Difference Between Aliphatic and Aromatic Hydrocarbons

Main Difference – Aliphatic vs Aromatic Hydrocarbons

Hydrocarbons are compounds that are composed only of carbon atoms and hydrogen atoms bonded to each other via covalent bonds. These compounds can be categorized into two groups depending on the arrangement of atoms. They are aliphatic hydrocarbons and aromatic hydrocarbons. Aliphatic hydrocarbons are organic compounds composed of carbon and hydrogen atoms, arranged in straight chains, branched structures or non-aromatic ring structures. Aromatic hydrocarbons are compounds composed of carbon and hydrogen atoms in ring structures with delocalized pi electrons. The main difference between aliphatic and aromatic hydrocarbons is that aliphatic hydrocarbons have a high carbon-to-hydrogen ratio whereas aromatic hydrocarbons have a low carbon-to-hydrogen ratio.

Key Areas Covered

1. What are Aliphatic Hydrocarbons
      – Definition, Different Types, General Properties
2. What are Aromatic Hydrocarbons
      – Definition, General Properties
3. What is the Difference Between Aliphatic and Aromatic Hydrocarbons
      – Comparison of Key Differences

Key Terms: Aliphatic, Aromatic, Covalent Bonds, Delocalized Pi Electrons, Hydrocarbons

What are Aliphatic Hydrocarbons

Aliphatic hydrocarbons are organic compounds composed of carbon and hydrogen atoms arranged in straight chains, branched or non-aromatic ring structures. Carbon and hydrogen atoms are bonded to each other via covalent bonds. Aliphatic hydrocarbons can be found in three types as alkanes, alkenes, and alkynes.

Aliphatic hydrocarbons can be subdivided into two groups as saturated aliphatic hydrocarbons and unsaturated aliphatic hydrocarbons depending on the presence or the absence of double bonds. Saturated hydrocarbons are only composed of single bonds. Therefore, they only have only sigma bonds. For example, alkanes are saturated hydrocarbons. Unsaturated hydrocarbons are composed of single bonds and double bonds; both sigma bonds and pi bonds are present in these molecules. Some molecules contain triple bonds as well. Alkenes and alkynes are unsaturated hydrocarbons.

Figure 1: Hexane is an Aliphatic Hydrocarbon

Most aliphatic hydrocarbons are flammable. These compounds can be found in crude oil and as natural gases. Cyclic compounds can be also considered as aliphatic hydrocarbons. This is because those cyclic structures are non-aromatic (no delocalized pi electrons).

What are Aromatic Hydrocarbons

Aromatic compounds are organic compounds composed of carbon and hydrogen atoms arranged in ring structures with delocalized pi electrons. Aromatic hydrocarbons are named as such due to their pleasant aroma. Aromatic hydrocarbons are essentially cyclic structures. These are also planar structures.

Aromatic compounds are highly stable due to the resonance effect. This means, aromatic compounds are often represented as resonance structures containing single and double bonds, but the actual structure has delocalized electrons shared between all the atoms of the ring.

Generally, aromatic compounds are nonpolar. Therefore, they are immiscible with water. The carbon-to-hydrogen ratio is less in aromatic compounds. Most aromatic compounds undergo electrophilic substitution reactions. Due to the presence of delocalized pi electrons, aromatic ring is rich with electrons. Therefore, electrophiles can attack this ring to share electrons.

Figure 2: Picene is an Aromatic Hydrocarbon

Most of the times, aromatic compounds are obtained from petroleum oil. Polyaromatic hydrocarbons (PAH) are considered as environmental pollutants and carcinogens.

Difference Between Aliphatic and Aromatic Hydrocarbons

Definition

Aliphatic Hydrocarbons: Aliphatic hydrocarbons are organic compounds composed of carbon and hydrogen atoms, arranged in straight chains, branched or non-aromatic ring structures.

Aromatic Hydrocarbons: Aromatic hydrocarbons are organic compounds composed of carbon and hydrogen atoms, arranged in ring structures with delocalized pi electrons.

Odor

Aliphatic Hydrocarbons: Aliphatic hydrocarbons do not have a pleasant odor.

Aromatic Hydrocarbons: Aromatic hydrocarbons have a pleasant odor.

Carbon-to-Hydrogen Ratio

Aliphatic Hydrocarbons: Carbon-to-hydrogen ratio of aliphatic hydrocarbons is high.

Aromatic Hydrocarbons: Carbon-to-hydrogen ratio of aromatic hydrocarbons is low.

Burning

Aliphatic Hydrocarbons: Aliphatic hydrocarbons burn with non-sooty flames.

Aromatic Hydrocarbons: Aromatic hydrocarbons burn with sooty flames.

Unsaturation

Aliphatic Hydrocarbons: Some aliphatic hydrocarbons are saturated whereas some are unsaturated.

Aromatic Hydrocarbons: All the aromatic hydrocarbons are unsaturated.

Delocalized Pi Electrons

Aliphatic Hydrocarbons: There are no delocalized pi electrons in aliphatic hydrocarbons.

Aromatic Hydrocarbons: There are delocalized pi electrons in aromatic hydrocarbons.

Conclusion

Aliphatic and aromatic hydrocarbons are organic compounds that are made out of only carbon and hydrogen atoms. These compounds can be found chiefly in crude oil and natural gases. The main difference between aliphatic and aromatic hydrocarbons is that aliphatic hydrocarbons have a high carbon-to-hydrogen ratio whereas aromatic hydrocarbons have a less carbon-to-hydrogen ratio.

References:

1. Helmenstine,  Anne Marie. “What Is an Aliphatic Hydrocarbon? Review Your Chemistry Concepts.” ThoughtCo, Available here.
2. “Aromatic hydrocarbon.” Wikipedia, Wikimedia Foundation, 25 Oct. 2017, Available here.
3. Carey, Francis A. “Hydrocarbon.” Encyclopædia Britannica, Encyclopædia Britannica, inc., 6 July 2017, Available here.

Image Courtesy:

1. “Hexane-3D-balls” By Ben Mills – Own work (Public Domain) via Commons Wikimedia
2. “Picene-3D-balls” By Jynto and Ben Mills – Derived from File:Benzene-aromatic-3D-balls.png (Public Domain) via Commons Wikimedia