Key Difference – Aliphatic vs Aromatic Hydrocarbons
Let us first briefly see what hydrocarbons are discussing the difference between aliphatic and aromatic hydrocarbons. Hydrocarbons are the organic compounds containing Carbon and Hydrogen atoms in their structure. The key difference between aliphatic and aromatic hydrocarbons is, aliphatic hydrocarbons do not contain a conjugated system of bond whereas aromatic hydrocarbons contain a conjugated bond system. However, both of these molecules are considered as organic compounds.
What are Aliphatic Hydrocarbons?
Aliphatic hydrocarbons are the organic molecules containing Carbon (C) and Hydrogen (H) atoms in their structure; in straight chains, branched chains or non-aromatic rings. Aliphatic hydrocarbons can be categorized into three main groups; alkanes, alkenes and alkynes.
What are Aromatic Hydrocarbons?
Aromatic hydrocarbons are sometimes known as “arenes” or “aryl hydrocarbons”. Most aromatic hydrocarbons contain a benzene ring in their structure; but there are non-benzene aromatic hydrocarbons called heteroarenes, which follow the “Huckle’s rule” (Cyclic rings which follow the Huckle’s rule have 4n+2 number of π-electrons; where n=0,1,2,3,4,5,6). Some aromatic hydrocarbons have more than one ring; they are called polycyclic aromatic hydrocarbons.
What is the difference between Aliphatic and Aromatic Hydrocarbons?
Structure of Aliphatic and Aromatic Hydrocarbons
Aliphatic Hydrocarbons: They have straight chains, branched chains or non-aromatic rings in their structure. This group has both saturated and unsaturated hydrocarbons. Alkanes are saturated hydrocarbons, alkenes and alkynes are unsaturated hydrocarbons.
Aromatic Hydrocarbons: Aromatic hydrocarbons have aromatic ring system in their structure. They are all unsaturated hydrocarbons, but relatively stable due to the conjugated bond system.
Categories of Aliphatic and Aromatic Hydrocarbons
There are three main groups in aliphatic hydrocarbons; alkanes, alkenes and alkynes. They are also known as allyl hydrocarbons.
Alkanes: In alkanes, Carbon and Hydrogen atoms are bonded together by single bonds. They do not have multiple bonds. Alkanes form ring structures, they are called cycloalkanes.
Alkenes: This group contains both single and double bonds between Carbon atoms. Hydrogen and Carbon atoms always form single bonds.
Alkynes: Alkynes have triple bonds between Carbon atoms in addition to single bonds.
Most of the aromatic hydrocarbons contain at least one benzene ring in their structure. But there are few non-benzene aromatic hydrocarbons, they are called “heteroarenes”. Aromatic hydrocarbons are called “aryl” hydrocarbons.
Biphenyl (An aromatic hydrocarbon with two benzene rings)
Bonding Pattern of Aliphatic and Aromatic Hydrocarbons
In aliphatic hydrocarbons; single, double or triple bonds can exist anywhere in the molecule. Sometimes, there can be several structures for one molecular formula by changing the position of the multiple bond(s). These molecules have a localized electron system.
In aromatic hydrocarbons, they have an alternative single and double bond system to form a conjugated bond system to delocalize some electrons. (Delocalized electrons can move from one bond to another).
Reactions of Aliphatic and Aromatic Hydrocarbons
Saturated hydrocarbons undergo substitution reactions; unsaturated hydrocarbons attain the stability by addition reaction. But, some reactions happen under controlled conditions without breaking multiple bonds.
Aromatic hydrocarbons are unsaturated, but have a stable conjugated electron system, so that they are more liable to substitution reactions rather than addition reactions.Image Courtesy: “Polycyclic Aromatic Hydrocarbons” by Inductiveload – Own work by uploader, Accelrys DS Visualizer. (Public Domain) via Commons