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Carbocation Rearrangement Reactions (2) – Alkyl Shifts
Last updated: November 29th, 2022 |
Alkyl Shifts In Carbocation Rearrangement Reactions, Including Ring Expansion
- Hydride shifts can sometimes occur when a more stable carbocation can be formed through migration of a C-H bond.
- If no hydride shift is possible that will result in a more stable carbocation, then it is possible that an alkyl shift can occur, where a C-C bond breaks and a C-C bond breaks. The net effect is the shifting over of the carbocation to the adjacent carbon.
- Alkyl shifts are most likely to occur when a quaternary carbon is adjacent to a secondary (or primary) carbocation
- Alkyl shifts adjacent to strained rings (e.g. cyclobutane) can result in ring expansion
Table of Contents
- Alkyl Shifts Can Lead To More Stable Carbocations, Too
- The Mechanism Of Alkyl Shift Reactions
- Example of An SN1 With Alkyl Shift
- Ring-Expansion Reactions Also Involve Alkyl Shifts
- Notes
1. Alkyl Shifts Can Lead To More Stable Carbocations, Too
In the previous post we saw how certain carbocations can sometimes rearrange (through hydride shifts) to give more stable carbocations (See post: Hydride Shifts)
However, sometimes there are situations where a hydride shift would not lead to a more stable carbocation, but an alkyl shift would!
Take a look at this carbocation. If a hydride shift occurred here, we’d be going to a less stable (primary) carbocation! Not going to happen.
You might note something with this example, however: it is possible for a more stable tertiary carbocation to be formed.
How? If an alkyl group migrates instead! (Look at that red methyl group! )
The most common situation where alkyl shifts can occur is when a quaternary carbon (that’s a carbon attached to 4 carbons) is adjacent to a secondary carbocation. (See post: Primary, Secondary, Tertiary, Quaternary)
2. The Mechanism Of Alkyl Shift Reactions In Carbocation Rearrangements
How does this work? First, the pair of electrons from the C-C bond must align with the empty p orbital on the carbocation (side note: this means they have to be aligned in the same plane in order for orbital overlap to occur).
Then, as the pair of electrons from the C–C bond is donated into the empty p-orbital, one C–C bond begins to break and the new C–C bond begins to form.
In the transition state, there are partial bonds between the carbon being transferred and each of the two adjacent carbon atoms. Then, as one bond shortens and the other lengthens, we end up with a (more stable) tertiary carbocation. (See post: 3 Factors That Stabilize Carbocations)
Rearrangements can potentially occur any time a carbocation is formed. That includes SN1 reactions (and as we’ll later see, elimination and addition reactions).
3. Example of An SN1 With Alkyl Shift
4. Ring-Expansion Reactions Also Involve Migration of Carbon
It doesn’t always have to be a methyl group that moves! One interesting example is when a carbocation is formed adjacent to a strained ring, such as a cyclobutane. (See: Ring Strain in Cyclopropane and Cyclobutane)
Even though the CH3 could potentially migrate in this case, it’s more favorable to shift one of the alkyl groups in the ring, which leads to ring expansion and the formation of a less strained, five-membered ring.
Here’s an example of an SN1 where migration of a carbon-carbon bond leads to ring expansion.
Notes
00 General Chemistry Review
01 Bonding, Structure, and Resonance
- How Do We Know Methane (CH4) Is Tetrahedral?
