Aromatic compounds are compounds that belong to a large class of unsaturated chemical compounds that are defined by the presence of one or more planar rings of atoms linked together by covalent bonds.
- The stability of aromatic compounds is referred to as the aromaticity of the compounds as a result of the delocalization of the shared electrons.
- The degree of aromaticity in different compounds might be different as it depends on the bonding arrangements of the atoms involved in the molecule.
- All aromatic compounds have a delocalized pi (π) cloud, which spreads uniformly on the atoms of the entire aromatic ring. The π cloud must contain (4n+ 2) π electrons as all aromatic compounds follow the Huckel’s ring where the n is an integer.
- The sharing of electrons between different atoms results in a resonance effect within the molecule. The resonance effect is responsible for the stability of the molecules.
- The structure of these compounds is often represented by resonance structures consisting of single and double bonds, but the actual structure has delocalized electrons shared between all the atoms in the molecule.
- Even though the term aromatic is used for these compounds, not all aromatic compounds have a sweet odor, and not all sweet-smelling compounds are aromatic.
- Aromatic hydrocarbons are the most important group of aromatic compounds. These are organic compounds that are composed of carbon and hydrogen atoms.
- The arrangement of carbon atoms in aromatic hydrocarbons is called a benzene ring, which is the simplest aromatic compound.
- Generally, aromatic compounds are nonpolar due to the delocalization of electrons between the atoms. The delocalized electrons result in an electron-rich aromatic ring which can then be attacked by electrophiles to share the electrons.
- Aromatic compounds are highly unsaturated compounds and are resistant to addition reactions.
- Most of these compounds are cyclic compounds with a ring and exist in a flat planar structure.
Aliphatic compounds are a group of hydrocarbons where the atoms are linked by single, double, or triple bonds to form nonaromatic structures.
- Aliphatic compounds are characterized by the presence of an open-chain structure that can be saturated or unsaturated.
- The most important group of aliphatic compounds includes aliphatic hydrocarbons composed of hydrogen and carbon atoms. These compounds can also contain other elements like oxygen, nitrogen, chlorine, and sulfur.
- Aliphatic hydrocarbons are of two types based on the number of bonds present between two carbon atoms. Saturated hydrocarbons are open-chain hydrocarbons with a single carbon-carbon bond.
- Unsaturated hydrocarbons, in turn, are open-chain hydrocarbons with double or triple bonds between the carbon atoms. Unsaturated hydrocarbons are termed alkenes (double bonds) or alkynes (triple bonds).
- Saturated hydrocarbons are often more reactive and less stable than unsaturated ones.
- Aliphatic compounds can, however, be cyclic, but they do not follow Huckel’s rule and do not demonstrate aromaticity.
- Similarly, aliphatic compounds usually do not demonstrate resonance like aromatic compounds due to the absence of cyclic aromatic ring structure.
- Most of the known aliphatic compounds are flammable due to high carbon content, which also allows the use of these compounds as fuels.
- The bonds in aliphatic compounds are weakly polar due to the small difference in electronegativities between carbon and hydrogen.
- Aliphatic compounds are involved in additional reactions due to the localization of electrons in particular atoms.
- The simplest aliphatic compound is methane (CH4) which results in the formation of all other aliphatic compounds via polymerization or substitution.
