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A Primer On Organic Reactions
What makes a good leaving group?
Last updated: March 29th, 2019 |
A leaving group is a nucleophile acting in reverse; it accepts a lone pair as the bond between it and its neighbor (usually carbon for our purposes) is broken.
So what makes a good leaving group?
The “happier” and more stable that lone pair is, the better a leaving group it will be. The most predictive rule for leaving group ability is….
Good leaving groups are weak bases.
Why?
Think about the dissociation of an acid H–A to give H+ and A– . The species A– is the conjugate base of HA. It accepts a pair of electrons from the H-A bond. It’s a base acting in reverse.
The more stable A- is, the greater the equilibrium constant will be that favors dissociation to give A- .
This turns out to be the very definition of acidity! The negative log of the equilibrium constant is the familiar pKa.
In other words pKa is a direct measurement of how “happy” and stable a lone pair of electrons is – the very definition of what we should be looking for when trying to quantify leaving group ability.
So it should be no surprise to find that very weak bases such as halide ions (I-, Br-, Cl-) water (OH2), and sulfonates such as p-toluenesulfonate (OTs) and methanesulfonate (OMs) are excellent leaving groups.
These are the conjugate bases of strong acids. (Recall that the stronger the acid, the weaker the conjugate base).
[One word of caution: pKa measures an equilibrium (think “differences in energy”), whereas leaving group ability is based on reaction rates. So although the correlation is very good, it isn’t perfect.]
On the other hand, strong bases are bad leaving groups. This is why alcohols don’t participate in SN2 reactions very much! the hydroxyl group (HO-) is a terrible leaving group.
If you’re not sure where a reaction is going to happen on a molecule, look for a good leaving group. That’s usually where the action is!
The trend is pretty clear – in general, the weaker the base, the better the leaving group. Furthermore, note how we (almost) never see alkanes or hydrogens as leaving groups. That’s because they’re strongly basic anions – and very unstable.
You might note that I have carefully avoided discussing fluorine. Fluorine tends to be a very poor leaving group for SN1/SN2/E1/E2 reactions. In Org 2, you may see some examples where F can act as a leaving group when it is attached to a carbonyl carbon or an aromatic ring. These reactions (addition-elimination reactions) are a little bit different in that the rate determining step is not so related to loss of the leaving group. There are some extra factors at work in these situations that we can discuss if you’re curious.
Next Post: Leaving Groups Are Nucleophiles Acting In Reverse
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00 General Chemistry Review
- Gen Chem and Organic Chem: How are they different?
- How Gen Chem Relates to Organic Chem, Pt. 1 - The Atom
- From Gen Chem to Organic Chem, Pt. 2 - Electrons and Orbitals
- From Gen Chem to Organic Chem, Pt. 3 - Effective Nuclear Charge
- From Gen Chem to Organic Chem, Pt. 4 - Chemical Bonding
- From Gen Chem to Organic Chem, Pt. 5 - Understanding Periodic Trends
- From Gen Chem to Org Chem, Pt. 6 - Lewis Structures, A Parable
- From Gen Chem to Org Chem, Pt. 7 - Lewis Structures
- From Gen Chem to Org Chem, Pt. 8 - Ionic and Covalent Bonding
- From Gen Chem to Org Chem, Pt. 9 - Acids and Bases
- From Gen Chem to Organic Chem, Pt. 10 - Hess' Law
- From Gen Chem to Organic Chem, Pt. 11 - The Second Law
- From Gen Chem to Org Chem Pt. 12 - Kinetics
- From Gen Chem to Organic Chem, Pt. 13 - Equilibria
- From Gen Chem to Organic Chem, Part 14: Wrapup
01 Bonding, Structure, and Resonance
- How Concepts Build Up In Org 1 ("The Pyramid")
- Review of Atomic Orbitals for Organic Chemistry
- How Do We Know Methane (CH4) Is Tetrahedral?
