From cooking to cosmetics, different types of oils are an essential part of our everyday life activities. But do you know the hidden chemistry behind the structure and polarity of oils?
Do you have a definite answer to the question, is oil polar or non-polar?
Well, your answer to the above question may be a yes or a no; in either case, let’s explore further all about the polarity of oil in this article.
Is oil polar or non-polar?
Oil is typically a non-polar chemical substance.
Chemically, oils are classified as lipids, more specifically triglycerides, i.e., fatty acid esters. These esters are formed by the condensation reaction of fatty acids (long-chain carboxylic acids) with polyols (alcohols containing multiple OH groups).
The ester (COO) linkage present in an oil molecule is moderately polar as a high electronegativity difference of 0.89 units exists between the covalently bonded carbon and oxygen atoms.
However, the long hydrocarbon chains present in an alcohol molecule comprise non-polar C-C and very weakly polar (almost non-polar) C-H bonds possessing zero to negligible electronegativity differences.
In the presence of these long, non-polar hydrocarbon chains, the slight polarity effect of the COO group is ignored. Consequently, an oil molecule is considered non-polar.
It is due to this non-polar nature and hydrophobic hydrocarbon chains that oil does not mix with water (a polar solvent).
Name of the substance
Oil
Chemical composition
Lipid (triglyceride)
Structure
Long-chain fatty acid esters
Bond type
Non-polar covalent (C-C and C-H bonds)
Polar covalent (C-O or C=O bonds)
Polar or non-polar?
Non-polar
Let us discuss the chemical composition of oil in detail so that we can better understand its non-polar nature.
As discussed above, oils fall under a sub-type of lipids, more specifically triglycerides which are esters of fatty acids containing long hydrocarbon chains.
A fatty acid consists of a long hydrocarbon chain and a carboxylic acid (COOH) functional group.
The long hydrocarbon chain may be saturated (containing all C-C single bonds) and/or unsaturated (containing one or more C=C or C ≡ C triple covalent bonds).
Similarly, the hydrocarbon can be straight-chained or a branched molecule.
The condensation reaction of a fatty acid molecule with a glycerol (tri-alcohol) molecule yields a triglyceride, as shown below.
Removal of 3 H-atoms from glycerol and 3 OH groups from COOH of fatty acid results in the formation of 3 ester functional groups; thus, the name triglyceride is given to this type of lipid, i.e., oil.
The following three factors mainly influence the polarity of a molecule:
The electronegativity difference between covalently bonded atoms
Dipole moments
The shape of the molecule
In the next section, we have discussed how oils are generally non-polar in light of the above three factors.
Factors affecting the polarity of oils
Electronegativity
Electronegativity is defined as the ability of an elemental atom to attract a shared pair of electrons from a covalent chemical bond.
The greater the electronegativity difference between bonded atoms in a molecule, the higher the bond polarity.
Triglycerides, the simplest oil molecules, consist of three main types of bonds, i.e., a C-C (or C=C) bond, a C-H bond, and a C-O (or a C=O) bond.
Atom
Electronic configuration
Valence electrons
Hydrogen (1H)
1s1
1
Carbon (6C)
1s2 2s2 2p2
4
Oxygen (8O)
1s2 2s2 2p4
6
As per Pauling’s electronegativity scale, a polar covalent bond is formed between two dissimilar atoms having an electronegativity difference between 0.4 to 1.6 units.
A C-C or C=C bond is purely non-polar as zero electronegativity difference exists between two identical carbon atoms.
A C-H bond is very weakly polar (almost non-polar as per Pauling’s electronegativity scale) as an electronegativity difference of only 0.35 units exists between a carbon (E.N = 2.55) and a hydrogen (E.N = 2.20) atom.
The C-O or C=O bond is moderately polar owing to an electronegativity difference of 0.89 units between the covalently bonded carbon and oxygen (E.N = 3.44) atoms.
Thus, in the oil molecule, the oxygen atoms present in the ester linkages gain partial negative charges (δ–) while the corresponding C-atoms obtain partial positive charges (δ+).
Dipole moments
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 charges 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 the oil molecule, the dipole moment of a C-O (or C=O) bond points from Cδ+ to Oδ-.
