The simplest organic compounds are saturated and unsaturated hydrocarbons. These include substances of the class alkanes, alkynes, alkenes.
Their formulas include hydrogen and carbon atoms in a specific sequence and quantity. They are often found in nature.
Determination of Alkenes
Their other name is olefins or ethylene hydrocarbons. That is what this class of compounds was called in the 18th century when the oily liquid, ethylene chloride, was discovered.
Alkenes include substances consisting of hydrogen and carbon elements. They relate to acyclic hydrocarbons. In their molecule there is a single double (unsaturated) bond connecting two carbon atoms to each other.
Alkenes Formulas
Each class of compounds has its own chemical designation. In them, the symbols and elements of the periodic system indicate the composition and structure of the bonds of each substance.
The general formula of alkenes is denoted as follows: C n H 2n , where the number n is greater than or equal to 2. When deciphering it, it is seen that for each carbon atom there are two hydrogen atoms.
The molecular formulas of alkenes from the homologous series are represented by the following structures: C 2 H 4 , C 3 H 6 , C 4 H 8 , C 5 H 10 , C 6 H 12 , C 7 H 14 , C 8 H 16 , C 9 H 18 , C 10 H 20 . It can be seen that each subsequent hydrocarbon contains one more carbon and 2 more hydrogen.
There is a graphic designation of the location and order of chemical compounds between atoms in a molecule, which shows the structural formula of alkenes. With the help of valence dashes, the bond of carbons with hydrogens is indicated.
The structural formula of alkenes can be depicted in an expanded form when all chemical elements and bonds are shown. With a more concise expression of olefins, the connection of carbon and hydrogen with the help of valence dashes is not shown.
The skeletal formula denotes the simplest structure. The broken line depicts the base of the molecule, in which the carbon atoms are represented by its tops and ends, and the links indicate hydrogen.
How are olefin names formed?
Based on the systematic nomenclature, the formulas of alkenes and their names are made up of the structure of alkanes belonging to saturated hydrocarbons. To do this, in the name of the latter, the suffix -an is replaced by -ylen or -en. An example is the formation of butylene from butane, and pentene from pentane.
To indicate the position of the double bond relative to carbon atoms, indicate the Arabic numeral at the end of the name.
The name of alkenes is based on the designation of the hydrocarbon with the longest chain in which a double bond is present. For the beginning of chain numbering, the end that is closest to the unsaturated compound of carbon atoms is usually chosen.
If the structural formula of alkenes has branches, then the names of the radicals and their quantity are indicated, and before them are numbers corresponding to the place in the carbon chain. Then follows the name of the hydrocarbon itself. After the numbers usually put a hyphen.
There are unsaturated radical branches. Their names can be trivial or are formed according to the rules of systematic nomenclature.
For example, HHC = CH- is referred to as ethenyl or vinyl.
Isomers
The molecular formulas of alkenes cannot indicate isomerism. However, for this class of substances, with the exception of the ethylene molecule, spatial modification is inherent.
The isomers of ethylene hydrocarbons can be along the carbon skeleton, according to the position of the unsaturated bond, interclass or spatial.
The general formula of alkenes determines the number of carbon and hydrogen atoms in the chain, but the presence and location of the double bond are not visible from it. An example is cyclopropane as an interclass isomer of C 3 H 6 (propylene). Other types of isomerism appear in C 4 H 8 or butene.
A different position of the unsaturated bond is observed for butene-1 or butene-2, in the first case the double compound is near the first carbon atom, and in the second in the middle of the chain. The carbon skeleton isomerism can be considered using methylpropene (CH 3 —C (CH 3 ) = CH 2 ) and isobutylene ((CH 3 ) 2C = CH 2 ) as an example.
Spatial modification is inherent in butene-2 in the trans- and cis-position. In the first case, the side radicals are located above and below the double bond main carbon chain; in the second isomer, the substituents are on the same side.
