The simplest organic compounds are saturated and unsaturated hydrocarbons. These include substances of the class of alkanes, alkynes, alkenes.
Their formulas include hydrogen and carbon atoms in a certain 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 an oily liquid, ethylene chloride, was discovered.
Alkenes are substances consisting of hydrogen and carbon elements. They belong to acyclic hydrocarbons. Their molecule contains a single double (unsaturated) bond connecting two carbon atoms to each other.
Alkene formulas
Each class of compounds has its own chemical designation. In them, the symbols of the 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: CH2n, where the number n is greater than or equal to 2. When decoding it, it can be seen that there are two hydrogen atoms for each carbon atom.
Molecular formulas of alkenes from the homologous series are represented by the following structures: C2H4, C3 H6, C4H8, C 5H10, C6H12, C 7H14, C8H16, C9 H18, C10H20. 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 lines, the bond of carbons with hydrogens is indicated.
The structural formula of alkenes can be displayed in expanded form, when all chemical elements and bonds are shown. With a more concise expression of olefins, the combination of carbon and hydrogen with the help of valence dashes is not shown.
The skeletal formula denotes the simplest structure. A broken line depicts the base of the molecule, in which carbon atoms are represented by its tops and ends, and hydrogen is indicated by links.
How olefin names are formed
Based on the systematic nomenclature, the formulas of alkenes and their names are made up of the structure of alkanes related to saturated hydrocarbons. To do this, in the name of the latter, the suffix -an is replaced by -ilen or -en. An example is the formation of butylene frombutane, 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.
Alkenes are named after the hydrocarbon with the longest chain containing a double bond. For the beginning of the numbering of the chain, the end is usually chosen, which is closest to the unsaturated compound of carbon atoms.
If the structural formula of alkenes has branches, then indicate the names of the radicals and their number, and they are preceded by numbers corresponding to the place in the carbon chain. Then follows the name of the hydrocarbon itself. The numbers are usually followed by a hyphen.
There are unlimited radical branches. Their names may be trivial or formed according to the rules of systematic nomenclature.
For example, HHC=CH- is called ethenyl or vinyl.
Isomers
Molecular formulas of alkenes cannot indicate isomerism. However, for this class of substances, with the exception of the ethylene molecule, spatial modification is inherent.
Isomers of ethylene hydrocarbons can be by carbon skeleton, by 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 it does not show the presence and location of the double bond. An example is cyclopropane as an interclass isomer of C3H6 (propylene). Other types of isomerism appear in C4H8 orbutene.
Different position of the unsaturated bond is observed in butene-1 or butene-2, in the first case, the double compound is located near the first carbon atom, and in the second - in the middle of the chain. Isomerism in the carbon skeleton can be considered using the example of methylpropene) and isobutylene ((CH3)2C=CH2).
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 main carbon chain with a double bond, in the second isomer, the substituents are on the same side.
Olefin characterization
The general formula of alkenes determines the physical state of all representatives of this class. Starting with ethylene and ending with butylene (from C2 to C4), substances exist in gaseous form. So colorless ethene has a sweet smell, low solubility in water, molecular weight is less than that of air.
In liquid form, hydrocarbons of the homologous range from C5 to C17 are presented. Starting from the alkene, which has 18 carbon atoms in the main chain, the transition of the physical state to the solid form occurs.
All olefins are considered to have poor solubility in an aqueous medium, but good solubility in organic solvents, 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 indicators during melting and boiling of these compounds.
Properties of olefins
Structural formula of alkenesshows the presence in the skeleton of a double bond of π- and σ-compound 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 destroy it with the formation of two new σ-bonds, which are obtained as a result of the addition of a pair of atoms. Unsaturated hydrocarbons are electron donors. They take part in electrophilic addition processes.
An important chemical property of all alkenes is the process of halogenation with the release of compounds similar to dihalogen derivatives. Halogen atoms are capable of attaching via a double bond to carbons. An example is the bromination of propylene with the formation of 1, 2-dibromopropane:
H2C=CH–CH3 + Br2 → BrCH 2–CHBr–CH3.
This process of color neutralization in bromine water with alkenes is considered qualitative proof of the presence of a double bond.
Important reactions include the hydrogenation of olifins with the addition of a hydrogen molecule under the action of catalytic metals such as platinum, palladium or nickel. The result is hydrocarbons with a saturated bond. The formulas of alkanes, alkenes are given below in the butene hydrogenation reaction:
CH3–CH2–CH=CH2 + H 2 Ni→ CH3–CH2–CH 2–CH3.
