Organic Chemistry, Heterocyclic Compounds, Lec.7 PDF

Organic Chemistry III Heterocyclic Compounds Furan | Lec.7 Organic Chemistry | Heterocyclic Compounds Contents : Properties 3 Aromaticity of Furan 4 Synthesis of Furan 5 Properties of Furan 13 Reactions Of Furan 15 Medicinal uses 26 Organic Chemistry | Heterocyclic Compounds Furan : Properties 1. Ar...

Organic Chemistry III Heterocyclic Compounds Furan | Lec.7 Organic Chemistry | Heterocyclic Compounds Contents : Properties 3 Aromaticity of Furan 4 Synthesis of Furan 5 Properties of Furan 13 Reactions Of Furan 15 Medicinal uses 26 Organic Chemistry | Heterocyclic Compounds Furan : Properties 1. Aromaticity Organic Chemistry | Heterocyclic Compounds Aromaticity of Furan : Furan have 4C and 1O, all are sp2 hybridized. sp2 hybridization is planar, it makes a planar furan ring structure. Each ring atom also contains unhybridized p orbital that is perpendicular to the plane of σ bonds (plane of ring). Here p orbitals are parallel to each other, so overlapping btwn p orbitals is possible. The total nu of non bonding e- are 6 (4 of four C, 2 from one O) The resonance of 6 e-follows the Hückel's rule. So the furan is aromatic. Note: nu = number Synthesis of Furan Organic Chemistry | Heterocyclic Compounds Synthesis of Furan: Industrially, furan is manufactured by the palladium-catalyzed decarboxylation of furfural, or by the copper-catalyzed oxidation of 1,3-butadiene. Organic Chemistry | Heterocyclic Compounds Synthesis of Furan: 1. Paal-Knorr synthesis of furan Acid catalysed¸ cyclising dehydration of 1.4- dicarbonyl compounds. Organic Chemistry | Heterocyclic Compounds Synthesis of Furan: Mechanism Organic Chemistry | Heterocyclic Compounds Synthesis of Furan: 2. Feist-Benary Synthesis Reaction of α-haloketones with ß-ketoesters in the presence of a base (not ammonia) to give furans. Note: Strong base like NaOH is required Organic Chemistry | Heterocyclic Compounds Synthesis of Furan: Mechanism Organic Chemistry | Heterocyclic Compounds Synthesis of Furan: 3. From carbohydrate Step 1: distillation of CH with sulfuric acid. Step 2: catalytic decomposition of furfural in steam. Organic Chemistry | Heterocyclic Compounds Synthesis of Furan: Wilson reported that 78% yield of furan was produced from Furoic acid at 200 °C without catalyst. Properties of Furan Organic Chemistry | Heterocyclic Compounds Properties of Furan: 1. Furan is a colourless volatile liquid. 2. Its boiling point is 31.4°C, it is highly volatile in room temperature. 3. Furan having odor of chloroform. 4. It is insoluble in ether but soluble in most organic solvents. 5. It is slightly soluble in water. Reactions Of Furan Organic Chemistry | Heterocyclic Compounds Characteristic reactions of furan: 1. Electrophilic substitution Furan, like pyrrole, undergoes electrophilic substitution reactions mainly at 2- positions. Substitution at 3-position occurs only when both of the 2positions (2&5) are already blocked. Organic Chemistry | Heterocyclic Compounds Reactions of Furan: Furan undergoes electrophilic substitution reaction at 2nd position. 2 reasons... C2 attack gives more resonance contributing structures than C3. Extra stable contributing structure generates upon C2 attack Organic Chemistry | Heterocyclic Compounds Organic Chemistry | Heterocyclic Compounds Formylation of furan: Gattermann Koch formylation Vilsmeyer formylation Organic Chemistry | Heterocyclic Compounds 2. Reduction Raney Ni: is a fine-grained solid composed mostly of nickel derived from a nickel–aluminium alloy. Organic Chemistry | Heterocyclic Compounds Bromination of furan in the presence of alcohol: If the bromination is conducted in an alcohol, trapping of the intermediate by C-5 addition of the alcohol, then alcoholysis of C-2- bromide. Produces 2,5-dialkoxy-2, 5-dihydrofurans: Organic Chemistry | Heterocyclic Compounds 3. Diels- Alder reaction O atom is highly electronegative, so the delocalization of lone pair e- (in aromatic system of furan) is not overly effective. Thus, Furan can behave as a diene and gives 4+2 cycloaddition. Organic Chemistry | Heterocyclic Compounds Reactions of Furan: Mercuration Mercuration takes place very readily with replacement of hydrogen, or carbon dioxide from an acid. Reaction with Oxidizing agents Organic Chemistry | Heterocyclic Compounds Reactions of Furan: Furan is used for pyrrole synthesis, it is an industrial method. Synthesis of pyrrole from Furans Industrial process Passing furan over ammonia in presence of alumina as catalyst at high temp. Organic Chemistry | Heterocyclic Compounds Order of aromaticity: Furan is not very aromatic therefore if there is a possibility of forming stable bonds such as C-O bonds by addition, this may be preferred to substitution, i.e. tendency to give addition products rather than substitution products increases as aromaticity decreases. Medicinal uses Organic Chemistry | Heterocyclic Compounds Medicinal uses: 1. Nitrofurans: Nitrofurazone, Furazolidone, Nitrofurantoin, Anti infective Agents: used as an antiprotozoal agent to treat trypanosomiasis and leishmaniasis. 2. Ranitidine: Antihistaminic Agents, used in peptic ulcer.