Is Alcohol polar or nonpolar? – (Polarity of Alcohol)
Alcohol is a generic term used to describe an organic compound that has one or more hydroxyl (OH) functional groups covalently bonded to an aliphatic hydrocarbon chain.
Methanol (CH3OH) and ethanol (CH3CH2OH) are the first two members of the alcohol family.
Alcohols are commonly used in industrial and pharmaceutical applications. Ethanol is being explored as an alternative fuel option in the modern scientific world.
In this article, we have discussed everything you need to know about the polarity of alcohol. So, continue reading to find out whether alcohols are polar or non-polar.
Is alcohol polar or non-polar?
Alcohols are polar chemical compounds.
The presence of a strongly polar hydroxyl (OH) functional group in an alcohol molecule leads to the formation of oppositely charged poles and, thus, a strong dipole moment value.
The O-H bond is strongly polar, having an electronegativity difference of 1.24 units between the covalently bonded oxygen (E.N = 3.44) and hydrogen (E.N = 2.20) atoms.
The C-C bonds are non-polar, while the C-H bonds present in alcohols are weakly polar.
The strong O-H dipole moments do not get canceled with the small C-H dipole moments due to the bent, angular or V-shape of an alcohol molecule w.r.t the O-atom.
The charged electron cloud stays non-uniformly distributed, yielding an overall polar molecule (net µ > 0).
A molecule is polar if there is a non-uniform charge distribution present in it. If the charge distribution gets equally balanced in different parts, then that molecule is considered non-polar.
The following three factors mainly influence the polarity of a molecule:
The electronegativity difference between two or more covalently bonded atoms
Dipole moment
Molecular geometry or shape
Now let us see how all the above factors contribute to making alcohol a polar molecule overall.
Factors affecting the polarity of alcohol
Electronegativity
It is defined as the ability of an elemental atom to attract a shared pair of electrons from a covalent chemical bond.
Electronegativity increases across a period in the Periodic Table while it decreases down the group.
Greater the electronegativity difference between bonded atoms in a molecule, the higher the bond polarity.
The three elemental atoms present in an alcohol molecule are carbon (C), hydrogen (H) and oxygen (O).
Hydrogen (H) lies at the top of the Periodic Table of elements in Group I A (or 1). Its electronic configuration is 1s1. It needs 1 more electron in order to gain a complete duplet electronic configuration.
Carbon (C) is present in Group IV A (or 14). Its electronic configuration is 1s2 2s2 2p2. As per this electronic configuration, it has a total of 4 valence electrons. Each C-atom thus needs 4 more electrons in order to gain a complete octet electronic configuration.
In contrast, oxygen (O) is present in Group VI A (or 16) of the Periodic Table. Its electronic configuration is 1s2 2s2 2p4. It has a total of 6 valence electrons, so it requires 2 more electrons for it to complete its octet.
Atom
Electronic configuration
Valence electrons
Hydrogen (1H)
1s1
1
Carbon (6C)
1s2 2s2 2p2
4
Oxygen (8O)
1s2 2s2 2p4
6
There are four main types of bonds present in a general alcohol structure, i.e., C-C, C-O, C-H, and O-H.
A C-C bond is purely non-polar, as it is composed of identical carbon atoms.
A C-H bond is very weakly polar (almost non-polar as per Pauling’s electronegativity scale) owing to an electronegativity difference of only 0.35 units between the bonded atoms.
In contrast, a C-O bond is polar, having an electronegativity difference of 0.89 units between the covalently bonded carbon (E.N = 2.55) and oxygen (E.N = 3.44) atoms.
Similarly, an O-H bond is strongly polar as per an electronegativity difference of 1.24 units between an oxygen and a hydrogen (E.N = 2.20) atom.
Oxygen being strongly electronegative, attracts the C-O and O-H bonded electrons largely towards itself. It thus gains a partial negative charge (δ–) while each of the C and H-atoms obtain strong partial positive charges (δ+ and δ++).
Thus, both C-O and O-H bonds are strongly polar in an alcohol molecule.
Dipole moment
Dipole moment (μ) is a vector quantity that points from the positive pole to the negative pole of a bond or a molecule.
It is mathematically calculated as a product of the magnitude of charge (Q) and charge separation (r). The dipole moment is expressed in a unit called Debye (D).
The dipole moment of a polar covalent bond conventionally points from the positive center to the center of the negative charge.
In an alcohol molecule, the strong dipole moment of an O-H bond points from Hδ++ to Oδ–. Conversely, the C-O dipole moment points from Cδ+ to Oδ–.