- Hybrid Orbitals
- 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
- Partial Charges Give Clues About Electron Flow
- 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
- Evaluating Resonance Forms (2): Applying Electronegativity
- Evaluating Resonance Forms: Factors That Stabilize Negative Charges
- Evaluating Resonance Forms (4): Positive Charges
- Exploring Resonance: Pi-Donation
- Exploring Resonance: Pi-acceptors
- In Summary: Resonance
- Drawing Resonance Structures: 3 Common Mistakes To Avoid
- How to apply electronegativity and resonance to understand reactivity
02 Acid Base Reactions
- Introduction to Acid-Base Reactions
- Walkthrough of Acid Base Reactions (1)
- Walkthrough of Acid Base Reactions (2): Basicity
- Walkthrough of Acid-Base Reactions (3) - Acidity Trends
- Five Key Factors That Influence Acidity
- Walkthrough of Acid-Base reactions (4) - 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
- Acid Base Reactions: What's the Point?
03 Alkanes and Nomenclature
- Summary Sheet - Alkane 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
- Common Mistakes: Drawing Tetrahedral Carbons
- Common Mistakes in Organic Chemistry: Pentavalent Carbon
- Table of Functional Group Priorities for Nomenclature
- Organic Chemistry IUPAC Nomenclature Demystified With A Simple Puzzle Piece Approach
04 Conformations and Cycloalkanes
- Conformations
- Newman Projections
- Putting the Newman into ACTION
- 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"
- The Ups and Downs of Cyclohexanes
- 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)
05 A Primer On Organic Reactions
- The Most Important Question To Ask When Learning a New Reaction
- The 4 Major Classes of Reactions in Org 1
- Learning New Reactions: How Do The Electrons Move?
- How (and why) electrons flow
- 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
- Common Mistakes: Formal Charges Can Mislead
- 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?
- Leaving Groups Are Nucleophiles Acting In Reverse
- What makes a good leaving group?
- 3 Factors That Stabilize Carbocations
- Three Factors that Destabilize Carbocations
- What's a Transition State?
- Hammond's Postulate
- Grossman's Rule
- Draw The Ugly Version First
- Learning Organic Chemistry Reactions: A Checklist (PDF)
- Introduction to Addition Reactions
- Introduction to Elimination Reactions
- 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
07 Stereochemistry and Chirality
- On Cats, Part 4: Enantiocats
- On Cats, Part 6: Stereocenters
- The Single Swap Rule
- Introduction to Assigning (R) and (S): The Cahn-Ingold-Prelog Rules
- Assigning Cahn-Ingold-Prelog (CIP) Priorities (2) - The Method of Dots
- Types of Isomers: Constitutional Isomers, Stereoisomers, Enantiomers, and Diastereomers
- 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
08 Substitution Reactions
- Introduction to Nucleophilic Substitution Reactions
- Walkthrough of Substitution Reactions (1) - Introduction
- Two Types of Substitution Reactions
- Common Blind Spot: Intramolecular Reactions
- 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
- The SN2 Mechanism
- The Conjugate Base is Always a Stronger Nucleophile
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
- Comparing the E1 and E2 Reactions
- The E2 Reaction and Cyclohexane Rings
- Bulky Bases in Elimination Reactions
- Comparing the E1 and SN1 Reactions
- Elimination (E1) Reactions With Rearrangements
10 Rearrangements
11 SN1/SN2/E1/E2 Decision
12 Alkene Reactions
- E and Z Notation For Alkenes (+ Cis/Trans)
- Addition Reactions: Elimination's Opposite
- Selective vs. Specific
- Regioselectivity In Alkene Addition Reactions
- Stereoselectivity In Alkene Addition Reactions: Syn vs Anti Addition
- Markovnikov Addition Of HCl To Alkenes
- Alkene Hydrohalogenation Mechanism And How It Explains Markovnikov's Rule
- Curved Arrows and Alkene Addition Reactions
- Addition Pattern #1: The "Carbocation Pathway"
- Rearrangements in Alkene Addition Reactions
- Bromination of Alkenes
- Bromination of Alkenes: The Mechanism
- Alkene Addition Pattern #2: The "Three-Membered Ring" Pathway
- Hydroboration - Oxidation of Alkenes
- Hydroboration Oxidation of Alkenes Mechanism
- Alkene Addition Pattern #3: The "Concerted" Pathway
- Bromonium Ion Formation: A (Minor) Arrow-Pushing Dilemma
- 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
13 Alkyne Reactions
- Acetylides from Alkynes, And Substitution Reactions of Acetylides
- Partial Reduction of Alkynes To Obtain Cis or Trans Alkenes
- Hydroboration and Oxymercuration of Alkynes
- Alkyne Reaction Patterns - Hydrohalogenation - Carbocation Pathway
- 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
14 Alcohols, Epoxides and Ethers
- Alcohols (1) - Nomenclature and Properties
- Alcohols Can Act As Acids Or Bases (And Why It Matters)
- Alcohols (3) - Acidity and Basicity
- The Williamson Ether Synthesis
- Williamson Ether Synthesis: Planning
- 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 Alcohol Oxidations
- Intramolecular Reactions of Alcohols and Ethers
- 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)
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
- Grignard Practice Problems: Synthesis (1)
- Grignard Reactions And Synthesis (2)
- Organocuprates (Gilman Reagents): How They're Made
- Gilman Reagents (Organocuprates): What They're Used For
- Common Mistakes with Carbonyls: Carboxylic Acids... Are Acids!
