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Selectivity in Free Radical Reactions: Bromination vs. Chlorination
Last updated: December 7th, 2022 |
The Selectivity of Free-Radical Bromination vs Chlorination. A Detailed Answer
In last blog post on radicals we saw this data that compares the chlorination of propane vs. the bromination of propane.
For chlorination, the reaction is selective for secondary C-H over primary C-H by a factor of 55/(45/3) = 3.6 to 1
Wow!!! Is bromination ridiculously more selective than chlorination, or what?? (Again, if you want to know how this was calculated, go back to the last post.)
Today we’re going to try to answer, “why is bromine more selective than chlorine” ? You might think that going from 4:1 to 97:1 will involve a huge difference in energies. But as we’ll see, it’s more subtle than you might expect. A few kcal/mol can make a huge difference!
Table of Contents
- The Selectivity-Generating Step Is Breakage Of The C–H Bond By A Halogen Radical (Propagation Step #1)
- Background: Activation Energy And The Arrhenius Equation (Yes, This Is Relevant)
- Heat Increases The Average Velocity (And Energy) Of Molecules
- As A Reaction Mixture Is Heated, A Larger Proportion Of Molecules Will Have Sufficient Activation Energy (Ea) To React
- Selectivity Is Proportional To Differences In Activation Energy For The Key Step
- Small Differences In Activation Energies (~ 3 kcal/mol) Can Mean LARGE Differences In Selectivity (97:1)
- So WHY Is The Difference For Activation Energies Greater For Bromination Than For Chlorination?
- The Transition State For Chlorination Resembles The Reactants (An “Early” Transition State) Which Are Close Together In Energy. So Selectivity Is Low.
- The Transition State For Bromination Resembles The Products (A “Late” Transition State) Which Are Farther Apart In Energy. So Selectivity Is High.
- Summary: Selectivity For Free-Radical Chlorination vs Bromination
- (Advanced) References and Further Reading
1. The Selectivity-Generating Step Is Breakage Of The C–H Bond By A Halogen Radical (Propagation Step #1)
The question that this post hopes to answer is “Why is bromine more “selective” for the secondary carbon than chlorine?”.
It’s kind of a long answer. This post goes through the data and makes the scientific argument. In the next post I’ll put forward a simple analogy that simplifies this idea for many students.
The first thing to note is formation of the different chloropropanes happens during the chain propagation step (that is, after initiation). So for our purposes here we are only going to analyze the two propagation steps and assume that initiation has already occurred. In other words, this step:
Before addressing the topic of selectivity directly, let’s first begin by talking about activation energy. You might recall that in order for a reaction to occur, the reactants must come into contact with each other with sufficient energy to overcome the repulsions between them (electron clouds). They also must collide in such a way that allows for transfer of electrons (aka “orbital overlap”)
In other words, the rate is equal to:
[concentration of molecules with sufficient energy] *[probability they collide in the “right way”]
This is dealt with by the Arrhenius equation:
Here, the “probability they collide in the right way” is dealt with by the pre-exponential factor A and is unique for each reaction. The exponential factor e-Ea/RT is what’s known as the “distribution function” and this is how we calculate the % of molecules in solution that have sufficient energy to react (heretofore known as the “activation energy”. )
Note the fact that this is temperature dependent. Why might that be? Well, as a collection of molecules is heated, the average speed of those molecules will increase. This is described by a related function known as the Boltzmann distribution, shown right here at three different temperatures [a=1, a=2, and a=5]. The y-axis shows the # of molecules (“distribution”) and the x-axis shows energy [thank you Wikipedia]
Heat Increases The Average Velocity Of Molecules
This shows the “distribution” (aka “number”) of molecules at specific energies. The hump in the middle is the “most probable” energy, and notice how it resembles a bell curve with tails at both ends. At the far right of the scale are the most energetic molecules. Note what happens when temperature is increased: on average, molecules have greater energy, and also the right-hand “tail” extends further out.
4. As A Reaction Mixture Is Heated, A Larger Proportion Of Molecules Will Have Sufficient Activation Energy (Ea) To React
Now imagine we have a reaction with activation energy Ea. At very cold temperatures, very few molecules have sufficient energy to react. But as the reaction mixture is heated, the proportion of those molecules increases. Ergo, the rate of the reaction increases with heat. Here is an example of how it works. Note how in this case below, the reaction doesn’t occur at 300K, but starts to occur at a reasonable rate at 330K!
Okay, that covers changing the reaction temperature. But what does this have to do with the selectivity of halogenation?
5. Selectivity For Bromination vs Chlorination Is Proportional To Differences In Activation Energy For The Key Step
Here, we’re keeping reaction temperature constant, but the activation energy for each reaction is slightly different.
We have four reactions in total to think about (two different halogens and two different C-H bonds). The activation energy for each reaction has been experimentally measured [Ref 2] (and here, we’re talking about the activation of C-H bond breaking [i.e. propagation] not initiation. Here’s what they look like.
This is actually all the information we need to be able to make a rough estimate of selectivities.
