The structural features of acetylene affect its properties, production and use. The symbol for the composition of a substance - С2Н2 - is its simplest and gross formula. Acetylene is formed by two carbon atoms, between which a triple bond occurs. Its presence is reflected by different types of formulas and models of the ethine molecule, which make it possible to understand the problem of the influence of the structure on the properties of a substance.
Alkynes. General formula. Acetylene
Alkyne hydrocarbons, or acetylenic hydrocarbons, are acyclic, unsaturated. The chain of carbon atoms is not closed; it contains single and multiple bonds. The composition of alkynes is reflected in the summary formula C H2n – 2. Molecules of substances of this class contain one or more triple bonds. Acetylene compounds are unsaturated. This means that only one valence of carbon is realized at the expense of hydrogen. The remaining three bonds are used when interacting with other carbon atoms.
The first - and most famous representative of alkynes - acetylene, or ethyne. The trivial name of the substance comes from the Latin word "acetum" - "vinegar" andGreek - "hyle" - "tree". The ancestor of the homologous series was discovered in 1836 in chemical experiments, later the substance was synthesized from coal and hydrogen by E. Davy and M. Berthelot (1862). At normal temperature and normal atmospheric pressure, acetylene is in a gaseous state. It is a colorless gas, odorless, slightly soluble in water. Ethine dissolves more easily in ethanol and acetone.
Molecular formula of acetylene
Etin is the simplest member of its homologous series, its composition and structure reflect the formulas:
- С2Н2 - ethine composition molecular record, which gives an idea that the substance is formed by two carbon atoms and the same the number of hydrogen atoms. Using this formula, you can calculate the molecular and molar masses of the compound. Mr (С2Н2)=26 a. e.m., M (С2Н2)=26.04 g/mol.
- Н:С:::С:Н - electron point formula of acetylene. Such images, called "Lewis structures", reflect the electronic structure of the molecule. When writing, it is necessary to follow the rules: the hydrogen atom, when forming a chemical bond, tends to have the configuration of the helium valence shell, other elements - an octet of external electrons. Each colon means a common for two atoms or lone pair of electrons of the outer energy level.
- H-C≡C-H - structural formula of acetylene, reflecting the order and multiplicity of bonds between atoms. One dash replaces one pair of electrons.
Acetylene molecule models
Formulas showing the distribution of electrons served as the foundation for the creation of atomic orbital models, spatial formulas of molecules (stereochemical). As early as the end of the 18th century, ball-and-stick models became widespread - for example, balls of different colors and sizes, denoting carbon and hydrogen, which form acetylene. The structural formula of the molecule is presented in the form of rods, symbolizing the chemical bonds and their number in each atom.
The ball-and-stick model of acetylene reproduces bond angles equal to 180°, but the internuclear distances in the molecule are reflected approximately. The voids between the balls do not create an idea of filling the space of atoms with electron density. The disadvantage is eliminated in Dreiding's models, which designate the nuclei of atoms not as balls, but as points of attachment of rods to each other. Modern volumetric models give a clearer picture of atomic and molecular orbitals.
Acetylene hybrid atomic orbitals
Carbon in an excited state contains three p-orbitals and one s with unpaired electrons. In the formation of methane (CH4) they take part in the creation of equivalent bonds with hydrogen atoms. The famous American researcher L. Pauling developed the doctrine of the hybrid state of atomic orbitals (AO). The explanation of the behavior of carbon in chemical reactions is the alignment of AO in form and energy, the formation of new clouds. hybridorbitals give stronger bonds, the formula becomes more stable.
Carbon atoms in the acetylene molecule, unlike methane, undergo sp-hybridization. The s- and p electrons are mixed in shape and energy. Two sp-orbitals appear, lying at an angle of 180°, directed on opposite sides of the nucleus.
Triple bond
In acetylene, hybrid electron clouds of carbon participate in the creation of σ-bonds with the same neighboring atoms and with hydrogen in C-H pairs. There remain two non-hybrid p-orbitals perpendicular to each other. In the ethine molecule, they are involved in the formation of two π bonds. Together with σ, a triple bond arises, which is reflected by the structural formula. Acetylene differs from ethane and ethylene by the distance between the atoms. A triple bond is shorter than a double bond, but has a larger energy reserve and is stronger. The maximum density of σ- and π-bonds is located in perpendicular regions, which leads to the formation of a cylindrical electron cloud.
Features of the chemical bond in acetylene
The ethine molecule has a linear shape, which successfully reflects the chemical formula of acetylene - H-C≡C-H. Carbon and hydrogen atoms are located along one straight line, 3 σ- and 2 π-bonds appear between them. Free movement, rotation along the C-C axis is impossible, this is prevented by the presence of multiple bonds. Other triple bond features:
- number of pairs of electrons that bind two carbon atoms - 3;
- length - 0.120 nm;
- break energy - 836kJ/mol.
For comparison: in ethane and ethylene molecules, the lengths of single and double chemical bonds are 1.54 and 1.34 nm, respectively, the C-C break energy is 348 kJ/mol, C=C - 614 kJ/mol.
Acetylene homologues
Acetylene is the simplest representative of alkynes, in the molecules of which there is also a triple bond. Propyne CH3C≡CH is an acetylene homologue. The formula of the third representative of alkynes - butyne-1 - CH3CH2C≡CH. Acetylene is the trivial name for ethine. The systematic nomenclature of alkynes follows the IUPAC rules:
- in linear molecules, the name of the main chain is indicated, which arose from the Greek numeral, to which the suffix -in and the number of the atom at the triple bond are added, for example, ethyne, propyne, butyne-1;
- numbering of the main chain of atoms starts from the end of the molecule closest to the triple bond;
- for branched hydrocarbons, the name of the side branch comes first, followed by the name of the main chain of atoms with the suffix -in.
- the final part of the name is a number indicating the location in the triple bond molecule, for example, butyne-2.
Isomerism of alkynes. Dependence of properties on the structure
Ethine and propyne do not have triple bond position isomers, they appear starting with butyne. Carbon skeleton isomers are found in pentine and the homologues following it. With respect to the triple bond, there is no spatialisomerism of acetylenic hydrocarbons.
The first 4 homologues of ethine are gases that are poorly soluble in water. Acetylene hydrocarbons C5 – C15 - liquids. The solids are ethine homologues, starting with the hydrocarbon C17. The chemical nature of alkynes is significantly influenced by the triple bond. Hydrocarbons of this type are more active than ethylene ones, they attach various particles. This property is the basis for the widespread use of ethine in industry and technology. When burning acetylene, a large amount of heat is released, which is used in gas cutting and welding of metals.
