Electronic Structure of Alkynes

The H–C≡C  bond angles in acetylene are exactly 180°. Alkynes have a rod-like shape as a result of the bond angle. Cyclic alkynes, on the other hand, are extremely uncommon. It is impossible to separate benzyne. 121 picometers is a much shorter distance than the C=C distance in alkenes (134 pm) or the C–C bond distance in alkanes (140 pm) for the C–C bond (153 pm).

The triple bond has a bond strength of 839 kJ/mol, which indicates that it is quite strong. The sigma bond contributes 369 kJ/mol of bond strength, the first pi bond contributes 268 kJ/mol, and the second pi-bond gives 202 kJ/mol of bond strength, with the sigma bond contributing the most. When we talk about bonding, we normally think about it in terms of molecular orbital theory, which acknowledges that triple bonds are formed when two orbitals of the same atom overlap. The carbon atoms in an alkyne bond are sp hybridised, which means that they each contain two unhybridized p orbitals and two sp hybrid orbitals, according to the terminology of valence bond theory. The overlap of a sp orbital from each atom results in the formation of a single sp–sp sigma bond. Three bonds are formed when the p orbitals of two atoms overlap and generate two pi bonds, for a total of three bonds between the atoms. It is possible for each atom’s remaining sp orbital to create a sigma bond with another atom, for example, with the hydrogen atoms in the parent acetylene. The two sp orbitals of the carbon atom are positioned on opposing sides of the atom.

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.

Electronic Structure

The sp hybridization of the carbon-carbon triple bond results in the perpendicular orientation of the sigma bond and two pi bonds as a result of the sp hybridization of the carbon-carbon triple bond. The tight closeness of the electrons in this geometric orientation results in molecules with reduced stability as a result of their close contact. When it comes to the chemical reactivity of alkynes and the acidity of terminal alkynes, the structure of the carbon-carbon triple bond has a significant impact. A ten-membered carbon ring is the smallest structure that may support this function without causing significant strain due to its linear arrangement (the bond angle of a sp-hybridised carbon is 180o).

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.

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) (153pm).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.