For a reaction at 300K, we can calculate RT using the gas constant (1.987 cal/ K mol) and plug in the activation energy for each reaction. By dividing the two equations by each other, the pre-exponential factor A will roughly cancel out and we can obtain estimates for selectivities.
The bottom line here is that due to the nature of the Arrhenius equation, the greater the difference between activation energies, the larger the selectivity. The effects can be dramatic, even when going from a difference of 1kcal/mol (for chlorination) to 3 kcal/mol (for bromination of primary vs. secondary)
6. Small Differences In Activation Energies (~ 3 kcal/mol) Can Mean LARGE Differences In Selectivity (97:1)
Just a bit of math using the Arrhenius equation, a rough calculation of selectivities based on differences in activation energy.
All this is fine – it’s based on experimental data for activation energies. But it just opens up another question.
WHY is the difference for activation energies greater for bromination (3 kcal/mol) than for chlorination (1 kcal/mol). Great question!
Understanding this point begins with understanding the energy profile of these two different reactions (chlorination and bromination).
We’ll do the math in a second, but the key difference is that in chlorination, the key propagation step is exothermic and in bromination, the key propagation step is endothermic. This is because chlorination forms a strong H-Cl bond (103 kcal/mol) and bromination forms a much weaker H-Br bond (87 kcal/mol).
The two reaction coordinates roughly look like this:
Look closely where the transition state is for each reaction.
In chlorination, the reaction is exothermic, and the transition state resembles the reactants. According to Hammond’s postulate, we could say that this transition state is “early”.
In bromination, the reaction is endothermic, and the transition state resembles the products. According to Hammond’s postulate we say that this transition state is “late”.
8. The Transition State For Chlorination Resembles The Reactants (An “Early” Transition State) Which Are Close Together In Energy. So Selectivity Is Low.
What this means is that for chlorination, the difference in activation energies between the two radical pathways (i.e. “secondary” and “primary”) will most closely resemble the reactants (which are identical in energy). So there will be a very small difference in activation energies between the two.
The difference in activation energies is small (1 kcal/mol) because of the early transition state. Note that even though there is a fairly large difference in energy between the products (3kcal/mol) it doesn’t affect the activation energies.
9. The Transition State For Bromination Resembles The Products (A “Late” Transition State) Which Are Farther Apart In Energy. So Selectivity Is High.
We can do the same analysis for bromination. Here, it’s a “late” transition state, so the difference in activation energies between primary and secondary will closely resemble the differences in energy between the two. So we would expect the activation energy difference to more closely resemble the difference between the energies of the products. And that is the case!
So the bottom line for today’s post is a few things:
1) going from an activation energy difference of 1kcal/mol to about 3 kcal/mol can mean the difference between a reaction with a selectivity of 3.5:1 and a reaction with a selectivity of 97:1. Wow!!
2) We compared a reaction with an “early” transition state and a “late” transition state and saw that the reaction with the “late” transition state was more selective. This is to be expected when we’re starting with identical reactants and there are significant differences in the energies of the products.
In the next post I’ll use a simple analogy to drive home the point in a way that has helped students to understand this point on a more intuitive level.
Next Post: Halogenation At Tiffany’s
- The interaction of free radicals with saturated aliphatic compounds
J. M. Tedder
Q. Rev. Chem. Soc., 1960, 14, 336-356
This review, though dated, contains a wealth of useful information, including the derivation of the activation energy of free-radical aliphatic bromination (13.3 kcal/mol), and the proposal that NBS can brominate alkanes by providing a steady low concentration of Br2.
- Competitive chlorination reactions in the gas phase: hydrogen and C1—C5 saturated hydrocarbons
John H. Knox and Robert L. Nelson
Faraday Soc., 1959, 55, 937-946
This paper is concerned with the free-radical chlorination of C1-C5 alkanes, and is a detailed kinetic analysis of the chlorination reaction using the Arrhenius equation. The experimentally determined activation energies are all rather low, on the order of 0.02-4.1 kcal/mol.
- Substitutions at Saturated Carbon-Hydrogen Bonds Utilizing Molecular Bromine or Bromotrichloromethane
Glen A. Russell and Charles DeBoer
Journal of the American Chemical Society 1963, 85 (20), 3136-3139
This paper studies benzylic bromination via a free-radical mechanism. As expected, reaction rates increase with increasing substitution at the benzylic position, but this paper provides experimental evidence for that.
- Directive Effects in Aliphatic Substitutions. XIX. Photobromination with N-Bromosuccinimide
Glen A. Russell and Kathleen M. Desmond
Journal of the American Chemical Society 1963, 85 (20), 3139-3141
This paper immediately follows Ref. 3 in the same issue of JACS, and provides evidence that NBS can also brominate via a free-radical mechanism (i.e. the Wohl-Ziegler reaction).
- Solvent Effects in the Reactions of Free Radicals and Atoms. VIII. The Photochlorination of Aralkyl Hydrocarbons
Glen A. Russell, Akihiko. Ito, and Dale G. Hendry
Journal of the American Chemical Society 1963, 85 (19), 2976-2983
Significant solvent effects are observed in the free-radical photochlorination of benzylic compounds. Complexation by solvent can significantly attenuate the reactivity of the chlorine radical, making it react more selectively.
