Aromatic Compounds and Lipid Chemistry Principles

Posted on Feb 12, 2026 in Chemistry

Benzene: Structure and Properties

Benzene is an organic, colorless chemical compound. It is a highly flammable liquid with a sweet smell and a high melting point. The chemical formula of benzene is C6H6; it contains 6 carbons and 6 hydrogens and is classified as a hydrocarbon. It has a boiling point of 80°C and is lighter than water. Natural sources of benzene include volcanoes and forest fires. It is also a natural part of crude oils. Benzene and all compounds that resemble benzene in their chemical behavior are known as Aromatic Compounds. Benzene is a highly toxic, carcinogenic chemical.

Structure of Benzene

The molecular formula of benzene (C6H6) shows a high degree of unsaturation. Sir August Kekulé was the first scientist to introduce the chemical structure of benzene as a 6-membered ring structure. He also proposed the presence of 3 alternate double bonds in the benzene ring. According to Kekulé, the 3 alternate double bonds in benzene change their position rapidly to satisfy the tetravalency of carbon.

Kekulé Structure of Benzene

Although the Kekulé structure satisfies the structural features of benzene and explains the equivalent nature of hydrogen, it fails to explain the unusual behavior of benzene. Since this structure was a significant approach by Sir Kekulé, it is still in use. Following are the characteristics not explained by Kekulé’s model:

  • Chemical Reaction: Benzene undergoes substitution reactions rather than addition reactions, unlike other unsaturated compounds.
  • C-C Bond Length: According to Kekulé, two kinds of bonds are present: single (1.54 Å) and double (1.34 Å). However, experimentally, the bond length between all carbon atoms is 1.39 Å.
  • Stability: Kekulé’s structure could not explain the stability of benzene towards oxidizing agents. While most unsaturated compounds undergo oxidation, benzene does not.

Evidences in the Structure of Benzene

Analytical and synthetic evidence supports the derivation of the benzene structure. From its elemental composition and molecular weight determination, it is found that benzene contains 6 carbons and 6 hydrogens.

Chemical Reactions of Benzene

Most commonly, benzene undergoes Electrophilic Aromatic Substitution Reactions. In these reactions, a hydrogen atom of the aromatic ring is replaced by an electrophile.

Types of Electrophilic Aromatic Substitution

  1. Halogenation: Benzene reacts with halogens (Br2, Cl2, I2) in the presence of a Lewis acid catalyst (AlCl3) to give Bromobenzene, Chlorobenzene, or Iodobenzene.
  2. Nitration: Benzene reacts with concentrated HNO3 in the presence of concentrated H2SO4 to form Nitrobenzene.
  3. Sulfonation: Benzene reacts with concentrated or fuming sulfuric acid to form Benzene Sulfonic Acid.
  4. Friedel-Crafts Alkylation: Named after Charles Friedel and James Crafts, benzene reacts with an alkyl group to form Alkyl Benzene.
  5. Friedel-Crafts Acylation: Benzene reacts with an acyl group to form aromatic ketones (Acetophenone) in the presence of AlCl3.

Effect of Substituents

Benzene contains 6 identical hydrogens. When 1 hydrogen is substituted, it becomes monosubstituted benzene. The nature of the substituent (X) decides the reactivity and the position of further substitutions.

Types of Substituents

  • Ring Activating Groups: These increase electron density and reactivity towards electrophilic substitution. They are Ortho-Para directing. Examples include -NH2, -OH (Strongly Activating), and -CH3 (Weakly Activating).
  • Ring Deactivating Groups: These decrease electron density and reactivity. They are Meta directing. Examples include -NO2, -CN, -SO3H (Strongly Deactivating), and Halogens (Weakly Deactivating).

Benzene Derivatives and Their Uses

  • DDT (Dichloro Diphenyl Trichloroethane): A toxic contact poison for insects that disorganizes their nervous system.
  • Saccharin: An artificial sweetening agent used in vitamins and medicines. Its soluble salt is Saccharin Sodium.
  • BHC (Benzene Hexachloride): Used as an insecticide in crops and pharmaceuticals.
  • Chloramine-T: Used as a disinfectant, for treating burns/wounds, and as an oral mouthwash.

