Alkynes — Alkenes & Alkynes | Organic Chemistry

Alkynes do everything alkenes do, twice over — but their real importance in a synthesis course is different. A terminal alkyne is acidic enough to deprotonate, and the resulting carbanion is one of the very few reliable carbon nucleophiles available at this stage. Alkynes are how you make carbon skeletons bigger.

sp hybridization and linear geometry

Each carbon of a C≡C is sp hybridized: two sp orbitals 180° apart form the sigma bonds, and two unhybridized p orbitals — perpendicular to each other and to the sp axis — form two separate pi bonds. A triple bond is one sigma plus two pi.

The 180° geometry makes every alkyne carbon and its attached atom perfectly linear, which has a structural consequence: a triple bond cannot be accommodated in a ring smaller than about eight members without severe strain. The C≡C is about 120 pm long and worth roughly 200 kcal/mol in total.

Because a triple bond counts as two degrees of unsaturation, an alkyne and a compound with two separate double bonds have the same molecular formula — which is exactly the kind of ambiguity spectroscopy is for.

Terminal alkyne C–H: unusually acidic

A terminal alkyne's C–H has a pKa of about 25, against roughly 44 for an alkene C–H and 50 for an alkane. That is nineteen orders of magnitude more acidic than an alkene, from nothing but hybridization.

This is the Orbital factor from the ARIO framework in Module 3. The conjugate base's lone pair sits in an sp orbital, which is 50% s in character, against 33% for sp² and 25% for sp³. More s-character holds the electron density closer to the nucleus, lowering the anion's energy and making the parent C–H more acidic.

pKa 25 is comfortably within reach of sodium amide (NaNH₂, conjugate acid pKa 38) or sodium hydride. It is not within reach of hydroxide or alkoxide, whose conjugate acids sit around pKa 16 — nine units on the wrong side, so the equilibrium would lie 10⁹ to one against deprotonation. When a question specifies NaNH₂ rather than NaOH, that choice is the whole point of the step.

Acetylide anions: making carbon–carbon bonds

Why the acidity of one hydrogen earns its own section. Nothing else in the first half of this course lets you deprotonate a carbon — an alkane C–H sits at pKa 50, out of reach of any reasonable base. A terminal alkyne sits at 25, because its sp carbon is half s-character and holds the leftover pair close in. Sodium amide takes that proton, and what you are left with is a carbon nucleophile: run it into an SN2 and two chains become one.This is the reaction that makes alkynes worth a chapter: it hands you a nucleophilic CARBON. Run it into an SN2 with a primary alkyl halide and you have stapled two carbon chains together — one of the few ways this course gives you to build a carbon skeleton rather than decorate one.

Deprotonating a terminal alkyne with NaNH₂ gives an acetylide anion — a small, strong, genuinely nucleophilic carbanion. Acetylides do exactly what any strong nucleophile does in Module 6: SN2 on an unhindered alkyl halide, forming a new carbon–carbon bond.

This matters more than it may look. Most of the reactions you have met so far rearrange or decorate an existing skeleton; this one extends it. Acetylide alkylation, together with the enolate and organometallic chemistry of later chapters, is one of a short list of dependable ways to build carbon frameworks.

Acetylide alkylation only works on methyl and primary halides. An acetylide is a strong base as well as a strong nucleophile, so on a secondary or tertiary substrate it does E2 instead and returns the alkyne unchanged along with an alkene. Planning a synthesis that alkylates an acetylide with tert-butyl bromide is a standard error; if you need that connection, install the branch differently.

Worked example — a two-step chain extension

From acetylene (HC≡CH) to hex-1-yne.

Step 1: NaNH₂ deprotonates one terminus, giving HC≡C⁻ Na⁺.

Step 2: add 1-bromobutane. SN2 gives HC≡C–CH₂CH₂CH₂CH₃.

Two carbons plus four carbons, joined by a C–C bond you made. Repeat the sequence on the remaining terminal C–H and you can build an internal alkyne with whatever two groups you like on either end.

Addition reactions: partial versus complete

Two pi bonds means addition can stop once, giving an alkene, or go twice, giving an alkane. Which one you get is a matter of reagent choice, and the pair of partial reductions is particularly valuable because they give opposite alkene geometries.

H₂ with Lindlar catalyst — palladium poisoned with lead and quinoline to make it deliberately less active — stops cleanly at the alkene and delivers both hydrogens to the same face. Syn addition gives the cis alkene.

Na in liquid NH₃, a dissolving-metal reduction, proceeds by a radical-anion mechanism instead, and the intermediate settles into its lower-energy trans arrangement before the second protonation. This gives the trans alkene.

H₂ with ordinary Pd or Pt does not stop; it goes all the way to the alkane.

So a single internal alkyne can be converted into the cis alkene, the trans alkene, or the alkane, purely by reagent choice — which is precisely why alkynes are so useful as synthetic intermediates. Alkene geometry is otherwise hard to control.

Hydration and the enol

Acid-catalyzed hydration of an alkyne, with Hg²⁺ as catalyst, adds water with Markovnikov selectivity to give an enol — a compound with OH attached directly to a C=C. Enols are not stable, and this one immediately tautomerizes: the proton migrates from oxygen to carbon and the pi bond shifts from C=C to C=O, giving a ketone.

The tautomerization is not a resonance form — atoms move, so keto and enol are genuinely different compounds in equilibrium, drawn with ⇌. The keto form is favoured by roughly 10⁵ to one for a simple ketone, because a C=O is substantially stronger than a C=C.

The regiochemistry can be flipped here too. Markovnikov hydration of a terminal alkyne gives a methyl ketone; hydroboration–oxidation of the same alkyne, using a bulky borane, puts the oxygen on the terminal carbon and gives an aldehyde instead.

The keto–enol relationship introduced here becomes central in Module 11. Every enolate reaction — aldol, Claisen, alpha-halogenation, alkylation — runs through the same enol or its anion. Meeting it first as a fleeting intermediate in alkyne hydration is a useful way in, because here it is doing something simple and visible.

What carries forward

Acetylide chemistry is one of the course's main carbon–carbon bond-forming tools and appears in synthesis problems constantly. The Lindlar/sodium-ammonia pair is the standard way to set alkene geometry deliberately. And keto–enol tautomerism, met here in passing, is the foundation of the entire enolate chapter.