In organic chemistry most substances consist mainly of carbon and hydrogen. These substances are called hydrocarbons and you encounter them all the time in everyday life. For example, in a lighter, hydrocarbons are responsible for creating a flame when it is ignited.
Definition of hydrocarbons
hydrocarbons are chemical compounds consisting exclusively of carbon and hydrogen.
Carbon atoms combine with hydrogen atoms to form hydrocarbons. The carbon atom uses its four outer electrons to form atomic bonds with the hydrogen atoms. A region is created in which the electrons overlap – a molecular orbital.
The regions around the atomic nucleus are called orbitals. This spatial structure indicates the probability of an electron being present. There are two electrons in each orbital.
Hydrocarbons are of particular interest in chemistry because they can contain carbon chains, carbon rings, or mixtures of both.
The hydrocarbons include some subgroups such as the alkanes, alkenes, alkynes and aromatics. Hydrocarbons are of great importance due to their use as fossil fuels and in organic synthesis.
There are different ways to describe hydrocarbons with formulas:
- Molecular formula: It reflects the composition of the molecule. The number of molecules of the same type is given.
- Structural formulas: They reflect the spatial arrangement of the atoms in a molecule.
- IUPAC nomenclature(International Union of Pure and Applied Chemistry) : she has the purpose of assigning chemical compounds to a clear structure using a specific name.
The classification of hydrocarbons
The hydrocarbons can be divided into different groups. A distinction is made between the aromatic and the aliphatic hydrocarbons. In turn, aliphatic hydrocarbons are divided into unsaturated and saturated compounds.
In addition, all chemical compounds with the same functional group are summarized in substance groups. These include the alkanes and alkenes, but also alcohols and aldehydes. An example of the functional groups is the amino group or the carboxyl group that makes up amino acids.
Alkanes, alkenes and alkynes
the saturated hydrocarbons contain only CC single bonds. These chain-like compounds are called alkanes designated. Each carbon atom is linked to four other atoms. The simplest alkane is methane. If additional CH2 groups are inserted into the CH bonds of methane, long, chain-like molecules are formed. The general molecular formula of these molecules is CnH2n+2, where n is a number greater than or equal to 1. This is how one forms homologous seriesthe simplest representatives of which are ethane, propane and butane.
A homologous series is a series of molecules that differ by consecutive members. In the homologous series of alkanes, the various molecules differ in the number of CH2 groups. You can find out more about the subject of homologous series in the corresponding explanation.
The homologous series of unbranched alkanes can be continued by repeatedly inserting CH2 molecules. as unbranched are called alkanes in which the carbon atoms are always bonded to only two other carbons. So these molecules form a simple chain with no side branches. at branched For alkanes that have side chains, naming is a bit more complex. The IUPAC nomenclature is used for this.
The IUPAC nomenclature gives chemical substances a unique name that can only be assigned to this single chemical structure. This is to prevent different substances being meant due to linguistic differences. You can find out more about the steps you need to follow for naming in the IUPAC nomenclature explanation.
The longer the chain length, the higher the boiling point of alkanes. The first four are gases, then liquids, becoming more viscous as chain length and molecular weight increase. From hexadecane (C16H34), they are in the solid state at room temperature.
Both unsaturated hydrocarbons distinguish between alkenes and alkynes. The alkenes contain chemical C=C double bonds. The simplest substance in this group of substances is ethene (C2H4), which is also known as ethylene. The general empirical formula for alkenes with only one double bond is for the homologous series CnH2n. Chemical compounds with at least two C=C double bonds are called polyenes. This includes, for example, 1,3-butadiene.
Both alkynes are hydrocarbons that have one or more CC triple bonds. This includes, for example, ethyne, also known as acetylene, with the chemical formula C2H2.
The general molecular formula of alkynes for the homologous series is CnH2n-2.
aromatic hydrocarbons
The aromatic hydrocarbons have aromaticity and usually have C6 rings, so-called arena. The aromatic hydrocarbons include, for example, benzene with the chemical formula C6H6.
There are four criteria for recognizing whether a substance is aromatic:
- The molecule is planar and thus arranged «flat».
- The double bonds are conjugated. This means that single and double bonds always alternate.
- The Hückel rule must be fulfilled. This rule states that the molecule has 4n+2 π electrons. where n ≥ 1.
