7 Isomers Examples in Daily Life


Our world is a manifestation of atoms, molecules, and ions. Their behavior determines the properties of matter that we encounter every day in our lives. Knowledge in chemistry provides us with the tool to deeply understand that behavior and differentiate the properties based on how these three-dimensional building blocks interact with each other. For instance, two molecules in a three-dimensional space, comprising a similar number of identical atoms, can arrange themselves differently in order to manifest different physical and chemical properties. In chemistry, such molecules or compounds are known as isomers. Let’s take an analogy of your hands for a better understanding of the concept. Both of your hands comprises four fingers, one thumb, and a palm. Now turn both the hands, facing palm down, and try to put one hand on the other. Despite being identical in appearance, both your hands cannot superimpose each other because of their orientation. Any such structures would be analogous to isomers. Isomers are molecules with the same molecular formulas but different arrangements of atoms. Isomerism is the existence or possibility of a molecule having isomers, and it can be categorized in two main forms:

  • Structural or constitutional isomerism: A structural isomer, also known as a constitutional isomer, is one in which two or more organic compounds have the same molecular formulas but different structures. In general, a structural isomer of a compound is another compound whose molecule has the same number of atoms of each element but with logically distinctive bonds between them. For example, butane and 2-methyl propane (isobutane) have the same molecular formula ({C}_{4}{H}_{10}) but are two distinct structural isomers, described as follow:
    Structural isomers
    As a result, the two molecules have different chemical properties (such as lower melting and boiling points for isobutane). Because of these differences, butane is typically used as a fuel for cigarette lighters and torches, whereas isobutane is often employed as a refrigerant or as a propellant in spray cans. You might find it interesting that the simplest hydrocarbons, methane ({CH}_{4}), ethane ({CH}_{3}{CH}_{3}), and propane ({CH}_{3}{CH}_{2}{CH}_{3}), have no constitutional isomers. This happens because there is no other way to connect the carbons and hydrogens of these molecules consistent with the tetravalency of carbon and the univalency of hydrogen. Furthermore, among the structural isomers, one can distinguish several classes, including skeletal isomers, positional isomers (or regioisomers), functional isomers, tautomers, and structural topoisomers.
  • Stereoisomerism or spatial isomerism: Stereoisomerism, or spatial isomerism, is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in space. There are different classifications of stereoisomers but they are often categorized mainly as enantiomers and diastereomers depending on how the arrangements differ from one another. Enantiomers, also known as optical isomers, are two stereoisomers that are non-superposable mirror images of each other. The human hand is a perfect analogy for optical isomerism. Two enantiomers are related through a reflection operator such that any stereogenic center in one has the opposite configuration in the other.
    The two enantiomers of lactic acid shown above have the same physical properties, except for the direction in which they rotate polarized light and how they interact with different optical isomers of other compounds. As a result, different enantiomers of a compound may have substantially different biological effects. Diastereomers, on the other hand, are defined as non-identical stereoisomers that have a non-mirror image. Hence, they occur when two or more stereoisomers of a compound have different configurations at one or more (but not all) of the equivalent (related) stereocenters and are not mirror images of each other. When two diastereoisomers differ from each other at only one stereocenter, they are epimers. Each stereocenter gives rise to two different configurations and thus typically increases the stereoisomerism by a factor of two.
    The cis- and trans- isomers in the above image are examples of diastereomeric and collectively known as Geometrical isomers.

Although isomerism is a molecular phenomenon, and therefore, can not be observed with the naked eye, there are a few examples that can be observed in daily life as the physical and chemical properties of molecules that show isomerism.

1. Carbohydrates


Carbohydrates are the main source of energy for human beings. They are a ubiquitous part of our diet from which we get calories. Most carbohydrates are digested and broken down into glucose before entering the bloodstream. Cells can convert carbohydrates into the fuel molecule ATP through a process called cellular respiration. In chemical terms, carbohydrates are polyhydroxy aldehydes or ketones or a molecule that yields a polyhydroxy aldehyde or ketone upon hydrolysis. The term “carbohydrate” comes from the observation that their apparent molecular formula is {C}_{n}({H}_{2}{O})_{n}. For example, in the case of glucose, the molecular formula of {C}_{6}{H}_{12}{O}_{6} can be understood as {C}_{6}({H}_{2}{O})_{6}. The presence of an asymmetric carbon atom (a carbon atom to which four different groups of atoms are attached) makes possible the formation of isomers of a compound. For instance, glucose and fructose have the same molecular formula ({C}_{6}{H}_{12}{O}_{6}), yet they show structural isomerism because of the presence of different functional groups.


