting it all together, here’s a summary of what we’ve talked about. The cool thing is that this exact pattern applies to HCl, HBr, and HI. It also applies to the addition of H3O(+) to alkenes, although there’s an extra book-keeping step at the end with this one to give neutral OH.
Tomorrow: the second important pattern.
Thanks for reading! James
P.P.S. Addition with rearrangement