An isomer is one of two or more substances made of exactly the same atoms, in the same numbers, but put together differently. That means one chemical formula can describe substances that boil at different temperatures, smell or taste different, or act differently in the body. The idea surprised chemists in the 1820s, when two substances with identical makeup turned out to be clearly different, and it became one of the foundations of modern chemistry.1Journal of Chemical Education. Liebig–Wöhler Controversy and the Concept of Isomerism
The Two Main Families of Isomers
The first split is whether the atoms are connected in a different order. A structural isomer has its atoms linked in a different sequence, like the same letters spelling different words. Structural isomers come in three common kinds: a carbon chain arranged differently, the same group moved to a different spot, or a different reactive group altogether.2Doc Brown’s Advanced Chemistry. Constitutional isomers of molecular formula C2H6O Chain differences usually need at least four carbon atoms, because shorter chains have only one way to line up.3Doc Brown’s Advanced Chemistry. Constitutional isomers of molecular formula C2H6O
The second split is same connections, different shape in space. A stereoisomer has every atom linked to the same neighbors, but parts of the molecule point in different directions. The two main kinds are cis–trans isomers, which differ in which side of a rigid bond groups sit on, and mirror-image isomers, which are like left and right hands.
To sort a pair, draw both molecules and list what each atom is bonded to. If any bond partner differs, they are structural isomers. If all partners match but you can’t lay one on top of the other, they are stereoisomers.
Counting atoms first saves time. If the two drawings have different numbers of any element, they aren’t isomers, however alike they look. Condensed formulas can also hide the difference: C4H10 alone doesn’t say whether the four carbons run in a line or one is branched off the middle, so you need the drawn structure or the name.
Some things look like isomers but aren’t. Two drawings of the same molecule, rotated or flipped on paper, are just one molecule. Molecules that only twist around single bonds are usually the same compound. Isotopes differ in neutrons, not arrangement, so that is a different idea.
Structural Isomers and Why They Behave Differently
Ethanol, the alcohol in drinks, and dimethyl ether share the formula C2H6O. Ethanol has an oxygen–hydrogen group attached to carbon, while in dimethyl ether the oxygen sits between two carbon groups.4Vaia. What Is an Isomer? Definition and Types That difference lets ethanol molecules cling to each other through hydrogen bonds, which dimethyl ether’s can’t do.5Doc Brown’s Advanced Chemistry. Constitutional isomers of molecular formula C2H6O The result: ethanol is a liquid at room temperature and dimethyl ether is a gas. The missing group also changes how dimethyl ether reacts in chemical reactions.6Vaia. What Is an Isomer? Definition and Types
A functional group is a small cluster of atoms that sets how a molecule reacts. Ethanol’s -OH group makes it act like an alcohol. Swap that group, as in the ether, and you get a different class of chemical. This is why functional group isomers differ more than chain isomers do.
Moving one group along a chain gives a position isomer with the same groups in a new spot. Propan-1-ol and propan-2-ol are an example, and the second is rubbing alcohol. Their properties are usually closer than those of functional group isomers.
Branching changes boiling point too. Among chain isomers, the more branched the molecule, the lower its boiling point tends to be.7Chemistry LibreTexts. What Is an Isomer? Definition and Types This is a trend among isomers of similar size, not an exact law for every pair. Long thin molecules lie close together and attract each other more, while compact branched ones can’t pack as closely.8Chemistry LibreTexts. What Is an Isomer? Definition and Types
Pentane has five carbons in a line, while neopentane has a central carbon with four carbons around it, making it nearly ball-shaped.
Cis and Trans Isomers, Including Trans Fats
A double bond between carbons can’t rotate, so the groups attached to it stay fixed on the same side (cis) or on opposite sides (trans). That is why the cis and trans versions are separate, stable compounds rather than one molecule in two poses. A pair only exists if each double-bond carbon has two different groups attached; if one carbon has two identical groups, there is no difference.
In a fat chain, the same side puts a kink in it, while opposite sides leave it nearly straight. Kinked chains can’t stack neatly, so oils rich in them stay liquid at room temperature. Straighter chains pack together tightly, so trans fat chains resemble saturated fats and melt at higher temperatures.9Pol. J. Food Nutr. Sci.. Formation and properties of trans fatty acids – A review In cell membranes, trans chains likewise behave more like saturated chains, while cis bonds disturb membranes far more.10PubMed. Comparison of cis and trans fatty acid containing phosphatidylcholines on membrane properties
Food makers liked that firmness and the longer shelf life it brought. Partially hydrogenating vegetable oil makes it solid, like margarine or shortening, and that process creates trans bonds as a side effect. Many countries now restrict them, but labels may still show “partially hydrogenated oil.”
