Organic Chemistry · Alkynes: An Introduction to Organic Synthesis
Oxidative Cleavage of Alkynes
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Oxidative cleavage Breaking a C–C bond while oxidizing both carbons Full entry → tears the alkyne apart at the triple bond, converting the two alkyne carbons into Carboxylic acid Compound with a COOH group Full entry → carbons. The standard reagents are ozone (O3) followed by a water workup, or hot, concentrated potassium permanganate (KMnO4). The general patterns are:
- Internal alkyne Triple bond between two nonterminal carbons Full entry → R-C ≡ C-R' → two carboxylic acids, RCOOH and R'COOH.
- Terminal alkyne Triple bond at the end of a chain, RC ≡ CH Full entry → R-C ≡ CH → one carboxylic acid RCOOH plus CO2 (the terminal CH carbon becomes Carbon dioxide CO2, the fate of a terminal alkyne carbon Full entry →).
Because each carbon of the triple bond becomes a carboxyl carbon, the products are like a "carbon-counting fingerprint" of the original alkyne. This makes oxidative cleavage a classic structure-determination tool: run the reaction, identify the acids, and deduce the alkyne. It also complements the cleavage of alkenes (which give ketones/aldehydes) and completes the picture of how carbon skeletons are broken down by oxidation.
Why this matters
Oxidative cleavage answers two questions chemists ask constantly: "What is this molecule?" and "How do I make a carboxylic acid?" When an unknown alkyne is cleaved, the identities of the carboxylic acids reveal the length and branching of the alkyl groups on each side of the triple bond — no spectroscopy required, just careful product identification. In synthesis, cleavage converts alkynes into valuable diacids and monoacids; adipic acid, a precursor to nylon, and many pharmaceutical carboxylic acids are built from this kind of carbon-skeleton logic. For exams, the carbon-counting rule (every alkyne carbon ends up as a COOH carbon, except a terminal CH which becomes CO2) is a favorite trap that rewards careful bookkeeping.
The college version
Core Concepts
The cleavage reaction with ozone
Ozonolysis Reaction with ozone followed by an aqueous workup Full entry → of an alkyne uses ozone followed by water. The triple bond is completely broken, and each alkyne carbon is oxidized to the +3 oxidation state of a carboxylic acid carbon:
R-C ≡ C-R' 1) O3 2) H2O⟶ R-COOH + R'-COOH
For a terminal alkyne, the terminal carbon is oxidized all the way to carbon dioxide:
R-C ≡ CH 1) O3 2) H2O⟶ R-COOH + CO2
The reaction works because ozone is a strong oxidizing agent; the unstable molozonide-type intermediate collapses in the aqueous workup to the acid products. Note the contrast with alkene ozonolysis, which stops at aldehydes or ketones because the alkene carbons are only oxidized to the +1 or +2 oxidation state.
Cleavage with potassium permanganate
Hot, concentrated KMnO4 (often with acid) also cleaves alkynes to carboxylic acids. The stoichiometry for an internal alkyne can be written with permanganate as the oxidant:
3 R-C ≡ C-R' + 8 KMnO4 + 4 H2O → 3 R-COOH + 3 R'-COOH + 8 KOH + 8 MnO2
Terminal alkynes again give a carboxylic acid plus CO2. Permanganate cleavage is harsher than ozonolysis — it can also oxidize other easily oxidized groups present in the molecule — so ozonolysis is often preferred for delicate substrates. Both reagents share the same product logic: each triple-bond carbon becomes a carboxyl carbon.
Carbon counting and structure determination
The single most useful skill in this topic is working backward: given the cleavage products, reconstruct the alkyne. The rule is simple:
- The two carbons of the triple bond become the two carboxyl carbons of the products.
- Every carbon attached to the triple bond stays attached to its carboxyl carbon.
- A terminal CH on the alkyne becomes CO2 (one carbon, no alkyl group).
For example, if cleavage of an unknown alkyne gives only acetic acid (CH3COOH), the alkyne must have been 2-butyne, CH3C ≡ CCH3. If it gives acetic acid and hexanoic acid, the alkyne was 2-octyne, CH3C ≡ C(CH2)4CH3. This bookkeeping converts an "unknown" problem into simple addition: the alkyne is the two acids minus their two OH groups, joined at the carboxyl carbons.
Why cleavage is useful in synthesis
Oxidative cleavage is a way to "cut and label" a carbon skeleton. Because the products are strongly functionalized acids, cleavage is used to prepare dicarboxylic acids (e.g., from cyclic or di-alkynes), to degrade natural products for structural analysis, and to generate short-chain acids from long-chain alkynes. The reaction also illustrates the general principle that strong oxidants push carbon to its highest common oxidation state (+3 in carboxylic acids; terminal carbons go to +4 in CO2).
