Are lipids polar or non-polar? – (Polarity of lipids)
Fat, oil, and waxes are everyday examples of lipids. These are present in food items (egg, butter, soya beans, peanuts) and many others.
Oil and water don’t mix. So what do you think; are lipids polar or non-polar?
Well, we are here to answer this question, so without any further ado, let’s find out whether lipids are polar or non-polar in nature.
Are lipids polar or non-polar?
Lipids are essentially non-polar in nature due to the presence of long hydrophobic hydrocarbon chains in their structures.
A hydrocarbon chain consists of numerous C-H bonds. Each C-H bond is weakly polar due to an electronegativity difference of only 0.35 units between the covalently bonded carbon and hydrogen atoms.
Some polar character may be present in a lipid molecule in the presence of an ester (COO) functional group (in the case of a triglyceride) or a phosphate (PO43-) group (in the case of a phospholipid).
Therefore, the polarity of a lipid molecule is largely controlled by its chemical composition and structure.
So, let us first discuss the chemical composition of lipids in order to understand their polarity better.
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).
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.
If two OH groups of a glycerol molecule are chemically reacted with 2 fatty acid molecules to yield 2 ester functional groups while the third OH group of glycerol is replaced with a phosphate (PO43-) group, it produces another type of lipids, phospholipids.
Steroids consist of four fused carbon rings.
Both phospholipids and steroids are considered complex lipids, while fats are simple lipid molecules.
Steroids are structurally very different from the other two types of lipids but nonetheless resemble them in terms of polarity.
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 lipids are generally non-polar in light of the above three factors.
Factors affecting the polarity of lipids
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 (fats), the simplest lipid molecules, consist of four main types of bonds i.e., a C-C bond, a C-H bond, C-O, and 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
A C-C or C=C 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.
Contrarily, in phospholipids, in addition to the above bonds, a P-O (or P=O) bond is also present.
The P-O bond is strongly polar as it consists of an electronegativity difference of 1.25 units between a phosphorus (E.N = 2.19) and an oxygen (E.N = 3.44) atom.
The P-atom thus gains a partial positive (δ+) charge, while each O-atom gains a partial negative (δ–) charge in the PO43- ion.
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.
Lipids consist of very small C-H dipole moments. The dipole moment of a C-O or C=O bond points from Cδ+ to Oδ– while the strong dipole moment of a P-O or P=O bond points from Pδ+ to Oδ– in phospholipids.
The shape of the molecule and overall molecular polarity
In triglycerides, 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.
However, complex lipids such as phospholipids are amphiphilic. They consist of a non-polar, hydrophobic tail and a polar, hydrophilic head i.e., the phosphate group where the strong P-O dipole moments stay uncancelled (net µ > 0), leading to a non-uniform electron cloud and a formal charge of -1.
That is why phospholipids form a bilayer in animal cell membranes, where the hydrophobic tails point inwards while the polar phosphate groups are extended outwards.
The phospholipid bilayer acts as a channel or passage for different ions and molecules to pass into and out of the human cell membrane.
Lipids are essentially non-polar molecules as they consist of a long hydrophobic, hydrocarbon chain. The small C-H dipole moments get canceled equally in the hydrocarbon chain.
Moderately polar COO groups are present in triglycerides (simplest lipids), but considering the long length of the hydrophobic chain, the small polarity effect is ignored.
Additionally, lipids such as oil are water-insoluble. Like dissolves like while repelling opposite polarity molecules. Thus, non-polar lipids are insoluble in polar water molecules.
Why are lipids non-polar but fatty acids polar?
Lipids are fatty acid esters with a long hydrophobic hydrocarbon chain; thus, they are non-polar.
In contrast, fatty acids are carboxylic acids having a strongly polar carboxyl (COOH) functional group hence they are polar in nature.
Are lipids soluble in a non-polar solvent?
Yes, lipids such as oil or grease are soluble in non-polar solvents such as benzene due to their non-polar nature.
Like dissolves like. Lipid molecules develop attractive forces with non-polar solvents via hydrophobic forces of attraction.
Do oil and water dissolve?
Oil and water don’t dissolve as water is a polar solvent, while oil or lipids consists of long, hydrophobic (water-repelling) non-polar hydrocarbon chains.
Why are lipids non-polar but carbohydrates polar?
Lipids are non-polar due to a long, non-polar hydrocarbon chain. Contrarily, carbohydrates are polyhydroxy compounds of aldehydes and ketones.
The strongly polar hydroxyl (OH) functional groups present in carbohydrates, such as sugars, make carbohydrates strongly polar molecules.
‘Lipids’’ (May 2nd, 2022). Retrieved from https://chem.libretexts.org
‘Lipids’’. Module 3: Important Biological Macromolecules. Retrieved from https://courses.lumenlearning.com/wm-biology1/chapter/reading-lipids-2/
‘Are Lipids polar or non-polar?’’ Retrieved from https://techniescientist.com/are-lipids-polar-or-non-polar/
‘Lipids: An introduction’’ (Nathan H Lentz). Retrieved from Nathanhttps://www.visionlearning.com/en/library/Biology/2/Lipids207.
‘Simultaneous non-polar and polar lipid analysis by on-line combination of HILIC, RP and high-resolution MS’’ (Rampler et.al, 2018). Retrieved from https://pubs.rsc.org/en/content/articlelanding/2018/an/c7an01984j.
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