How to Find Oxidation Numbers (Rules and Examples)
How to Find Oxidation Numbers (Rules and Examples)

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Calculating the oxidation state of a carbon

Last updated: November 12th, 2022 |

The Oxidation State Formalism

By the end of Gen Chem, calculating oxidation states of different metals should be pretty familiar.

Here’s what you do for a typical compound like FeCl3 for instance.

• Treat every bond between the metal and a different atom as if it were an ionic bond.
• That means the more electronegative elements (like chlorine, say, or oxygen) bear negative charges, and the less electronegative element (such as the metal) bears the positive charge.
• If the compound is neutral, the sum of the oxidation states also has to be neutral. (If the compound has a charge, you adjust the oxidation states accordingly so that their sum equals the charge).

1. Calculating Oxidation States In Inorganic Compounds

Now here’s a fun exercise. Try applying the same rules to carbon.

It’s going to feel a little bit weird. Why? Because there are two key differences.

• First, carbon is often more electronegative (2.5) than some of the atoms it’s bound to (such as H, 2.2). So what do you do in this case?
• Secondly, unlike metal-metal bonds, carbon-carbon bonds are ubiquitous. So how do you deal with them?

1. In a C-H bond, the H is treated as if it has an oxidation state of +1. This means that every C-H bond will decrease the oxidation state of carbon by 1.
2. Any two bonds between the same atom do not affect the oxidation state (recall that the oxidation state of Cl in Cl-Cl (and that of H in H-H) is zero. So a carbon attached to 4 carbons has an oxidation state of zero.

So unlike metals, which are almost always in a positive oxidation state, the oxidation state of carbon can vary widely, from -4 (in CH4) to +4 (such as in CO2). Here are some examples.

2. Calculating The Oxidation State Of Carbon

(Don’t forget that this is called a “formalism” for a reason. The charge on the carbon isn’t really +4 or –4. But the oxidation state formalism helps us keep track of where the electrons are going, which will come in handy very soon).

With an understanding of how to calculate oxidation states on carbon, we’re ready for the next step: understanding changes in the oxidation state at carbon, through reactions known as oxidations (where the oxidation state is increased), and reductions (where the oxidation state is reduced). More on that next time.

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
• 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
• 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”
• 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)
• Newman Projection Practice
• Cycloalkanes Practice Problems
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?
• What makes a good leaving group?
• 3 Factors That Stabilize Carbocations
• Equilibrium and Energy Relationships
• 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 Elimination Reactions
• Introduction to Free Radical Substitution Reactions
• Introduction to Oxidative Cleavage Reactions
• Bond Dissociation Energies = Homolytic Cleavage
• 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
• Synthesis (2) – Reactions of Alkanes
07 Stereochemistry and Chirality
• Types of Isomers: Constitutional Isomers, Stereoisomers, Enantiomers, and Diastereomers
• How To Draw The Enantiomer Of A Chiral Molecule
• 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
• On Cats, Part 4: Enantiocats
• On Cats, Part 6: Stereocenters
• Stereochemistry Practice Problems and Quizzes
08 Substitution Reactions
• Introduction to Nucleophilic Substitution Reactions
• Walkthrough of Substitution Reactions (1) – 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
• The Conjugate Base is Always a Stronger Nucleophile
• 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
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
• Selective vs. Specific
• Regioselectivity In Alkene Addition Reactions
• Markovnikov Addition Of HCl To Alkenes
• Alkene Hydrohalogenation Mechanism And How It Explains Markovnikov’s Rule
• Arrow Pushing 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
• Alkene Reactions: Ozonolysis
• Summary: Three Key Families Of Alkene Reaction Mechanisms
• Palladium on Carbon (Pd/C) for Catalytic Hydrogenation
• OsO4 (Osmium Tetroxide) for Dihydroxylation of Alkenes
• m-CPBA (meta-chloroperoxybenzoic acid)
• 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 or Na/NH3 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
• 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
• 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 The Mechanisms of 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
• SOCl2 Mechanism For Alcohols To Alkyl Halides: SN2 versus SNi
• 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?
• 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
• 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 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
• 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” ?
• 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

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