By: Dr. Shatha Alaqeel 1

 The alkynes comprise a series of carbon and hydrogen based compounds that contain one triple bond. This group of compounds is a homologous series with the general molecular formula of C n H 2n—2  The alkyne triple bond is composed of one σ and two π covalent bonds.  The simplest alkyne, ethyne (also known as acetylene), has two carbon atoms and the molecular formula of C 2 H 2. The structural formula for ethyne is: 2

This involves the mixing of one s- and one p-orbital forming two sp-hybrid orbitals. The two sp-hybrid orbitals are oriented in a linear arrangement and bond angle is 180°. 3

Derived from acetylene 4

1. Identify the longest continuous chain of carbon atoms that contains the carbon- carbon triple bond. the - ane ending is changed to – yne 2. Number the carbon atoms of the longest continuous chain, starting at the end closest to the triple bond. 3. The position of the triple bond is indicated by placing the lower of the pair of numbers assigned to the triple-bonded carbon atoms in front of the name of the alkyne. 4. The location and name of any substituent atom or group is indicated. 5. Double and triple bonds are considered to have equal priority: thus in a molecule with both a double and triple bond, whichever is close to the end of the chain determines the direction of numbering. 6. In case where double and triple bonds would have the same position number, the double bond takes the lower number. 7. In writing the final name ‘’ene’’ comes before ‘’yne’’ regardless which takes the lower number (i.e. alphabetical order). 5

IUPAC : Propyne Common : Methyl acetylene 5-bromo-2-pentyne IUPAC :2,6-Dimethyl-3-heptyne Common :Isobutylisopropylacetylene 6

 Nonpolar, insoluble in water.  Soluble in most organic solvents.  Boiling points similar to alkane of same size.  Less dense than water.  Up to 4 carbons, gas at room temperature.  Terminal alkynes, R-C  C-H, are more acidic than other hydrocarbons. 8

1. Dehydrohalogenation of alkyl dihalides 9

2. Reaction of sodium Acetylide with Primary Alkyl Halides 10

Reactions of alkynes 1. Addition of hydrogen ( Hydrogenation ) 11 Alkynes can be reduced to trans-alkenes using Na or Li in liquid NH 3 Alkynes can be partially reduced to cis-alkenes with H 2 in the presence of poisoned catalysts. Alkynes can be partially reduced to cis-alkenes with H 2 in the presence of poisoned catalysts.

3. Addition of hydrogen halide 12