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____________________________________________________________________________________________________ Subject Chemistry Paper No and Title 14: Organic Chemistry- IV (Advanced Organic Synthesis, supramolecular chemistry and carbocyclic rings) Module No and Title 30: Azulenes Module Tag CHE_P14_M30 CHEMISTRY Paper no. 14: Organic Chemistry- IV (Advanced Organic Synthesis, supramolecular chemistry and carbocyclic rings) Module no. 30: Azulenes ____________________________________________________________________________________________________ TABLE OF CONTENTS 1. Learning Outcomes 2. Introduction of Azulenes 3. Synthesis of Azulnes 4. Properties of Azulenes 5. Summary CHEMISTRY Paper no. 14: Organic Chemistry- IV (Advanced Organic Synthesis, supramolecular chemistry and carbocyclic rings) Module no. 30: Azulenes ____________________________________________________________________________________________________ 1. Learning Outcomes After studying this module, you shall be able to understand: Basics of azulenes. Synthesis of azulenes. Physical properties of azulenes and Chemical properties of azulenes 2. Introduction of Azulenes The name ‘Azulene’ is derived from the Spanish word ‘azul’, meaning ‘blue’. Azulene (1) is a dark blue colour organic compound. It is an isomer of naphthalene (2) having molecular formula, C10H8. Azulene is a completely conjugated non-benzenoid fused ring hydrocarbon, which consists of a seven membered ring fused with a five membered ring. While naphthalene is a benzenoid hydrocarbon consisting of two benzene rings fused together at ortho-position. Azulene (bicycle[5.3.0]decapentaene, 1) is a non-benzenoid aromatic compound, while naphthalene is a benzenoid aromatic compound. The higher stability of naphthalene over azulene is due to resonance stabilization, since only two resonance structures can be written for azulene compared to three for naphthalene (Figure 1). CHEMISTRY Paper no. 14: Organic Chemistry- IV (Advanced Organic Synthesis, supramolecular chemistry and carbocyclic rings) Module no. 30: Azulenes ____________________________________________________________________________________________________ Figure 1. Resonance structures of azulene and naphthalene Azulene is a natural product and function as pigment in living organisms like plants, fungi etc. This compound was first isolated from the German Chamomile and later from yarrow, wormwood and other plants as oily liquid. This compound is used in skin care products as biocolour. This imparts blue tint and is an important ingredient in beauty products like soaps, moisturizers etc. This also reduces inflammation in the skin tissue and also has the potential to protect the skin from free radical damage. 3. Synthesis of Azulenes There are several methods available for the synthesis of azulenes. Few are discussed here. (i) From adipic acid: Two molecules of adipic acid combine together by the loss of one molecule of water and one molecule of carbon dioxide resulting in the formation of 6-oxo-undecanedioic acid. Loss of one water molecule from 6-oxo-undecanedioic acid results in the formation of 2-(4- carboxy-butyl)-cyclopent-1-enecarboxylic acid. Again loss of one molecule of water and one molecule of carbon dioxide from2-(4-carboxy-butyl)-cyclopent-1-enecarboxylic acid results in CHEMISTRY Paper no. 14: Organic Chemistry- IV (Advanced Organic Synthesis, supramolecular chemistry and carbocyclic rings) Module no. 30: Azulenes ____________________________________________________________________________________________________ the formation of 2,3,5,6,7,8-hexahydro-1H-azulen-4-one. Further dehydration and dehydrogenation from it results in the formation of azulene (Figure 2). Figure 2: Synthesis of azulene from adipic acid (ii) From indane (ring-expansion method): This is the most widely used method for the azulene synthesis. In this method, indane (or indane derivatives) is treated drop-wise with ethyl diazoacetate at 130-135°C for about 2 hr. Upon completion of the addition, the temperature is raised to 160-165°C for several hours. During the process, first carbene is generated in situ through nitrogen extrusion reaction (Figure 3). - H - H H .. + + .. - N2 .. N N C N N C C COOCH2CH3 COOCH2CH3 COOCH2CH3 Ethyl diazoacetate Carbene CHEMISTRY Paper no. 14: Organic Chemistry- IV (Advanced Organic Synthesis, supramolecular chemistry and carbocyclic rings) Module no. 30: Azulenes ____________________________________________________________________________________________________ Figure 3: Generation of carbene The carbene further reacts with one of the double bonds of benzene of indane to form the cyclopropane ring (Figure 4). After the reaction, the unreacted indane and ethyl diazoacetate are distilled off. The addition product is dehydrogenated