# Lecture 15: Alkynes: Structure and Reactions
## Organic Chemistry I
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## Learning Objectives
By the end of this lecture, students will be able to:
1. Describe the structure and bonding of alkynes (sp hybridization, two pi bonds)
2. Name alkynes using IUPAC nomenclature
3. Explain the acidity of terminal alkynes and their deprotonation by strong bases
4. Describe the synthesis of alkynes from alkyl halides and acetylide anions
5. Predict products of electrophilic addition reactions to alkynes (HX, X2)
6. Describe hydration of alkynes (Markovnikov and anti-Markovnikov)
7. Explain reduction of alkynes to cis-alkenes (Lindlar catalyst) and trans-alkenes (Na/NH3)
8. Use alkynes as versatile intermediates in organic synthesis
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## Lecture Content
### I. Structure and Bonding of Alkynes
Alkynes are hydrocarbons containing at least one carbon-carbon triple bond, with acyclic monoalkynes following the general formula CnH2n-2. Each triple bond represents two degrees of unsaturation. The carbons of the triple bond are sp hybridized, with two sp hybrid orbitals forming sigma bonds (one to the other triple-bond carbon and one to a substituent) in a linear geometry with 180-degree bond angles. Two unhybridized p orbitals on each carbon overlap to form two pi bonds that are perpendicular to each other, creating a cylinder of electron density surrounding the C-C axis.
The C-triple-bond-C bond is 1.20 angstroms in length, shorter than the C=C double bond at 1.34 angstroms, and has a bond energy of approximately 839 kJ/mol. The sp C-H bond is correspondingly shorter and stronger than sp2 or sp3 C-H bonds. Terminal alkynes bear at least one hydrogen directly on a triple-bond carbon (R-C-triple-bond-C-H), while internal alkynes have carbon substituents on both sides (R-C-triple-bond-C-R').
### II. Nomenclature of Alkynes
IUPAC naming follows the same logic as for alkenes: identify the longest chain containing the triple bond, change the -ane suffix to -yne, and number to give the triple bond the lowest locant. Examples include ethyne (the IUPAC name for acetylene), propyne, 1-butyne, 2-butyne, and 3-methyl-1-pentyne. When both a double bond and a triple bond are present, the suffix -en-yne is used, and the lowest set of locants is assigned to the multiple bonds as a group, with the double bond receiving the lower number in case of a tie. The common name acetylene for ethyne (HC-triple-bond-CH) and the term propargyl for the HC-triple-bond-C-CH2- group remain in widespread use.
### III. Acidity of Terminal Alkynes
Terminal alkynes are remarkably acidic for hydrocarbons, with pKa values of approximately 25. This compares favorably to alkene C-H bonds (pKa approximately 44) and alkane C-H bonds (pKa approximately 50), although terminal alkynes are still far less acidic than water (pKa 15.7) or alcohols.
The enhanced acidity is a direct consequence of hybridization. The sp carbon has 50% s character, meaning the conjugate base (the acetylide anion) holds its electron pair in an orbital that is closer to the nucleus than sp2 (33% s) or sp3 (25% s) orbitals. This proximity to the nucleus stabilizes the negative charge, making the anion more energetically accessible.
To deprotonate a terminal alkyne, one needs a base whose conjugate acid has a pKa greater than 25. Sodium amide (NaNH2, conjugate acid NH3 with pKa 38), n-butyllithium (conjugate acid butane with pKa approximately 50), and sodium hydride (conjugate acid H2 with pKa 35) all work. Sodium hydroxide (conjugate acid water with pKa 15.7) does not, because the equilibrium would be unfavorable. The resulting acetylide anions (RC-triple-bond-C:-) are powerful nucleophiles and bases, and their use in SN2 reactions with primary alkyl halides provides a valuable method for forming new carbon-carbon bonds.
Panel A: Orbital diagram comparing C-H bond character across hybridizations: sp3 (25% s, least acidic), sp2 (33% s), and sp (50% s, most acidic). The electron density of the conjugate base orbital is shown progressively closer to the carbon nucleus with increasing s character. A pKa scale on the right shows ~50 for sp3, ~44 for sp2, and ~25 for sp. Panel B: The deprotonation reaction of a terminal alkyne by NaNH2: RC≡CH + NaNH2 → RC≡C-Na+ + NH3, with pKa values shown for the acid on each side of the equation (25 and 38), demonstrating favorable equilibrium.
### IV. Synthesis of Alkynes
Alkynes can be synthesized through acetylide alkylation, in which the acetylide anion acts as a nucleophile in an SN2 reaction with a primary or methyl alkyl halide: RC-triple-bond-C:- + R'CH2-X yields RC-triple-bond-C-CH2R' + X-. This method is limited to primary and methyl halides because secondary and tertiary halides undergo E2 elimination instead. Sequential double alkylation from acetylene itself can build internal alkynes: deprotonation with NaNH2, alkylation with one alkyl halide, a second deprotonation, and alkylation with a second alkyl halide.
Alkynes can also be synthesized by double dehydrohalogenation, in which a geminal or vicinal dihalide is treated with two equivalents of a strong base such as NaNH2 at elevated temperatures. The reaction proceeds through a vinyl halide intermediate before arriving at the alkyne.
