How Are Triple Bonds Formed and Why Are They Important?
In chemistry, a triple bond is defined as a covalent linkage, where two atoms share three pairs of electrons, as in the nitrogen molecule, acetylene, C2H2, or N2. One of the main electron pairs exists in a sigma bond, which is concentrated in the region along the line joining the two nuclei; the remaining two pairs are available in pi bonds, each of which occupies the two parallel regions of space on the opposite sides of the line that is determined by the two atoms.
The triple bond symbol is ☰.
Bonding
The bonding types may be explained in terms of orbital hybridization. In the acetylene case, every carbon atom contains two p-orbitals and two sp-orbitals. The two p-orbitals are perpendicular on both the y-axis and z-axis. At the same time, the two sp-orbitals lie linear with 180° angles and occupy the x-axis (cartesian coordinate system). When the carbon atoms approach each other, the sp orbitals overlap to make an sp-sp sigma bond. The Pz-orbitals, at the same time, approach, and together, they produce a Pz-Pz pi-bond. Similarly, the other pair of Py-orbitals produce a Py-Py pi-bond. The result is the formation of one sigma-bond and two pi-bonds.
Example of a Triple Bond Formation
Formation of Triple bond in Carbon
One of the perfect triple bond examples representing the value of hybrid orbitals is Carbon. The ground state configuration of Carbon is given as follows:
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As per the Valence Bond Theory, carbon should produce two covalent bonds by resulting in a CH2 due to the reason it has two unpaired electrons in its electronic configuration. However, some experiments have already shown that CH2 is highly reactive and it cannot exist outside of a reaction. Thus, this does not explain how the CH4 can exist. To produce four bonds, the carbon’s configuration must contain four unpaired electrons.
CH4 can be explained in one way, one 2s and three 2p orbitals combine to form four equal energy sp3 hybrid orbitals. That would give the configuration as follows:
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Now that the carbon compound has four unpaired electrons so that it can have four equal energy bonds. The orbital hybridization is favored due to the hybridized orbitals being more directional that leads to greater overlap when forming the bonds. Thus the bonds formed are stronger. This results in very stable compounds when hybridization takes place.
Let us see the explanation of various types of hybridization further and how every type helps explain the structure of certain molecules.
sp3 Hybridization
sp3 hybridization may explain the tetrahedral structure of the molecules. In it, both the 2s orbitals and all the three of 2p orbitals hybridize to make four sp3 orbitals, each consisting of 75% of p character and 25% of s character. And, the frontal lobes align themselves in a manner as represented below. In this particular structure, electron repulsion is minimized.
Energy changes occurring in the hybridization is given below:
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The s-orbital’s hybridization with all the three p orbitals (Px, Py, and Pz) results in four sp3 hybrid orbitals. And, the sp3 hybrid orbitals are oriented at a bond angle of 109.5° from each other. This particular 109.5° arrangement gives the tetrahedral geometry as figured above.
Stability of Triple Bond Compared to a Single Bond
In the case of covalent molecules, more is the sharing of electrons between the atoms; stronger is: a single bond 2 electrons are shared, in a manner, 4 in double bond and 6 in a triple bond. Therefore, a triple bond is the strongest and most difficult to break. Now, the stronger the bond between the two atoms, the stabler (or more stable) the molecule. Thus, a triple bond is said to be more stable.
In terms of strength, the order is Triple >> Double >> Single,
In terms of bond length, the order is Single >> Double >> Triple,
In terms of stability, However, it is Single >> Double >> Triple.
Why? Because mostly, it’s much harder to break the sigma bonds compared to break the pi bonds, and single bonds contain zero pi bonds, whereas the triple bonds have two of them, which makes them easier to break.
Rotation of a Triple Bond
Yes, the rotation of a triple bond takes place. Only a single p-orbital is involved in the sp hybridization in acetylene. The other two mutually perpendicular P-orbitals, say Pz, Py can rotate, replacing each other's axis (on rotation, Pz becomes Py and Py becomes Pz). However, practically it is meaningless and merely detected because there is no formation of a new compound and the groups attached to do carbon just to rotate about their axis, bringing no change in the overall symmetry and geometry. (The angle of rotation should be 90°** or its multiples)
Example
Let us take an example of 2-butyne. Rotating the triple bond raises many questions. First, it is not said to be meaningful. Raman or Infrared spectroscoPy are the two methods to notice the rotations. The end methyl groups’ rotations are visible. So, What visible is the stretching of a triple bond. If you grab the two methyl groups on a speculative paper experiment, the triple bond could rotate, but the main thing is how you would see it.