- Hybrid Orbitals and Hybridization
- How To Determine Hybridization: A Shortcut
- Orbital Hybridization And Bond Strengths
- Sigma bonds come in six varieties: Pi bonds come in one
- A Key Skill: How to Calculate Formal Charge
- The Four Intermolecular Forces and How They Affect Boiling Points
- 3 Trends That Affect Boiling Points
- How To Use Electronegativity To Determine Electron Density (and why NOT to trust formal charge)
- Introduction to Resonance
- How To Use Curved Arrows To Interchange Resonance Forms
- Evaluating Resonance Forms (1) - The Rule of Least Charges
- How To Find The Best Resonance Structure By Applying Electronegativity
- Evaluating Resonance Structures With Negative Charges
- Evaluating Resonance Structures With Positive Charge
- Exploring Resonance: Pi-Donation
- Exploring Resonance: Pi-acceptors
- In Summary: Evaluating Resonance Structures
- Drawing Resonance Structures: 3 Common Mistakes To Avoid
- How to apply electronegativity and resonance to understand reactivity
- Bond Hybridization Practice
- Structure and Bonding Practice Quizzes
- Resonance Structures Practice
02 Acid Base Reactions
- Introduction to Acid-Base Reactions
- Acid Base Reactions In Organic Chemistry
- The Stronger The Acid, The Weaker The Conjugate Base
- Walkthrough of Acid-Base Reactions (3) - Acidity Trends
- Five Key Factors That Influence Acidity
- Acid-Base Reactions: Introducing Ka and pKa
- How to Use a pKa Table
- The pKa Table Is Your Friend
- A Handy Rule of Thumb for Acid-Base Reactions
- Acid Base Reactions Are Fast
- pKa Values Span 60 Orders Of Magnitude
- How Protonation and Deprotonation Affect Reactivity
- Acid Base Practice Problems
03 Alkanes and Nomenclature
- Meet the (Most Important) Functional Groups
- Condensed Formulas: Deciphering What the Brackets Mean
- Hidden Hydrogens, Hidden Lone Pairs, Hidden Counterions
- Don't Be Futyl, Learn The Butyls
- Primary, Secondary, Tertiary, Quaternary In Organic Chemistry
- Branching, and Its Affect On Melting and Boiling Points
- The Many, Many Ways of Drawing Butane
- Wedge And Dash Convention For Tetrahedral Carbon
- Common Mistakes in Organic Chemistry: Pentavalent Carbon
- Table of Functional Group Priorities for Nomenclature
- Summary Sheet - Alkane Nomenclature
- Organic Chemistry IUPAC Nomenclature Demystified With A Simple Puzzle Piece Approach
- Boiling Point Quizzes
- Organic Chemistry Nomenclature Quizzes
04 Conformations and Cycloalkanes
- Staggered vs Eclipsed Conformations of Ethane
- Conformational Isomers of Propane
- Newman Projection of Butane (and Gauche Conformation)
- Introduction to Cycloalkanes (1)
- Geometric Isomers In Small Rings: Cis And Trans Cycloalkanes
- Calculation of Ring Strain In Cycloalkanes
- Cycloalkanes - Ring Strain In Cyclopropane And Cyclobutane
- Cyclohexane Conformations
- Cyclohexane Chair Conformation: An Aerial Tour
- How To Draw The Cyclohexane Chair Conformation
- The Cyclohexane Chair Flip
- The Cyclohexane Chair Flip - Energy Diagram
- Substituted Cyclohexanes - Axial vs Equatorial
- Ranking The Bulkiness Of Substituents On Cyclohexanes: "A-Values"
- Cyclohexane Chair Conformation Stability: Which One Is Lower Energy?
- Fused Rings - Cis-Decalin and Trans-Decalin
- Naming Bicyclic Compounds - Fused, Bridged, and Spiro
- Bredt's Rule (And Summary of Cycloalkanes)
- Newman Projection Practice
- Cycloalkanes Practice Problems
05 A Primer On Organic Reactions
- The Most Important Question To Ask When Learning a New Reaction
- Learning New Reactions: How Do The Electrons Move?
- The Third Most Important Question to Ask When Learning A New Reaction
- 7 Factors that stabilize negative charge in organic chemistry
- 7 Factors That Stabilize Positive Charge in Organic Chemistry
- Nucleophiles and Electrophiles
- Curved Arrows (for reactions)
- Curved Arrows (2): Initial Tails and Final Heads
- Nucleophilicity vs. Basicity
- The Three Classes of Nucleophiles
- What Makes A Good Nucleophile?
- What makes a good leaving group?
- 3 Factors That Stabilize Carbocations
- Equilibrium and Energy Relationships
- What's a Transition State?
- Hammond's Postulate
- Learning Organic Chemistry Reactions: A Checklist (PDF)
- Introduction to Free Radical Substitution Reactions
- Introduction to Oxidative Cleavage Reactions
06 Free Radical Reactions
- Bond Dissociation Energies = Homolytic Cleavage
- Free Radical Reactions
- 3 Factors That Stabilize Free Radicals
- What Factors Destabilize Free Radicals?
- Bond Strengths And Radical Stability
- Free Radical Initiation: Why Is "Light" Or "Heat" Required?