Characterization of olefins
The general formula of alkenes determines the physical condition of all representatives of this class. Starting from ethylene and ending with butylene (from C 2 to C 4 ), the substances exist in gaseous form. So the colorless ethene has a sweet smell, low solubility in water, the molecular weight is less than that of air.
In liquid form, hydrocarbons of the homological gap from C 5 to C 17 are represented. Starting from an alkene having 18 carbon atoms in the main chain, a physical state transitions to solid form.
All olefins are considered poor solubility in the aquatic environment, but good in solvents of an organic nature, such as benzene or gasoline. Their molecular weight is less than that of water. An increase in the carbon chain leads to an increase in temperature indices during melting and boiling of these compounds.
Olefin Properties
The structural formula of alkenes shows the presence in the skeleton of a double bond of the π and σ compounds of two carbon atoms. This structure of the molecule determines its chemical properties. The π bond is considered not very strong, which makes it possible to break it with the formation of new two σ bonds, which result from the addition of a pair of atoms. Unsaturated hydrocarbons are electronic donors. They take part in the processes of joining according to the electrophilic type.
An important chemical property of all alkenes is the halogenation process with the release of compounds similar to dihalogenated substances. Halogen atoms are capable of attaching to the carbons via a double bond. An example is the bromination of propylene with the formation of 1,2-dibromopropane:
H 2 C = CH – CH 3 + Br 2 → BrCH 2 –CHBr – CH 3 .
This process of color neutralization in bromine water by alkenes is considered as qualitative evidence of the presence of a double bond.
Important reactions include hydrogenation of olifins with the addition of a hydrogen molecule under the influence of catalytic metals such as platinum, palladium or nickel. The result is a saturated bond hydrocarbon. The formulas of alkanes, alkenes are given below in the hydrogenation of butene:
CH 3 –CH 2 –CH = CH 2 + H 2 Ni → CH 3 –CH 2 –CH 2 –CH 3 .
The process of attaching a hydrogen halide molecule to olefins is called
hydrohalogenation, following the rule discovered by Markovnikov. An example is the hydrobromination of propylene to form 2-bromopropane. In it, hydrogen combines in a double bond with carbon, which is considered the most hydrogenated:
CH 3 –CH = CH 2 + HBr → CH 3 –BrCH – CH 3 .
The reaction of alkene addition of water under the action of acids is called hydration. The result is a propanol-2 alcohol molecule:
CH 3 –HC = CH 2 + H 2 O → CH 3 –OHCH – CH 3 .
When exposed to alkenes with sulfuric acid, a sulfonation process occurs:
CH 3 –HC = CH 2 + HO − OSO − OH → CH 3 –CH 3 CH – O – SO 2 −OH.
The reaction proceeds with the formation of acid esters, for example, isopropylsulfuric acid.
Alkenes are susceptible to oxidation during their combustion under the action of oxygen with the formation of water and carbon dioxide:
2CH 3 –HC = CH 2 + 9O 2 → 6CO 2 + 6H 2 O.
The interaction of olefin compounds and diluted potassium permanganate in the form of a solution leads to the formation of glycols or alcohols of a diatomic structure. This reaction is also oxidative with the formation of ethylene glycol and discoloration of the solution:
3H 2 C = CH 2 + 4H 2 O + 2KMnO 4 → 3OHCH – CHOH + 2MnO 2 + 2KOH.
Alkenes molecules can be involved in the polymerization process with a free radical or cation-anion mechanism. In the first case, under the influence of peroxides, a polymer such as polyethylene is obtained.
According to the second mechanism, acids act as cationic catalysts, and organometallic substances with the release of a stereoselective polymer are anionic.
What are alkanes?
They are also called paraffins or saturated acyclic hydrocarbons. They have a linear or branched structure, which contains only saturated simple bonds. All representatives of the homologous series of this class have the general formula C n H 2n + 2 .
They contain only carbon and hydrogen atoms. The general formula of alkenes is formed from the designation of saturated hydrocarbons.