- The molecule is ring-shaped and there are mesomeric boundary structures.
The properties of hydrocarbons
are hydrocarbons hydrophobic and lipophilic. That means many non-polar Hydrocarbons cannot be dissolved in water, but are soluble in many organic solvents. When carbon occurs in gaseous form, it is highly flammable and burns with a very hot flame. A lot of energy is released in the process. Liquid hydrocarbons, which have a low boiling point, also vaporize easily and can therefore quickly start a fire.
When a fabric is hydrophobic, it means it is «water repellent». This means that water molecules are repelled. Lipophilic substances are «fat-loving» and easily soluble in oils and fats.
Whether a substance is polar or non-polar has something to do with electronegativity. This is a measure of the strength with which bonding electrons are attracted to an element. You can use the difference in electronegativity between two elements that are bonded together to determine whether the bond is polar or non-polar.
When two of the same elements are bonded together, the electronegativity difference is 0 and the bond is polar. In the case of dissimilar elements, such as the CH bond in hydrocarbons, the difference in electronegativity is very small, which is why the bond is nonpolar.
When hydrocarbon burns completely, water and carbon dioxide are formed. In the event of incomplete combustion, carbon monoxide or carbon in the form of soot can also be produced.
Butane in particular can burn. This reaction in the presence of oxygen produces water and carbon dioxide.
The reactivity of the alkanes is influenced by the chain length. A long chain tends to make the alkanes less reactive, a short chain tends to make them more reactive. So if a hydrocarbon burns with a sooty flame, this is a possible sign of a high carbon content in the chemical compound and thus a long chain length.
The great variety of hydrocarbons is due to their high tendency to form CC chains. This so-called concatenation or categorization is in the high binding energy the covalent CC bonds justified.
Binding energy is the energy required to break a covalent bond. Covalent bonds are bonds between nonmetallic elements where the electronegativity is not too far apart.
Typical reactions in chemistry
Hydrocarbons can undergo many different reactions in which atomic bonds are broken and rebuilt.
Next to the redox reaction alkanes also go when burned substitution reactions a. Here, hydrogen atoms are exchanged for other atoms and groups of atoms. Mainly these are halogens.
A simple example is the reaction of ethane with chlorine. In this substitution reaction, exposure to light leads to a halogenation. A hydrogen atom in the ethane is replaced by a chlorine atom. Chloroethane and hydrogen chloride are formed.
Alkenes and alkynes, on the other hand, are very reactive. They react with many substances that attach themselves to the CC multiple bond. This is the so-called addition reaction. In this reaction, the double bond is broken and a single bond is formed. There is no by-product here.
In the electrophilic addition For example, halogens such as chlorine, bromine, iodine or hydrogen halides can be added to the double bond of alkenes. For example, propene can react with hydrogen chloride to form 2-chloropropane.
Figure 13: Electrophilic addition of hydrogen chloride to propene
Occurrence of hydrocarbons
In nature, you can find large amounts of hydrocarbons primarily in oil and natural gas. These two fuels are also called fossil hydrocarbons.
Hydrocarbons also occur in the chemical industry. There they are primarily used for plastics. For example, the plastic polypropylene consists of several propene molecules arranged in a row. Many plants also contain hydrocarbons such as terpenes, carotenoids and rubber.
Some microorganisms also produce simple hydrocarbons, especially methane, as metabolic products. You can also find methane and ethane in space. These hydrocarbons are found on comets, planets, moons and in interstellar matter.
Hydrocarbons are considered harmful to the environment. For example, methane gas plays a major role in the greenhouse effect and thus promotes climate change.
use of hydrocarbons
Hydrocarbons are used in many areas. Alkanes are mostly used in mixtures as fossil fuels. These include biogas, liquid gas, petrol, diesel fuel, heating oil, kerosene and petroleum. Alkanes such as n-butane, isopentane, hexane and cycloalkane are also found in petrol, which is used in engines.
In chemical synthesis processes, hydrocarbons are used in industry. Alkenes such as cyclohexene and ethene as well as alkynes such as ethyne and polyenes such as 1,3-butadiene, isoprene and cyclopentadiene play a role here. Aromatic hydrocarbons such as benzene, toluene, xylene and styrene are also of great importance in industry.
The polymerisation products of hydrocarbons such as polystyrene, polyethylene,…