Aromatic Compounds, Diazonium Chemistry and Lipid Analysis
Chemistry Notes: Aromatic Compounds and Lipid Analysis
Resorcinol: Structure and Uses
Write the structure and uses of Resorcinol. Resorcinol has the chemical formula C6H4(OH)2. It is a dihydroxybenzene with hydroxyl groups at positions 1 and 3 (1,3-dihydroxybenzene). It is used in the production of resins, dyes, and as an antiseptic in skin ointments. It also serves as an intermediate in organic synthesis.
Hückel’s Rule of Aromaticity
Write Hückel’s rule of aromaticity with suitable example. Hückel’s rule states that a molecule is aromatic if it contains (4n+2) π electrons, where n is a non-negative integer. Benzene is a classic example with 6 π electrons (n = 1), which makes it aromatic. The compound must also be planar and cyclic with a conjugated π system.
Diphenylmethane: Two Preparation Methods
Write any two methods for preparation of Diphenylmethane. Diphenylmethane can be synthesized using:
- Reaction of benzyl chloride with benzene in the presence of AlCl3 (Friedel–Crafts alkylation).
- Grignard reagent method, where phenylmagnesium bromide reacts with benzaldehyde followed by acid workup.
Coulson and Moffitt Modification
Explain Coulson and Moffitt’s modification. Coulson and Moffitt modified valence bond theory by introducing the concept of partial ionic character in covalent bonds. They proposed that resonance hybrids are more stable than individual resonance structures. This helped explain molecular stability and bond lengths in aromatic compounds.
Saponification Value and Its Significance
Define saponification value. Give its significance. Saponification value is the amount of potassium hydroxide (KOH) required to saponify one gram of fat or oil. It is expressed in mg KOH/g. It indicates the average molecular weight of fatty acids in fats/oils: a high value indicates shorter-chain fatty acids.
Naphthalene: Structure and Uses
Write the structure and medicinal uses of Naphthalene. Naphthalene is a bicyclic aromatic hydrocarbon with formula C10H8. It is used as an insect repellent, particularly against moths. Medicinally, it has antiseptic and antiparasitic properties. It is also used in the synthesis of dyes and plastics.
Synthetic Uses of Aryl Diazonium Salts
Write synthetic uses of aryl diazonium salts. Aryl diazonium salts are used in the Sandmeyer reaction to form aryl halides, in azo coupling reactions to synthesize dyes, and in replacement reactions to introduce –OH, –CN, or –NO2 groups. They are key intermediates in aromatic substitution reactions.
Limitations of Friedel–Crafts Alkylation
Discuss limitations of Friedel–Crafts alkylation. Friedel–Crafts alkylation suffers from limitations like polyalkylation, carbocation rearrangement, and deactivation of the aromatic ring by electron-withdrawing groups. It is also not suitable for aromatic compounds with strong deactivating groups.
BHC (Hexachlorocyclohexane): Structure and Uses
Draw structure and write uses of BHC. BHC (benzene hexachloride) refers to isomeric hexachlorocyclohexanes; the common insecticidal form is lindane (γ-hexachlorocyclohexane). It has six chlorine atoms on a cyclohexane ring. It is used as an agricultural insecticide and has been used in treatment of lice and scabies.
Halogen Directing Effects and Deactivation
Give reason: Though halogens are o,p-directors, they are ring deactivators. Halogens are ortho/para directors because their lone pairs can donate electron density into the aromatic ring by resonance (+R). However, their high electronegativity causes an inductive electron-withdrawing effect (–I), which overall reduces the reactivity of the ring toward electrophilic substitution, making them deactivators.
Reactions of Cyclopropane
Write any two reactions of cyclopropane.
- Cyclopropane undergoes ring-opening reactions with halogens such as Br2, giving haloalkyl products.
- It reacts with hydrogen in the presence of catalysts like Ni to form propane (hydrogenation). Due to ring strain, it is comparatively highly reactive.
