Ethyne is the simplest alkyne with the chemical formula C2H2. It is a colourless gas with a faint garlic odour at room temperature. The preferred IUPAC name for C2H2 is acetylene.
Ethyne is highly flammable and used in oxy-acetylene flames for welding purposes. It is also utilized to manufacture various plastics and polymers, such as neoprene.
So what is the polarity of this highly flammable gas ethyne (C2H2)? Is it a polar or a non-polar molecule? Let’s find out in this article.
Is C2H2 polar or non-polar?
Ethyne/acetylene (C2H2) is a non-polar molecule. Each C-H bond in the C2H2 molecule is non-polar due to a slight electronegativity difference between them. The C≡C bond is also non-polar due to no electronegativity difference between the bonded C atoms.
The carbon (C) atom’s electronegativity is slightly higher than the hydrogen (H) atom. There is almost equal sharing of the electron cloud in the C-H bond due to their slight electronegativity difference.
Thus, each C-H and C≡C bond in the C2H2 molecule is non-polar and possesses no dipole moment value.
Also, due to the symmetrical linear geometry of C2H2, the net dipole moments (µ), if any, will be canceled. As a result, C2H2 is a non-polar molecule with a net dipole moment equal to 0.
A molecule with an equal charge distribution between different centers of bonded atoms is a non-polar molecule, such as a C2H2 molecule.
It is formed by the covalent bond between two same atoms leading to a symmetric electron density in the molecule overall.
In the case of symmetric molecules, If the dipole moments of individually polar bonds are canceled, the molecule will be non-polar.
Hence a non-polar molecule has an equal distribution of the electronic charge. Contrarily, if the electronic charge is not evenly distributed over the molecule, in that case, it will be a polar molecule.
The following three factors influence the polarity of any covalent molecule:
Electronegativity.
Dipole moment.
Molecular geometry or shape.
In the next section, we will discuss how these factors control the polarity of the C2H2 molecule.
Factors affecting the polarity of C2H2
Electronegativity
It is defined as the ability of an atom to attract a shared pair of electrons from a covalent chemical bond.
Electronegativity decreases down the group in the Periodic Table of elements while it increases across a period.
Greater the electronegativity difference between the bonded atoms in a molecule, the higher the bond polarity.
Hydrogen belongs to group 1-A (or 1) of the Periodic Table. The electronic configuration of hydrogen is 1s1, so it has 1 valence electron.
On the other hand, carbon belongs to group IV-A (or 14) of the Periodic Table. The electronic configuration of carbon is 1s22s22p2, so it has 4 valence electrons available for bonding.
The two H-atoms thus form single covalent bonds with the two C-atoms on each side of the C2H2 molecule. The carbon atoms also form a triple covalent bond (≡) with each other to complete their octet configuration.
Atom
Electronic configuration
Valence electrons
Hydrogen (1H)
1s1
1
Carbon (6C)
1s22s22p2
4
According to the Pauling scale, a bond is polar covalent if the molecule’s electronegativity difference in bonded atoms ranges from 0.5 to 1.6 units.
An electronegativity difference of less than 0.5 indicates the non-polarity of a covalent bond.
Carbon (E.N = 2.55) is slightly more electronegative than hydrogen (E.N = 2.2). An electronegativity difference of only 0.35 units is present between the C-H bonds in the C2H2 molecule.
Also, no electronegativity difference exists between the carbon atoms in the C≡C bond (∆E.N = 0).
Due to the slight electronegativity difference, there is almost equal sharing of the electron cloud between the bonded C and H atoms in the C2H2 molecule.
The C≡C bond also has an equal sharing of electrons due to no electronegativity difference between C-atoms.
Consequently, each C-H and C≡C bond in the C2H2 molecule is non-polar, leading to the overall non-polarity of the molecule.
Dipole Moment
The dipole moment is the product of electrical charge (Q) and bond length (r) between two bonded atoms. It is a vector quantity expressed in Debye (D) units.
It is represented by a Greek symbol µ and measures the polarity of a bond.
The dipole moment of any molecule depends on the difference in electronegativity between the bonded atoms. The greater the electronegativity difference, the higher the bond polarity, resulting in a high dipole moment value.
It points from the partial positive (δ+) center to the partial negative (δ–) center of a bond or molecule.
As each C-H and C≡C bond in the C2H2 molecule is non-polar due to very little to no electronegativity difference, each bond has no dipole moment value.
Thus, the C2H2 molecule is non-polar due to zero dipole moment overall.
Molecular geometry
As discussed earlier, an ethyne (C2H2) molecule consists of two single C-H covalent bonds and one triple C≡C covalent bond. There are a total of 10 valence electrons in the overall molecule.
