Which pairs are isomers

Despite sharing the same molecular formulae, isomers may have very different physical properties, such as boiling point, melting point, and chemical reactivity. Take cyclohexane b. No matter how different their physical properties, or reactivities, their common molecular formula makes them isomers of each other. Likewise, propionic acid and 1-hydroxypropanone share the same molecular formula, C 3 H 6 O 2 , making them isomers of each other but not isomers of cyclohexane or 1-hexene, of course!

This leads us to the next question. What kind of isomer are they? Isomers divide neatly in to two categories: constitutional isomers different connectivity and stereoisomers same connectivity, different arrangement in space. So what does that actually mean? Constitutional isomers have the same molecular formula, but different connectivities.

The same parts, but arranged in different ways. To take this oldie-but-goodie example, switch a tail and a leg and you make isocats:. Yes — from nomenclature. If two molecules with the same molecular formula have different connectivity, it will be obvious either in the locant e.

By way of an example, these 5 molecules are all constitutional isomers of each other. They have the same empirical formula C 6 H 12 but different connectivity. There is only one way to connect C 6 H 12 together to form cyclohexane, and only one way to connect the same atoms together to get 1-hexene. But there are two ways to connect C 6 H 12 to give molecules with the names 2-hexene, and 3-methylpentene!

And four ways to connect C 6 H 12 to give 1-ethylmethylcyclopropane! For example: there are two ways to arrange the hydrogens on the double bond of 2-hexene; when they are on the same side, we refer to it as cis or Z ; on the opposite side, trans E.

Since free rotation about the double bond is not possible, these are completely distinct molecules. They can be separated, put in different flasks, left on the shelf for years, and never interconvert.

What kind of isomers are these? We need another name. Since they differ in the arrangement of their groups in space about the double bond we call them stereoisomers. Stereoisomers can also arise from tetrahedral carbon atoms that are attached to four different substituents i. There are 2 and only 2! These molecules may look the same, but they are actually non-superimposable mirror images more on that a few paragraphs below. This is not unlike the distinction between diastereomers stereoisomers that ARE NOT non-superimposable mirror images and enantiomers stereoisomers that ARE non-superimposable mirror images.

In organic chemistry, two molecules that can be superimposed on each other, through rotation of bonds i. A mole, after all, is 6. Later that evening, after a few drinks, things got awkward. One has a scar over his left eye and the other has a scar over his right. No amount of twisting and turning on the floor in pain can possibly make them superimposable now. Since they are no longer superimposable, in chemistry terms they are no longer the same.

Similar, yes! But not the same molecule. No more. Now the left half of each twin is different from the right half. With molecules, the most common way to impart chirality is with a carbon bearing 4 different groups, as in 4-methylpentene, above. There are two and only two! So a molecule with a single asymmetric center will exist as a pair of stereoisomers. To be more specific, it will exist as a pair of non-superimposable mirror images: enantiomers. Separating these two isomers was hell on wheels, since they have identical solubilities, melting points, and other physical properties.

Pasteur was only able to accomplish it through observing minute differences in the appearances of their salts, and picked them apart using tweezers and a magnifying glass. The only physical property which differentiates these two isomers is that they rotate the plane of polarized light in equal and opposite directions. With hindsight, we now know the structures of these two isomers of tartaric acid, and using the Cahn-Ingold-Prelog rules, have named them R,R and S,S tartaric acid.

This is an important clue in identifying enantiomers and one we will discuss further in a future post :. These should also be stereoisomers, right? When we draw out the structures of 2R,3S and 2S,3R tartaric acid, however, something quickly becomes apparent. While they are indeed mirror images of each other, they are mirror images of each other in the same way that our pre-Voldemort identical twins are mirror images of each other:.

They are superimposable mirror images , and therefore considered to be identical molecules. Therefore 2 R, 3 S -tartaric acid and 2 S , 3 R -tartaric acid are not enantiomers. They are actually two different ways of describing the same molecule, and tartaric acid only has three stereoisomers overall.