Acid Value and Its Significance
Define Acid value and give its significance. Acid value is the number of mg of KOH required to neutralize the free fatty acids in 1 gram of fat or oil. It indicates rancidity: a higher acid value shows deterioration and the presence of free fatty acids in oils.
DDT: Structure and Uses
Structure and uses of DDT. DDT (dichloro-diphenyl-trichloroethane) has two benzene rings connected by a trichloromethyl-bearing carbon. It has been widely used as an insecticide. However, due to its persistence and toxicity, its use is now restricted or banned in many countries.
Qualitative Test for Phenol
Write a qualitative test for phenol. Liebermann’s test is used for phenol detection. Phenol is treated with sodium nitrite and concentrated sulfuric acid, forming a deep blue or green color upon dilution and addition of alkali.
Anthracene: Structure and Medicinal Uses
Structure and medicinal uses of Anthracene. Anthracene is a tricyclic aromatic hydrocarbon (three fused benzene rings). It is used in the production of dyes such as alizarin. Medicinally, some anthracene derivatives have been used in topical treatments for skin diseases like psoriasis due to cytotoxic properties.
Hückel’s Rule for Naphthalene and Anthracene
What is Hückel’s rule for naphthalene and Anthracene? Naphthalene has 10 π electrons and anthracene has 14 π electrons; both satisfy Hückel’s rule (4n+2) and are aromatic. They exhibit electrophilic substitution reactions similar to benzene, with positions of greater reactivity determined by resonance and intermediate stability.
Aromaticity and Why Benzene Is Aromatic
What is Aromaticity? Why is benzene aromatic? Aromaticity is a property of cyclic, planar compounds with delocalized π electrons that obey Hückel’s rule (4n+2 π electrons). Benzene is aromatic because it has 6 π electrons, is planar and cyclic, and has delocalized electrons across the ring.
Mechanism of Diazotization Reaction
Write the mechanism for diazotization reaction. Diazotization involves converting primary aromatic amines to diazonium salts using sodium nitrite and hydrochloric acid at 0–5 °C. The mechanism proceeds via formation of nitrous acid (HNO2), generation of nitrosonium ion (NO+), and reaction with the amine to form the diazonium salt.
Basicity of Amines
Explain the basicity of amines. Amines act as bases due to the lone pair of electrons on nitrogen which can accept a proton. Aromatic amines are less basic than aliphatic ones because the lone pair on nitrogen is partially delocalized into the aromatic ring, reducing its availability for protonation.
Rancidity and Its Types
Define rancidity along with its types.
Rancidity refers to the spoilage of oils and fats resulting in unpleasant smell and taste. Types include:
- Hydrolytic rancidity — breakdown of triglycerides to free fatty acids and glycerol.
- Oxidative rancidity — reaction with oxygen forming hydroperoxides and secondary oxidation products.
Rancidity can be prevented using antioxidants or by storing oils in airtight, dark containers.
Reichert–Meissl (RM) Value and Significance
Define Reichert Meissl Value and give its significance. The Reichert–Meissl (RM) value measures the amount of volatile water-soluble fatty acids in fats and oils. It is significant in distinguishing butter from adulterated products, as genuine butter has a high RM value due to butyric acid content.
Difference Between Fats and Oils
Write difference between fats & oils. Fats are solid at room temperature and are typically rich in saturated fatty acids. Oils are liquid at room temperature and are typically rich in unsaturated fatty acids. Fats usually come from animal sources, whereas oils are plant-derived. Both are triglycerides.
Analytical Constants of Fats and Oils
Enlist the analytical constants of fats and oils. Analytical constants include:
- Acid value
- Saponification value
- Iodine value
- Reichert–Meissl (RM) value
- Polenske value
These constants help determine the quality, stability, and type of fats and oils and assist in identifying adulteration.
Diphenylmethane Synthesis via Grignard Reagent
Write the reaction of synthesis of Diphenylmethane from Grignard reagent. Phenylmagnesium bromide reacts with benzaldehyde followed by hydrolysis to form diphenylmethanol (diphenylcarbinol). Upon acid-catalyzed dehydration or subsequent reduction, diphenylmethane is obtained.