Aromatic Character and Huckel’s Rule

Aromaticity is decided by Huckel’s Rule (1931). For a compound to be aromatic, it must be:

  • Cyclic
  • Conjugated (delocalized electrons)
  • Planar
  • Contain (4n + 2) π electrons (where n = 0, 1, 2…)

Phenols: Properties and Acidity

Phenols (Carbolic Acid) are aromatic compounds with a hydroxyl group (-OH) attached to the ring. The general formula is C6H5OH. They are white or colorless crystalline solids.

Types of Phenols

  • Monohydric: One -OH group.
  • Dihydric: Two -OH groups (e.g., Catechol, Resorcinol, Hydroquinone).
  • Trihydric: Three -OH groups.

Acidity of Phenols

Phenols are weak acids, more acidic than alcohols but less than carboxylic acids. They form a stable Phenoxide Ion after releasing H+. This ion is stabilized by resonance (delocalization of π bonds). Phenols are more acidic than alcohols because the phenoxide ion is more stable than the alkoxide ion due to resonance and the sp2 hybridization of the carbon atom.

Basicity of Aromatic Amines

Aromatic amines are basic due to the lone pair of electrons on the nitrogen atom (Lewis bases). However, they are less basic than aliphatic amines and ammonia because the lone pair is delocalized into the benzene ring via resonance, reducing its availability.

Aromatic Acids: Benzoic Acid

Aromatic acids contain a carboxylic group (-COOH) attached to the ring. Benzoic Acid is a colorless crystalline solid, non-polar, and soluble in hot water. It is used in medicines, food preservatives (as sodium salt), and cosmetics.

Chemistry of Fats and Oils

Fats and oils are triglycerides (triesters of fatty acids and glycerol). Fats are generally solids at room temperature (saturated), while oils are liquids (unsaturated).

Analytical Values of Fats and Oils

  • Acid Value: Milligrams of KOH required to neutralize free fatty acids in 1g of fat/oil. It measures rancidity.
  • Saponification Value: Milligrams of KOH required to saponify 1g of fat/oil. It indicates the average molecular weight of fatty acids.
  • Ester Value: The difference between Saponification Value and Acid Value.
  • Iodine Value: Grams of iodine absorbed by 100g of fat/oil. It measures the degree of unsaturation.
  • Acetyl Value: Indicates the number of free hydroxyl groups.
  • Reichert-Meissl (RM) Value: Measures soluble, volatile fatty acids. Useful for testing the purity of butter.

Polynuclear Hydrocarbons

Anthracene

Anthracene (C14H10) consists of 3 linear fused benzene rings. It is more reactive than benzene and undergoes resonance across 4 structures. It can be synthesized via Friedel-Crafts reactions or Haworth synthesis.

Phenanthrene

An isomer of anthracene with 3 fused rings in a non-linear arrangement. It is a colorless crystalline solid synthesized via Haworth or Pschorr synthesis.

Diphenyl Methane

Consists of methane where two hydrogens are replaced by phenyl groups. It is used in soaps, pesticides, and as an antihistamine agent.

Theories of Ring Stability

Baeyer’s Strain Theory

Proposed by Adolf von Baeyer (1885), it suggests that deviation from the ideal tetrahedral angle (109°28′) causes Angle Strain, leading to instability. Cyclopropane has the highest strain and is the most reactive.

Sachse-Mohr’s Theory

Also known as the Theory of Strainless Rings, it explains that higher cycloalkanes like cyclohexane are stable because they are not planar; they exist in puckered Chair and Boat forms to retain tetrahedral angles.

Coulson and Moffit’s Modification

Known as the Banana Bond Theory, it explains the stability of cyclopropane through bent bonds. These bonds are weaker than sigma bonds but stronger than pi bonds, explaining the reactivity of small rings.