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.