The small C-H dipole moments in the hydrocarbon chain point from Hδ++ to Cδ+.
The shape of the molecule and overall molecular polarity
In oils, the shape of the molecule is tetrahedral w.r.t each C-atom in the hydrocarbon chain, while it is trigonal planar w.r.t COO bonded C-atom.
The small C-H dipole moments get canceled equally in opposite directions to give a non-polar hydrocarbon tail.
Contrarily, there is some effect of the C-O and C=O dipole moments; however, it gets negated in comparison to the long non-polar hydrocarbon chain.
The insolubility of lipids or oil in polar molecules such as water further endorses the non-polarity of the latter.
FAQ
Why is oil non-polar?
Oil is non-polar on account of the non-polar, hydrophobic, hydrocarbon chains present in its structure.
Oils are chemically defined as fatty acid esters.
A fatty acid ester consists of one or more polar ester (COO) linkages and long, non-polar hydrocarbon chains.
The charged electron cloud stays uniformly disturbed over an alcohol molecule and is not concentrated at a single point.
The small C-H dipole moments get canceled equally. The C=O and C-O dipole moments stay intact, but their effect is ignored in the presence of the major non-polar region. Therefore, oil molecules are typically non-polar in nature.
Is vegetable oil polar or non-polar?
Vegetable oil is a triglyceride in nature and is thus non-polar. An example of a vegetable oil is Canola (rapeseed) oil. It is a triglyceride formed by a mixture of oleic acid, linoleic acid and linolenic acid.
The 18-carbon oleic acid, linoleic acid and linolenic acid chains are non-polar. Therefore, their ester is also non-polar with a balanced electron cloud distribution.
Is mineral oil polar or non-polar?
Mineral oil (or paraffin oil) is referred to as a mixture of paraffins, napthenes and aromatic oils.
Paraffins are alkanes or saturated hydrocarbons containing C-C and C-H bonds only. The C-C bonds are non-polar, while the C-H bonds are negligibly polar, yielding an overall non-polar paraffin molecule.
Napthenes refers to conjugated aromatic hydrocarbons containing two fused carbon rings, again non-polar in nature.
In contrast, aromatic oils are essential oils containing terpenoids, non-terpenoid hydrocarbons and their oxygenated derivatives.
The C-O and O-H bonds present in essential oils induce a slightly polar character; however, in the predominantly non-polar, complex mixture, this slight polarity effect is ignored. Hence, mineral oils are non-polar.
Is salad oil polar or non-polar?
Salad oil denotes the oils used for salad dressings. Their examples include olive oil and vegetable oil.
Vegetable oil is non-polar (as discussed in the previous question), while olive oil is a complex mixture containing non-polar and polar components.
The non-polar components of olive oil include triglycerides and sterols, while the multiple polar components include free fatty acids, aliphatic alcohols, tocopherols (containing a polar head and non-polar tails), phenolic compounds, etc.
The COOH and OH functional groups present in these polar components possess strong dipole moment values.
Therefore, salad oils can be both polar and non-polar.
Why can’t we remove an oil stain with plain water without using any soap or detergent?
Oil and water are two opposite polarity substances.
The polar water (H2O) molecules cannot develop any attractive forces, such as H-bonding with non-polar oil molecules. Therefore, a mediatory such as soap is required.
Soap consists of a polar head and non-polar tails. The non-polar tails develop attractive forces with oily stains, while the polar head develops Van der Waal’s forces of attraction with polar H2O molecules.
In this way, the oil particles break down into small pieces and get washed away with water easily.
Summary
Oil is a non-polar substance.
Chemically, oils are triglycerides, i.e., long-chain fatty acid esters.
The ester (COO) linkage in an oil molecule is polar, while the hydrocarbon chains are essentially non-polar.
It is due to the non-polar nature of long hydrocarbon chains that the slight polarity effect of the small ester bonds is ignored.
The charged electron cloud stays uniformly distributed; thus, oils are marked non-polar. Oil and water, when shaken together, form two separate layers, which further endorses the non-polarity of oils.
References
Scientific American. ‘Mix it up with oil and water’’ by Megan Arnett (May 24th, 2018). https://www.scientificamerican.com/article/mix-it-up-with-oil-and-water/
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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