Nomenclature, Structure and Acidic Character of Alkynes

Alkynes are named on the same longest-chain, lowest-locant principle as alkanes and alkenes, but

with the suffix '-yne': the parent chain must include both triple-bonded carbons, numbered from

whichever end gives that triple bond the lower locant, e.g. but-1-yne (triple bond C1–C2) versus

but-2-yne (triple bond C2–C3).

Structure of the triple bond. At each triple-bonded carbon, one $s$ and one $p$ orbital

combine to give two collinear $sp$ hybrid orbitals, oriented at exactly $180°$ to each other, plus

two unhybridised $p$ orbitals left mutually perpendicular to each other and to the $sp$ axis. One

$sp$ orbital on each carbon forms the $\text{C--C}$ sigma bond (head-on overlap with the other

carbon's $sp$ orbital); the second $sp$ orbital on each carbon forms a sigma bond to the attached H

(or R group) -- so for ethyne, $\text{H--C}{\equiv}\text{C--H}$, the two carbons and both

hydrogens all lie on one straight line, giving the molecule its characteristic linear geometry

with $180°$ bond angles at each carbon. The two pairs of unhybridised $p$ orbitals, one pair per

carbon, then overlap sideways in two mutually perpendicular planes to give two pi bonds. The

triple bond is therefore one sigma bond plus two pi bonds: the shortest ($\approx 120$ pm) and

strongest of the three types of carbon-carbon bond, though — as with the alkene's single pi bond —

the exposed pi electron density is what alkynes' addition reactions attack first.

Acidic character. A terminal alkyne's $\equiv\text{C--H}$ hydrogen is measurably more

acidic than a comparable hydrogen on an alkene or alkane -- roughly $\text{p}K_a \approx 25$ for

ethyne, against $\approx 44$ for ethene and $\approx 50$ for ethane. The reason lies in the

hybridisation of the carbon bearing the hydrogen: an $sp$ orbital has $50\%$ $s$-character,

against $33\%$ for an $sp^2$ orbital and $25\%$ for an $sp^3$ orbital, and because an $s$ orbital

holds its electron density much closer to (and more tightly bound by) the positively charged

nucleus than a $p$ orbital does, the more $s$-character a hybrid orbital has, the more

electronegative the carbon effectively behaves and the more stable (lower in energy, less basic)

the lone pair left behind on the carbanion after the $\text{C--H}$ bond is broken. This is why

acidity of a $\text{C--H}$ bond runs $sp > sp^2 > sp^3$, i.e. alkyne $>$ alkene $>$ alkane.

Because it is (mildly) acidic, a terminal alkyne's hydrogen can be removed by a sufficiently

strong base -- sodamide, $\text{NaNH}_2$, in liquid ammonia, deprotonates it cleanly to give a

sodium acetylide (used synthetically to build longer alkyne chains by reaction with a primary alkyl …