- The Heck, Suzuki, and Olefin Metathesis Reactions (And Why They Don't Belong In Most Introductory Organic Chemistry Courses)
- Reaction Map: Reactions of Organometallics
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
- Homotopic, Enantiotopic, Diastereotopic
- 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
- How Concepts Build Up In Org 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 Control
- 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
- Regiochemistry In The Diels-Alder Reaction
18 Aromaticity
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
- Synthesis (7): Reaction Map of Benzene and Related Aromatic Compounds
20 Aldehydes and Ketones
- Weird Nomenclature In Carbonyl Chemistry
- Aldehydes and Ketones: 14 Reactions With The Same Mechanism
- Wittig Reaction
- Imines and Enamines
- On Acetals and Hemiacetals
- Carbonyl Chemistry: 10 Key Concepts (Part 1)
- Carbonyls: 10 key concepts (Part 2)
- Acid Catalysis Of Carbonyl Addition Reactions: Too Much Of A Good Thing?
- Breaking Down Carbonyl Reaction Mechanisms: Anionic Nucleophiles (Part 1)
- Breaking Down Carbonyl Reaction Mechanisms: Reactions of Anionic Nucleophiles (Part 2)
21 Carboxylic Acid Derivatives
- Simplifying the reactions of carboxylic acid derivatives (part 1)
- Carbonyl Mechanisms: Neutral Nucleophiles, Part 1
- Carbonyl chemistry: Anionic versus Neutral Nucleophiles
- Proton Transfers Can Be Tricky
- Let's Talk About the [1,2] Elimination
- Carbonyl Chemistry: Learn Six Mechanisms For the Price Of One
- Summary Sheet #5 - 9 Key Mechanisms in Carbonyl Chemistry
- Summary Sheet #7 - 21 Carbonyl Mechanisms on 1 page
- How Reactions Are Like Music
- Making Music With Mechanisms (PADPED)
- The Magic Wand of Proton Transfer
- The Power of Acid Catalysis
22 Enols and Enolates
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
24 Carbohydrates
- D and L Notation For Sugars
- What is Mutarotation?
- Reducing Sugars
- Pyranoses and Furanoses: Ring-Chain Tautomerism In 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
25 Fun and Miscellaneous
- Organic Chemistry and the New MCAT
- A Gallery of Some Interesting Molecules From Nature
- The Organic Chemistry Behind "The Pill"
- Maybe they should call them, "Formal Wins" ?
- Planning Organic Synthesis With "Reaction Maps"
- Organic Chemistry Is Shit
- The 8 Types of Arrows In Organic Chemistry, Explained
- The Most Annoying Exceptions in Org 1 (Part 1)
- The Most Annoying Exceptions in Org 1 (Part 2)
- 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
- The Marriage May Be Bad, But the Divorce Still Costs Money
- Why Do Organic Chemists Use Kilocalories?
- What Holds The Nucleus Together?
- 9 Nomenclature Conventions To Know
This is gold !!.. Thanks alot
Thanks!