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FAQs on Triple Bond: Structure, Stability & Hybridization
1. What is a triple bond and how is it formed?
A triple bond is a type of covalent bond where two atoms share three pairs of electrons, for a total of six bonding electrons. It consists of one sigma (σ) bond and two pi (π) bonds. The strong sigma bond is formed by the direct, head-on overlap of hybrid orbitals, while the two weaker pi bonds are formed by the sideways overlap of the remaining unhybridized p-orbitals above and below the sigma bond axis.
2. What is the hybridization of atoms involved in a triple bond?
Atoms that participate in a triple bond, such as the carbon atoms in acetylene (C₂H₂), typically exhibit sp hybridization. In this process, one s-orbital and one p-orbital combine to form two sp hybrid orbitals. These orbitals arrange themselves in a linear geometry with a 180° angle between them to minimize repulsion. The two remaining p-orbitals are unhybridized and are used to form the two pi bonds.
3. Can you provide some common examples of molecules with triple bonds?
Certainly. Several common molecules feature triple bonds, which are crucial to their structure and properties. These include:
- Nitrogen (N₂): The diatomic nitrogen molecule in the air consists of two nitrogen atoms connected by a triple bond.
- Acetylene (Ethyne, C₂H₂): The simplest alkyne, it has a carbon-carbon triple bond.
- Hydrogen Cyanide (HCN): This molecule contains a triple bond between the carbon and nitrogen atoms.
- Propyne (C₃H₄): An alkyne that contains one carbon-carbon triple bond in its three-carbon chain.
4. How does a triple bond compare to a single or double bond in terms of strength and length?
When comparing bonds between the same two elements, a triple bond is both the strongest and the shortest.
- Strength: Triple bonds have the highest bond enthalpy (energy required to break the bond) because they involve the sharing of six electrons. The order is: Triple bond > Double bond > Single bond.
- Length: The strong attraction between the nuclei and the six shared electrons pulls the atoms closer together, resulting in the shortest bond length. The order is: Single bond > Double bond > Triple bond.
5. Why do molecules with a triple bond, like acetylene (C₂H₂), have a linear shape?
Molecules with a triple bond have a linear shape because the central atoms are sp hybridized. According to VSEPR theory, the two sp hybrid orbitals on each carbon atom position themselves as far apart as possible to minimize electron-pair repulsion. This results in a perfect bond angle of 180°, forcing the atoms into a straight line and giving the molecule a linear geometry.
6. For determining molecular geometry, how many electron domains does a triple bond count as?
When using VSEPR (Valence Shell Electron Pair Repulsion) theory to predict molecular geometry, a triple bond is counted as only one electron domain. Although it consists of three electron pairs, they are all located in the same region between the two bonded atoms. Therefore, for the purpose of geometry determination, a triple bond exerts a similar repulsive effect as a single bond or a double bond, counting as a single region of electron density.
7. How does the presence of a triple bond affect the stability and reactivity of a molecule?
The effect is twofold:
- Stability: A triple bond has a very high bond dissociation energy, making it very strong and stable. It requires a large amount of energy to break all three bonds completely.
- Reactivity: Despite its strength, a triple bond is a site of high reactivity. The two pi bonds create a region of high electron density, making it susceptible to electrophilic addition reactions, where the pi bonds break to form new single bonds.
8. Why is there no free rotation around a carbon-carbon triple bond?
Free rotation is restricted around a triple bond because of the specific orientation required for the two pi (π) bonds. These bonds are formed by the sideways overlap of p-orbitals that must remain parallel to each other. Any rotation around the central sigma bond axis would misalign these p-orbitals, breaking the pi bonds. This process would require a significant amount of energy, effectively locking the atoms into a rigid, non-rotating structure.
