1. By Dr. Nahed Nasser 1

2. CONTENTS Structure of alkynes Hybridization of alkynes Nomenclature Physical properties of alkynes Preparation of alkynes Reactions of alkynes THE CHEMISTRY OF ALKYNES 2

3. Alkynes: Molecular and Structural Formulas The alkynes comprise a series of carbon- and hydrogen- based compounds that contain at least one triple bond. This group of compounds is a homologous series with the general molecular formula of Cn H2n—2 The alkyne triple bond is composed of one σ and two 2  covalent bonds, the triple bond can be terminal or internal. 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: 3

4. sp Hybridization of carbon atom This involves the mixing of one s- and one p-orbital forming two sp-hybrid orbitals of equivalent energy. The two sp-hybrid orbitals are oriented in a linear arrangement and bond angle is 180° to minimize the replusion between them. The remaining two p orbitals (py and Pz) are unaltered 4

5. Molecular formula of ethyne is C 2 H 2. In ethyne, each carbon atom is sp-hybridized. In this way, four sp-orbital are generated. One sp- orbital of each carbon atom by overlapping forms a sigma bond between carbon atoms. Remaining one sp-orbital of each carbon atom overlap with 1s-orbital of a hydrogen atom to produce two sigma bonds. Py-orbital and Pz-orbitals of each carbon by parallel overlapping form two pi-bonds between the two carbon atoms. Geometry (shape) of ethyne molecule is linear in which bond angles are 180 o. 5

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7. Summary sp hybridisation occurs when a C has 2 sigma bonds only sp hybridised orbitals has 50% s and 50% p character The 2 sp hybrids point in opposite directions at 180 o to each other Each sp hybrid is involved in a( )sigma bond The remaining p orbitals forms the 2pi bonds The triple bond is one ( )bond and two pi bonds. 7

8. Alkynes are named in the same general way that alkanes and alkenes are named Find the longest chain containing both atoms of the triple bond; this gives the name of the parent alkane Change -ane ending to -yne. Number the chain, starting at the end closest to the triple bond. Give branches or other substituents a number to locate their position. The simplest alkyne, H-C  C-H, named in the IUPAC system as ethyne, but it is more often called acetylene, its common name Therefore the common names of alkynes are derived from acetylene ( e.g Methyl acetylene) IUPAC Nomenclature 8

9. IUPAC : Propyn Common : Methyl acetylene IUPAC :2,6-Dimethyl-3-heptyne Common :Isobutylisopropylacetylene 9 5-Bromo-2-pentyne Not 1-Bromo-3-pentyne

10. Exercise 1)Give the IUPAC and common names of the following compounds: 10

11. Physical Properties 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. Acetylene can be converted to acetylide anion by NH 2 - (NH 3 /Na) but not by OH - or RO -. 11

12. Preparation of alkynes 1)Dehydrohalogenation of alkyl dihalides (- 2 HX) (- 2 HX) 12

13. Reaction of sodium Acetylide with Primary Alkyl Halides 2. Reaction of sodium Acetylide with Primary Alkyl Halides 13

14. Reactions of alkynes 1.Addition of hydrogen ( Hydrogenation) 14 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.

15. Addition of halogen 2. Addition of halogen 3Addition of hydrogen halide 3. Addition of hydrogen halide 4. Addition of water: Hydration 15