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“Hydrocarbons” are natural chemicals that contain both carbon and hydrogen. Aliphatic hydrocarbons are compounds with saturated single bonds (alkanes) or unsaturated double or triple bonds in their molecules. Alkenes have the structural formula CnH2n and include carbon–carbon double bonds. Alkynes are unsaturated hydrocarbons with carbon–carbon triple bonds that are structurally represented by the formula CnH2n-2.
The most basic alkyne is ethyne, often known as acetylene, a colourless gas that burns at high temperatures and is used as a welding fuel. Alkynes can be found in a wide range of natural and manufactured chemicals. Natural alkynes, for example, may be found in the venom of South American tree frogs. Synthetic alkyne-containing chemicals, on the other hand, play a major role in medications like ethynylestradiol, an oral contraceptive. Other alkyne-based medicines, such as selegiline, are used to treat Parkinson’s disease in conjunction with synthetic dopamine or L-dopa.
Hybridisation Of Alkynes
Alkynes undergo hybridization when the sp orbitals of the carbon–carbon (C–C) bond overlap to produce a sigma bond, and the lateral overlapping of the 2py and 2pz orbitals to form the two pi bonds, respectively. The carbon–hydrogen (C–H) sp–1s sigma bond is formed by the overlap of the carbon sp orbital with the hydrogen atom’s 1s orbital. Alkynes have a 50 percent s character and display a sp hybridization. Because electrons in the s orbital have lower energies than electrons in the p orbital, increasing the s character of an atom improves its electronegativity.
Molecular Geometry: Bond length, Bond angles, and Bond Strengths
Alkynes have a linear shape with a bond angle of 180° due to sp hybridization involving the lateral overlap of orbitals. Due to the higher number of bonds binding the two carbons together, the length of the C–C triple bond in acetylene is 121 pm or 1.21, which is less than that of alkenes (134 pm or 1.34), and alkanes (153 pm or 1.53).
Due to the higher s-character of the sp hybridised carbon orbital creating the sp-1s bond with the hydrogen, the C–H bond in alkynes (1.06 in acetylene) is likewise shorter than in alkenes and alkanes.Electrons in the s-orbital are closer to the atomic nucleus and are bonded more closely than those in the p orbitals, in addition to having lower energies. As a result, a higher s-character boosts the bond’s strength. As a result, alkynes’ C-C triple bonds and C-H bonds are shorter and stronger than those of alkenes and alkanes, owing to their higher s character. Due to the existence of these shorter and stronger bonds, the bond dissociation energy of alkynes is 966 kJ or 231 kcal/mol, which is greater than that of alkanes and alkenes.
Physical Properties
Unsaturated hydrocarbons with at least one triple bond between the carbon atoms are known as alkynes. Terminal alkynes and internal alkynes are the two forms of alkynes. Terminal alkynes are triple-bonded compounds in which the carbon atom and the carbon at the end of the chain share a triple bond. Internal alkynes are those in which the triple bond is formed between two carbon atoms that are not terminal.CnH2n–2 is the typical molecular formula for alkynes. Alkynes have physical properties that are quite similar to alkenes’ physical qualities. The physical features of alkynes will be discussed in depth in this article.
Because of the existence of hybridization, the structure of the alkyne is unique. The acidity, nonpolar bonding strength, and linearity of alkynes are all due to the triple bonds in the alkynes. In polar solvents, the alkyne compounds are marginally soluble, but entirely insoluble in water. Alkynes have the ability to dissolve in organic solvents since their density is lower, which is another property of alkenes. Alkynes, for example, have the capacity to dissolve in ether solutions.
The boiling point of these triple-bonded alkyne compounds is somewhat higher than that of alkanes and alkenes. Ethane, for example, has a boiling point of -88.6 C, whereas ethene has a boiling point of -103.7 C. The boiling point of ethyne is somewhat higher, at -84 C. The boiling point of alkynes rises as the number of carbon atoms increases.
These triple alkyne bound compounds have a greater acidity than their counterparts, alkanes and alkenes. The most acidic alkynes are those that have been hybridised with sp. They can only be deprotonated by using strong bases. For example, ethane has a pKa value of 62, making it the least acidic, while ethene has a pKa value of 45, making it the most acidic. With a pKa of 26, ethyne is the most acidic of the bunch.
Alkynes are relatively recognised to be unstable compounds with a high energy due to electron repulsion. The quantity of energy contained in the alkyne molecules is responsible for a considerable amount of heat production.
Conclusion
The H–C≡C bond angles in the compound acetylene are 180°. Alkynes have a rod-like shape due to this bond angle. Cyclic alkynes, on the other hand, are uncommon. Benzyne is a very volatile compound. The C–C bond in alkanes is shorter than the C=C bond in alkenes (134 pm), and the C–C bond in alkanes is shorter than the C=C bond in alkenes (134 pm) (153 pm).Alkynes have physical characteristics that are identical to those of their parent alkane or alkene. They are insoluble in water and polar solvents because they are nonpolar compounds with a lower density than water. They do, however, dissolve well in nonpolar organic solvents. At normal temperature, lower-molecular-weight alkynes like ethyne and propyne exist as gases, but higher-molecular-weight alkynes like 1-octyne and 1-decyne exist as liquids.
