Alkynes: Structure, Properties, and Nomenclature
Explore the fundamental aspects of alkynes, focusing on their structure, reactivity, and nomenclature. Learn about the carbon-carbon triple bond, acetylene, higher alkynes, physical properties, and industrial sources of alkynes. Discover the unique characteristics that make alkynes essential in organic synthesis.
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Alkyne Second stage Assist. Professor : Wasanabdul razzaq mahmood 2024-2025
Introdaction 1.1 Now we come to carbon-carbon triple bond, the functional group of the family called alkyne. Like the double it is unsaturated and highly reactive toward the reagents that double bond s react with, and toward some others besides .Through a combination of its characteristic properties, the carbon carbon triple band plays a special role one of increasing importance in organic synthesis . 2.2 Structure of acetylene .The carbon carbon triple bond The simplest member of the alkyne family is acetylene, C2H2 .We arrive at a structure in which the carbon atoms share three pairs of electrons, that is, are joined by a triple bond . The carbon-carbon triple bond is the distinguishing of the alkyne structure. HC::: CH H-C C-H Acetylene Acetylene linear molecule, it have two orbitals on one carbon (doughnut shaped cloud) can overlap two orbitals on the other carbon.
The carbon-carbon triple bond is thus made up of ne strong bond and weaker bonds ;it has a total strength of 198 Kcal .
_ Shape: linear Bond angle: 108 Bond length: 1.2 A S character: 50% P character : 50% _ _ _ _ Because of their greater character ,sp orbitals are smaller than sp2orbitals,and sp hybridized carbon forms shorter bonds than sp2 hybridized carbon .the C-H bond dissociation energy in acetylene is not known ,but we would expect it to be even greater than in ethylene .the same sp hybridization that almost certainly makes cleavage of the C-H Bond form free radicals (hemolysis) more difficult, makes cleavage e to form ion (heterolysis) .
1.3 Higher alkynes . Nomenclature The alkynes are named according to tow systems. In one ,they considered to be derived from acetylene by replacement of one or both hydrogen atoms by alkyl groups . H-C C-C2H5 CH3-C C-CH3 CH3-C C-CH(CH3)2 Ethyl acetylene Dimethylacetlylene Isopropylmethylacetylene 1-Butyne 2-Butyne 4-Methyl-2-pentyne IUPAC names are used ,the rules are exactly the same as for the naming of alkenes ,except that the ending -yne replaces -ene 1-The parent structure is the longest continuous chain that contains the triple bond. 2- The positions both of substituents and of triple bond are of indicated by numbers. 3-The triple bond is given the number the first triply bonded carbon encountered, starting from the end of the chain nearest the triple bond. 4- The rules are exactly the same as for the naming of alkenes, except that the ending -yne replaces ene.
2.4 Physical properties of alkynes Being compounds of low, the alkynes have physical properties that are essentially the same as those of the alkanes and alkenes. They are insoluble in water but soluble in the usual organic solvents of low polarity: ligroin, ether, benzene, carbontatrachloride .They are less dense than water. Their boiling points show the usual increase with increasing carbon number, and the usual effects of chain branching; they are very nearly the same as the boiling points of alkanes or alkanes or alkene with the carbon skeletons . 1.5 Industria sourece of acetylene 2-The alkyne of chief industrial impotence is the member of the family, acetylene. It is made by the controlled, high temperature partial oxidation of methane 6CH4 + O2 2CH CH + CO2 + 10H2 1500C