- Initiation, Propagation, Termination
- Monochlorination Products Of Propane, Pentane, And Other Alkanes
- Selectivity In Free Radical Reactions
- Selectivity in Free Radical Reactions: Bromination vs. Chlorination
- Halogenation At Tiffany's
- Allylic Bromination
- Bonus Topic: Allylic Rearrangements
- In Summary: Free Radicals
- Synthesis (2) - Reactions of Alkanes
- Free Radicals Practice Quizzes
07 Stereochemistry and Chirality
- Types of Isomers: Constitutional Isomers, Stereoisomers, Enantiomers, and Diastereomers
- How To Draw The Enantiomer Of A Chiral Molecule
- How To Draw A Bond Rotation
- Introduction to Assigning (R) and (S): The Cahn-Ingold-Prelog Rules
- Assigning Cahn-Ingold-Prelog (CIP) Priorities (2) - The Method of Dots
- Enantiomers vs Diastereomers vs The Same? Two Methods For Solving Problems
- Assigning R/S To Newman Projections (And Converting Newman To Line Diagrams)
- How To Determine R and S Configurations On A Fischer Projection
- The Meso Trap
- Optical Rotation, Optical Activity, and Specific Rotation
- Optical Purity and Enantiomeric Excess
- What's a Racemic Mixture?
- Chiral Allenes And Chiral Axes
- Stereochemistry Practice Problems and Quizzes
08 Substitution Reactions
- Nucleophilic Substitution Reactions - Introduction
- Two Types of Nucleophilic Substitution Reactions
- The SN2 Mechanism
- Why the SN2 Reaction Is Powerful
- The SN1 Mechanism
- The Conjugate Acid Is A Better Leaving Group
- Comparing the SN1 and SN2 Reactions
- Polar Protic? Polar Aprotic? Nonpolar? All About Solvents
- Steric Hindrance is Like a Fat Goalie
- Common Blind Spot: Intramolecular Reactions
- Substitution Practice - SN1
- Substitution Practice - SN2
09 Elimination Reactions
- Elimination Reactions (1): Introduction And The Key Pattern
- Elimination Reactions (2): The Zaitsev Rule
- Elimination Reactions Are Favored By Heat
- Two Elimination Reaction Patterns
- The E1 Reaction
- The E2 Mechanism
- E1 vs E2: Comparing the E1 and E2 Reactions
- Antiperiplanar Relationships: The E2 Reaction and Cyclohexane Rings
- Bulky Bases in Elimination Reactions
- Comparing the E1 vs SN1 Reactions
- Elimination (E1) Reactions With Rearrangements
- E1cB - Elimination (Unimolecular) Conjugate Base
- Elimination (E1) Practice Problems And Solutions
- Elimination (E2) Practice Problems and Solutions
10 Rearrangements
11 SN1/SN2/E1/E2 Decision
- Identifying Where Substitution and Elimination Reactions Happen
- Deciding SN1/SN2/E1/E2 (1) - The Substrate
- Deciding SN1/SN2/E1/E2 (2) - The Nucleophile/Base
- SN1 vs E1 and SN2 vs E2 : The Temperature
- Deciding SN1/SN2/E1/E2 - The Solvent
- Wrapup: The Key Factors For Determining SN1/SN2/E1/E2
- Alkyl Halide Reaction Map And Summary
- SN1 SN2 E1 E2 Practice Problems
12 Alkene Reactions
- E and Z Notation For Alkenes (+ Cis/Trans)
- Alkene Stability
- Alkene Addition Reactions: "Regioselectivity" and "Stereoselectivity" (Syn/Anti)
- Stereoselective and Stereospecific Reactions
- Hydrohalogenation of Alkenes and Markovnikov's Rule
- Hydration of Alkenes With Aqueous Acid
- Rearrangements in Alkene Addition Reactions
- Halogenation of Alkenes and Halohydrin Formation
- Oxymercuration Demercuration of Alkenes
- Hydroboration Oxidation of Alkenes
- m-CPBA (meta-chloroperoxybenzoic acid)
- OsO4 (Osmium Tetroxide) for Dihydroxylation of Alkenes
- Palladium on Carbon (Pd/C) for Catalytic Hydrogenation of Alkenes
- Cyclopropanation of Alkenes
- A Fourth Alkene Addition Pattern - Free Radical Addition
- Alkene Reactions: Ozonolysis
- Summary: Three Key Families Of Alkene Reaction Mechanisms
- Synthesis (4) - Alkene Reaction Map, Including Alkyl Halide Reactions
- Alkene Reactions Practice Problems
13 Alkyne Reactions
- Acetylides from Alkynes, And Substitution Reactions of Acetylides
