Physical and Chemical Properties — Core Principles

Core Principles

Alkynes are unsaturated hydrocarbons with a carbon-carbon triple bond ($C equiv C$), having $sp$ hybridized carbons and a linear geometry. Physically, they are nonpolar, insoluble in water but soluble in organic solvents.

Their boiling and melting points increase with molecular weight, and lower members are gases. A key property is the acidity of terminal alkynes, where the hydrogen attached to the $sp$ carbon is weakly acidic, allowing reactions with strong bases and heavy metal ions (e.

g., Tollens' reagent, ammoniacal cuprous chloride) to form acetylides. Chemically, the electron-rich triple bond undergoes electrophilic addition reactions. These include hydrogenation (complete to alkanes, partial to cis-alkenes with Lindlar's catalyst, or trans-alkenes with Na/liq.

NH3), halogenation (to tetrahaloalkanes), hydrohalogenation (to geminal dihalides following Markovnikov's rule), and hydration (to enols that tautomerize to ketones/aldehydes, also following Markovnikov's rule).

Alkynes also undergo oxidation, with strong agents like hot $KMnO_4$ leading to oxidative cleavage and formation of carboxylic acids or $CO_2$. Polymerization can also occur, such as the trimerization of ethyne to benzene.

Important Differences

vs Alkenes

Aspect	This Topic	Alkenes
Hybridization of C-C multiple bond carbons	$sp$ (Alkynes)	$sp^2$ (Alkenes)
Geometry around multiple bond	Linear ($180^circ$) (Alkynes)	Trigonal planar ($120^circ$) (Alkenes)
Acidity of C-H bond (if terminal)	Weakly acidic (Terminal Alkynes)	Non-acidic (Alkenes)
Number of $pi$ bonds	Two (Alkynes)	One (Alkenes)
Hydration product	Ketones/Aldehydes (via enol tautomerism) (Alkynes)	Alcohols (Alkenes)
Partial hydrogenation stereochemistry	Can be controlled (cis with Lindlar's, trans with Na/liq. NH3) (Alkynes)	Not applicable (already one $pi$ bond) (Alkenes)

Alkynes and alkenes are both unsaturated hydrocarbons, but their distinct hybridization ($sp$ vs. $sp^2$) leads to significant differences. Alkynes have a linear geometry and two pi bonds, making them more reactive towards electrophilic addition than alkenes. Crucially, terminal alkynes exhibit weak acidity due to the $sp$ carbon, a property absent in alkenes, allowing them to form metal acetylides. Their hydration reactions yield carbonyl compounds via tautomerism, while alkenes yield alcohols. Furthermore, alkynes offer unique stereochemical control in partial hydrogenation, forming either cis or trans alkenes, a feature not applicable to alkenes themselves.