Alkines are saturated hydrocarbons that have in their structure a triple bond, in addition to a single bond. The general formula is identical to the alkadiene formula — C n H 2n-2 . The triple bond is fundamental in the characterization of a given class of substances, its isomerism and structure.
General characteristics of triple bond
Carbon atoms forming a triple bond have sp hybridization. Based on the method of localized electron pairs, this bond is known to be formed by the overlap of two p-orbitals that are in the perpendicular position and one s-orbital that connects the atoms. Thus, the overlapping of the hybrid orbital provides the formation of one sigma bond, and two non-hybrid ones - the formation of two pi bonds. It is worth noting that the triple bond is shorter than the double one, and the energy released during its breaking is much greater. Therefore, the triple bond is much stronger.
So, above we examined the structure of alkynes, isomerism and nomenclature will be studied in the following paragraphs.
Nomenclature
The nomenclature and isomerism of alkynes plays an important role in the designation of substances of this class of compounds.
We give various examples of the names of alkynes, based on the systematic and substitutional (UPAC) nomenclatures. For example, the simplest representative of the homologous series of alkynes - C 2 H 2 according to the systematic nomenclature is called ethine, and according to the nomenclature proposed by UPAC, it is called acetylene.
We give an example of how to name compounds according to systematic nomenclature. The suffix -in indicates the presence of a triple bond, and its location in the chain is determined by the number. To begin with, we choose a connection, we find the main circuit in it. It must necessarily have more carbon and a triple bond. Then we write the name of the chain, indicating all the substituents in front, indicating their location with the corresponding numbers. Next, we assign the suffix -in and at the end through the dash we add a figure showing the position of the triple bond.
The designation of compounds according to the nomenclature proposed by UPAC is also not difficult. Two hydrocarbons with a triple bond are called acetylene, and the subsequent attached hydrocarbons are denoted by their respective names. For example: propine will be called methylacetylene, and hexin-1 - butylacetylene. If hydrocarbons connected by a triple bond are used as a substituent, then their names will be ethynyl (2 carbon), propynyl (3 carbon), and increase the amount of hydrocarbons, respectively.
Isomerism of alkynes
Isomerism is a phenomenon that consists in the ability to form substances that are identical in composition and molecular weight, but different in structural structure. The alkane isomerism also has a place to be, however, it is limited by the ability of multiple bonds. As mentioned above, the triple bond is more saturated, it very tightly pulls together positively charged atoms and provides a more dense contact of neighboring carbons, which is very difficult to ignore.
Consider the inherent alkynes types of isomerism.
The first, inherent in all hydrocarbons, is structural isomerism. This type of isomerism of alkynes is subdivided into the isomerism of the carbon skeleton and multiple bonds. According to the carbon skeleton, it is determined by the different positions of the bonds in the molecule. The simplest alkine that can use this type is pentin-1. It can be modified to 2-methylbutin-1.
Multiple bond isomerism is due to the different positions of the triple bond. The simplest alkine capable of applying isomerism of multiple bonds is butyl-1. It can be modified in butyl-2.
The second type, characteristic of the isomerism of alkynes, is interclass. It is due to the presence in different classes of compounds of the same general formula. Not surprisingly, such compounds drastically vary in structure. This type of isomerism of alkynes is found due to the same formula with dienes and cycloalkenes. For example, both hexin-1, hexadiene-2,3, and cyclohexene have the formula C 6 H 10 .
Geometric isomerism of alkynes
Geometric isomerism due to different positions of the molecule in space (-cis, -trans) is not found in alkynes due to the fact that under the influence of a triple bond the hydrocarbon chain takes only a linear position.
However, a linear fragment of this chain containing a triple bond can be included in large closed carbon cycles that can undergo geometric (spatial) isomerism. These cycles should contain enough carbon so that the spatial stress caused by the strong triple bond is not noticeable.
Cyclononin is the first stable cycloalkine compound. He is the most stable among others like him. With an increase in the number of carbons, these compounds lose their strength.
The effect of the triple bond on the properties of alkynes
Alkines having a triple bond at the end (terminal) have an increased dipole moment when compared with other hydrocarbons with an equal number of carbon atoms. This indicates a higher polarizability of the triple bond under the action of alkyl groups. Alkines are more durable than other classes of substances. They are not soluble in water, but soluble in non-polar or slightly polar solvents (esters, benzene).
The presence of a triple bond largely determines the properties of alkynes. Naturally, they are characterized by the reaction of addition of hydrogen halides, water, alcohols, carboxylic acids, they are easily oxidized and reduced. A distinctive feature of alkynes with a terminal triple bond is their CH-acidity.
Alkines have an electrophilic addition reaction . Based on the fact that the degree of unsaturation in them is higher than in alkenes, the reactivity of the former should be higher, but, most likely, because of the strength of the triple bond, the reactivity of electrophilic addition of alkenes and alkynes is almost identical.
conclusions
So, in this article, alkynes, their structural features, nomenclature in a systematic manner and form proposed by UPAC were considered. Both of these nomenclatures are used to refer to compounds worldwide, that is, any of the names will be correct. Different types of isomers of alkynes reflect their properties and subtleties, which largely depend on multiple bonds. This feature is characteristic not only for alkynes, but also for any carbon chains.