Acetylene or C2H2 is the simplest alkyne and a hydrocarbon that is colorless and has a garlic-like odor. It is highly reactive to atmospheric temperature and lacks oxygen being an unsaturated compound due to the presence of two carbon atoms bonded with a triple bond. As acetylene is reactive, unstable, and lighter than the air, the gas is highly flammable and leads to an explosion.
Irrespective of being toxic, acetylene is used for welding purposes as it is flammable. To human beings, this compound is no less than an element of risk as to the existence of it in the atmosphere reduces the level of oxygen. Not only it affects human beings but other living species as well disturbing various natural atmospheric cycles for whom oxygen is an integral component.
In light of the same, the recommended airborne exposure limit (REL) of acetylene is set to 2500 ppm (Ceiling) where an amount greater than this can kill human beings by becoming an asphyxiant gas.
With this, it becomes crucial to understand the behavioral chemical properties of acetylene to understand why it behaves in such a specific manner. To initiate, let’s begin with studying the Lewis structure of acetylene.
Lewis Structure of Acetylene (C2H2)
Lewis Structure is the pictorial representation showing how the valence electrons are participating in bond formation.
To study this, first, it is crucial to know the electronic configuration of the participating elements. Carbon (C) has atomic number 6 where its electronic configuration is 1s2 2s2 2p2.
On the other hand, the atomic number of Hydrogen (H) is 1 where its electronic configuration is 1s1.
As per the octet rule, each atom tries to achieve a stable condition by stabilizing the number of valence electrons, which is 8 for Carbon and 2 for Hydrogen. So, the number of valence electrons for Carbon is 4 and the same for Hydrogen is 1.
What are the valence electrons?
The electrons which are the farthest from the nucleus within an atom are called the valence electrons. The farthest two shells of an atom consist of the valence electrons who participate in a bond formation either by getting shared or by donating themselves completely.
Moreover, their number within the shell depends on the octet rule which says a maximum of 8 valence electrons is the most stable condition for an atom.
Now, let’s make the Lewis structure of Acetylene step-by-step:
Step 1: Search for the total number of valence electrons one molecule of acetylene already has: It is 10 for a single acetylene (C2H2) molecule.
Step 2: Search for how many more valence electrons one molecule of acetylene requires: It is 10 for a single acetylene (C2H2) molecule.
Step 3: Find the central atom to begin drawing the structure: As both the elements (carbon and hydrogen) are participating in equal numbers, there will be no central atom. This explains that the structure will be linear in the shape.
Step 4: Search for the type of bond forming between the participating atoms: A triple bond is formed between the Carbon (C) atoms and a single bond between the Hydrogen (H) and Carbon (C) atoms.
Now assemble all the aforementioned points and draw the structure:
Why the number of valence electrons is different for Hydrogen and Carbon as per the octet rule?
Filling the number of valence electrons in outermost shells depends on the maximum capacity of a shell and its flexibility in exceptional conditions.
As Hydrogen can withhold a maximum of two electrons and carbon can eight, so is the case. It might be interesting for you to realize that there are certain elements, like sulfur, which do not obey the octet rule and can accommodate ten to twelve valence electrons.
Molecular Geometry of Acetylene (C2H2)
Studying the molecule geometry of a molecule is a fundamental step in chemistry to analyze the behavioral properties of any molecule.
It helps with determining polarity, phase of matter, magnetism, reactivity, color, and biological activity of a molecule, in short, anything and everything about a molecule can be studied through molecular geometry.
Acetylene (C2H2) is a tetra atomic molecule having two different atoms bonding in equal numbers. Moreover, carbon is bonding to carbon which gives acetylene (C2H2) a linear structure and a bond angle of 180°.
The molecular geometry of acetylene (C2H2) can be studied with the help of the Valence Shell Electron Pair Repulsion (VSEPR) theory which says the valence electrons surrounding an atom in the pair tend to repel each other till they reach an arrangement where this repulsion is minimized the most.
We will study this more while discussing the polarity of acetylene (C2H2) in the next sub-heading.
C2H2 (Acetylene) Polarity
Polarity is a chemical property of elements through which they develop poles separating negative and positive charges.
Due to this separation of charges, the molecule tends to develop a strong attraction and repulsion behavior with the help of a hydrogen bond. Unlike unstable polar molecules, the non-polar molecules are comparatively stable as no separation of charges occurs in them.
