Important Concepts
1. The rules for naming alkynes are essentially the same as those formulated for
alkenes. Molecules with both double and triple bonds are called alkenynes, the double bond receiving
the lower number if both are at equivalent positions. Hydroxy groups are given
precedence in numbering alkynyl alcohols (alkynols).
2. The electronic structure of the triple bond reveals two π bonds, perpendicular to each other,
and a σ bond, formed by two overlapping sp hybrid
orbitals. The strength of the triple bond is about 229 kcal mol-1;
that of the alkynyl C – H bond is 131 kcal mol-1. Triple bonds form
linear
structures with respect to other attached
atoms, with short C – C (1.20 Å) and C – H (1.06 Å) bonds.
3. The high s character at C1 of a
terminal alkyne makes the bound hydrogen relatively acidic (pKa <25 o:p="">
4. The chemical shift of the alkynyl hydrogen is low (δ = 51.7 – 3.1 ppm)
compared with that of alkenyl hydrogens because of the shielding effect of
an induced electron current around the molecular axis caused by the external
magnetic field. The triple bond allows for long-range coupling. IR
spectroscopy indicates the presence of the C≡C and ≡C – H bonds in
terminal alkynes through bands at 2100 – 2260 cm-1 and 3260 –
3330 cm-1, respectively.
5. The elimination reaction with vicinal dihaloalkanes proceeds
regioselectively and stereospecifically to give alkenyl halides.
6. Selective syn dihydrogenation of
alkynes is possible with Lindlar catalyst, the surface of which is less
active than palladium on carbon and therefore not capable of hydrogenating
alkenes. Selective anti hydrogenation
is possible with sodium metal dissolved in liquid ammonia because simple
alkenes cannot be reduced by one-electron transfer. The stereochemistry is set
by the greater stability of a trans disubstituted alkenyl radical
intermediate.
7. Alkynes generally undergo the same
addition reactions as alkenes; these reactions may take place twice in
succession. Hydration of alkynes is unusual. It requires an Hg(II) catalyst,
and the initial product, an enol,
rearranges to a ketone by tautomerism.
8. To stop the hydroborationof terminal
alkynes at the alkenylboron intermediate stage, modified dialkylboranes —
particularly dicyclohexylborane — are used. Oxidation of the resulting
alkenylboranes produces enols that tautomerize to aldehydes.
9. The Heck reaction links alkenes to alkenyl halides in a metal-catalyzed
process.
From "Organic Chemistry" Textbook of VOLLHARDT & SCHORE
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