Org_Lect-Alkyne (1).pdf

Alkynes An alkyne is a hydrocarbon that contains a carbon–carbon triple bond. Only relatively few alkynes are found in nature. Examples include capillin, which has fungicidal activity, and ichthyothereol, a convulsant used by the in...

Alkynes An alkyne is a hydrocarbon that contains a carbon–carbon triple bond. Only relatively few alkynes are found in nature. Examples include capillin, which has fungicidal activity, and ichthyothereol, a convulsant used by the indigenous people of the Amazon for poisoned arrowheads. Because of its triple bond, an alkyne has four fewer hydrogens than an alkane with the same number of carbons. Therefore, while the general molecular formula for an acyclic alkane is CnH2n+2 , the general molecular formula for an acyclic alkyne is CnH2n-2 and that for a cyclic alkyne is CnH2n-4. The systematic name of an alkyne is obtained by replacing the “ane” ending of the alkane name with “yne.” Compounds with other functional groups are named, the longest continuous chain containing the carbon–carbon triple bond is numbered in the direction that gives the functional group suffix as low a number as possible. If the triple bond is at the end of the chain, the alkyne is classified as a terminal alkyne. Alkynes with triple bonds located elsewhere along the chain are internal alkynes. In common nomenclature, alkynes are named as substituted acetylenes. The common name is obtained by stating the names of the alkyl groups (in alphabetical order) that have replaced the hydrogens of acetylene. If counting from either direction leads to the same number for the functional group suffix, the correct systematic name is the one that contains the lowest substituent number. If the compound contains more than one substituent, the substituents are listed in alphabetical order. 1  Q6/ Draw the structure for each of the following: or Name the following: Q7/ Draw the structures and give the common and systematic names for the seven alkynes with molecular formula C 6H10. 2  The Physical Properties Of Unsaturated Hydrocarbons All hydrocarbons—alkanes, alkenes, and alkynes—have similar physical properties. They are all insoluble in water but soluble in nonpolar solvents. They are less dense than water and, like other homologous series, have boiling points that increase with increasing molecular weight. Alkynes are more linear than alkenes, and a triple bond is more polarizable than a double bond. These two features cause an alkyne to have stronger van der Waals interactions and, therefore, a higher boiling point than an alkene with the same number of carbons. What orbitals are used to form the carbon–carbon s bond between the highlighted carbons? 3 j Reactions of alkynes: 1-addition of hydrogen halide to an alkyne: 4  1-addition of hydrogen halide to an alkyne: 1-addition of hydrogen halide to an alkyne: 5 1-addition of hydrogen halide to an alkyne: 6 7  Reactions of alkynes: 2-addition of water: 1-addition of hydrogen halide to an alkyne: 6 2-addition of water: 2-addition of water: 6 Reactions of alkynes: 2-addition of hydrogen 6