Due to this, the molecule does not easily bond with nearby atoms to form a new molecule altogether. Non-polar molecules are formed of weak Van der Waal forces which are not as strong as hydrogen bonds so no strong bond formation takes place with new atoms.
The behavior of polarity solely depends upon the electronegativity values of the participating atoms. Electronegativity is the ability of atoms that determines how strongly it will attract the electrons towards itself to complete its octet. The higher the electronegativity value is, the stronger the attraction will be, and vice-versa. The electronegativity values of Carbon (C) and Hydrogen (H) are 2.55 and 2.20.
So, the electronegativity difference between Carbon-Carbon (C-C) bond is 0, and that between Carbon-Hydrogen (C-H) is 0.35.
Through this, it can be analyzed that both the values are lower than 0.4 which should have made acetylene (C2H2) non-polar, but Carbon-Hydrogen (C-H) is slightly polar than Carbon-Carbon (C-C) bond as the value of it is a bit higher.
But, in a bigger picture, overall acetylene (C2H2) is non-polar because the electronegativity values are lower than 0.4 which cancels out the net dipole moment completely.
You must also go through the article written specifically on the polarity of C2H2.
C2H2 (Acetylene) Hybridization
The concept of hybridization waves path for the molecular orbital diagram influencing the idea that the atomic orbitals combine and overlap to fuse and form hybrid orbitals which directly affects the molecular geometry and the bonding behavior of the newly produced molecule.
This can be studied with the help of the Valence Bond Theory (VBT) which says bonding takes place in such a manner that each molecule reaches a stable energy level with no strong repulsion, what-so-ever.
The hybridization of carbon atoms in the acetylene (C2H2) molecule is sp, whereas the hydrogen atoms have unhybridized 1s atomic orbitals. In the case of sp hybridization, the s orbital of the central atom only binds with one of its p orbitals.
Atoms that show sp hybridization always have a linear molecular geometry where two sp orbitals will be held at 180° to each other. So, linear molecular geometry is congruent to the sp hybridization where finding one will help with concluding the other.
From the Lewis structure, it can be seen that both the carbon atoms are associated with one another through a triple bond which is formed by one sigma (σ) bond and 2 pi (π) bonds.
This means that both the carbon atoms have two sets of unhybridized p atomic orbitals which undergo overlapping to produce two pi bonds in between the sigma (σ) bonded sp-hybridized carbon atoms.
When the carbon reaches the excited state, one electron from the 2s orbital moves to the 2pz orbital where all 2px 2py 2pz orbitals consist of one electron each. Whereas, in the case of a carbon-hydrogen bond, only 2s1 and 2pz1 orbitals get hybridized.
This hybridization leads to the formation of new 4 sp hybridized orbitals where the carbon-hydrogen bonding will produce 2 new sp hybridized orbitals.
C2H2 Molecular Orbital (MO) Diagram
The Molecular Orbital (MO) Diagram is a pictorial representation of bonding taking place between the electrons of the participating atoms to produce new molecules.
The basic principle this diagram follows is the atomic orbitals combine and overlaps in a certain manner to produce a similar number of molecular orbitals.
This occurs when electrons move to different orbitals as per the excitation level to get distributed and redistributed within the participating orbitals.
When these electrons move from their original positions, bonding and antibonding orbitals are produced which gives birth to the molecular orbital diagram specific to each molecule.
The above-mentioned molecular orbital diagram of acetylene (C2H2) is specifically showing the Carbon-Carbon bond.
You can see how the sp hybridized orbitals combine and overlap to form a bonding sigma (σ) orbital and an antibonding sigma (σ*) orbital.
Moreover, it can be seen that the four p orbitals combine and overlap to produce two π and two π* orbitals. If we try drawing the energy sequence from the lowest, reaching to the highest molecular orbital, it will be: σ < π(y) = π(z) < π(y)* = π(z)* < σ*.
As only 6 electrons are available to fill the orbitals, the first and only bonding orbitals are filled.
Acetylene (C2H2) is a toxic molecule for human beings as it can reduce the concentration of oxygen in the air.
A lot can be studied about the molecule through the Lewis structure which says acetylene (C2H2) is an unsaturated compound making it compatible and reactive enough to bond with atmospheric molecules and become toxic to human health.
Such behavior can be well understood with the help of its molecular geometry, hybridization, polarity, and molecular orbital (MO) diagram.