Alkynes Lec.6

Hydrocarbons that contain a carbon–carbon triple bond are alkynes. General formula is C n H 2n-2 and for cyclic alkyne is C n H 2n-4 Alkynes are unsaturated, because they contain fewer hydrogens per carbon than alkanes (C n H 2n+2 ) The systematic name of an alkyne is obtained by replacing the “ane” ending of the alkane name with “yne.” The simplest alkyne is ethyne or acetylene

It can be classified the structure according to the relative positions of the multiple bonds(double or triple). Double bonds are said to be cumulated when they are right next to one another. When multiple bonds alternate with single bonds, they are called conjugated. When more than one single bond comes between multiple bonds, the latter are isolated or nonconjugated.

Problem: What are all of the structural possibilities for the compound C 3 H 4 ???????????????? Nomenclature of Alkynes a)Common Names Common names for alkynes are derived by adding the names of substituents on the carbon-carbon triple bond to the word acetylene.

b)IUPAC Names 1. The ending -yne is used for a triple bond (-diyne for two triple bonds and so on). 2. Select the longest chain that includes both carbons of the triple bond. For example, 3.Number the chain from the end nearest the triple bond so that the carbon atoms in that bond have the lowest possible numbers. 4. And as the same rules of alkenes.

Problem(1): Name each of the following structure by the IUPAC system: 2-heptyne or hept-2-yne Problem(2): Draw the structures and give the common and systematic names for the seven alkynes with molecular formula C 6 H 10 (assignment) CH 3 CH ═ CHCH 2 C≡CCH 3 5- when we have double and triple bond in the same compound, we write the number Of carbon atom that has the bond beside the reference end, for example : Hept-2-en-5-yne

Important note: 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 called internal alkyne. For example: 1-butyne is a terminal alkyne, but 2-pentyne is an internal alkyne.

Physical Properties of Alkynes The physical properties of alkynes are similar to those of alkanes and alkenes with analogous carbon skeletons. The lower-molecular-weight alkynes are gases at room temperature. Those that are liquids at room temperature have densities less than 1.0 g/mL (less dense than water). There are nonpolar compounds, they are insoluble in water and other polar solvents. They are soluble in each other and in other nonpolar organic solvents. An alkyne has a higher boiling point than an alkene containing the same number of carbon atoms.

Preparation of alkynes Thermal decomposition of ethene From methane by heating to 1500°C for a short time: Dehydrohalogenation of a haloalkene(alkyne from alkene)

Note: Dehydrohalogenation means removal of HX, where X is a halogen