The structural features of acetylene affect its properties, production and use. The symbol of the composition of the substance - C 2 H 2 - is its simplest and gross formula. Acetylene is formed by two carbon atoms, between which a triple bond occurs. Its presence is reflected in various 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 matter.
Alkines. The general formula. Acetylene
Hydrocarbons alkynes, or acetylenic, are acyclic, unsaturated. The chain of carbon atoms is not closed, it contains simple and multiple bonds. The composition of alkynes is reflected in the general formula C n H 2n - 2 . One or several triple bonds are present in the molecules of substances of this class. Acetylene compounds are unsaturated. This means that only one valency of carbon is realized due to hydrogen. The remaining three bonds are used in interaction with other carbon atoms.
The first - and the most famous representative of alkynes - acetylene, or ethine. The trivial name of the substance comes from the Latin word "acetum" - "vinegar" and the Greek - "hyle" - "tree". The founder 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 ordinary temperature and normal atmospheric pressure, acetylene is in a gaseous state. It is a colorless gas, odorless, sparingly soluble in water. Ethin is more easily soluble in ethanol and acetone.
Acetylene molecular formula
Etin is the simplest member of his homologous series, its composition and structure reflect the formulas:
- C 2 H 2 is a molecular record of the composition of ethine, which gives an idea that the substance is formed by two carbon atoms and the same number of hydrogen atoms. Using this formula, the molecular and molar masses of the compound can be calculated. Mr (C 2 H 2 ) = 26 a. E. m., M (C 2 H 2 ) = 26.04 g / mol.
- H: C ::: C: H is the electron point formula of acetylene. Similar images, called the “Lewis structure”, reflect the electronic structure of the molecule. When writing, you must follow the rules: the hydrogen atom tends to have the configuration of the helium valence shell when forming a chemical bond, other elements - an octet of external electrons. Each colon means a common for two atoms or a lone pair of electrons of the external energy level.
- H — C≡C — H is the 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 far back as the end of the 18th century, ball-rod models became widespread - for example, balls of different colors and sizes, representing carbon and hydrogen, which form acetylene. The structural formula of the molecule is presented in the form of rods symbolizing chemical bonds and their quantity at each atom.
The spherical rod model of acetylene reproduces valence angles of 180 °, but the internuclear distances in the molecule are reflected approximately. The voids between the balls do not create the idea of filling the space of atoms with electron density. The disadvantage is eliminated in the Drading models, which designate atomic nuclei not by balls, but by the points of attachment of the rods to each other. Modern volumetric models give a more vivid idea of atomic and molecular orbitals.
Acetylene Hybrid Atomic Orbitals
Carbon in the excited state contains three p-orbitals and one s with unpaired electrons. In the formation of methane (CH 4), they take part in creating 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 to equalize AO in shape and energy, the formation of new clouds. Hybrid orbitals give stronger bonds, the formula becomes more stable.
Carbon atoms in an acetylene molecule, unlike methane, undergo sp-hybridization. In shape and energy s- and p-electrons are mixed. Two sp-orbitals appear, lying at an angle of 180 °, directed on opposite sides of the nucleus.
Triple bond
In acetylene, hybrid electron carbon clouds participate in the creation of σ bonds with the same neighboring atoms and with hydrogen in C – H vapors. There remain two non-hybrid p-orbitals perpendicular to each other. In the ethine molecule, they participate in the formation of two π-bonds. Together with σ, a triple bond arises, which is reflected in the structural formula. Acetylene differs from ethane and ethylene in the distance between atoms. The triple bond is shorter than the double, but has a large supply of energy, is more durable. 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. The carbon and hydrogen atoms are located along one straight line, 3 σ- and 2 π-bonds arise between them. Free movement, rotation along the C – C axis is impossible, this is prevented by the presence of multiple bonds. Other features of triple bond:
- the number of pairs of electrons connecting two carbon atoms is 3;
- length - 0.120 nm;
- burst energy - 836 kJ / mol.
For comparison: in the molecules of ethane and ethylene, the length of the single and double chemical bonds is 1.54 and 1.34 nm, respectively, the C – C breaking 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. Propine CH 3 ≡ - Acetylene homolog. The formula of the third representative of alkynes is butin-1 - CH 3 CH 2 CH≡CH. Acetylene is the trivial name for ethine. The systematic nomenclature of alkynes obeys the IUPAC rules:
- in linear molecules, the name of the main chain arises from the Greek numeral, to which is added the suffix -in and the number of the atom with a triple bond, for example, ethine, propine, butyn-1;
- the numbering of the main chain of atoms begins at the end of the molecule closest to the triple bond;
- for branched hydrocarbons, the name of the side branch is first, followed by the name of the main chain of atoms with the suffix -in.
- the final part of the name is a figure indicating the location in the molecule of a triple bond, for example, butyn-2.
Isomerism of alkynes. The dependence of properties on the structure
Ethine and propine do not have isomers of the triple bond position; they appear starting from butine. Pentines and subsequent homologues have isomers of the carbon skeleton. With respect to the triple bond, the spatial isomerism of acetylene hydrocarbons does not appear.
The first 4 homologues of ethine are gases that are poorly soluble in water. Acetylene hydrocarbons C 5 - C 15 - liquids. Solids are ethine homologues starting with C 17 hydrocarbon. The chemical nature of alkynes is significantly affected by the triple bond. Hydrocarbons of this type are more active than ethylene, attach various particles. The widespread use of ethine in industry and technology is based on this property. When burning acetylene, a large amount of heat is released, which is used in gas cutting and welding of metals.