using Pd-C to form the corresponding possible mixture of azulene carboxylic esters. These esters are further hydrolyzed to the corresponding carboxylic acids which are decarboxylated and dehydrogenated to azulene (Figure 4). CHEMISTRY Paper no. 14: Organic Chemistry- IV (Advanced Organic Synthesis, supramolecular chemistry and carbocyclic rings) Module no. 30: Azulenes ____________________________________________________________________________________________________ Figure 4: Synthesis of azulene from Indane (iii) From methylamine derivative (Demjanow ring expansion): This method of ring expansion involves a chemical reaction between C-(octahydro-inden-5-yl)-methylamine with nitrous acid. The nitrous acid is formed in situ by the reaction between sodium nitrite and hydrochloric acid at o + 0 C. This reaction gave –CH2 carbocation which facilitated the ring expansion and further reaction with water gave decahydro-azulen-5-ol. This alcohol is oxidized with the help of an oxidizing agent chromium trioxide to form decahydro-azulen-5-one. Further reaction with methyl magnesium iodide (Grignard reagent) followed by hydrolysis and aromatization results in the formation of 5- methyl-azulene (Figure 5). CHEMISTRY Paper no. 14: Organic Chemistry- IV (Advanced Organic Synthesis, supramolecular chemistry and carbocyclic rings) Module no. 30: Azulenes ____________________________________________________________________________________________________ Figure 5. Synthesis of azulene using Demjanow ring expansion 4. Properties of Azulenes Azulene is a cyclic and planar molecule with 10 -electrons. It obeys Huckel’s rule of aromaticity (4n +2, where n = 2) and hence is aromatic in nature. Dipole moment: It has a substantial dipole moment (1.0 D), which suggests that there exists a charge separation in the molecule. The each ring approximates to six -electron system. After the charge separation, it is regarded as a combination of aromatic cyclopentadienyl anion and aromatic cycloheptatrienyl cation (Figure 6). CHEMISTRY Paper no. 14: Organic Chemistry- IV (Advanced Organic Synthesis, supramolecular chemistry and carbocyclic rings) Module no. 30: Azulenes ____________________________________________________________________________________________________ Figure 6. Ionic structure of azulene Thus in valence bond terms, the ionic structure of azulene is an important contributor to the resonance bond. Isomerization: Azulene can be isomerized quantitatively to naphthalene on heating above 350oC in the absence of air (Figure 7). Figure 7. Isomarization in azulene Electrophilic aromatic substitution reaction: Since azulene is aromatic in nature, hence, it undergoes electrophilic aromatic substitution reactions like halogenations, nitration, Friedel Craft alkylation, etc. preferentially in the five membered ring since it is more electron rich than the seven membered ring. The quantum mechanical calculations suggested that the electrophilic aromatic substitution reactions take place at 1-position of azulene (Figure 8). CHEMISTRY Paper no. 14: Organic Chemistry- IV (Advanced Organic Synthesis, supramolecular chemistry and carbocyclic rings) Module no. 30: Azulenes ____________________________________________________________________________________________________ Figure 8. Carbon position in azulene Further reaction gives product both at 1 and 3-position. If positions 1 and 3 are occupied, the reaction takes place at 5 and 7 positions. For example, Friedel craft acylation of azulene using acetyl chloride and aluminium chloride leads to the formation of 1-acetyl azulene and 1,3-diacetylazulene (Figure 9). Figure 9. Friedel craft acylation of azulene The electrophilic substitution reactions of azulene with isocyanates (R-N=C=O) and nitrile oxides (RCNO) in the presence of Lewis acid like ZnCl2 took place at C-1 and gave azulene-1- carboxamides, and 1-aroylazulene oximes, respectively. A 1-nitroazulene was obtained on treating azulene with cupric nitrate and acetic anhydride in red colour needles. CHEMISTRY Paper no. 14: Organic Chemistry- IV (Advanced Organic Synthesis, supramolecular chemistry and carbocyclic rings) Module no. 30: Azulenes ____________________________________________________________________________________________________ (v) With acid and bases: Azulenes reacts differently to acids and bases. They are not soluble in alkali and soluble in strong acids. The solubility in acid is a valuable one and is employed for the purification and characterization of azulenes. Azulene get protonated in the presence of strong acid at 1-position leading to the formation of azulinium ion a derivative of relatively stable cycloheptatrienyl ion (tropylium ion) (Figure 10). Figure 10. Protonation of Azulene 5. Summary Azulene is a non-benzenoid
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