Just in the same way as our pre-Voldemort Property Brother had chiral left and right ears, but was achiral overall due to the internal mirror plane. Only chiral molecules can have enantiomers. A molecule with an internal mirror plane — a plane of symmetry — is achiral and will not have an enantiomer. Likewise, 2 R, 3 S -tartaric acid has chiral centers, but possesses an internal mirror plane.

The chiral center with the S configuration is the mirror image of the chiral center with the R configuration, and the other substituents are arranged symmetrically. So if 2R, 3R -tartaric acid and 2S, 3S -tartaric acid are enantiomers, how do we describe the relationship between each of these molecules and meso- tartaric acid? Therefore… they are the same! Actually, they are different conformations of the same molecule, and we make the assumption that all conformations of the same molecule are interconvertible, unless told otherwise.

In the next instalment we will learn a technique that — with practice — will allow you to quickly determine whether molecules are enantiomers, diastereomers, or the same. Thanks again to Matt for co-authoring. Generally we make the assumption that conformational isomers interconvert quickly on the timescale necessary to measure optical rotation.

For example, the two chair forms of cis -1,2-dimethylcyclohexane are actually enantiomers, but since they interconvert so quickly at room temperature, they are treated as if they are the same. These two conformations are non-superimposable mirror images of each other in the same way that a left-handed and right-handed screw are non-superimposable mirror images of each other.

The barrier between the two conformers is large enough that conformer A and conformer B can be resolved separated and put in different bottles. They are correct. I double-checked! A stereoisomer will have the same connectivity among all atoms in the molecule. With a molecule such as 2-butene, a different type of isomerism called geometric isomerism can be observed. Geometric isomers are isomers in which the order of atom bonding is the same but the arrangement of atoms in space is different.

The double bond in an alkene is not free to rotate because of the nature of the bond. Therefore, there are two different ways to construct the 2-butene molecule see figure below.

The image below shows the two geometric isomers, called cis butene and trans butene. The cis isomer has the two single hydrogen atoms on the same side of the molecule, while the trans isomer has them on opposite sides of the molecule. In both molecules, the bonding order of the atoms is the same. In order for geometric isomers to exist, there must be a rigid structure in the molecule to prevent free rotation around a bond. This occurs with a double bond or a ring.

In addition, the two carbon atoms must each have two different groups attached in order for there to be geometric isomers. Propene see figure below has no geometric isomers because one of the carbon atoms the one on the far left involved in the double bond has two single hydrogens bonded to it.

Physical and chemical properties of geometric isomers are generally different. As with alkenes, alkynes display structural isomerism beginning with 1-butyne and 2-butyne.

However, there are no geometric isomers with alkynes because there is only one other group bonded to the carbon atoms that are involved in the triple bond.

Stereoisomers that are not geometric isomers are known as optical isomers. Optical isomers differ in the placement of substituted groups around one or more atoms of the molecule. They were given their name because of their interactions with plane-polarized light.

Optical isomers are labeled enantiomers or diastereomers. Enantiomers are non-superimposable mirror images. A common example of a pair of enantiomers is your hands. Your hands are mirror images of one another but no matter how you turn, twist, or rotate your hands, they are not superimposable.

Objects that have non-superimposable mirror images are called chiral. When examining a molecule, carbon atoms with four unique groups attached are considered chiral. Look at the figure below to see an example of a chiral molecule. Note that we have to look beyond the first atom attached to the central carbon atom.

The four circles indicate the four unique groups attached to the central carbon atom, which is chiral. Another type of optical isomer are diastereomers , which are non-mirror image optical isomers. Diastereomers have a different arrangement around one or more atoms while some of the atoms have the same arrangement. As shown in the figure below, note that the orientation of groups on the first and third carbons are different but the second one remains the same so they are not the same molecule.

Epimers are a sub-group of diastereomers that differ at only one location. All epimers are diastereomers but not all diastereomers are epimers. Allison Soult , Ph.