### V. Reactions of Alkynes: Addition of HX
Alkynes undergo electrophilic addition reactions analogous to those of alkenes, but because they possess two pi bonds, they can add one or two equivalents of reagent. Addition of one equivalent of HX follows Markovnikov's rule, producing a vinyl halide: the hydrogen adds to the terminal carbon and the halide to the internal carbon. For example, propyne plus HBr gives 2-bromopropene. The vinyl halide product is less reactive toward further addition than the original alkyne because the electron-withdrawing halogen reduces the electron density of the remaining double bond.
Addition of a second equivalent of HX adds across the remaining double bond, again following Markovnikov's rule, to produce a geminal dihalide in which both halogens reside on the same carbon. Propyne plus two equivalents of HBr gives 2,2-dibromopropane.
### VI. Reactions of Alkynes: Hydration
Markovnikov hydration of alkynes uses an acid catalyst (H2SO4) with mercuric sulfate (HgSO4) as a co-catalyst. Water adds across the triple bond following Markovnikov's rule, placing the hydroxyl group on the more substituted carbon. The initial product is an enol (a vinyl alcohol), which rapidly tautomerizes to the more stable keto form through keto-enol tautomerism. The keto form is thermodynamically favored (approximately 99.9%), and interconversion is rapid in acid or base. Terminal alkynes give methyl ketones by this method.
Anti-Markovnikov hydration is accomplished through hydroboration-oxidation, using a bulky borane such as disiamylborane or 9-BBN to stop at monoaddition. After oxidation with H2O2/NaOH, the hydroxyl ends up on the less substituted carbon. The resulting enol tautomerizes to give an aldehyde rather than a ketone, providing complementary regiochemistry to the mercury-catalyzed method.
Panel A: Markovnikov hydration of 1-hexyne: treatment with H2O, H2SO4, HgSO4 produces the enol intermediate (shown in brackets with OH on C2), which tautomerizes via keto-enol tautomerism to 2-hexanone (methyl ketone product). The tautomerism is shown with curved arrow mechanism (protonation at C1, deprotonation of OH). Panel B: Anti-Markovnikov hydration of 1-hexyne: treatment with disiamylborane then H2O2/NaOH gives the enol with OH on C1, which tautomerizes to hexanal (an aldehyde). Both products are drawn and boxed, with labels indicating Markovnikov vs. anti-Markovnikov regiochemistry.
### VII. Reduction of Alkynes
Alkynes can be selectively reduced to alkenes (partial reduction) or fully to alkanes, and the choice of reducing conditions determines the stereochemistry of the alkene product. Catalytic hydrogenation with Lindlar's catalyst (palladium on calcium carbonate, poisoned with lead acetate and quinoline) delivers H2 in a syn fashion to produce the cis (Z) alkene. The catalyst's "poison" reduces its activity enough to halt reduction at the alkene stage. For example, 2-butyne plus H2 over Lindlar's catalyst gives cis-2-butene.
Dissolving metal reduction with sodium or lithium in liquid ammonia at -33 degrees C produces the trans (E) alkene through a radical anion mechanism. The metal donates an electron to the alkyne, forming a radical anion that preferentially adopts a trans geometry. Protonation by ammonia, a second electron addition, and a second protonation complete the sequence, delivering the trans alkene as the product. For example, 2-butyne treated with Na in liquid NH3 gives trans-2-butene. Complete reduction to the alkane requires standard catalytic hydrogenation without a poisoned catalyst, adding two equivalents of H2.
### VIII. Alkynes in Synthesis
Alkynes serve as versatile synthetic intermediates because of several complementary capabilities: the acidity of the terminal C-H allows deprotonation and use of the acetylide as a nucleophile for C-C bond formation; selective reduction gives access to either cis or trans alkenes; and electrophilic addition produces vinyl halides, ketones, or aldehydes depending on the reagents chosen.
Retrosynthetic thinking highlights these connections. To make a cis-alkene, one envisions an alkyne reduced with Lindlar's catalyst. To make a trans-alkene, one plans dissolving metal reduction. Internal alkynes can be assembled by double SN2 alkylation from acetylene. Methyl ketones point to Markovnikov hydration of terminal alkynes, while aldehydes suggest hydroboration-oxidation. A representative synthesis of cis-3-hexene from acetylene and bromoethane illustrates the power of this approach: deprotonation, alkylation to form 1-butyne, a second deprotonation, a second alkylation to form 3-hexyne, and finally Lindlar reduction to deliver cis-3-hexene.
A synthetic roadmap centered on an alkyne (R-C≡C-R'). Arrows radiate outward to show all transformations: upward to cis-alkene (H2/Lindlar, labeled "syn addition") and trans-alkene (Na/NH3, labeled "anti addition"), downward to ketone (H2O/H2SO4/HgSO4, "Markovnikov") and aldehyde (disiamylborane then H2O2/NaOH, "anti-Markovnikov"), leftward to vinyl halide (1 eq HX) and geminal dihalide (2 eq HX), rightward to alkane (H2/Pd, excess). A separate box shows the synthesis of alkynes: terminal alkyne + NaNH2 then R-X (SN2) or vicinal dihalide + 2 NaNH2 (double elimination).
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