Sky Of Chemistry

The Bischler–Napieralski synthesis This synthesis was first suggested by Bischler and Napieralski and has been subjected to a number of improvements later on. This method involves the cyclodehydration of an acyl derivative of B-phenylethylamine to give 3,4-dihydro isoquinoline, in the presence of Lewis acids such as polyphosphoric acid, zinc chloride or phosphorous pentoxide. The 3,4-dihydro isoquinoline is then dehydrogenated by Pd( Palladium ) at 160°C to Isoquinoline. It must be noted that the yield of this reaction is excellent if electron-donating groups are present on the benzene ring however if the electron-withdrawing groups are present on the benzene ring the yields are very poor. This is because of the electrophilic ring closure nature of the ring.

Isoquinoline Isoquinoline is a heterocyclic aromatic organic compound. It is a structural isomer of quinoline. Isoquinoline is also obtained by ring fusion of pyridine and with a benzene ring. It was first isolated by Hoogewerff and Drop from the quinoline fraction of coal tar in 1885. Several derivatives of Isoquinoline also occur in coal tar. Isoquinoline does not occur free in nature but founds frequently in several alkaloids. It is called 2-azanaphthalene or benzo[b]pyridine. The numbering of the atoms in Isoquinoline is similar as followed in quinoline; however, the nitrogen atom is assigned position-2. Isoquinoline has close similarities in the structure with quinoline; therefore both have a close relationship in their physical and chemical properties.

This is one of the most important methods for preparation of quinoline. In this method the aniline and its derivatives having vacant ortho position is when heated with glycerol, concentrated H2SO4 and an oxidizing agent the resultant product is obtained as quinoline or its derivatives. The nitrobenzene is generally used as mild oxidizing agent in Skraup synthesis. Glycerol when heated with concentrated H2SO4 it gives the acroline after dehydration. Condensation of acroline thus obtained with aniline or its derivatives followed by oxidation gives the quinoline. The reaction is shown follow. Mechanism: The step wise mechanism of Skraup synthesis of quinoline is given as follow.

Quinoline Quinoline is a heterocyclic aromatic organic compound with the chemical formula C9H7N. It is a colourless hygroscopic liquid with a strong odor. It is also called 1-azanaphthalene or phenazopyridine.  Quinoline was first extracted from coal tar in 1834 by German chemist Friedlieb Ferdinand Rung; he called quinoline leukol ("white oil" in Greek). Coal tar remains the principal source of commercial quinoline. In 1842, French chemist Charles Gerhardt Obtained a compound by dry distilling quinine., strychnine, or chinchonine with potassium hydroxide; he called the compound Chinoilin or Chinolein. Runge's and Gephardt's compounds seemd to be distinct isomers beacuse they reacted differently. However, the German chemist August Hoffmann eventually recognized that the differences in behaviors were due to the presence of contaminants and that the two compounds were actually identical. Like other nitrogen heterocyclic compounds, such as pyridine derivatives, quino

Reduction of Pyridine Under catalytic hydrogenation of pyridine hexahydro pyridone is formed. It is also known as Piperidine.

Basicity of Pyridine, Pyrrole From Experimental studies, it is observed that the pKb values of Pyrrole, Pyridine and Piperidine are ~14, ~8.7 and ~2.7, respectively. Based on the suggested pKb values the priperidine in found as a stronger base than pyridine and pyrrole. Pyrrole is the weakest base among these three heterocyclic bases. The order of basicity of pyrrole, pyridine and piperidine is as given below: The above order of basicity of pyrrole, pyridine and piperidine can be justified in terms of the structure of these compounds. As we know that the basicity of nitrogen compounds depends upon the availability of lone pair of electrons on the nitrogen atoms. In pyrrole, the lone pair of electron on nitrogen atom exists in the sp2 hybridized orbital of nitrogen and participates in the delocalization, hence does not freely available to cause the basic character of pyrrole. Similar to pyrrole, the lone pair of electrons on the nitrogen atom of pyridine also exists in the sp2 hyb

Nucleophilic Substitution Reactions of Pyridine As we have discussed in the previous section that pyridine generally deactivated the aromatic ring towards electrophilic substitution reaction. The deactivation of the aromatic rings towards electrophilic substitution resulted due to the electron-withdrawing nature of the nitrogen atoms. Due to such deactivation, Pyridine also gives nucleophilic substitution reaction. Nucleophilic substitution in pyridine ring occurs at position C-2. Approach of the nucleophilic at position C-2 leads to the formation of three resonating structures(I, II and III); similarly, the approach of nucleophilic at position C-3 also leads to the formation of three resonating structures (IV, V and VI). The resonating structures for intermediate resulting from the attack of the nucleophile at position C-2 are more stable than those of position C-3 since more electronegative nitrogen atoms hold a -ve charge in one of the resonating structures (III) obtained from th