Awesome! This clarifies a lot! Thanks!!
Glad you found it useful!
I love your site!!! I have been on this more than my text, increasing my quiz score.
Thank you so much!
Glad you find it useful!
this was super clear!
you’re too awesome, thnx a million!
Which is the good living group phenoxy ( PhO) or chloride (Cl)?
What’s a weaker base?
Which is the good leaving group phenoxy (PhO) or Chloride Cl?
Procedure for you figuring this out for yourself: Go to a pKa table and figure out the pKa of Phenol (PhOH) and HCl. Which is a stronger acid? Now: the stronger the acid, the weaker the conjugate base. . So which of these would have the weaker conjugate base? The weaker the base, the better the leaving group.
phenol has a pKa of about 16 and HCl has a pKa of about -5 so “strong acid weak conjugate base” would imply that Cl- is the better leaving group.
anyone knows HCl is a strong acid!! STUPID
hcl is a strong acid and it is the better leaving group
how did you get through general chemistry not knowing that?!
phenol at about 10 pka, HCl at about -7 pka. So HCl is the stronger acid=weaker conj. base = more stable as an anion = better leaving group. Do I have this thought process correct?
The most understandable way I have ever seen! Thank you very much, please keep up the good work.
Glad you find it helpful Ivana! thanks
Thank you so much. Very lucid and informative. Please keep it up.
Awesome blog!!!
I have my MCAT exam in 2 days, (well 1.5 days if you start counting from now), wish I stumbled on your blog 3 months ago! well, still 1.5 x 12 hours left! :)
Glad you’re finding it useful. When you’re done, tell your friends :-)
Yes, please discuss the extra factors you mentioned regarding F- in a-e rxns.
Thanks so much for always making it clear and interesting!
What happens if the leaving group is positive, making the charge on the carbon neutral, like S+(CH3)2?
Ah… well the leaving group would be S(CH3)2 which is a very weak base and an excellent leaving group ! (much like OH2(+)]
This is just what I need,Organic chemistry has been a part of chemistry that seemed to be impossible to understand. But with the help of your explanations I see my self getting a straight A,beyond the shadow of doubt. Thanks to you James.
this is a great website, veyr helpful and much clearer than my text or the professor. thank you.
Thanks for this amazing resource to help me through orgo. It’s people like you who need to be in Universities teaching students because anyone can see based off this website you’ve made, that you actually care about the people learning and understanding orgo….thank you.
Just wanted to say thank you. Wish I had found this website months ago. All the information is so accessible and very well organized. Keep up the tremendous work. You’ve gained a loyal visitor to your website. I’ll be back next semester for organic II as well.
This website is so useful! Thank you very much!
There is a mistake in the second paragraph, I think. An atom can never be neutral and charged at the same time. A neutral atom’s atomic number(number of protons) is the same as the number of electrons. I recommend you to say an atom becomes an ion(negatively charged) to reach the full octect.
No he means it BECOMES charged when the electrons move onto the leaving group. I don’t think there’s a mistake.
keep up your good work. your site really helped me and my friends most of the concepts in organic chemistry.
That’s so helpful! You made very simple. Are there other factors beside PKa? what about the bond strength effect in case of a hydroxy gp attched to a carbon…
Thank James
Is CN- a good leaving group
No, it’s quite a poor leaving group
Thanks! But can u explain plz why weak bases are better leaving groups?
The more stable the leaving group the better it’s ability to leave right? So, if the base is weak, it will not be in a hurry to accept protons and react. Hence, as the base is not readily reacting, it is stable and a good leaving group.
Weak bases are very stable as anions in solution on their own. A weak base has a strong conjugate acid (e.g. HCl is the strong conjugate acid of the weak base Cl-). In the acid, you can think of the base as being a “leaving group” that allows the H+ to go off on it’s own (Like Cl- rapidly leaving H+ in aqueous solution). Since the acid is strong, we know that the base must very readily leave.
