Chapter 2. Cycloalkanes (alicyclic hydrocarbons)

In the composition of cyclic alkanes, carbon is also in the state of sp3 hybridization, however, the degree of unsaturation is higher there. The general formula of hydrocarbons in a series of cycloalkanes is СnH2n. (Not to be confused with alkenes, which also have the same gross formula).

2.2.1. Classification and nomenclature

Cycloalkanes differ in ring size as well as in the number of rings in a molecule.

Among polycycloalkanes, there are compounds:

- with detached rings (A);

- with rings connected by a simple bond (B);

- rings having one common carbon atom (C);

- rings having two or more common carbon atoms (D):

A is 1-cyclohexyl-6-cyclopropylheptane;

B- 1-methyl-2-cyclopentylcyclohexane;

C - Spiro [4.5] decane;

D- 2-methylbicyclo [2.2.2] octane

When naming monocyclic compounds, the carbon atoms of the ring are numbered so that the sum of the numbers is minimal, after which the available substituents are listed and the name of the hydrocarbon corresponding to the number of carbon atoms in the ring is added with the prefix cyclo- Compounds containing more than one cycle have a more complex naming system.

So, if the rings are isolated (A), the name is formed from an open-chain hydrocarbon, and the rings act as substituents.

If the cycles are connected by a simple bond, one of them is called radical (B).

If the cycles have one common carbon atom (variant C), such molecules are called spirains, in square brackets indicate the number of carbon atoms in one and the other cycle, not counting the common atom. The brackets are followed by the name according to the total number of carbon atoms in the cycles, as if it were an open chain:

Polycyclic hydrocarbons with rings sharing two or more carbon atoms also have carbon atoms called bridge heads. Numbering is carried out from the head of the bridge along the path of the largest cycle, then the smaller cycle. The substituents are listed with the numbers of the atoms of the rings to which they are attached, the word bicyclo (for bicyclic hydrocarbons) or tricyclo (for tricyclic) is added. The numbers of carbon atoms in the cycle bridges are listed in square brackets, separated by a dot, after which the name of the hydrocarbon is added corresponding to the number of carbon atoms in the cycles (in total).

2.2.2. Isomerism of cycloalkanes

Cycloalkanes with the same number of carbon atoms in a molecule can have not only structural isomers (by ring size and arrangement of substituents), but also spatial isomers if the ring has two or more substituents.

Isomers differ in the arrangement of substituents relative to the plane of the ring.

The figure shows the projection model of 1,2-dimethylcyclohexane. The cyclohexane ring is in the chair conformation. (Red and blue in the figure show the C-C bonds of the ring and substituents. Black - bonds with hydrogen atoms. Hydrogen atoms are not shown).

It can be seen that, depending on the position of the methyl substituents relative to the ring, the existence of two isomers is possible: when both substituents are located on one side of the ring, and when they are located on opposite sides. The figure shows the trans isomer of 1,2-dimethylcyclohexane.

2.2.3. Cyclanes conformations

Due to the fact that the normal bond angles of the sp3-hybridized carbon are 109 degrees with a small degree, only the cyclopropane ring can be flat, where the angular stresses are very pronounced. Other cycles are far from the plane state due to the fact that they adapt to the minimum angular stresses of the bonds. So, for example, cyclohexane can be in two conformational states - chair and bath:

These states are capable of transforming into each other at ordinary temperatures, and cannot be individually separable, therefore it makes no sense to talk about the presence of isomers here. The state of the chair is energetically more favorable, especially if there are substituents in positions 1 and 4.

2.2.4. Chemical properties of cycloalkanes

The chemical properties of cycloalkanes depend on the size of the cycle (up to a certain size). Small cycles, due to strong angular stress, have increased internal energy and tend to react with ring opening. As the angular stresses decrease (from cyclopropane to cyclobutane), this reactivity drops sharply, and compounds such as cyclopentane or cyclohexane strongly resemble alkanes in chemical ability.

Thus, compounds with the number of carbon atoms in the cycle of 7 or more do not practically differ from cyclohexane either in specific heat of hydrogenation or in other characteristics.

They are not inclined to enter into reactions accompanied by ring opening; they are also characterized by substitution reactions proceeding by a radical mechanism.

So, for cyclopropane (and to a lesser extent for cyclobutane) the following reactions are characteristic:

Under the action of chlorine, along with the rupture of the cycle, the reaction of hydrogen substitution also takes place:

The presence of electron-withdrawing substituents (-CN, -Cl, -NO2, -COOH, etc.) in the ring makes it difficult to open the rings, and the presence of electron donor (alkyl groups) facilitates the opening of the rings.

2.2.5. Methods for the synthesis of cycloalkanes

The effect of sodium metal on 1,3-dihalide derivatives of alkanes (intramolecular Wurtz reaction):

The same transformation proceeds even better with the action of zinc dust in an alcoholic solution.

An important way to obtain three-membered rings is also the addition of carbenes to unsaturated compounds:

If diazo compounds are used for this purpose, heterocycles are first formed, which, when heated in the presence of copper, release nitrogen and form cyclopropane derivatives:

The processes of dehydrocyclization of normal and branched alkanes lead to the production of 5- and 6-membered cycloalkanes:

The production of rings with a large number of carbon atoms turned out to be an unexpectedly trivial task. In any case, these methods cannot be obtained. They were obtained much later in various indirect ways.