- Partial Reduction of Alkynes With Lindlar's Catalyst
- Partial Reduction of Alkynes With Na/NH3 To Obtain Trans Alkenes
- Alkyne Hydroboration With "R2BH"
- Hydration and Oxymercuration of Alkynes
- Hydrohalogenation of Alkynes
- Alkyne Halogenation: Bromination, Chlorination, and Iodination of Alkynes
- Alkyne Reactions - The "Concerted" Pathway
- Alkenes To Alkynes Via Halogenation And Elimination Reactions
- Alkynes Are A Blank Canvas
- Synthesis (5) - Reactions of Alkynes
- Alkyne Reactions Practice Problems With Answers
14 Alcohols, Epoxides and Ethers
- Alcohols - Nomenclature and Properties
- Alcohols Can Act As Acids Or Bases (And Why It Matters)
- Alcohols - Acidity and Basicity
- The Williamson Ether Synthesis
- Ethers From Alkenes, Tertiary Alkyl Halides and Alkoxymercuration
- Alcohols To Ethers via Acid Catalysis
- Cleavage Of Ethers With Acid
- Epoxides - The Outlier Of The Ether Family
- Opening of Epoxides With Acid
- Epoxide Ring Opening With Base
- Making Alkyl Halides From Alcohols
- Tosylates And Mesylates
- PBr3 and SOCl2
- Elimination Reactions of Alcohols
- Elimination of Alcohols To Alkenes With POCl3
- Alcohol Oxidation: "Strong" and "Weak" Oxidants
- Demystifying The Mechanisms of Alcohol Oxidations
- Protecting Groups For Alcohols
- Thiols And Thioethers
- Calculating the oxidation state of a carbon
- Oxidation and Reduction in Organic Chemistry
- Oxidation Ladders
- SOCl2 Mechanism For Alcohols To Alkyl Halides: SN2 versus SNi
- Alcohol Reactions Roadmap (PDF)
- Alcohol Reaction Practice Problems
- Epoxide Reaction Quizzes
- Oxidation and Reduction Practice Quizzes
15 Organometallics
- What's An Organometallic?
- Formation of Grignard and Organolithium Reagents
- Organometallics Are Strong Bases
- Reactions of Grignard Reagents
- Protecting Groups In Grignard Reactions
- Synthesis Problems Involving Grignard Reagents
- Grignard Reactions And Synthesis (2)
- Organocuprates (Gilman Reagents): How They're Made
- Gilman Reagents (Organocuprates): What They're Used For
- The Heck, Suzuki, and Olefin Metathesis Reactions (And Why They Don't Belong In Most Introductory Organic Chemistry Courses)
- Reaction Map: Reactions of Organometallics
- Grignard Practice Problems
16 Spectroscopy
- Degrees of Unsaturation (or IHD, Index of Hydrogen Deficiency)
- Conjugation And Color (+ How Bleach Works)
- Introduction To UV-Vis Spectroscopy
- UV-Vis Spectroscopy: Absorbance of Carbonyls
- UV-Vis Spectroscopy: Practice Questions
- Bond Vibrations, Infrared Spectroscopy, and the "Ball and Spring" Model
- Infrared Spectroscopy: A Quick Primer On Interpreting Spectra
- IR Spectroscopy: 4 Practice Problems
- 1H NMR: How Many Signals?
- Homotopic, Enantiotopic, Diastereotopic
- Diastereotopic Protons in 1H NMR Spectroscopy: Examples
- C13 NMR - How Many Signals
- Liquid Gold: Pheromones In Doe Urine
- Natural Product Isolation (1) - Extraction
- Natural Product Isolation (2) - Purification Techniques, An Overview
- Structure Determination Case Study: Deer Tarsal Gland Pheromone
17 Dienes and MO Theory
- What To Expect In Organic Chemistry 2
- Are these molecules conjugated?
- Conjugation And Resonance In Organic Chemistry
- Bonding And Antibonding Pi Orbitals
- Molecular Orbitals of The Allyl Cation, Allyl Radical, and Allyl Anion
- Pi Molecular Orbitals of Butadiene
- Reactions of Dienes: 1,2 and 1,4 Addition
- Thermodynamic and Kinetic Products
- More On 1,2 and 1,4 Additions To Dienes
- s-cis and s-trans
- The Diels-Alder Reaction
- Cyclic Dienes and Dienophiles in the Diels-Alder Reaction
- Stereochemistry of the Diels-Alder Reaction
- Exo vs Endo Products In The Diels Alder: How To Tell Them Apart
- HOMO and LUMO In the Diels Alder Reaction
- Why Are Endo vs Exo Products Favored in the Diels-Alder Reaction?
