Source: LT5 Worksheet, Brown textbook Ch. 6
Tags: redox, oxidation, reduction, organic chemistry, NAD+, NADH, electron transfer, bonds to hydrogen, bonds to oxygen
Difficulty: Introductory to Intermediate | Prerequisites: General Chemistry (electron transfer, oxidation states). Helpful to review functional groups and hybridisation before starting.
Redox reactions are one of the most important classes of organic transformations. Every living cell runs on them: glycolysis, the citric acid cycle, and fatty-acid metabolism all depend on controlled oxidations and reductions. In organic chemistry, the concept shifts from the electron-transfer model you learnt in general chemistry to a bond-counting model, where you track bonds to hydrogen versus bonds to oxygen (or other heteroatoms). This topic is the gateway to understanding alcohol oxidations, alkene cleavage, catalytic hydrogenation, and carbonyl reductions, all of which appear in CHEM 212's later learning targets.
In organic chemistry, oxidation means increasing bonds to oxygen (or nitrogen/halogen) and decreasing bonds to hydrogen. Reduction is the reverse. You can classify any organic transformation as oxidation, reduction, or neither by counting those bonds, and you can link each redox change to whether the reaction is an addition, elimination, or substitution based on hybridisation changes.
Reduction
In general chemistry: the gain of electrons. In organic chemistry: an increase in the number of C–H bonds and/or a decrease in bonds to oxygen, nitrogen, or halogen (heteroatoms).
Think of it as: the molecule is gaining hydrogen or losing oxygen.
Oxidation
In general chemistry: the loss of electrons. In organic chemistry: an increase in bonds to oxygen, nitrogen, or halogen and/or a decrease in C–H bonds.
Think of it as: the molecule is gaining oxygen or losing hydrogen.
Reducing agent
The species that donates electrons (or hydrogen) and is itself oxidised in the process. Common lab examples include H₂/Pd, LiAlH₄, and NaBH₄. The biological reducing agent in many metabolic pathways is NADH (or NADPH).
In simple terms: the reducing agent is the helper that hands over hydrogen or electrons to another molecule.
Oxidizing agent
The species that accepts electrons (or hydrogen) and is itself reduced. Common lab examples include chromic acid (H₂CrO₄), PCC, Dess-Martin periodinane, and mCPBA. The biological oxidising agent in many metabolic pathways is NAD⁺ (or NADP⁺).
In simple terms: the oxidising agent is the helper that pulls hydrogen or electrons away from another molecule.
Forget oxidation states for now. In organic chemistry you count bonds:
Reduction = increase in bonds to H, decrease in bonds to O/N/X (heteroatoms)
Oxidation = increase in bonds to O/N/X, decrease in bonds to H
A reaction that swaps one heteroatom for another (e.g. Br for Cl) without changing the total count of bonds to hydrogen or heteroatoms is neither oxidation nor reduction.
A practical tip from the worksheet: always draw in the implicit hydrogens on both reactant and product. This makes the bond count obvious and removes guesswork.
Worked example from the worksheet:
Ethanol → acetaldehyde (catalysed by alcohol dehydrogenase, ADH). Ethanol loses two C–H bonds and gains a C=O bond. That is an increase in bonds to oxygen and a decrease in bonds to hydrogen, so ethanol has been oxidised. The oxidising agent is NAD⁺, which is itself reduced to NADH.
Note that this reaction is reversible. The enzyme catalyses both directions, and the equilibrium depends on substrate concentrations. NAD⁺ is an aromatic, positively charged ring (very stable), and accepting a hydride (H⁻) converts it to the neutral, non-aromatic NADH. The thermodynamic drive comes from the stability difference between these two forms.
The worksheet walks through several biological transformations. For each one, the method is the same: look at the functional group that changes, count bonds to H versus bonds to O/N/X, and classify.
Glycerol phosphate dehydrogenase
Dihydroxyacetone phosphate (a ketone, C=O) → (R)-glycerol phosphate (an alcohol, C–OH). The carbonyl gains two C–H bonds (addition of H). This is a reduction. NADPH is the reducing agent, and it is oxidised to NADP⁺ in the process.
The reaction type is addition (sp² ketone → sp³ alcohol, increased saturation).
Lysine catabolism
A saturated C–H bond is lost, and a new C=O pi bond forms. Bonds to H decrease. This is an oxidation. The reaction type is elimination (new pi bond = increased unsaturation).
Glutamate breakdown (amine → imine)
An amine (C–NH₃⁺) is converted to an imine (C=NH₂⁺). Bonds to nitrogen increase (single bond → double bond), and bonds to H decrease. This is an oxidation. The reaction type is elimination.
