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Nomenclature of Alkynes
Alkynes are hydrocarbons that contain a carbon–carbon
triple bond. Noncyclic
alkynes have the molecular formula CnH2n-2.
The systematic name of an alkyne is
obtained by replacing the -ane ending of the alkane
name with -yne. Both acetylene and
ethyne are acceptable IUPAC names for HC≡CH.
Structure
and bonding of Alkynes
Each carbon of the alkynes have sp hybridized, so each has two sp orbitals and
two p orbitals. One sp orbital overlaps the s orbital of a hydrogen, and the
other overlaps an sp orbital of the other carbon. Each of the two p
orbitals on one carbon
overlaps the parallel p orbital on the other carbon to form two π bonds
The acidity of terminal
Alkynes.
The hydrogen atoms of ethyne are considerably more acidic than those of
ethane
or ethane. Solutions of sodium acetylide (HC≡CNa) may be prepared by
adding sodium amide (NaNH2) to acetylene in liquid ammonia as the
solvent
Anions of acetylene and terminal alkynes are nucleophilic and
react with methyl and primary alkyl halides to form carbon–carbon bonds by
nucleophilic substitution.
Stabilities of Alkynes
Like alkenes, alkynes are stabilized by electron-donating alkyl groups.
Internal
alkynes, therefore, are more stable than terminal alkynes.
Synthesis of Alkynes
Alkynes may be prepared by a double dehydrohalogenation of dihaloalkanes.
The dihaloalkane may be a geminal dihaloalkane, one in which both halogens are
on the same carbon, or it may be a vicinal dihaloalkane, one in which the
halogens are on adjacent carbons.
Reactions of Alkynes
Alkynes undergo
the acid-catalyzed addition of water. The product of the reaction is an
enol, wich immediately rearranges to a ketone.
Hydrogen adds
to an alkyne in the presence of a metal catalyst such as palladium,
platinum, to yield alkanes.
Metals in
liquid ammonia as the solvent convert alkynes to trans alkenes. The
reaction proceeds by a four-step sequence in which electron-transfer and
proton-transfer steps alternate.
Addition
of 1 equivalent of chlorine or bromine to an alkyne yields a trans
dihaloalkene |