Alkynes

Alkynes - hydrocarbons with carbon-carbon 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 2,3-dibromobutane. Ex. 2. Dehydrohalogenate 2,2-dibromobutane.

Ex. 3. Synthesize propyne from propene.