Alkynes
• Alkynes - hydrocarbons with carboncarbon triple bonds
• Alkadiynes - hydrocarbons with two
carbon-carbon triple bonds
• Alkenynes - hydrocarbons with both
double and triple bonds
Nomenclature of Alkynes
• 1. Find the longest continuous chain of
carbon atoms containing the multiple bonds
and name it using –yne, -diyne, or –en-yne.
• 2. Number the chain so the multiple bonds
have the lowest possible number
– **if numbering from either end gives the same
number for the multiple bonds, then the double
bond gets preference over the triple bond (the
double bond gets the lower number)
• 3. Name, number and alphabetize the
constituent groups.
Alkyne Nomenclature Examples
• 1.
• 2.
• 3.
• 4.
• 5.
Reactions of Alkynes
• Alkynes undergo similar reactions to the
alkenes (addition reactions)
• 1. Hydrogenation – addition of H2
• -the usual product is an alkane, BUT…with
the use of a special catalyst known as
Lindlar’s catalyst (Pd), it is possible to stop
the reaction at the alkene
• -this is especially useful for preparing cis
alkenes, since both hydrogens add to the
triple bond from the same side
• Hydrogenation Examples
• 1. Hydrogenate 2-butyne with Lindlar’s
reagent.
• 2. Hydrogenate 2-butyne with Pt or Ni.
• 2. Halogenation – addition of Cl2 or Br2
– this is a trans addition
– Ex.; Brominate ethyne.
• 3. Hydrohalogenation - addition of HI,
HBr, or HCl
• - follows Markovnikoff’s rule – H
attaches to the carbon with the greater
number of hydrogens
• Ex.; React propyne with HCl.
• 4. Hydration - addition of H-OH
• - follows Markovnikoff’s rule
• - catalyst is a mixture of dilute sulfuric acid
(H2SO4) and mercuric sulfate (HgSO4)
• -product is an unstable enol, which
rearranges itself to form an aldehyde or a
ketone
• -enols have a double bond (-ene) and an
alcohol (-ol) on the same carbon atom
• Hydration examples.
• 1. Hydrate propyne
• 2. Hydrate ethyne
Preparation of Alkynes
• Dehydrohalogenation of a Dihalide - removal
of HCl or HBr (two of them) using KOH and
ethanol (C2H5OH)
• - most commonly used to convert an alkene
to an alkyne in a synthesis sequence
• Ex. 1. Dehydrohalogenate 2,3dibromobutane.
• Ex. 2. Dehydrohalogenate 2,2dibromobutane.
• Ex. 3. Synthesize propyne from
propene.