The Special Chemistry of Alkynes

Why a Triple Bond Is Its Own Animal

You have spent this rung treating carbon-carbon multiple bonds as electron-rich clouds that electrophiles love. An [[alkyne|alkyne]] — a molecule with a carbon-carbon triple bond — has even more pi electrons stuffed into the same short bond, so it does undergo electrophilic addition much like an alkene, often twice in a row. But the headline of this guide is the part that has no alkene analogue at all. Tuck the difference into one fact about geometry and you can predict almost everything else: the two carbons of a triple bond are sp-hybridized, held in a rigid straight line at 180 degrees, with the triple bond made of one sigma bond and two pi bonds.

Here is the crucial split. A terminal alkyne ends in a C-H sitting directly on the sp carbon (think of acetylene, H-C(triple)C-H, or 1-butyne, CH3CH2-C(triple)C-H). An internal alkyne has carbon groups on both ends and no such C-H. That single terminal hydrogen, perched on an sp carbon, is the doorway to everything special — because it can be pulled off as a proton far more easily than any ordinary C-H you have met. An internal alkyne, lacking it, simply cannot play this game.

An Unusually Acidic C-H

Back in the acids-and-bases rung you learned that acidity is really a question about the stability of the anion left behind: the more comfortable the conjugate base, the more willing the proton is to leave, the lower the pKa. Apply that lens to a terminal alkyne. Yank off the terminal proton and you are left with a carbanion — a carbon bearing a lone pair and a full negative charge — but a remarkably content one, because that lone pair sits in an sp orbital.

Why does the sp orbital matter so much? An sp hybrid is built from one s orbital and only one p orbital, so it is fully 50 percent s-character — versus 33 percent for sp2 (an alkene carbon) and 25 percent for sp3 (an alkane carbon). The s orbital hugs the nucleus closely, so a lone pair with more s-character sits lower in energy, nearer the positive nucleus, where a negative charge is happiest. More s-character means a more stable anion, which means a more acidic C-H. That is the whole story: the terminal alkyne C-H is acidic because its conjugate base parks its electrons in a low, tightly-held sp orbital.

  acidity of C-H  (lower pKa = more acidic)

     H-C(triple)C-H      pKa ~ 25     sp  carbon, 50% s
     H2C=CH2             pKa ~ 44     sp2 carbon, 33% s
     CH3-CH3             pKa ~ 50     sp3 carbon, 25% s

  more s-character  ->  lower lone pair  ->  weaker acid... stronger acid
  (compare: water pKa ~ 16, so OH- canNOT deprotonate an alkyne)

Keep the comparison honest, though. A pKa around 25 makes a terminal alkyne dramatically more acidic than an alkene (about 44) or an alkane (about 50) — but it is still a weaker acid than water (about 16) or an alcohol. So ordinary bases like hydroxide or alkoxide barely touch it; pull on the equilibrium and almost nothing deprotonates. To actually rip off that proton you need a much stronger base, classically sodium amide (NaNH2), whose conjugate acid is ammonia (NH3, pKa about 38). Because 38 is well above 25, amide grabs the alkyne's proton essentially completely. The rule of thumb you met earlier holds: a base deprotonates an acid cleanly only when the base's own conjugate acid is weaker than the acid being removed.

The Acetylide Anion: A Carbon That Attacks

Deprotonate a terminal alkyne and the carbanion you make has a name: the [[acetylide-anion|acetylide anion]] (R-C(triple)C:-). This is where the alkyne earns its keep. Most of the nucleophiles you have used so far were heteroatoms — oxygen in hydroxide, a halide, nitrogen in an amine. The acetylide is something rarer and more powerful: a nucleophile whose attacking atom is carbon itself. When a carbon nucleophile attacks a carbon electrophile, a brand-new carbon-carbon bond is born — and forging C-C bonds is the central craft of building bigger organic molecules from smaller ones.

The classic move is to let the acetylide attack a primary alkyl halide in a clean SN2 reaction. The acetylide's lone pair comes in from behind, the halide leaves from the opposite face, and you stitch the alkyne chain onto the alkyl chain at exactly that carbon. Acetylide plus CH3CH2Br, for instance, gives R-C(triple)C-CH2CH3: the molecule has grown by two carbons, and the triple bond is still sitting there, now internal, ready for whatever you want to do next. This is the workhorse method for lengthening a carbon skeleton a controlled chunk at a time.

1. Deprotonate: a strong base (NaNH2) removes the terminal proton, giving the acetylide anion R-C(triple)C:- with its lone pair on the end carbon.
2. Attack: that carbon lone pair reaches for the back of a primary alkyl halide R'-CH2-X, opposite the leaving group.
3. Invert and expel: the halide leaves with its electrons as the new C-C bond forms, the SN2 backside attack turning that carbon inside-out like an umbrella in a gust.
4. Result: an internal alkyne, the chain lengthened, with the original triple bond intact and available for the reductions below.