- Diels-Alder Reaction: Kinetic and Thermodynamic Control
- The Retro Diels-Alder Reaction
- The Intramolecular Diels Alder Reaction
- Regiochemistry In The Diels-Alder Reaction
- The Cope and Claisen Rearrangements
- Electrocyclic Reactions
- Electrocyclic Ring Opening And Closure (2) - Six (or Eight) Pi Electrons
- Diels Alder Practice Problems
- Molecular Orbital Theory Practice
18 Aromaticity
- Introduction To Aromaticity
- Rules For Aromaticity
- Huckel's Rule: What Does 4n+2 Mean?
- Aromatic, Non-Aromatic, or Antiaromatic? Some Practice Problems
- Antiaromatic Compounds and Antiaromaticity
- The Pi Molecular Orbitals of Benzene
- The Pi Molecular Orbitals of Cyclobutadiene
- Frost Circles
- Aromaticity Practice Quizzes
19 Reactions of Aromatic Molecules
- Electrophilic Aromatic Substitution: Introduction
- Activating and Deactivating Groups In Electrophilic Aromatic Substitution
- Electrophilic Aromatic Substitution - The Mechanism
- Ortho-, Para- and Meta- Directors in Electrophilic Aromatic Substitution
- Understanding Ortho, Para, and Meta Directors
- Why are halogens ortho- para- directors?
- Disubstituted Benzenes: The Strongest Electron-Donor "Wins"
- Electrophilic Aromatic Substitutions (1) - Halogenation of Benzene
- Electrophilic Aromatic Substitutions (2) - Nitration and Sulfonation
- EAS Reactions (3) - Friedel-Crafts Acylation and Friedel-Crafts Alkylation
- Intramolecular Friedel-Crafts Reactions
- Nucleophilic Aromatic Substitution (NAS)
- Nucleophilic Aromatic Substitution (2) - The Benzyne Mechanism
- Reactions on the "Benzylic" Carbon: Bromination And Oxidation
- The Wolff-Kishner, Clemmensen, And Other Carbonyl Reductions
- More Reactions on the Aromatic Sidechain: Reduction of Nitro Groups and the Baeyer Villiger
- Aromatic Synthesis (1) - "Order Of Operations"
- Synthesis of Benzene Derivatives (2) - Polarity Reversal
- Aromatic Synthesis (3) - Sulfonyl Blocking Groups
- Birch Reduction
- Synthesis (7): Reaction Map of Benzene and Related Aromatic Compounds
- Aromatic Reactions and Synthesis Practice
- Electrophilic Aromatic Substitution Practice Problems
20 Aldehydes and Ketones
- What's The Alpha Carbon In Carbonyl Compounds?