The process of adding a hydrogen halide molecule to olefinsis called
hydrohalogenation, proceeding according to the rule discovered by Markovnikov. An example is the hydrobromination of propylene to form 2-bromopropane. In it, hydrogen combines at a double bond with carbon, which is considered the most hydrogenated:
CH3–CH=CH2 + HBr → CH3–BrCH– CH3.
The reaction of addition of water by alkenes under the action of acids is called hydration. The result is a molecule of alcohol propanol-2:
CH3–HC=CH2 + H2O → CH 3–OHCH–CH3.
When exposed to alkenes with sulfuric acid, the process of sulfonation occurs:
CH3–HC=CH2 + HO−OSO−OH → CH3 –CH3CH–O−SO2−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 to form water and carbon dioxide gas:
2CH3–HC=CH2 + 9O2 → 6CO 2 + 6H2O.
The interaction of olefinic compounds and dilute potassium permanganate in the form of a solution leads to the formation of glycols or dihydric alcohols. This reaction is also oxidative, producing ethylene glycol and discoloring the solution:
3H2C=CH2 + 4H2O+ 2KMnO 4 → 3OHCH–CHOH+ 2MnO2 +2KOH.
Alkene molecules can be involved in the polymerization process with a free radicalor 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 are anionic catalysts with the release of a stereoselective polymer.
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 homological series of this class have the general formula C H2n+2.
They contain only carbon and hydrogen atoms. The general formula for 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 root of the numeral in Greek, to which the suffix -an is added. The names of alkanes are listed in the IUPAC nomenclature.
The general formula of alkenes, alkynes, alkanes includes only two types of atoms. These include the elements carbon and hydrogen. The number of carbon atoms in all three classes is the same, the difference is observed only in the number of hydrogen, which can be split off or added. Unsaturated compounds are obtained from saturated hydrocarbons. Representatives of paraffins in the molecule contain 2 more hydrogen atoms than olefins, which confirmsgeneral formula of alkanes, alkenes. The alkene 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.
From methane to butane (from C1 to C4), substances exist in gaseous form.
In liquid form, hydrocarbons of the homologous range from C5 to C16 are presented. Starting from the alkane, which has 17 carbon atoms in the main chain, the transition of the physical state to the solid form occurs.
They are characterized by isomerism in the carbon skeleton and optical modifications of the molecule.
In paraffins, carbon valences are considered to be completely occupied by neighboring carbons or hydrogens with the formation of a σ-type bond. From a chemical point of view, this causes their weak properties, which is why alkanes are called saturated or saturated hydrocarbons, devoid of affinity.
They enter into substitution reactions associated with radical halogenation, sulfochlorination or nitration of the molecule.
Paraffins undergo a process of oxidation, combustion or decomposition at high temperatures. Under the action of reaction accelerators, the elimination of hydrogen atoms or the dehydrogenation of alkanes occurs.
What are alkynes
They are also called acetylenic hydrocarbons, which have a triple bond in the carbon chain. The structure of alkynes is described by the generalformula C H2n–2. It shows that, unlike alkanes, acetylenic 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 (H2C꞊CH2) and a triple (HC≡CH) ties.
Name of alkynes and their characteristics
The simplest representative is acetylene or HC≡CH. It is also called ethin. It comes from the name of a 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 is the third option.
From acetylene to butane (from C2 to C4), substances are gaseous in nature.
In liquid form there are hydrocarbons of the homologous interval from C5 to C17. Starting from the alkyne, which has 18 carbon atoms in the main chain, the transition of the physical state to the solid form occurs.
They are characterized by isomerism in the carbon skeleton, in the position of the triple bond, as well as interclass modifications of the molecule.
Pochemical characteristics of acetylenic hydrocarbons are similar to alkenes.
If alkynes have a terminal triple bond, then they act as an acid with the formation of alkynide s alts, for example, NaC≡CNa. The presence of two π-bonds makes the sodium acetyledine molecule a strong nucleophile that enters into substitution reactions.
Acetylene undergoes chlorination in the presence of copper chloride to obtain dichloroacetylene, condensation under the action of haloalkynes with the release of diacetylene molecules.
Alkynes participate in electrophilic addition reactions, the principle of which underlies halogenation, hydrohalogenation, hydration and carbonylation. However, such processes proceed more weakly than in alkenes with a double bond.
For acetylenic hydrocarbons, addition reactions of the nucleophilic type of the molecule of alcohol, primary amine or hydrogen sulfide are possible.