Dialing the Reduction: Cis or Trans, You Choose

An internal alkyne is a fork in the road. Reduce it all the way and you get a plain alkane — that is what ordinary catalytic hydrogenation over a metal like platinum or palladium does, slamming on two molecules of H2 and leaving no double bond at all. But the alkyne's two stages of reduction can be stopped halfway, and depending on how you stop it you can deliver the alkene as either its cis or its trans geometric isomer. That level of stereochemical control — choosing which side of the new C=C the two new hydrogens end up on — is one of the most elegant payoffs in this whole rung.

For the cis (Z) alkene, use the Lindlar catalyst — palladium deliberately poisoned (with lead and a little quinoline) so it is too feeble to push past the first reduction. The triple bond lies flat on the metal surface and picks up both hydrogens from the same face, so they land on the same side: a cis double bond, with the two R groups crowded together. The poisoning is the clever part. A normal palladium catalyst would gleefully reduce the resulting alkene onward to the alkane; Lindlar's is just strong enough to do the alkyne and just weak enough to leave the alkene alone.

For the trans (E) alkene, ditch the metal surface entirely and use a dissolving-metal reduction — sodium metal in liquid ammonia (Na/NH3). The mechanism is completely different: it is not surface chemistry but a sequence of single electrons. The sodium hands one electron to the alkyne, making a radical anion; that picks up a proton from ammonia; a second electron and a second proton finish the job. At the radical-anion stage the two halves arrange themselves on opposite sides to keep their charges and radicals as far apart as possible, and that geometry is locked in as the hydrogens add — so you get the trans alkene. One molecule, two reagents, two opposite stereochemical outcomes, on demand.

1. H2 over ordinary Pt or Pd (full hydrogenation): both pi bonds fall and you get the plain alkane R-CH2-CH2-R', no double bond left.
2. H2 with the Lindlar catalyst (poisoned Pd, too feeble to over-reduce): both hydrogens add to the same face, giving the cis (Z) alkene.
3. Na in liquid NH3 (dissolving-metal, single electrons rather than a surface): the two halves splay apart, giving the trans (E) alkene.
4. Same starting alkyne, three different destinations — the reagent you pick decides the outcome.

From the Lab Bench to Industry: Addition Polymers

Everything so far has built one molecule at a time. Now scale the same electron-hungry chemistry up by a factor of millions and you get the industrial payoff: addition polymerization. The idea is disarmingly simple. Take a small molecule with a carbon-carbon double bond — a monomer like ethylene (CH2=CH2) — and let its pi bond open up to grab the next monomer, whose pi bond opens to grab the next, and so on, thousands of times. Each pi bond becomes a sigma bond linking one unit to the following one. No atoms are lost along the way; the monomers simply add together into one gigantic chain. That is why the product is called an [[addition-polymer|addition polymer]].

This is exactly the addition chemistry of this rung, just run as a chain reaction. The growing end can be a carbon radical (the same kind of radical you met in radical halogenation) or a cation, and each step is the now-familiar move of a reactive center attacking a double bond and opening its pi bond. Polyethylene shopping bags, the polypropylene in a yogurt tub, the PVC in plumbing pipe, the polystyrene in a coffee-cup lid, the PTFE (Teflon) on a frying pan — all of them are made this way, by chaining up alkene monomers through their double bonds. The reaction you learned to draw with two carbons is, at industrial scale, one of the most economically important transformations on Earth.

Putting the Toolkit Together

Step back and see what the alkyne gives you that nothing else in this rung does: a programmable building block. You can grow a carbon chain by deprotonating to the acetylide and alkylating with a primary halide (forming a C-C bond), then sculpt the leftover triple bond into precisely the alkane, cis-alkene, or trans-alkene you need. A two-carbon acetylene unit, alkylated twice and then half-reduced one way or the other, can be walked into a remarkable range of target molecules. The triple bond is less a destination than a versatile waypoint.

And notice how little of this was actually new. The acidity argument was just your conjugate-base-stability lens from the acids rung, applied to an sp carbon. The acetylide alkylation was a plain SN2 with a carbon nucleophile. The reductions added H across a multiple bond, the mirror of the additions you studied all rung. Even polymerization was electrophilic (or radical) addition run a million times. The alkyne feels special not because it breaks the rules but because it lets you apply the rules you already own in a sharper, more deliberate way — which is exactly the mark that you are starting to think like a chemist rather than memorize like a student.