Carbohydrates are primarily classified into three subtypes: monosaccharides, disaccharides, and polysaccharides. A monosaccharide is the most basic form of carbohydrates. Monosaccharides can be polymerized through glycosidic bonds to form larger carbohydrates, known as disaccharides or polysaccharides. Conversely, through several catabolic processes, polysaccharides can be broken down into monosaccharides. Carbohydrates have a wide range of classifications and show several types of isomerism. To get a vague idea of isomerism in carbohydrates, let’s discuss isomerism in monosaccharides. In organic chemistry, Le Bel-van’t Hoff rule states that for a structure with n asymmetric carbon atoms, there is a maximum of {2}^{n} different stereoisomers possible. Therefore, as glucose has n=6, it alone can form 32 different stereoisomers. The most common of them is D-glucose, where D- symbolizes the optical isomerism shown by glucose. This nomenclature is based on Fischer projection designates D– when it rotates the plane-polarized light in the clockwise direction. L– is when it rotates the plane-polarized light in a counterclockwise direction.

2. Retinal

TED-Ed - Gifs worth sharing — In the retina, there are two different types of...

Have you ever wondered how our eye can change the light photons into a signal that tells our brain the appearance of the object we are watching? Retinal (also known as retinaldehyde) is a chemical called polyene chromophore, which is bound to proteins called opsins and forms the basis of animal vision. It allows certain microorganisms to convert light into metabolic energy. There are two different isomers of retinal responsible for converting the energy in light photons into electrical impulses in the retina, 11-cis-retinal and all-trans-retinal, where the number 11 refers to the double bond at the 11th carbon in the ring. Most of us are familiar with the fact that lack of Vitamin-A can cause night-blindness, or in general, it can be of concern to our eye-sight. The precursor of 11-cis-retinal is the alcohol all-trans-retinol, commonly known as Vitamin A. This molecule cannot be synthesized by mammals and has to be acquired through the diet. All-trans-retinol is converted to 11-cis-retinal in two steps, both involving enzymes. First, the alcohol group is oxidized to an aldehyde, then the double bond between C-11 and C-12 is isomerized from a trans to a cis configuration. The mechanism of vision involves a photosensitive molecule called rhodopsin (also known as visual purple) joined to 11-cis-retinal via a protonated Schiff base on one of its lysine side-chains. chemistry-of-vision-15-638

Opsin does not absorb visible light on its own, but when it is bonded with 11-cis-retinal to form rhodopsin, the new molecule has a very broad absorption band in the visible region of the spectrum. When a photon of light falls onto rhodopsin, the molecule absorbs the energy to temporarily convert the cis-double-bond between C-11 and C-12 in the retinal into a single bond. Thus the light isomerizes the molecule from cis to trans, and as it did so, it changed the energy in a photon atomic motion. This generates nerve impulses that travel along the optic nerve to the brain, and we perceive them as visual signals – sight. The free all-trans-retinal is then converted back into the cis form by a series of enzyme-catalyzed reactions, whereupon it re-attaches to another opsin ready for the next photon to begin the process again.

3. Vitamin C


Vitamin C is one of the most vital nutrients for human health. It is necessary for human beings to maintain adequate levels of vitamin C in their body. Not only it prevents scurvy and helps our immune system to function properly, but vitamin C is also an essential nutrient involved in the repair of tissue and the enzymatic production of certain neurotransmitters. Chemically, it is also known as ascorbic acid or ascorbate with the chemical formula {C}_{6}{H}_{8}{O}_{6}. The source of Vitamin C for humans is food, primarily fruits, and vegetables. Ascorbic acid is a water-soluble micronutrient required for multiple biological functions. Ascorbic acid exists as two enantiomers (mirror-image isomers), commonly denoted “L” (for “Levo”) and “D” (for “Dextro”). The name “vitamin C” essentially refers to the L-enantiomer of ascorbic acid and its oxidized forms. It functions as a cofactor in enzymatic reactions, such as collagen synthesis, and as an antioxidant. In addition to L-ascorbic acid itself, there are three other stereoisomers: D-ascorbic acid, D-isoascorbic acid, and L-isoascorbic acid.