- Reactions of radicals. 42. Hydrogen abstraction by the p-nitrophenyl radical
A. Pryor, K. Smith, J. T. Echols, and D. L. Fuller
The Journal of Organic Chemistry 1972, 37 (11), 1753-1758
Besides bromine and chlorine, other radicals can be generated and used in free-radical reactions. Organic radicals can also be used, and in this case, the phenyl and p-nitrophenyl radicals can be generated from decomposition of the respective azo precursors. The p-nitrophenyl radical is observed to be more selective than the phenyl radical in free-radical substitution reactions.
- Inertia and driving force of chemical reactions
M. G. Evans and M. Polanyi
Trans. Faraday Soc., 1938, 34, 11-24
This is a very important paper, introducing what is now known as the Bell-Evans-Polanyi principle. This observes that the difference in activation energy between two reactions of the same family is proportional to the difference of their enthalpy of reaction, thus allowing comparisons of similar reactions. Interestingly, while Michael Polanyi (this paper), was elected to the Royal Society, he did not receive a Nobel Prize, but his son, J. C. Polanyi, received the Nobel Prize in Chemistry in 1986 for his work in chemical physics.
- A Correlation of Reaction Rates
George S. Hammond
Journal of the American Chemical Society 1955, 77 (2), 334-338
Hammond’s Postulate is commonly taught to chemistry undergraduates worldwide and is a common mental tool which is also employed by experienced scientists. Exergonic reactions will have ‘early’ transition states resembling the starting materials, while endergonic reactions will have ‘late’ transition states resembling the products.
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
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- A Key Skill: How to Calculate Formal Charge
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- 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
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- Exploring Resonance: Pi-Donation
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- In Summary: Evaluating Resonance Structures
- Drawing Resonance Structures: 3 Common Mistakes To Avoid
- How to apply electronegativity and resonance to understand reactivity
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02 Acid Base Reactions
- Introduction to Acid-Base Reactions
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03 Alkanes and Nomenclature
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05 A Primer On Organic Reactions
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06 Free Radical Reactions
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- Selectivity In Free Radical Reactions
- Selectivity in Free Radical Reactions: Bromination vs. Chlorination
- Halogenation At Tiffany's
- Allylic Bromination
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07 Stereochemistry and Chirality
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08 Substitution Reactions
- Introduction to Nucleophilic Substitution Reactions
- Walkthrough of Substitution Reactions (1) - Introduction
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- 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
- The Conjugate Base is Always a Stronger Nucleophile
- Substitution Practice - SN1
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09 Elimination Reactions
- Elimination Reactions (1): Introduction And The Key Pattern
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- The E1 Reaction
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- 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
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
- Deciding SN1/SN2/E1/E2 (3) - The Solvent
- Deciding SN1/SN2/E1/E2 (4) - The Temperature
- Wrapup: The Quick N' Dirty Guide To 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
- Addition Reactions: Elimination's Opposite
- Selective vs. Specific
- Regioselectivity In Alkene Addition Reactions
- Stereoselectivity In Alkene Addition Reactions: Syn vs Anti Addition
- Hydrohalogenation of Alkenes and Markovnikov's Rule
- Hydration of Alkenes With Aqueous Acid
- Rearrangements in Alkene Addition Reactions
- Addition Pattern #1: The "Carbocation Pathway"
- Halogenation of Alkenes and Halohydrin Formation
- Oxymercuration Demercuration of Alkenes
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- Hydroboration Oxidation of Alkenes
- m-CPBA (meta-chloroperoxybenzoic acid)
- OsO4 (Osmium Tetroxide) for Dihydroxylation of Alkenes
- Palladium on Carbon (Pd/C) for Catalytic Hydrogenation
- Alkene Addition Pattern #3: The "Concerted" Pathway
- 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
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13 Alkyne Reactions
- Acetylides from Alkynes, And Substitution Reactions of Acetylides
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- Alkyne Reactions - The "Concerted" Pathway
- Alkenes To Alkynes Via Halogenation And Elimination Reactions
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14 Alcohols, Epoxides and Ethers
- Alcohols - Nomenclature and Properties
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- What's An Organometallic?
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- The Heck, Suzuki, and Olefin Metathesis Reactions (And Why They Don't Belong In Most Introductory Organic Chemistry Courses)
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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
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- Diels-Alder Reaction: Kinetic and Thermodynamic Control
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- The Pi Molecular Orbitals of Benzene
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- Frost Circles
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19 Reactions of Aromatic Molecules
- Electrophilic Aromatic Substitution: Introduction
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- 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
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- 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
- 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
- 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
- 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
- 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
- Organic Chemistry GIFS - Resonance Forms
- 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" ?
- Intramolecular Reactions of Alcohols and Ethers
- 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)
- Reproducibility In Organic Chemistry
- 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
- How Reactions Are Like Music