Conversely, if the acid is weak (like CH3COOH), we know that it holds the H+ relatively tightly. The base (CH3COO-) is relatively unstable in solution, so it doesn’t want to leave the H+. It’s very similar when that same base is attached to a carbon skeleton. Cl will leave the carbon skeleton readily because it’s fairly happy just being Cl- in solution, whereas CH3COO- is not so happy to do so. Moderately strong bases, like NH3 are poor leaving groups, and strong bases like OH- are terrible leaving groups. If you take a very strong base like CHC- (acetylide) it will never leave, so we don’t even consider that a leaving group.
Where would O-Ph rank as a leaving group? Is it better than the halides?
the site is excellent, thank you very much, you had solved most of my problems in organic chemistry.
By far the most coherent explanation of nucleophiles and leaving groups I have seen so far. Better than Youtube and most text books for that matter!
What if there are two identical leaving groups on different parts of the molecule? Say, two bromines. What factors do you look at when choosing the leaving group then?
Then you would examine the carbon that the leaving groups are attached to. Primary, secondary, or tertiary? Then you would look at the reaction conditions (i.e. what nucleophile is present?). This then leads into the SN1/SN2/E1/E2 decision. You might want to check my “Quick N’ Dirty Guide to SN1/SN2/E1/E2 for more information there because it’s a lot to summarize in a blog comment.
I ve seen alkoxides getting removed during sn2th reactions (related to carbonyls) . I’m in a mess and get choked up in such reactions.please help
Thank you so much!
Minor fix needed on the first image: the reactants side is neutral while the products side is positive (need to either add a negative formal charge to Nu or take off the negative formal charge from L).
Could u explain why F can become a leaving group when it is attached to a carbonyl carbon or an aromatic ring?
Yes, F- is an OK leaving group in those situations (nucleophilic aromatic substitution) because the rate limiting step is attack of the aromatic ring (disrupting aromaticity). Loss of the leaving group is slow compared to that step. Also, because F- is so electron withdrawing, F actually helps to accelerate the reaction by removing electron density from the ring and making it easier to attack.
James, someone mentioned the difference for F- between protic and aprotic solvents, do you mind shedding some light and clarity on this, please?
Thank you in advance
No matter the solvent, F will be a poor leaving group in nucleophilic substitution and elimination reactions.
this is a great website which is helping for my exams very much clarifies my doubts in organic chemistry can you tell me why flurine is a exeption and in a carbonyl carbon or an aromatic ring acts as a good leaving group compared to other halogens
I- is a BLG than F- in aprotic solvents.
F- is a BLG than I- in protic solvents .
If something is a poor nucleophile it has to be BLG.
I am right , right ?
asking this because I read poor bases are BLG which I accept ,but I think that it should differ in solvents.
(Ex:F- in protic is much more stable than I- due h bond.)
someone please tell me if I am right or not.
F-
why iodine is a good leaving group as well as a good nucleophile
Thank you so much!
These articles really help. Continue the good work. Cheers from India !
+NR3 is the best leaving group.
Could you please explain the fluoride ion exception?
Oh. My. Goodness. This is absolutely amazing. I will never be able to thank you enough or express how much this has helped me. Seriously, you are a god in my eyes right now.
Awesome website, explains the any topic with ease. Whenever I have any doubt I refer MOC, it helps me a lot. Thank you Mr. James for your remarkable efforts. You have made learning organic chemistry interesting through this website.
the best explanations can be found here…thank u so much
OK, thank you ankit!
in which solvent is this pka table given.
Q. does the leaving tendency depend upon solvent for example if F- leaves in polar protic medium it would stabilize its charge through hydrogen bonding which might not be possible in polar aprotic medium
The C-F bond is very strong, and it’s unlikely to be a leaving group in substitution or elimination regardless of solvent.
Why are leaving group weak bases, any explanation for this rule??
Think about what’s happening in a reaction where a leaving group is created. You need to break a bond and to form a relatively stable species. So if the bond is too strong (e.g. C-F) or if the leaving group is not very stable by itself (stongly basic) then the activation energy for this process will be too high for it to happen.
I’m confused about HF and its conjugate base. Does F- make an excellent leaving group? So for an E2 reaction it would leave?
No, F- makes a terrible leaving group. Part of the reason is that the C-F is extremely strong.
It would definitely not leave in an E2 reaction.