Stereoisomers of ascorbic acid: (1a) L-ascorbic acid [Vitamin C]; (1b) D-ascorbic acid; (2a) L-isoascorbic acid; (2b) D-isoascorbic acid or D-erythorbic acid.

4. The Myth of Oranges and Lemons


How can you differentiate between an orange and a lemon when you are blindfolded and not allowed to touch either of them? By the sense of smell, right? There is a common misconception that oranges and lemons smell distinctive because of the presence of two enantiomers of a chemical compound called limonene. This molecule comes in two different chemical arrangements which are the mirror images of one another. One type of limonene is responsible for the smell of lemon, while the other is responsible for the smell of an orange. However, this assertion is completely false. In reality, oranges and lemons have both the R- and S- limonene enantiomers present. The R-limonene is present in the majority of 96-99% and the S-limonene is present around 1% to 4%. The S-limonene indeed has a slightly-lemon scent to itself, but that’s not the reason why oranges and lemons smell different. Rather, there are a lot of other chemical compounds that work together to gives the oranges and lemons their distinctive smell.


5. The Smell of Roses


Roses are not only famous for their beautiful appearance but also for the incredible aroma they endure. In fact, William Shakespeare, in his famous play Romeo and Juliet, quoted, “A rose by any other name would smell as sweet.” The smell of a rose that we encounter in cosmetics, perfumes, and food additives probably comes from a single species of rose called damask rose. The chemical responsible for the fragrance of rose is known as rose oxide. However, it does not exclusively has a sweet odor, but it can also have a fruity, minty, and even citrus aroma. You might wonder that how can a single chemical have so many different odors. The reason is it’s not a single chemical but a molecule that has four different isomers. All four isomers of rose oxide have 10 carbon atoms, 18 hydrogen atoms, and one oxygen atom, all connected in the same order to form {C}_{10}{H}_{18}{O}. The compound has cis- and trans- isomers, each with a (+)- and (−)-stereoisomer, but only the (−)-cis isomer (odor threshold 0.5 ppb) is responsible for the typical rose (floral green) fragrance. This slight change in the arrangement of few atoms affects how each rose oxide interacts with an olfactory receptor in our nose, and therefore, they all smell differently. 


6. Stereomerism and Odor

Stereomerism is very closely linked to our perception of a smell. For instance, the chemical responsible for the minty odor of the spearmint leaves is a stereoisomer of the chemical responsible for the spicy aroma of the caraway seeds. Carvone is a chemical found naturally in many essential oils but is most abundant in the oils from seeds of caraway (Carum carvi), spearmint (Mentha spicata). It forms two mirror-image forms or enantiomers. While R-(–)-carvone, or L-carvone, has a sweetish minty smell, its mirror image, S-(+)-carvone, or D-carvone, has a spicy aroma with notes of rye. The fact that the two enantiomers are perceived as smelling different is evidence that olfactory receptors present in our nose must contain chiral groups, allowing them to respond more strongly to one enantiomer than to the other. However, not all enantiomers have distinguishable odors.

spearmint vs caraway

7. Unsaturated Fats

In nutrition, biology, and chemistry, fat usually means any ester of fatty acids or a mixture of such compounds; most commonly those that occur in living beings or in food. The molecule of a fatty acid consists of a carboxyl group HO(O=)C− connected to an unbranched alkyl group –({CH}_{x})_{n}{H}. The difference between saturated and unsaturated fats is that the former only have single bond hydrocarbon chains while the latter has at least one double bond in the hydrocarbon chain. Due to the presence of double bonds, unsaturated fatty acids exhibit geometrical isomerism, which depends on the orientation of groups around the double bond. The designation “cis” means that the acyl chains are on the same side. Whereas, “trans” means the acyl chains are on the opposite side of the double bond. Unsaturated fatty acids can exist in either the cis or trans form depending on the configuration of the hydrogen atoms attached to the carbon atoms joined by the double bonds. Under conditions of partial hydrogenation, a double bond may change from a cis to a trans configuration (geometric isomerization) or move to other positions in the carbon chain (positional isomerization). Both types of isomerization frequently occur in any fatty acid undergoing hydrogenation.



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