Names of alkanes and their characteristics
The simplest representative of this class is methane. It is followed by substances such as ethane, propane and butane. Their name is based on the numeral root in Greek, to which the suffix -an is added. Alkane names are listed in the IUPAC nomenclature.
The general formula of alkenes, alkynes, alkanes includes only two types of atoms. These include elements of carbon and hydrogen. The number of carbon atoms in all three classes coincides, the difference is observed only in the number of hydrogen, which can be split off or attached. Unsaturated compounds are obtained from saturated hydrocarbons . Representatives of paraffins in the molecule contain 2 hydrogen atoms more than olefins, which is confirmed by the general formula of alkanes, alkenes. Alkenes structure is considered unsaturated due to the presence of a double bond.
If we correlate the number of hydrogen and carbon atoms in alkanes, then the value will be maximum in comparison with other classes of hydrocarbons.
Starting from methane and ending with butane (from C 1 to C 4 ), substances exist in a gaseous form.
In liquid form, hydrocarbons of the homological gap from C 5 to C 16 are represented. Starting from an alkane having 17 carbon atoms in the main chain, a physical state transitions to solid form.
They are characterized by carbon skeleton isomerism and optical modifications of the molecule.
In paraffins, carbon valencies are considered to be fully occupied by adjacent carbons or hydrogens with the formation of a σ-type bond. From a chemical point of view, this leads to their weak properties, which is why alkanes are called saturated or saturated hydrocarbons, lacking affinity.
They enter into substitution reactions associated with radical type halogenation, sulfochlorination or nitration of the molecule.
Paraffins undergo oxidation, combustion, or decomposition at high temperatures. Under the action of reaction accelerators, the removal of hydrogen atoms or the dehydrogenation of alkanes occurs.
What is alkynes
They are also called acetylene hydrocarbons, in which a triple bond is present in the carbon chain. The structure of alkynes is described by the general formula C n H 2 n – 2 . It can be seen from it that, unlike alkanes, acetylene hydrocarbons lack four hydrogen atoms. They are replaced by a triple bond formed by two π-compounds.
Such a structure determines the chemical properties of this class. The structural formula of alkenes and alkynes clearly shows the unsaturation of their molecules, as well as the presence of a double (H 2 C꞊CH 2 ) and triple (HC≡CH) bond.
Name of alkynes and their characteristics
The simplest representative is acetylene or HC≡CH. It is also called ethine. It comes from the name saturated hydrocarbon, in which the suffix -an is removed and -in is added. In the names of long alkynes, the number indicates the location of the triple bond.
Knowing the structure of saturated and unsaturated hydrocarbons, it is possible to determine under which letter the general formula of alkynes is indicated: a) CnH2n; c) CnH2n + 2; c) CnH2n-2; d) CnH2n-6. The correct answer would be the third option.
Starting with acetylene and ending with butane (from C 2 to C 4 ), the substances are gaseous in nature.
In liquid form, there are hydrocarbons of the homological gap from C 5 to C 17 . Beginning with an alkyne having 18 carbon atoms in its main chain, a physical state transitions to solid form.
They are characterized by isomerism along the carbon skeleton, by the position of the triple bond, as well as interclass modifications of the molecule.
By chemical characteristics, acetylene hydrocarbons are similar to alkenes.
If alkynes have a triple bond end, then they perform the function of acid with the formation of salts of alkynides, for example, NaC≡CNa. The presence of two π-bonds makes the sodium acetyledin molecule a strong nucleophile, which enters into the substitution reaction.
Acetylene is subjected to chlorination in the presence of copper chloride to obtain dichloroacetylene, condensation under the action of halogenalkines with the release of diacetylene molecules.
Alkines participate in electrophilic addition reactions , the principle of which is the basis of halogenation, hydrohalogenation, hydration and carbonylation. However, such processes are weaker than that of double-bonded alkenes.
For acetylene hydrocarbons, nucleophilic type addition reactions of an alcohol molecule, a primary amine, or hydrogen sulfide are possible.