Source: Organic Chemistry I, University of Minnesota Twin Cities
Tags: alkynes, triple bond, sp hybridisation, terminal alkyne, internal alkyne, alkyne nomenclature, alkyne acidity, organic chemistry I
Difficulty: Intermediate | Prerequisites: Ch. 1–2 (bonding, hybridisation, acid–base), Ch. 7 (alkene structure and nomenclature), Ch. 8 (alkene addition reactions).
Alkynes are hydrocarbons that contain a carbon–carbon triple bond (C≡C). They sit one step up in unsaturation from the alkenes you met in Chapters 7 and 8, and much of the chemistry here runs in parallel: addition of HX, X₂, H₂, water, and ozone all work on the same electrophilic-addition logic, just applied to a triple bond instead of a double bond. The key differences come from the sp hybridisation of the triple-bond carbons, which makes alkynes shorter, stronger, more acidic, and less reactive toward electrophiles than alkenes. If you are comfortable with alkene addition reactions, nomenclature, and Markovnikov's rule, this chapter is a natural extension. If you are not, revisit Ch. 7–8 first.
Alkynes have a triple bond made of one sigma bond and two pi bonds, with sp-hybridised carbons that are linear and more electronegative than sp³ carbons. This makes terminal alkyne C–H bonds acidic enough (pKa ~26) to deprotonate with strong bases like NaNH₂, which opens up reactions (alkylation) that alkenes cannot do. Most alkyne addition reactions, including hydrohalogenation, halogenation, hydrogenation, hydration, and ozonolysis, mirror alkene chemistry but can stop at one or two equivalents of reagent.
Alkyne
A hydrocarbon containing at least one carbon–carbon triple bond (C≡C). In simple terms, think of it as an even more unsaturated version of an alkene.
sp Hybridisation
The orbital mixing that produces two sp hybrid orbitals with 50% s character, arranged at 180°. This is what gives alkynes their linear geometry and higher electronegativity compared to sp² or sp³ carbons.
Terminal Alkyne
An alkyne where the triple bond sits at the end of the chain, with at least one hydrogen directly on a triple-bond carbon (H–C≡C–). Think of it as the exposed end of the triple bond, and the reason the C–H is acidic.
Internal Alkyne
An alkyne where the triple bond is flanked by carbon groups on both sides (C–C≡C–C). No acidic proton on the triple bond here.
Acetylide Anion (⊖C≡C–)
The conjugate base formed when a terminal alkyne loses its proton. It is both a strong base and a good nucleophile, which is why it participates in Sₙ₂ alkylation reactions.
Keto-Enol Tautomerization
An equilibrium between an enol (C=C–OH) and a ketone (CH–C=O), catalysed by acid. The ketone form is more stable and predominates. In simple terms, the –OH on a double bond rearranges to give a C=O with a hydrogen shift.
Lindlar Catalyst
Pd on CaCO₃, deactivated ("poisoned") with lead. It reduces a triple bond to a cis-alkene (stops after the first addition of H₂) rather than going all the way to an alkane.
Markovnikov’s Rule
In an addition across an unsymmetrical alkyne, the hydrogen adds to the carbon that already bears more hydrogens (or, equivalently, the electrophile adds to give the more substituted intermediate). Same principle as with alkenes.
The triple bond consists of 1 sigma (σ) bond and 2 pi (π) bonds
Bond strength: 961 kJ/mol, which is stronger than a single bond (375 kJ/mol) but not three times as strong
Bond length: 120 pm, shorter than a C=C double bond (134 pm)
The pi bonds are less polarisable than those in alkenes, making alkynes less reactive toward electrophiles
Triple-bond carbons are sp hybridised: 50% s character (compared to 25% in sp³)
Geometry is linear, with a bond angle of 180°
Higher s character means electrons are held closer to the nucleus, so sp carbons are more electronegative than sp³ carbons
The C–H bond on a terminal alkyne is relatively acidic: pKa ≈ 26
Compare: sp³ C–H has pKa ≈ 60, and NH₃ has pKa ≈ 38
NaNH₂ (pKa of NH₃ = 38) is strong enough to deprotonate a terminal alkyne, forming the acetylide anion (⊖C≡C–)
The acetylide anion is a good nucleophile: basic, unhindered, and useful for forming new C–C bonds
C–H bond with sp carbon: 106 pm, 547 kJ/mol (shorter and stronger)
C–H bond with sp³ carbon: 111 pm, 421 kJ/mol (longer and weaker)
Alkynes are not very polar, so intermolecular forces are limited to London dispersion forces
Consequence: low boiling points, low melting points, and no appreciable water solubility
Behave similarly to alkanes and alkenes of comparable molecular weight in terms of physical state
Replace the "-ane" suffix with "-yne"
If the molecule also contains a double bond, the "-ene" suffix comes before "-yne" (e.g., pent-1-en-3-yne)
The main chain must include the triple bond
Number the chain to give the triple bond the lowest possible locant
When a double bond and triple bond tie for the lowest number, the double bond gets priority in numbering
Terminal alkyne: the triple bond is at the end of the chain, with a hydrogen on one of the sp carbons (H–C≡C–R)
Internal alkyne: the triple bond is flanked by carbon substituents on both sides (R–C≡C–R)
This distinction matters for reactivity: only terminal alkynes have an acidic C–H and can undergo alkylation
Pent-1-en-3-yne: five-carbon chain with a double bond at C1 and a triple bond at C3
4-Methylpent-2-yne: five-carbon chain with a triple bond at C2 and a methyl branch at C4
Acetylene (the simplest alkyne, HC≡CH) is the fuel in oxyacetylene welding torches, where its high-energy triple bond produces an extremely hot flame. The acidity of terminal alkynes and the nucleophilicity of acetylide anions are used in pharmaceutical synthesis to build complex carbon skeletons, for example in the synthesis of ethynylestradiol (a component of oral contraceptives).