Eating trans fats raises LDL, the “bad” cholesterol, as saturated fat does.11Pol. J. Food Nutr. Sci.. Formation and properties of trans fatty acids – A review They also lower HDL, the protective cholesterol,12Pol. J. Food Nutr. Sci.. Formation and properties of trans fatty acids – A review which makes them worse for the blood-fat profile than many other fats. Higher trans fat intake has been linked to heart disease and hardened arteries.13PubMed. Comparison of cis and trans fatty acid containing phosphatidylcholines on membrane properties The concern is factory-made trans fat eaten regularly; small natural amounts from cattle, sheep and dairy are not considered a major health concern.14Pol. J. Food Nutr. Sci.. Formation and properties of trans fatty acids – A review Checking an ingredient list for “partially hydrogenated” oils is more reliable than trusting a front-of-pack claim.
The same idea applies beyond food. In the cancer drug cisplatin, a metal compound, the two chlorine groups sit on the same side. Its partner transplatin has identical atoms in a different layout, and cisplatin is the far stronger drug.
Mirror-Image Isomers and Chirality
A chiral molecule can’t be laid perfectly over its mirror image, just as a left glove won’t fit a right hand. Such a molecule exists as two mirror-image twins, called enantiomers. The test is simple: if no rotation in space lines the molecule up with its reflection, it is handed. The twins share boiling point, melting point and other physical properties, with one exception: they twist a beam of polarized light by equal amounts in opposite directions, which is why they’re also called optical isomers.15Master Organic Chemistry. Optical Rotation, Optical Activity, and Specific Rotation
Handedness usually comes from a carbon bonded to four different groups. Swapping any two groups gives the mirror twin. A carbon with two identical groups doesn’t count, because swapping those changes nothing. That single-carbon rule is a starting guide, not a guarantee, since some molecules with several such carbons are still symmetric inside.
You’ll see labels R and S, or + and −, on drug and flavor names. R and S describe the actual arrangement of groups around the carbon, set by a ranking rule chemists apply to the four attached groups. The + and − signs, sometimes written d and l, describe which way the twin rotates polarized light, measured in the lab. The two systems don’t line up automatically: an R molecule may rotate light either way.
Stereoisomers that aren’t mirror images of each other are called diastereomers, and they can have quite different properties. Tartaric acid, found in grapes, has one form with a mirror-symmetric inside, so it doesn’t twist light and has a distinctly different melting point from its handed relatives.16Chemistry LibreTexts. What Is an Isomer? Definition and Types
Living things care about handedness because body proteins are themselves handed, built almost entirely from one hand of amino acids. That makes the body’s receptors and enzymes handed surfaces. A twin that matches the shape of a binding site fits and triggers a response, like a hand in a glove, while its mirror image fits poorly or not at all. In a flask with no handed partner, twins act identically, so the difference only appears in a handed environment.
Why Isomers Matter in Medicine, Smell and Fuel
Thalidomide, a sedative given to pregnant women around 1960, caused thousands of birth defects. Its two mirror twins act differently: the R form sedates while the S form harms developing babies.17Wikipedia. Thalidomide The S form fits a particular body protein better, which is mostly what causes the harm.18PubMed. The structure of isolated thalidomide as reference for its chirality-dependent biological activity: a laser-ablation rotational study Yet giving only the R form wouldn’t have helped, because the body converts each twin into the other.19Wikipedia. Thalidomide
Thalidomide was sold as a mixture of both twins. The case pushed regulators and drug makers to study each twin’s behavior on its own when a medicine is handed, so many modern drugs are tested twin by twin. Names starting with es- or lev-, like esomeprazole, often mean a single twin, which may need lower doses. A pharmacist can say whether two brands are the same isomer before you switch.
Smell shows the same split. Most mirror-image pairs smell only a little different.20PubMed Central. Structural determinants of a conserved enantiomer-selective carvone binding pocket in the human odorant receptor OR1A1 Carvone’s twins are distinct: one smells like spearmint, the other like caraway.21PubMed Central. Structural determinants of a conserved enantiomer-selective carvone binding pocket in the human odorant receptor OR1A1 About 8% of people can’t smell the spearmint form, a sign noses have handed scent receptors.22PubMed Central. Structural determinants of a conserved enantiomer-selective carvone binding pocket in the human odorant receptor OR1A1
Fuel depends on branching. Refineries rearrange straight-chain molecules in gasoline into branched isomers to raise octane.23Energy & Fuels. Hydroisomerization and Cracking of n-Octane and n-Hexadecane over Zirconia Catalysts Straight chains tend to ignite too early under compression, causing the knocking sound, while branched isomers resist this.
In physics, an energized form of an atom’s nucleus is also called an isomer. Chemical isomers differ in atom arrangement, while nuclear isomers differ in the energy state inside one atom’s nucleus. Technetium-99m is the most used medical radioactive tracer, in tens of millions of scans a year.24Wikipedia. Technetium-99m Technetium-99m releases that energy as rays a scanner can see.