Alkynes contain a carbon–carbon triple bond — one sigma and two pi bonds — between two sp-hybridized carbons. That hybridization sets all of their distinctive properties. The two sp carbons and their attached atoms are arranged in a linear, 180° geometry, and the triple bond is shorter and stronger than a double bond.

The most conceptually important feature is the acidity of terminal alkynes. A terminal alkyne (one with an ≡C–H at the end of the chain) has a C–H that is unusually acidic for a hydrocarbon, with a pKa around 25 — dramatically more acidic than an alkene (~44) or an alkane (~50). The explanation ties back to hybridization and Track 4's acid–base logic: the more s-character in the orbital holding the electron pair, the closer those electrons sit to the positively charged nucleus, the more stable the resulting anion. An sp carbanion (50% s-character) is far more stable than an sp² (33%) or sp³ (25%) carbanion, so the sp C–H gives up its proton most readily.

This acidity is not a trivia point — it is synthetically central. A strong enough base (commonly sodium amide, NaNH₂) can deprotonate a terminal alkyne to form an acetylide anion, a genuine carbon nucleophile. Note the base must be strong: NaNH₂ works because ammonia (pKa ~38) is a much weaker acid than the alkyne, so the equilibrium lies toward the acetylide; hydroxide (water pKa ~16) is not strong enough. Acetylides are one of the few readily available carbon nucleophiles in Orgo 1, which makes them the workhorse of the next track.

Alkynes are doubly valuable: they undergo their own addition chemistry (with an extra pi bond to work through), and — uniquely among the hydrocarbons so far — they let you form new carbon–carbon bonds, which is the heart of building larger molecules.

Acetylide alkylation (C–C bond formation). The acetylide anion from Track 20 is a strong nucleophile, and it performs an SN2 reaction on a methyl or primary alkyl halide, forging a new carbon–carbon bond and lengthening the carbon skeleton. This is a landmark reaction because most of Orgo 1 rearranges existing bonds; here you are building the skeleton. The SN2 restrictions from Track 14 apply strictly: because the acetylide is also a strong base, secondary and tertiary halides give elimination (E2) instead of substitution, so the reaction only works cleanly on unhindered (methyl/1°) halides.

Addition reactions. Because an alkyne has two pi bonds, reagents can add once (giving an alkene) or twice (giving an alkane or a geminal/dihalide product), and controlling that is part of the skill:

Hydrogenation can be tuned. Full hydrogenation (H₂, Pt/Pd) reduces all the way to the alkane. Lindlar's catalyst (a poisoned Pd) stops at the alkene and, being a syn addition, gives the cis (Z) alkene. A dissolving-metal reduction (Na or Li in NH₃) also stops at the alkene but by an anti-style radical-anion pathway, giving the trans (E) alkene. This complementary pair — Lindlar → cis, Na/NH₃ → trans — is a classic exam point because it lets you choose the alkene geometry deliberately.
Hydrohalogenation and halogenation follow Markovnikov and can be run once or twice (e.g., HBr once → vinyl bromide; twice → geminal dibromide).
Hydration of an alkyne is especially important because the initial product, an enol (a C=C bearing an –OH), is unstable and spontaneously tautomerizes to a carbonyl. Standard acid-catalyzed hydration (H₂O, H₂SO₄, HgSO₄) follows Markovnikov and yields a ketone; hydroboration–oxidation of a terminal alkyne follows anti-Markovnikov and yields an aldehyde. This is your first encounter with keto–enol tautomerism, an equilibrium (favoring the keto form) that will return prominently in the enol/enolate chemistry of Orgo 2.

"Triple Threat"