"A triple bond is three times as strong as a single bond." It is not. The triple bond (961 kJ/mol) is less than three times the single bond (375 kJ/mol) because the two pi bonds are individually weaker than the sigma bond.
"Alkynes are more reactive toward electrophiles than alkenes because they have more pi electrons." The opposite is true. The pi bonds in alkynes are less polarisable, making alkynes less reactive toward electrophilic addition.
"All alkynes can be deprotonated by NaNH₂." Only terminal alkynes have an acidic C–H (pKa ~26). Internal alkynes have no proton on the triple bond to remove.
"sp hybridisation means the carbon is less electronegative." The higher s character (50%) in sp actually makes the carbon more electronegative than sp² or sp³ carbons, because s orbitals hold electrons closer to the nucleus.
⚠️ Be able to explain why terminal alkyne C–H is acidic (sp hybridisation → 50% s character → electrons closer to nucleus → more stable conjugate base)
⚠️ Know the pKa comparison: sp C–H (~26) vs. NH₃ (~38) vs. sp³ C–H (~60). This is a favourite exam comparison.
⚠️ Be ready to assign correct IUPAC names, especially for molecules with both double and triple bonds (en-yne naming, with double bond getting numbering priority in a tie)
⚠️ Distinguish terminal from internal alkynes and know which reactions require a terminal alkyne (alkylation)
True or False: A triple bond is exactly three times as strong as a single C–C bond. (False)
The bond angle around an sp-hybridised carbon is _____. (180°)
True or False: Internal alkynes can be deprotonated by NaNH₂. (False – no acidic C–H)
The pKa of a terminal alkyne C–H is approximately _____. (26)
True or False: When naming an enyne, if the double bond and triple bond tie for the lowest locant, the double bond gets the lower number. (True)
Q: Why is the C–H bond of a terminal alkyne (pKa ~26) more acidic than an sp³ C–H bond (pKa ~60)?
A: The sp-hybridised carbon has 50% s character, which holds the bonding electrons closer to the carbon nucleus. The resulting conjugate base (acetylide anion) is more stable because the negative charge sits in an orbital with high s character, closer to the nucleus.
Q: Name the compound CH₃–C≡C–CH₂–CH₃ using IUPAC rules.
A: Pent-2-yne. The longest chain includes all five carbons and the triple bond, which is given the lowest possible locant (C2).
Q: A molecule has both a double bond and a triple bond. In a tie for lowest locant, which bond receives the lower number?
A: The double bond. IUPAC rules give the lower locant to the double bond when there is a tie between "-ene" and "-yne."
Q: Explain why alkynes are less reactive toward electrophilic addition than alkenes, despite having more pi electrons.
A: The pi bonds in a triple bond are less polarisable than those in a double bond. Polarisability determines how easily the electron cloud distorts to interact with an electrophile, so alkynes react more slowly.
Q: What is the hybridisation and geometry of the carbon atoms in the triple bond of propyne?
A: Both carbons of the triple bond are sp hybridised, giving a linear geometry with 180° bond angles at those carbons.
This material connects directly to Ch. 7–8 (alkene structure and reactions). Every addition reaction here, including hydrohalogenation, halogenation, hydrogenation, hydration, and ozonolysis, is the triple-bond analogue of an alkene reaction you have already seen. The concept of hybridisation ties back to Ch. 1–2 bonding theory. Acid–base chemistry (pKa comparisons, deprotonation by NaNH₂) draws on the same framework introduced in Ch. 3. Looking ahead, the ability of alkynes to form new C–C bonds via alkylation becomes a key synthetic tool in later chapters on multi-step synthesis.
Alkynes, triple bond, C≡C, sp hybridisation, sp hybridization, linear geometry, 180 degrees, terminal alkyne, internal alkyne, acetylide, acetylide anion, alkyne acidity, pKa 26, alkyne nomenclature, enyne naming, IUPAC alkyne naming, alkyne physical properties, dispersion forces, Lindlar catalyst, Markovnikov rule alkynes, organic chemistry I, Ch. 9 alkynes