The small C-H dipole moments point from Hδ++ to Cδ+ in the hydrocarbon chain (R), as shown in the figure drawn below.
Molecular shape
W.r.t the most electronegative O-atom, the shape of an alcohol molecule is bent, angular or V-shaped.
The presence of 2 lone pairs of electrons on the O-atom leads to strong lone pair-lone pair and lone pair-bond pair electronic repulsions. This strong repulsive effect distorts the shape and geometry of the molecule.
In this distorted molecular shape, the strong O-H and C-O dipole moments stay uncancelled to yield an overall polar molecule with an asymmetric electron cloud distribution (net µ > 0).
FAQ
Why is alcohol polar?
Alcohol is polar due to the presence of strongly polar C-O and O-H bonds in it.
A C-O bond is polar, having an electronegativity difference of 0.89 units between the bonded atoms, while an O-H bond is strongly polar as per an electronegativity difference of 1.24 units between the bonded atoms.
The molecular shape of alcohol w.r.t an O-atom is bent, angular or V-shaped.
The strong C-O and O-H dipole moments stay uncancelled with the weak C-H dipole moments to yield an overall polar alcohol molecule.
Why is alcohol both a polar and non-polar solvent?
Alcohol is essentially a polar solvent owing to the presence of a strongly polar hydroxyl (OH) functional group in it.
The O-H bond can develop strong hydrogen bonding with polar solvents such as water (H2O) and get solubilized. It can also dissolve other polar chemical compounds through dipole-dipole interactions.
A slightly non-polar character is present in polar alcohol molecules owing to the presence of a long hydrocarbon chain.
The longer the hydrocarbon chain (such as in pentanol, hexanol, decanol etc) in an alcohol molecule, the greater the non-polarity effect in an otherwise polar solvent i.e., alcohol.
Is isopropyl alcohol polar or non-polar?
Isopropyl alcohol is polar.
The IUPAC name for isopropyl alcohol is propan-2-ol. 2 methyl (CH3) groups are attached to the carbon carrying the hydroxyl (O-H) functional group.
The small C-H dipole moments get canceled equally in opposite directions in the methyl groups.
However, the strong O-H dipole moment does not get canceled with the weak C-H dipole moment in the bent shape of isopropyl alcohol w.r.t O-atom. So it is overall polar.
Is rubbing alcohol polar or non-polar?
Rubbing alcohol is the commercial name for the diluted solution of isopropyl alcohol. Polar isopropyl alcohol molecules get readily dissolved in polar H2O by developing strong intermolecular forces of attraction (H-bonding).
This results in an overall polar alcoholic aqueous solution, i.e., rubbing alcohol.
It is known as rubbing alcohol because it was primarily used as a liniment for massage on the skin and for surface cleaning.
The dipole moment of a strongly polar hydroxyl (OH) functional group stays uncancelled in the bent, angular or V-shape of the molecule w.r.t O-atom.
Thus, methyl alcohol is overall polar (net µ > 0).
Is ethyl alcohol polar?
Ethyl alcohol, another name for ethanol, represents a strongly polar molecule. It consists of a polar hydroxyl (O-H) functional group chemically bonded to an ethyl group i.e., CH2CH3.
The strong O-H and C-O dipole moments do not get canceled with the weak C-H dipole moments due to the bent shape w.r.t oxygen atom. Thus, ethyl alcohol is overall polar (net µ > 0).
However, ethyl alcohol is less polar than methyl alcohol due to a relatively longer non-polar hydrocarbon chain.
Is cetyl alcohol polar?
Cetyl alcohol is the common name for hexadecane-1-ol, a 16-carbon alcohol molecule. It has a polar head (i.e., the O-H group) and a long non-polar hydrocarbon chain (i.e., C16H33).
In this way, cetyl alcohol has both polar and non-polar parts.
Summary
Alcohols are polar in nature.
Alcohol is an organic compound possessing a polar hydroxyl (O-H) functional group in it. The O-H group is bonded to an aliphatic carbon chain containing a C-O bond and more than one C-H bond.
The O-H bond is strongly polar due to a high electronegativity difference of 1.24 units between the bonded atoms.
The C-O bond is also polar, while the C-C and C-H bonds are typically non-polar.
The strong dipole moments of O-H and C-O bonds stay uncancelled in the bent molecular shape to yield an overall polar alcohol molecule (net µ > 0).
References
Vedantu. ‘Is alcohol polar? And why?’’ Questions and Answers. https://www.vedantu.com/question-answer/is-alcohol-polar-and-why-class-12-chemistry-cbse-6112949aa947c07fce1f5f47
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