A triple bond is a form of chemical bond between two atoms that is mediated via electron pairs (→ electron pair bond ). Three pairs of binding electrons between the atoms ensure the cohesion of the molecule built on them .
Triple bond reactivity
|Nitrogen , N 2
A triple bond has a very high electron density and should therefore easily enter into electrophilic addition reactions . In the case of carbon-carbon triple bonds, this is also true. A key factor here is that C≡C triple bonds are more energetic than C = C double or single bonds (when a triple bond is formed, an average of 811 kJ mol −1 are released, for each double bond 615 kJ mol −1 and for each single bond 345 kJ mol −1 ). This shows that the energy output is higher when three individual bonds are formed instead of a single triple bond. In most cases, the differences are easily applied by the newly formed bonds, and there is usually a considerable gain in energy with additions to C≡C triple bonds.
With nitrogen , the situation is exactly the opposite. The triple bond in the nitrogen molecule N 2 , with a binding energy of 945 kJ mol −1, is relatively much lower in energy (= more stable) than an NN double bond (when it is formed 466 kJ mol −1 ) or even a single bond (when it is formed only 159 kJ mol −1 is released). This shows that the energy output is lower when three individual bonds are formed instead of a single triple bond. These energy differences cannot normally be applied by forming additional bonds with the reactants, and therefore molecular nitrogen is a well-known example of the occurrence of a strong triple bond with high stability and sluggish reactivity.
The molecule carbon monoxide (carbon monoxide, CO) has the highest binding energy (1077 kJ · mol −1 ) of a triple bond , since a weak ionic bond is added to the triple bond.
Orbital model of the triple bond
From a quantum chemical point of view, bonds are created by overlapping atomic orbitals to form a molecular orbital . The most common description of the triple bond in alkynes is via a sigma bond made up of sp hybrid orbitals , which lies between the core compound axis and two pi bonds , which form an angle of 90 degrees with one another and both lie outside the core compound axis. An alternative, completely equivalent description uses three equivalent “banana bonds” that are formed by the overlap of sp 3 hybrid orbitals.
Bond length according to Pauling
According to Pauling, covalent bond lengths can be estimated as the sum of two atomic radii. On the basis of experimental and quantum chemical data, additive covalent radii for atoms in triple bonds have been published for the elements beryllium to copernicium . The data set used is self-consistent and contains only one radius for all oxidation states and coordination numbers of the elements considered. By simply adding the atomic radii, a prediction of the triple bond length can be made.
Triple Bond Molecules
Molecules that contain C – C triple bonds belong to the alkynes group. The chemistry of alkynes is characterized by the addition of electrophiles to this bond. A carbon atom involved in a triple bond is sp-hybridized, so alkyne anions are correspondingly stable and can be used as nucleophiles. The bond length of a C – C triple bond in alkynes is 120 pm .
Triple bond with boron
In 2012, a working group at the University of Würzburg succeeded for the first time in producing a compound that contains a triple bond between two boron atoms, stable up to around 200 ° C.
Triple bond with sulfur
In 1984, a working group at the Free University of Berlin succeeded in synthesizing a carbon-sulfur triple bond in the form of trifluoroethylidine-sulfur trifluoride for the first time. It is a colorless gas that oligomerizes quickly.
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