Tautomers

Tautomers are a particular class of isomers, which belong to the constitutional isomers. They are molecules characterized by differ among themselves usually in the position of a hydrogen atom. Thus, reactions involving the transfer of a H + to another position in the same molecule (i.e., intramolecular transfer of a proton), are tautomerization reactions.

The tautomers have a high rate of interconversion, and due to this fact often both tautomeric forms are "ignored" and they are considered as the same molecule.
The most common situation is called keto-enol tautomerism. But what does this mean? When we say "keto", we are referring to a molecule which has a carbonyl group, whilst when we say "enol", it is a molecule having one hydroxyl group and the carbon which has this group is simultaneously involved in a double bond. Hence the name enol (en = double bond; ol = hydroxyl group). Normally the form "keto" is the most favored, but in certain (bio)chemical contexts, it may originate the enol form.

In Biochemistry, tautomers have an important role in the metabolism of nitrogen bases, in particular in the pairing between nitrogen bases which occurs in DNA and mainly in RNA.

 

Diastereoisomers

The diastereomers are a class of isomers which, together with the enantiomers, belong to the family of optical isomers. Therefore, they differ in the configuration of chiral carbons. However, in the case of diastereoisomers, the molecules must have at least two chiral carbons which means that whenever there is only one chiral carbon, the molecule do not present diastereoisomers. As I mentioned in a previous post here on the blog, the enantiomers differ in the configuration of ALL chiral carbons. The diastereomers are characterized by presenting AT LEAST ONE chiral carbon with different configuration and AT LEAST ONE chiral carbon with the same configuration.

Therefore, if a molecule with two chiral carbon presents one with the same configuration in both isomers, these are diastereoisomers.

If the molecule has 100 carbons chiral, an enantiomer has to have an opposite configuration at all of them. All the intermediate situations, where at least one configuration remains the same, these are diastereoisomers.
Interestingly, despite the difference between the diastereomers can be in the configuration of only one chiral carbon, they have distinct properties. Please see what happens in the following curious molecules ...

Both have two chiral carbons, one of which has the same configuration and other features different configuration. However, this "small" difference causes one of the diastereoisomers to have a sweet taste (molecule from left), while the other has a bitter taste (right molecule).

Enantiomers

Enantiomers are a class of isomers belonging to the optical isomers, taht means, they are characterized by differencies in regions containing at least one chiral carbon.
The enantiomers are considered as the mirror image of one another, like it happens with our hands. Therefore, they are symmetric, non-superimposable molecules.

However, when trying to compare two potential enantiomers, it is not always easy to imagine the rotation of the molecules in space, in order to check whether one is dealing with mirror images of one another. So, the best thing to do is to use the definition of enantiomer. This definition tells us that the enantiomers are molecules that differ in the configuration of ALL chiral carbons. Thus, what should be done is:
1. Identify all the chiral carbons present in the molecule.
2. Verify if the configuration of each one differs or stands in the possible enantiomer.
When I speak in different configuration, I am referring to the situation where it is impossible to overlap all substituents of a chiral carbon in the two molecules. If the configuration of ALL chiral carbons is different, they are enantiomers. If there is at least one chiral carbon with the same configuration in both molecules, they are no longer enantiomers.
Finally, by definition, when there is only one chiral carbon in a molecule, and the corresponding configuration is different in the two isomers, these are enantiomers.

Geometric isomers

The geometric isomers belong to the family of the stereoisomers, more precisely, the configurational isomers. So, as the name indicates, they differ in the configuration of one or more carbons. The particularity of this kind of isomerism is the fact that it involves non-tetraedric carbons, that means, carbons that do not establish 4 single bonds. Putting it more simply, it involves carbons that establish double bounds. 

 The classic example of this kind of isomerism is the cis and trans isomers, where the configuration of one of the carbons involved in the double bound changes. It is important to note that these isomers are not conformational ones, because the double bonds cannot rotate. 

Finally, it is noteworthy that one should be careful when comparing two molecules that differ in a region that contains double bonds. In this case, the molecules are geometric isomers only if the constitutional isomerism is not applicable to the molecules. For example, in the following image the first two molecules are geometric isomers, while the molecules 1 and 3, or 2 and 3 are constitutional isomers.

Conformational isomers

Today I will post about conformational isomers, a class that belongs to the family of stereoisomers. This type of isomerism is somehow controversial, since there are people that consider it, indeed, as a particular type of isomerism, while others consider that we are talking about different structures of the same molecule.

In order to understand the concept of conformational isomer, it is recommended to highlight a property that is observed only in single bounds – its capacity to rotate, functioning as an axis.

In this context, regions of a molecule that contain single bounds are characterized by their high rotational flexibility.

When two molecules are compared, if it is possible to convert one in the other through rotation on one (or more) single bounds, they are conformational isomers.

Since, in fact, the two molecules are converted without the need to break or create new covalent bonds (this is the definition of conformation, as it was explained here in a previous post…), there are people that claim that those molecules don’t have to be considered isomers.

Constitutional isomers

In the coming weeks I will devote a few posts to the different types of isomers that molecules can present. I'll start with the constitutional isomers...
This type of isomerism involves changes in the pattern of the covalent bonds that exist in a molecule. That is, when we compare two isomers in order to try to understand what kind of isomerism exists between them, the first thing to do is to look at their covalent bonds skeleton. Basically the idea is this ... we have to see if each atom of a molecule establishes exactly the same kind of bonds with the same substituents, than the corresponding isomer. If there is at least one difference in those bonds, these are constitutional isomers.

In this context, there are several situations that can occur, being the most common:
 

1. Changes in the identity of the functional groups
In some cases, changing the pattern of covalent bonds may lead to changes in the identity of functional groups such as the following examples.

2. Changes in the position of the functional groups
In this particular case, the isomers are designated as positional isomers.

3. Changes in the localization of double bonds
In this situation the double bonds of the molecules remain in the same amount, the only change is their localization within the molecule.

4. Cyclization of alkenes
Sometimes, constitutional isomers appear when the alkenes undergoes cyclization, losing the double bond during the process.

Conformation vs. configuration

Before I start posting on the different types of isomers, I will devote a post to an aspect that is very important when studying the isomerism of molecules. Despite its great importance, this can be sometimes confusing ... I'm talking about the difference between conformation and configuration. J
These two concepts are often used interchangeably, but represent very different things. The conformation regards the relative spatial orientation of a portion of a molecule relative to another. Thus, it is an aspect that is not directly related to the covalent bonds that are established within the molecule, but with their possible rotation.

When we talk about rotation around covalent bonds, we are only referring to the single bonds, as they are the only ones that can suffer rotation. Basically this concept is easily understood if we think that the bounds work as an axis...

It should be noted that when we speak of different conformations, it does not necessarily involve all the covalent bonds of a molecule, it can account only for one or few of them.

Taken together, it is possible to convert one conformation to another without cleaving or forming chemical bonds, simply by rotating some simple covalent bonds.

The configuration is a concept that is related to the order by which different substituents linked to the same central atom establish covalent bonds. That means, in this case it is clearly an aspect that is a direct consequence of the covalent backbone of molecules.

To change the configuration, you must always cleave and form new covalent bonds...

 
In conclusion, the concept conformation encompasses portions of a molecule which are not directly linked to the same atom and do not involve the covalent backbone of the molecules, while the configuration comprehends parts of the molecule which are bound to the same atom, which means that there is a direct involvement of the covalent bounds of the molecule.