Once you have classified a transformation as oxidation or reduction, you can also identify the reaction type by looking at the hybridisation change:
Addition (A): sp or sp² → sp³. The molecule becomes more saturated. Reductions are often additions (e.g. hydrogenation of an alkene, reduction of a ketone to an alcohol).
Elimination (E): sp³ → sp² or sp. A new pi bond forms, and the molecule becomes more unsaturated. Oxidations are often eliminations (e.g. alcohol → aldehyde or ketone).
Substitution (S): no change in hybridisation. One atom or group is swapped for another. A substitution can be oxidation, reduction, or neither, depending on what is swapped (e.g. replacing H with OH is an oxidation; replacing Br with Cl is neither).
This framework is a quick diagnostic. When you see a reaction, ask: did the hybridisation change? If yes, which direction? That tells you both the reaction type and whether it is likely an oxidation or a reduction.
Students often think oxidation always involves oxygen. It does not. In organic chemistry, increasing bonds to nitrogen or halogen also counts as oxidation. The name is historical.
Students confuse the oxidising agent with the species being oxidised. The oxidising agent is the one doing the oxidising; it is itself reduced. NAD⁺ is the oxidising agent, NADH is the reduced form.
Adding HCl across a double bond is sometimes mistaken for a redox reaction. It is not. It is an addition, but neither oxidation nor reduction, because you gain a bond to H and a bond to Cl simultaneously (the changes cancel).
Students forget that substitution reactions can be redox reactions. Swapping –H for –OH is an oxidation, even though hybridisation does not change.
⚠️ You will be asked to classify biological transformations as oxidation or reduction. The bond-counting method (bonds to H vs. bonds to O/N/X) is the fastest way.
⚠️ Expect questions that pair redox classification with reaction type (addition, elimination, substitution). Know the hybridisation diagnostic.
⚠️ Be able to identify the oxidising and reducing agent in a reaction, including biological cofactors (NAD⁺/NADH, NADP⁺/NADPH).
⚠️ The ethanol/acetaldehyde/ADH/NAD⁺ example is a classic exam question. Know that ethanol is oxidised, NAD⁺ is the oxidising agent, and the reaction is reversible.
True or false: In organic chemistry, reduction means an increase in bonds to hydrogen. (True)
Fill in the blank: NAD⁺ acts as the ______ agent when ethanol is converted to acetaldehyde. (oxidising)
True or false: Adding HBr across a double bond is a redox reaction. (False, it is neither oxidation nor reduction.)
Fill in the blank: An elimination reaction forms a new ______ bond and makes the molecule more unsaturated. (pi)
True or false: Tertiary alcohols cannot be oxidised by chromium reagents because they lack an alpha hydrogen. (True)
Q: A ketone (C=O) is converted to a secondary alcohol (C–OH) using NaBH₄. Is this an oxidation or a reduction? What type of reaction is it?
A: This is a reduction (bonds to H increase, bonds to O decrease from a double bond to a single bond). The reaction type is addition (sp² → sp³).
Q: In the metabolism of ethanol by ADH, identify the oxidising agent and explain why the reaction is reversible.
A: NAD⁺ is the oxidising agent. It accepts a hydride (H⁻) from ethanol and is reduced to NADH. The reaction is reversible because the enzyme catalyses both directions, and the equilibrium depends on substrate and cofactor concentrations.
Q: Bromination of an alkene (adding Br₂ across a C=C) is an addition reaction. Is it an oxidation, reduction, or neither?
A: Neither. The molecule gains bonds to bromine (a heteroatom, which would suggest oxidation) but loses a C–C pi bond without changing bonds to H. By the strict bond-to-H-vs-bond-to-heteroatom count, this is neither oxidation nor reduction (sometimes called a "lateral" transformation).
Q: Classify the conversion of an amine (R–NH₂) to an imine (R=NH). What is the reaction type?
A: This is an oxidation (bonds to N increase from single to double, bonds to H decrease). The reaction type is elimination (new pi bond, sp³ → sp²).
This material connects directly to LT5's later sections on alcohol oxidation reagents (Jones, PCC, Dess-Martin, Swern) and alkene oxidation (ozonolysis, epoxidation), as well as reduction reactions (hydrogenation, metal-ammonia reduction, carbonyl reduction with LiAlH₄/NaBH₄). Understanding the bond-counting definition here is essential for predicting products in all of those transformations.
The biological cofactors NAD⁺/NADH and NADP⁺/NADPH reappear throughout biochemistry, especially in glycolysis, the citric acid cycle, and oxidative phosphorylation. The concept of a hydride source (H⁻ delivery) links directly to LT7 and LT8 material on nucleophilic addition to carbonyls.
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