According to the Valence Shell Electron Pair Repulsion Theory (VSEPR) theory of chemical bonding, C2H2 is an AX2-type molecule. There are 2 electron density regions around the central C-atom, and there is no lone pair of electrons on this atom. As a result, the ethyne (C2H2) molecule experiences no distortion in its geometry.
Due to the absence of disorientation as there are no lone pairs on the central C-atom, the C2H2 molecule has a linear shape with a mutual bond angle of 180°.
As a result, the dipole moments, if any, on each side along the C-H and C≡C bond will cancel out each other, making C2H2 a non-polar molecule.
In conclusion, the linear C2H2 molecule is non-polar with a dipole moment value equal to zero.
Difference between polar and nonpolar?
Polar molecule
Non-polar molecule
Atoms must have a difference in
electronegativity
Atoms may have the same or different electronegativity values
Unequal charge distribution overall
Equal charge distribution overall
Net dipole moment greater than zero
Net dipole moment equals to zero
Examples include water (H2O), ethanol (CH3CH2OH), ammonia (NH3), sulfur dioxide (SO2), bromine trifluoride (BrF3), nitric oxide (NO), bromine pentafluoride (BrF5), etc.
Examples include oxygen (O2), nitrogen (N2), methane (CH4), carbon disulfide (CS2), ethane (C2H6), propane (C3H8), Ethyne (C2H2), etc.
According to the Pauling scale, a bond is non-polar covalent if a molecule’s electronegativity difference in bonded atoms ranges less than 0.5 units.
The individual C-H bonds in the C2H2 molecule are non-polar due to an electronegativity difference of only 0.35 units between C and H atoms.
The C≡C bond is also non-polar, as there is no electronegativity difference between the bonded C atoms.
The molecule (C2H2) overall is non-polar because it has a symmetric linear geometry.
The individual dipole moments, if any, get canceled in opposite directions. So, net µ becomes zero in the molecule.
Thus, C2H2 is a non-polar molecule with a zero net dipole moment.
C2H2 and CO have the same linear geometry, but C2H2 is non-polar, while CO is polar. Why?
The C2H2 molecule is non-polar because it has non-polar C-H and C≡C bonds due to a very little to no electronegativity difference between the bonded atoms.
In addition, the individual dipole moments, if any, will be canceled in opposite directions in the linear molecule. As a result, C2H2 is non-polar.
The CO molecule has a polar C≡O covalent bond because of a great electronegativity difference of 0.89 units between bonded C and O atoms. Thus, the linear molecule has a dipole moment value leading to the polarity of the overall molecule.
Thus, C2H2 is a non-polar while CO is a polar molecule, even though both have a linear geometry.
Is acetylene (C2H2) soluble in water?
It is a general solubility principle that “Like dissolves like”.
Water is a polar molecule, while acetylene is a non-polar molecule.
Water molecules are attracted to other molecules through hydrogen bonding. Non-polar molecules like acetylene do not have a dipole moment and therefore do not engage in hydrogen bonding. As a result, they are not soluble in water.
Is there a formal charge on the bonded atoms in the C2H2 molecule?
Formal charge of an atom = [ valence electrons – non-bonding electrons- ½ (bonding electrons)]
For hydrogen atoms
Valence electrons = 1
Bonding electrons = 2
Non-bonding electrons = 0
∴ The formal charge on the hydrogen atoms = 1-0-2/2 = 1-1 = 0
For carbon atoms
Valence electrons = 4
Bonding electrons = 8
Non-bonding electrons = 0
∴ The formal charge on the central carbon atoms = 4-0-8/2 = 4-4 = 0
Thus, no formal charge is present on the ethyne/acetylene (C2H2) molecule.
Summary
Ethyne/acetylene (C2H2) is a non-polar molecule.
The individual C-H bonds are non-polar due to an electronegativity difference of only 0.35 units between the bonded atoms.
The C≡C bond is also non-polar, as there is no electronegativity difference between the bonded atoms (∆E.N = 0).
Due to the symmetric linear molecular geometry of the C2H2 molecule, the dipole moments of the individual C-C and C-H bonds, if any, will be canceled in opposite directions.
In conclusion, C2H2 is a non-polar molecule with net µ =0.
Vishal Goyal is the founder of Topblogtenz, a comprehensive resource for students seeking guidance and support in their chemistry studies. He holds a degree in B.Tech (Chemical Engineering) and has four years of experience as a chemistry tutor. The team at Topblogtenz includes experts like experienced researchers, professors, and educators, with the goal of making complex subjects like chemistry accessible and understandable for all. A passion for sharing knowledge and a love for chemistry and science drives the team behind the website. Let's connect through LinkedIn: https://www.linkedin.com/in/vishal-goyal-2926a122b/
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