- Nucleophilic Addition To Carbonyls
- Aldehydes and Ketones: 14 Reactions With The Same Mechanism
- Sodium Borohydride (NaBH4) Reduction of Aldehydes and Ketones
- Grignard Reagents For Addition To Aldehydes and Ketones
- Wittig Reaction
- Hydrates, Hemiacetals, and Acetals
- Imines - Properties, Formation, Reactions, and Mechanisms
- All About Enamines
- Breaking Down Carbonyl Reaction Mechanisms: Reactions of Anionic Nucleophiles (Part 2)
- Aldehydes Ketones Reaction Practice
21 Carboxylic Acid Derivatives
- Nucleophilic Acyl Substitution (With Negatively Charged Nucleophiles)
- Addition-Elimination Mechanisms With Neutral Nucleophiles (Including Acid Catalysis)
- Basic Hydrolysis of Esters - Saponification
- Transesterification
- Proton Transfer
- Fischer Esterification - Carboxylic Acid to Ester Under Acidic Conditions
- Lithium Aluminum Hydride (LiAlH4) For Reduction of Carboxylic Acid Derivatives
- LiAlH[Ot-Bu]3 For The Reduction of Acid Halides To Aldehydes
- Di-isobutyl Aluminum Hydride (DIBAL) For The Partial Reduction of Esters and Nitriles
- Amide Hydrolysis
- Thionyl Chloride (SOCl2)
- Diazomethane (CH2N2)
- Carbonyl Chemistry: Learn Six Mechanisms For the Price Of One
- Making Music With Mechanisms (PADPED)
- Carboxylic Acid Derivatives Practice Questions
22 Enols and Enolates
- Keto-Enol Tautomerism
- Enolates - Formation, Stability, and Simple Reactions
- Kinetic Versus Thermodynamic Enolates
- Aldol Addition and Condensation Reactions
- Reactions of Enols - Acid-Catalyzed Aldol, Halogenation, and Mannich Reactions
- Claisen Condensation and Dieckmann Condensation
- Decarboxylation
- The Malonic Ester and Acetoacetic Ester Synthesis
- The Michael Addition Reaction and Conjugate Addition
- The Robinson Annulation
- Haloform Reaction
- The Hell–Volhard–Zelinsky Reaction
- Enols and Enolates Practice Quizzes
23 Amines
- The Amide Functional Group: Properties, Synthesis, and Nomenclature
- Basicity of Amines And pKaH
- 5 Key Basicity Trends of Amines
- The Mesomeric Effect And Aromatic Amines
- Nucleophilicity of Amines
- Alkylation of Amines (Sucks!)
- Reductive Amination
- The Gabriel Synthesis
- Some Reactions of Azides
- The Hofmann Elimination
- The Hofmann and Curtius Rearrangements
- The Cope Elimination
- Protecting Groups for Amines - Carbamates
- The Strecker Synthesis of Amino Acids
- Introduction to Peptide Synthesis
- Reactions of Diazonium Salts: Sandmeyer and Related Reactions
- Amine Practice Questions
24 Carbohydrates
- D and L Notation For Sugars
- Pyranoses and Furanoses: Ring-Chain Tautomerism In Sugars
- What is Mutarotation?
- Reducing Sugars
- The Big Damn Post Of Carbohydrate-Related Chemistry Definitions
- The Haworth Projection
- Converting a Fischer Projection To A Haworth (And Vice Versa)
- Reactions of Sugars: Glycosylation and Protection
- The Ruff Degradation and Kiliani-Fischer Synthesis
- Isoelectric Points of Amino Acids (and How To Calculate Them)
- Carbohydrates Practice
- Amino Acid Quizzes
25 Fun and Miscellaneous
- A Gallery of Some Interesting Molecules From Nature
- Screw Organic Chemistry, I'm Just Going To Write About Cats
- On Cats, Part 1: Conformations and Configurations
- On Cats, Part 2: Cat Line Diagrams
- On Cats, Part 4: Enantiocats
- On Cats, Part 6: Stereocenters
- Organic Chemistry Is Shit
- The Organic Chemistry Behind "The Pill"
- Maybe they should call them, "Formal Wins" ?
- Why Do Organic Chemists Use Kilocalories?
- The Principle of Least Effort
- Organic Chemistry GIFS - Resonance Forms
- Reproducibility In Organic Chemistry
- What Holds The Nucleus Together?
- How Reactions Are Like Music
- Organic Chemistry and the New MCAT
26 Organic Chemistry Tips and Tricks
- Common Mistakes: Formal Charges Can Mislead
- Partial Charges Give Clues About Electron Flow
- Draw The Ugly Version First
- Organic Chemistry Study Tips: Learn the Trends
- The 8 Types of Arrows In Organic Chemistry, Explained
- Top 10 Skills To Master Before An Organic Chemistry 2 Final
- Common Mistakes with Carbonyls: Carboxylic Acids... Are Acids!
- Planning Organic Synthesis With "Reaction Maps"
- Alkene Addition Pattern #1: The "Carbocation Pathway"
- Alkene Addition Pattern #2: The "Three-Membered Ring" Pathway
- Alkene Addition Pattern #3: The "Concerted" Pathway
- Number Your Carbons!
