2.
Classification of Hydrocarbons
Nineteenth-century chemists classified hydrocarbons as either aliphatic or aromatic on the basis
of their sources and properties. Aliphatic (from Greek aleiphar, “fat”) described hydrocarbons
derived by chemical degradation of fats or oils. Aromatic hydrocarbons constituted a group of
related substances obtained by chemical degradation of certain pleasant-smelling plant extracts.
The terms aliphatic and aromatic are retained in modern terminology, but the compounds they
describe are distinguished on the basis of structure rather than origin [1].
Fig. 2.1 Classification of Hydrocarbons
2.1 Types of Hydrocarbons
Saturated Hydrocarbons: In these compounds, carbon-carbon atoms and carbon-
hydrogen atoms are held together by single bonds. These single-bonded compounds are
the simplest hydrocarbons. These types of hydrocarbons don’t have double or triple
bonds. In terms of hybridization, they have Sp3 hybridised carbon atoms with no Sp2 or
1
Sp hybridised carbon atoms. They are together called alkanes which have a general
formula of CnH2n+2. For example, CH4C3H6.
Unsaturated Hydrocarbons: These compounds consist of a single, double or triple bond
between carbon-carbon atoms. The double-bonded compounds are called alkenes, and the
triple-bonded compounds are called alkynes. The general formula for alkenes is CnH2n,
and for alkynes, the general formula is CnH2n-2.
Cycloalkanes: These hydrocarbons possess one or multiple carbon rings. The hydrogen
atom is attached to the carbon ring.
Aromatic Hydrocarbons: They are also called arenes. Arenes are compounds which
consist of at least one aromatic ring.
Aliphatic Hydrocarbons: They are straight chain structures having no rings in them.
Alicyclic Hydrocarbons: They are hydrocarbons having a ring structure in them. The
carbons atoms can be Sp, Sp2, or Sp3 hybridised.
2.1.1 Aliphatic Hydrocarbons
Aliphatic hydrocarbons are hydrocarbons based on
chains of C atoms. There are three types of aliphatic
hydrocarbons. Alkanes are aliphatic hydrocarbons with
only single covalent bonds. Alkenes are hydrocarbons
that contain at least one C–C double bond, and alkynes
are hydrocarbons that contain a C–C triple bond.
Occasionally, we find an aliphatic hydrocarbon with a
ring of C atoms; these hydrocarbons are called
cycloalkanes (or cycloalkenes or cycloalkynes). Because
alkanes have the maximum number of H atoms possible Fig. 2.2 Structure of representative
hydrocarbons
according to the rules of covalent bonds, alkanes are
also referred to as saturated hydrocarbons [2].
2
2.1.1.1 Alkanes
Alkanes, hydrocarbons in which all the bonds are single, have molecular formulas that satisfy the
general expression CnH2n + 2 (where n is an integer). Carbon is sp3 hybridized (three electron pairs
are involved in bonding, forming a tetrahedral complex), and each C—C and C—H bond is
a sigma (σ) bond. In order of increasing number of carbon atoms, methane (CH4), ethane (C2H6),
and propane (C3H8) are the first three members of the series.
Fig. 2.3 First three members of alkanes
2.1.1.2 Alkenes and Alkynes
Alkenes (also called olefins) and alkynes (also called acetylenes) belong to the class of
unsaturated aliphatic hydrocarbons. Alkenes are hydrocarbons that contain a carbon-carbon
double bond, whereas alkynes have a carbon-carbon triple bond. Alkenes are characterized by the
general molecular formula CnH2n, alkynes by CnH2n − 2 . Ethene (C2H4) is the
simplest alkene and ethyne (C2H2) the simplest alkyne [3].
Fig. 2.4 The simplest alkene and alkyne
3
Ethylene is a planar molecule with a carbon-carbon double bond length (1.34 angstroms) that is
significantly shorter than the corresponding single bond length (1.53 angstroms) in ethane.
Acetylene has a linear H―C≡C―H geometry, and its carbon-carbon bond distance (1.20
angstroms) is even shorter than that of ethylene.
2.1.2 Aromatic Hydrocarbons
Aromatic compounds, also known as "mono- and polycyclic aromatic hydrocarbons"[4], or
arenes, are organic compounds containing one or more aromatic rings [5]. The word "aromatic"
originates from the past grouping of molecules based on odor, before their general chemical
properties were understood [6]. The current definition of aromatic compounds does not have any
relation to their odor. Aromatic compounds are now defined as cyclic compounds satisfying
Hückel's Rule [7].
2.1.2.1 Arenes
These compounds are hydrocarbons that contain a benzene ring as a structural unit. In addition to
benzene, other examples include toluene and naphthalene.
Fig. 2.5 Ring structure of benzene, toluene,
and naphthalene
(Hydrogen atoms connected to the benzene ring are shown for completeness in the Fig. 2.5. The
more usual custom, which will be followed hereafter, omits them.)
4
References
[1] Chemistry (IUPAC), The International Union of Pure and Applied. "IUPAC -
(A00435.)". goldbook.iupac.org. Retrieved 2023-11-29.
[2] "8.12: Aromatic Compounds: Structure & Nomenclature". Chemistry LibreTexts.
2014-08-16. Retrieved 2023-11-29.
[3] "Aromatic Compounds - Definition, Example, Properties & Nomenclature with
Videos". BYJUS. Retrieved 2023-11-29.
[4] "Aromatic Compounds - Definition, Example, Properties & Nomenclature with
Videos". BYJUS. Retrieved 2023-11-29.
[5] "4.7: Chemical Properties of Aromatic Compounds". Chemistry LibreTexts. 2020-06-
21. Retrieved 2023-11-29.
[6] "22.11: Sources and Uses of Aromatic Hydrocarbons". Chemistry LibreTexts. 2014-
11-26. Retrieved 2023-11-29.
[7] "What does "aromatic" really mean?". Chemistry LibreTexts. 2013-10-02. Retrieved
2023-11-29.
