Cyclic connections have already been mentioned before, but cyclones are not the only ones. Amines and esters, both simple and complex, can also be cyclic. In general, they are similar in properties to their brethren, but in some ways, they differ quite noticeably. For example, tetrahydrofuran can be considered as an analog of diethyl ether, but due to its geometry negative charge of the oxygen atom is no longer in the center, and the molecule becomes more polar. This is manifested in the fact that tetrahydrofuran is mixed with water (as opposed to the ether), and boils at a noticeably higher temperature, as all polar compounds are entitled to.
Of course, there are also horologists of tetrahydrofuran (otherwise called oxolan): three-, four- and six-membered. They are called oxirane, oxetane and tetrahydropyran (oxane), respectively
Three-dick cycles are always very unstable. Oxyran (ethylene oxide) is very reactive, which is why it is recognized as a carcinogen. However, it is used to sterilize medical equipment (in particular disposable syringes) - this is safe because it is subsequently volatilized or decomposed. It is also so combustible that it is filled with bulk blast ammunition.
Compounds containing an oxirane fragment are called epoxies. Yes, the same epoxies as the epoxy resin - it is polymerized ("hardens") due to the reaction of the epoxy fragment with phenols present in the resin (in particular, bisphenol A). Catalysts in this reaction are amines such as polyethylene polyamine.
Cyclic esters have their own name: lactones. Cyclic amides are called lactates. Gamma-butyrolactone and gamma-butyrolactone (pyrrolidone) are important solvents and intermediates in synthesis. There is a class of compounds, pyrrolidone derivatives, which are used as drugs called "racetams". Everybody must have heard of them under the name "Piracetam". Another famous representative of cyclic amides is caprolactam, a raw material for Capron polymer production.
A number of the simplest nitro cycle heterocycles are constructed in the same way as several cyclic esters. Get acquainted with: aziridine, azetidine, pyrrolidine, piperidine. Five- and six-membered cycles are always more stable and more frequent in nature.
Because of their geometry, the nitrogen atom is more accessible (less obstructed by substitutes), so they are more reactive and more basic. We all know that tertiary amines are the weakest bases, but DABCO (Diazabicyclooctane) is not considered to be the same as quinuclidine - these two compounds are very strong bases and are used as catalysts in organic synthesis.
Now for a brief discussion of aromatic compounds. It turns out that cyclic compounds containing alternating with single double bonds, manifested themselves in reactions rather unusual - they almost exclusively enter into a substitution reaction instead of joining, to preserve this system of double bonds. This phenomenon is called aromaticity. The prototype of all aromatic compounds is benzene: it is so stable that the addition of hydrogen is so difficult (and dichlorobenzene under the influence of oxygen is easily oxidized back!), and in reactions with halogens formed hydrogen bromide and bromobenzene - almost like with aliphatic compounds!
Benzene is rare because of its toxicity, but toluene (methylbenzene) and its other derivatives - everywhere.
Aromatic" such compounds are called because for the first time they were isolated from aromatic compounds: benzene resin (benzene) and Toluene balsam (toluene). Both of them do not smell very nicely in their pure form, but the name is settled. Subsequently, other aromatic compounds were found, such as piperidine (pyridine), pyrrolidine (pyrrole) and oxolane (furan).
Depending on the substituent, we can get the corresponding alcohol (phenol), amine (aniline), acid (benzoic acid) and so on. Aromatic compounds differ significantly from aliphatic ones in terms of their properties - for example, phenol has acidic properties and easily forms salts, unlike alcohols, and aniline is noticeably weaker than aliphatic primary amines (for example, methylamine). Reactions involving aromatic compounds are often complex and unpredictable, although more than one multivolume book is written on this topic.
Almost all natural compounds (especially alkaloids) contain this or that aromatic cycle in their structure. These are, for example, nicotine, quinine, as well as aromatic amino acids phenylalanine and tryptophan.