15 Major Differences (Aromatic Compounds vs Aliphatic Compounds)
|Characteristics||Aromatic Compounds||Aliphatic Compounds|
|Definition||Aromatic compounds are compounds that belong to a large class of unsaturated chemical compounds that are defined by the presence of one or more planar rings of atoms linked together by covalent bonds.||Aliphatic compounds are a group of hydrocarbons where the atoms are linked by single, double, or triple bonds to form nonaromatic structures.|
|Benzene ring||A benzene ring is present in aromatic compounds.||A benzene ring is absent in aliphatic compounds.|
|Reactivity||Aromatic compounds are less reactive.||Aliphatic compounds are more reactive.|
|Stability||Aromatic compounds are more stable due to the presence of resonance.||Aliphatic compounds are less stable due to the absence of such resonance.|
|Delocalization of electrons||Shared electrons between atoms in an aromatic compound are delocalized and remain equally shared among all atoms.||Shared electrons between atoms in an aliphatic compound remain localized with distinct atoms and are not shared equally.|
|Polarity||Aromatic compounds are nonpolar.||Aliphatic compounds are weakly polar in nature.|
|Structure||Aromatic compounds are always cyclic.||Aliphatic compounds can be linear as well as cyclic.|
|Resonance||Aromatic compounds demonstrate resonance and, thus, can exist in a particular resonance structure.||Aliphatic compounds do not demonstrate resonance.|
|Saturation||Aromatic compounds are always unsaturated as the compounds often have double or triple bonds.||Aliphatic compounds can either be saturated or unsaturated, depending on the bonding of the compound.|
|Conjugation||Aromatic compounds are conjugated due to the presence of alternating double bonds.||Most of the aliphatic compounds are not conjugated.|
|Electrophilic||Aromatic compounds are not electrophilic due to the electron-rich aromatic ring.||Aliphatic compounds are electrophilic in nature.|
|Physical properties||Aromatic compounds often have a pleasant odor.||Aliphatic compounds do not have such odor.|
|Aromatic compounds are immiscible in water.||Many aliphatic compounds are miscible in water.|
|Carbon to hydrogen ratio||Aromatic hydrocarbons have a low carbon to hydrogen ratio.||Aliphatic compounds have a high carbon to hydrogen ratio.|
|Sooty flames||Aromatic compounds burn with sooty flames due to the higher carbon percentage.||Aliphatic compounds burn with non-sooty flames due to a lower carbon percentage.|
|Examples||Examples of aromatic compounds include benzene, toluene, naphthalene, anthracene, etc.||Examples of aliphatic compounds include propane, methane, ethanol, acetaldehyde, etc.|
Examples of aromatic compounds
- Benzene is the simplest aromatic hydrocarbon that was discovered by Faraday in 1823 but was first synthesized by Berthelot in 1870.
- It is a six-carbon compound with alternating double bonds between the carbon atoms. It exists in a ring structure, which can be represented by different resonance structures. The molecular formula of benzene is C6H6.
- Benzene has a cyclic, planar, and hexagonal structure where the resonance structure is the most convincing structure of benzene.
- Even though it is an unsaturated compound, it behaves like a saturated compound in that it doesn’t decolorize bromine solution and Baeyer’s reagent.
- This property of benzene is due to the delocalization of π electrons throughout the ring and the exchange of double bonds between the carbon atoms.
- Benzene and its derivatives have important industrial applications and can be synthesized or obtained from petroleum products in nature.
Examples of aliphatic compounds
- Methane is the simplest aliphatic hydrocarbon that exists in the atmosphere in the gaseous form and is a by-product of different human activities.
- It is a one-carbon compound where a carbon atom is bonded singly-bonded to four hydrogen atoms. The molecular formula of methane is CH4.
- Methane is slightly soluble in water as it is weakly polar but is lighter than air with a specific gravity of 0.554.
- Methane is a highly flammable gas that burns with a pale, slightly luminous flame. It is a stable gas on its own, but it can be explosive when mixed with air at 5-14% by volume.
- Methane also has an important role as fossil fuel as it is produced as biogas for cooking purposes. It is a member of greenhouse gas and is known to be one of the prime gases involved in global warming.
References and Sources
- National Center for Biotechnology Information. “PubChem Compound Summary for CID 297, Methane” PubChem, https://pubchem.ncbi.nlm.nih.gov/compound/Methane. Accessed 21 February, 2021.
- National Center for Biotechnology Information. “PubChem Compound Summary for CID 297, Benzene” PubChem, https://pubchem.ncbi.nlm.nih.gov/compound/Benzene. Accessed 21 February, 2021.
- Gautam AD, Pant M and Adhikari NR (2017). Comprehensive Chemistry Part 2. Heritage Publishers and Distributors. Kathmandu, Nepal.
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