- The 4 Major Classes of Reactions in Org 1
- How (and why) electrons flow
- Grossman's Rule
- Three Exam Tips
- A 3-Step Method For Thinking Through Synthesis Problems
- Putting It Together
- Putting Diels-Alder Products in Perspective
- The Ups and Downs of Cyclohexanes
- The Most Annoying Exceptions in Org 1 (Part 1)
- The Most Annoying Exceptions in Org 1 (Part 2)
- The Marriage May Be Bad, But the Divorce Still Costs Money
- 9 Nomenclature Conventions To Know
- Nucleophile attacks Electrophile
27 Case Studies of Successful O-Chem Students
- Success Stories: How Corina Got The The "Hard" Professor - And Got An A+ Anyway
- How Helena Aced Organic Chemistry
- From a "Drop" To B+ in Org 2 – How A Hard Working Student Turned It Around
- How Serge Aced Organic Chemistry
- Success Stories: How Zach Aced Organic Chemistry 1
- Success Stories: How Kari Went From C– to B+
- How Esther Bounced Back From a "C" To Get A's In Organic Chemistry 1 And 2
- How Tyrell Got The Highest Grade In Her Organic Chemistry Course
- This Is Why Students Use Flashcards
- Success Stories: How Stu Aced Organic Chemistry
- How John Pulled Up His Organic Chemistry Exam Grades
- Success Stories: How Nathan Aced Organic Chemistry (Without It Taking Over His Life)
- How Chris Aced Org 1 and Org 2
- Interview: How Jay Got an A+ In Organic Chemistry
- How to Do Well in Organic Chemistry: One Student's Advice
- "America's Top TA" Shares His Secrets For Teaching O-Chem
- "Organic Chemistry Is Like..." - A Few Metaphors
- How To Do Well In Organic Chemistry: Advice From A Tutor
- Guest post: "I went from being afraid of tests to actually looking forward to them".
Thanks for this wonderful website. I am not able to go for the membership plan, but all the information you’ve laid out for free is staggering and truly wonderful. I have learnt (and am learning) a lot from here.
I have a question too. If we take this molecule (https://www.wolframalpha.com/input?i=smiles+identifier+C1C%28%5BC%2B%5D%28C%29%28C%29%29C1), will it do ring expansion? Or will sigma-resonance stabilize it enough?
If there are possibilities of both tertiary carbonation and ring expansion then which one gets more preference?…which one forms?
In real life, both can occur.
In an exam-type situation, if you are faced with expanding a 4-membered ring to a 5-membered ring *or* doing a methyl shift to give a tertiary carbocation, you should expand out the 4-membered ring due to relief of ring strain.
There is very little relief of ring strain on going from 5- to 6-, and for that reason the two possibilities are a lot less clear cut in that case.
hi James if there were possibilities to form a tertiary carbocation in two ways in one way it’s an alkyl shift and another is a hydride shift, both leading to a stable carbocation (tertiary) which is likelyto occur …?
Almost certainly the hydride would migrate preferentially. It’s just easier from a principle of least motion standpoint.
Can ring expansion occur the other way around? I mean in a ring of Methyl cyclopentane. If there is a plus charge on the adjacent position of methyl. Will ring expansion still take place?
Do you mean ring contraction? Unlikely to go from five to four since four membered rings have a lot of strain.
If there is competing alkyl vs. hydride shifts that give equivalently stable carbocation, hydride shift will generally be favored since less molecular motion is involved.
Would you describe a ring expansion as exothermic or endothermic?
Generally, if one is going from a more strained ring to a less strained ring, the reaction should be exothermic since ring strain is being released.
Who is James why he is answering one?
Hi James, love your website and its worth every penny. We just had an interesting discussion in our study group regarding rearrangements and need an expert to weigh in. The BIG question: do rearrangements occur sequentially after carbocation formation or does it happen so fast that it appears as just one step? As an example: Hypothetically, imagine a substitution reaction at a primary carbon connected to a secondary carbon which is then connected to tertiary carbon. If you could form the primary carbocation, would the secondary then form, followed by the tertiary in a series of steps? In our discussion we imagined a reaction energy diagram with three humps, one for the activation energy of each carbocation intermediate and then a hodgepodge of products. We also considered that if this was the case a secondary carbocation would never result from a primary since they “never form”. On the other hand we also considered that the rearrangements happen simultaneously and so rapidly that they are coincident with the formation of “the most stable carbocation possible”. Finally, our Buddhist teammate said that we should not think in terms of duality so we broke it off and grabbed some beers and decided to ask you. (By the way we noted your example of lanosterol synthase. If it does happen sequentially that must be a real mess of side products)
Hi Len – it’s reasonable to postulate the existence of a transient secondary carbocation, but likely not a primary one. If rearrangement is happening to a primary site, likely that 1,2- shift is happening at the same time that the leaving group is being displaced so that at no point is there a “free” primary carbocation.
for a vinylcyclopentane reacting with H3O+, would it be better to do a hydride shift, or expand the ring structure?