Common Confusions
| Do Not Confuse | With | The Difference |
|---|---|---|
| Alkyne cleavage products | Alkene cleavage products | Alkynes → carboxylic acids (+ CO₂ if terminal); alkenes → ketones/aldehydes |
| Terminal alkyne fate | Internal alkyne fate | Terminal CH carbon becomes CO₂ (lost as gas); internal carbons both stay as COOH |
| Ozonolysis of alkynes | Ozonolysis of alkenes | Same reagent, different oxidation levels: alkynes go further, to acids |
| CO2 carbon | Carboxylic acid carbon | The terminal carbon is oxidized to +4 (CO₂); the other carbons end at +3 (COOH) |
| "Cleavage" | "Hydrogenation" | Cleavage breaks the C–C bond and oxidizes; hydrogenation adds H₂ without breaking the skeleton |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Imagine a triple bond is a stick with a candy on each end, and the stick is cut exactly in the middle. Each cut end grows a "COOH" tag, which is what a carboxylic acid looks like. If one end of the stick had no candy (that's a terminal alkyne), that bare end turns into fizzy carbon dioxide gas instead. If you know what the two tagged pieces look like, you can figure out how long the original stick was — that's how chemists use cleavage to identify unknown molecules.
Worked example
Worked example 1 (product prediction). Cleave 4-octyne, CH3CH2CH2C ≡ CCH2CH2CH3, with ozone followed by water. Each side of the triple bond is a butyl group, so the products are two molecules of butanoic acid:
CH3CH2CH2C ≡ CCH2CH2CH3 1) O3 2) H2O⟶ 2 CH3CH2CH2COOH
Worked example 2 (structure determination, backward reasoning). An unknown alkyne is cleaved to give only acetic acid, CH3COOH. Since both carboxyl carbons came from the triple bond and each bears a methyl group, the alkyne must be 2-butyne, CH3C ≡ CCH3. If instead the products are propanoic acid (CH3CH2COOH) and CO2, the alkyne was 1-butyne, CH3CH2C ≡ CH: the propyl side becomes propanoic acid and the terminal CH carbon becomes CO2.
Worked example 3 (stoichiometry with dimensional analysis). How many grams of acetic acid can be produced (theoretically) by cleaving 10.0 g of 2-butyne, CH3C ≡ CCH3? Each mole of alkyne gives 2 mol of acetic acid:
n(2-butyne) = 10.0 g54.09 g/mol = 0.185 mol
n(acetic acid) = 0.185 mol × 2 mol1 mol alkyne = 0.370 mol
m(acetic acid) = 0.370 mol × 60.05 g/mol = 22.2 g
The theoretical yield is 22.2 g of acetic acid (actual yield will be lower; the calculation assumes complete cleavage and recovery).
Key takeaways
- Internal alkyne cleavage → two carboxylic acids (one from each side of the triple bond).
- Terminal alkyne cleavage → one carboxylic acid + CO₂ (the terminal CH carbon becomes CO2).
- Reagents: O3 then water, or hot concentrated KMnO4.
- Carbon-counting rule: each triple-bond carbon becomes a COOH carbon; count carbons carefully in both directions (product → alkyne and alkyne → product).
- Alkene vs. alkyne cleavage: alkenes give ketones/aldehydes; alkynes give carboxylic acids (+ CO2 for terminal) because alkynes are oxidized further.
- Symmetric alkynes give one acid twice — a useful simplification in problems.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
What two products form when an internal alkyne R-C ≡ C-R' is subjected to ozonolysis with water workup?
Show answer
Two carboxylic acids: RCOOH and R'COOH.
What happens to the terminal carbon of a terminal alkyne during oxidative cleavage?
Show answer
It is oxidized to carbon dioxide, CO2. So a terminal alkyne gives one acid plus CO₂.
An unknown alkyne gives only butanoic acid on cleavage. What was the alkyne?
Show answer
4-octyne, CH3CH2CH2C ≡ CCH2CH2CH3 — both sides are butyl groups, so cleavage gives two molecules of butanoic acid.
Why do alkynes give carboxylic acids on ozonolysis while alkenes give aldehydes or ketones?
Show answer
The alkyne carbons are more oxidized to begin with (each carries fewer hydrogens), and the strongly oxidizing conditions carry both alkyne carbons all the way to the carboxylic acid oxidation state; alkene carbons stop at the aldehyde/ketone level.
How many moles of acetic acid form from 1 mol of 2-butyne on complete cleavage?
Show answer
2 mol — each alkyne carbon becomes one carboxyl carbon, so 1 mol of CH3C ≡ CCH3 yields 2 mol of CH3COOH.
Name two reagent systems that cleave alkynes to carboxylic acids.
Show answer
Ozone followed by water, or hot concentrated potassium permanganate (KMnO4).
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Oxidative cleavage
- Breaking a C–C bond while oxidizing both carbons
- Ozonolysis
- Reaction with ozone followed by an aqueous workup
- Carboxylic acid
- Compound with a COOH group
- Carbon dioxide
- CO2, the fate of a terminal alkyne carbon
- Internal alkyne
- Triple bond between two nonterminal carbons
- Terminal alkyne
- Triple bond at the end of a chain, RC ≡ CH
Sources & references
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