HI Raeanne – I would start with the hydride shift, since the 5-membered ring is not very strained relative to (say) a 4-membered ring.
If we have choice,then which alkyl group we will shift,suppose we have methyl,ethyl and propyl,among this option wjow we will shift.
Hello sir. If we have cyclohexyl methyl bromide and we react it to water, will it go as per Sn1 reaction and cause a rearrangement( a hydride shift) or will it proceed directly via Sn2 mechanism?? In general, if a compound is 1° but it can go rearrangement, will it proceed via Sn1 or Sn2 mechanism??
The short answer is that if it’s primary, then you’re generally looking at an SN2 process since primary carbocations are unstable and difficult to form.
Sir,
If suppose during ring expansion, in the figure you have uploaded we have an oxygen atom at 4th position.Then which of the following bonds C3-C6 or C3-O4 will be broken for ring expansion?
What you are describing is the ring expansion of an oxetane into a tetrahydrofuran derivative. I would expect the C-C bond should migrate with simultaneous formation of an intermediate C=O pi bond. This would be a variant of the pinacol rearrangement.
Hey
Can you tell about ethyl shifts …
If there are two groups on carbon one methyl and second ethyl then which group will shift to carbonation
Good question. Ethyl (and other primary carbons) will migrate preferentially over methyl.
Hi! Why did another water molecule appear in the first example of a alkyl shift? I thought you already attached one molecule to the carbocation. Shouldn’t the “leftovers” be H+Cl-? I understand the extra hydrogen makes the other water molecule turn into hydronium ion but where did the other water come in the first place? Is this a water characteristic in reactions?
Thank you!
This would be with water as solvent (an ionizing solvent). Without the water, no substitution reaction would occur.
Why does the alkyl shift cause C2-C4 bond and not C2-C7?
Why rearrangement doesn’t occur at 1 butylcarbocation. If occurs what is the product when 1butene reacts with Hcl?
Once the tertiary carbocation forms, no rearrangement can occur that will make it more stable.
How do you decide between ring expansion,alkyl shift and hydride shift
is it necessary that such shifts occur only from the carbon atom adjacent to the positive charge??
Yes.
Please explain why the reaction of neopentane yields 1- chloro- 2,2-dimethyl propane when treated with chlorine molecule during free radical mechanism ?
why not 1,2 methyl shift to give more stable tertiary free radical as compared to less stable primary free radicle????
please reply.
1,2-shifts do not occur with radical substrates. Only carbocations.
Hi James,
Greetings.
How would you explain the methyl and ethyl carboxylate groups attached to the same carbon and only the ester group migrates as the 1,2 shift?
Is the migration via a radical or to the carbanion? I do not think that to the carbocation center. You said there are no 1,2-shifts in radicals.
Best,
Helena
It would help to see the specific example you’re thinking of, but it wouldn’t be a radical shift. There are two lone pairs on oxygen that could come down to help stabilize any positive charge buildup on the carbonyl carbon during the shift.
The 1,2-hydride shift occurs because of the hydrogen atom moving to the adjacent carbon with both of the electrons in the bond, according to what my teacher explained. He said that even alkyl groups can shift (though alkyls are much larger than hydrogen) since only the electron cloud is actually shifting.
Is it then possible to have any other shift that stabilizes the carbocation intermediate in a reaction, e.g. a -OH shift? You mentioned “allowed” rearrangement reactions, so what reactions are disallowed and why?
Thanks in advance!
Both reduction amd rearrangement occur in reaction…???
Replacing Cl with OH is not a reduction. Both are more electronegative than carbon.
I think in the very last reaction the attack by the water is shown at a wrong place.
no, i think it’s correct. When you expand you realize that the C that shifted is satisfied with 4 bonds C-H, C-CH2, C-CH3, C-C+. Whereas the C to which OH later gets attached has only 3 bonds C-CH2, C-CH3 and C-CH. Moreover if the OH had attached to the C you think it should’ve, it wouldn’t have enough free valence e- to another atom.
I hope I haven’t confused you.
Fixed, thank you Satyajit.
Why the attack of water molecule is at C-2 instead of C -3 carbocation.please explain.drklbajaj
It was an error. Thanks for correcting me Krishnan.