Electrophilic substitution in Pyridine Pyridine is also an aromatic compound. It is less aromatic than benzene and pyrrole. Pyridine is usually considered a highly deactivated aromatic nucleus towards electrophilic substitution reactions. Therefore highly vigorous reaction conditions should be used for these reactions to take place. The low reactivity of pyridine towards the electrophilic substitution reactions is due to the following reasons: The higher electronegativity of the nitrogen atom reduces electron density on the ring, thus deactivate the ring. Pyridine is highly sensitive to acidic medium; it readily forms pyridinium cation with a positive charge on the nitrogen atom. Similarly, electrophile itself may also react with pyridine from the corresponding pyridinium ion. This positive charge on the nitrogen atom decreases the electron density of the nitrogen atom, consequently, the electron density on the ring also decreases. However, the effect of such deactivation is c

Pyridine Structure of Pyridine : The structure of Pyridine is completely analogous to that of benzene, being related by replacement of CH by N. The key differences are : (i) the going away from perfectly regular hexagonal geometry caused by the presence of the heteroatom, in particular the shorter carbon-nitrogen bonds, (ii) the replacement of a hydrogen in the plane of the ring with an unshared  electron pair, likewise in the plane of the ring, located in an sp2 hybrid orbital and not at all involved in the aromatic π - electron sextet; It is this nitrogen lone pair which is responsible for the basic properties of Pyridines, and (iii) a strong permanent diople,  discovered to the greater electronegativity of nitrogen compared with carbon. It is important to realise that the electronegative nitrogen causes inductive polarisation, mainly in the σ - bond framework, and additionally stabilize those polarised mesomeric contributors in which nitrogen is negatively charged - 8,9, a

Reactivity and Orientation of Pyrrole, Furan, Thiophene Pyrrole, Thiophene and Furan give electrophilic aromatic substitution reactions. These Compounds are more reactive compared to Benzene. Electrophiles majorly attack the 2nd position rather than the 3rd position in these heterocyclic compounds. The reason behind it is the more number of the resonating intermediate structure are possible to accommodate the positive charge when electrophile attacks on 2nd position (Three resonating intermediate structure) That makes the intermediate carbocation more stable while if electrophile attacks on the 3rd position then only two resonating intermediate structures are possible as shown in the figure which is comparatively less stable. That's why the attack of electrophiles takes place at the 2nd position in Pyrrole, Furan, Thiophene.

Electrophilic Substitution of Thiophene Thiophene undergoes electrophilic substitution reactions at position C-2. i. Halogenation: Thiophene reacts with halogens  [X2, Where X2= Cl2, Br2 And I2] to give 2-halothiophene. For example, the reaction of bromine with Thiophene in absence of any halogen carrier gives 2,5-dibenzothiophene. However, the Iodination of thiophene in presence of yellow mercuric oxide gives 2-iodothiophene. ii. Nitration: 2-Nitrothiophene is obtained when nitration of thiophene is performed by reacting it with fuming HNO3 in anhydride. The reaction of HNO3 and acetic anhydride resulted in acetyle nitrate in which -NO2 acts as an electrophile. iii. Sulphonation: Sulphonation of thiophene is achieved by reacting it with cold concentrated H2SO4. Thiophene-2-sulphonic acid is obtained as a product. iv. Friedel-Crafts Acylation: Reaction of thiophene with acetic anhydride in presence of H3PO4 gives 2-acetyl thiophene. Thank you

Electriphilic Substitution in Furan Furan undergoes electrophilic substitution reactions at position C-2. i. Halogenation: Furan reacts with halogens [X2, Where X2= Cl2, Br2 And I2] to give 2-halofuran. For example, the reaction of bromine with furan gives 2-bromofuran. ii. Nitration: Nitration of furan is achieved by reacting it with HNO3 in acetic anhydride. The reaction of HNO3 and anhydride resulted in acetyl nitrate in which -NO2 acts as an elec trophile. iii. Sulphonation: Sulphonation of Furan is achieved by reacting it with sulfur trioxide (SO3)- pyridine mixture in ethylene chloride at  100° C. iv. Friedel-Crafts Acylation: Reaction of furan with acetic anhydride in presence of BF3 gives 2-acetyl furan.

Electriphilic substitution in Pyrrole Pyrrole and furan react under very mild conditions α- Substitution favoured over β-substitution, because of more resonance forms for intermediate and so the charge is less localized (also applies to the transition state) Some β-substitution usually observed - depends on X and substituents Nitration of Furans Pyrrole undergoes electrophilic substitution reactions at position C-2 i. Halogenation: Pyrrole reacts with halogens  [ X2 ( X2 = Cl2, Br2 and I2 ) ] to give tetrahalopyrrole. For example, the Reaction of bromine with pyrrole gives tetrabromopyrrole.   ii. Nitration: Nitration of pyrrole is achieved by reacting it with  HNO3 in acetic anhydride. The reaction of HNO3 and anhydride resulted in acetyl nitrate in which -NO2 acts as an electrophile. iii. Sulphonation: Sulphonation of pyrrole is achieved by reacting it with sulfur trioxide (  SO3 ) - pyridine mixture in ethylene chloride. iv. Friedel-Craft

Source of Pyrrole & Thiophene Generally, Pyrrole and Thiophene both are obtained from Coal tar.                                                                                       Coal Tar                    Furan : Furan is also obtained from decarbonylation of Furfuraldehyde and source is,: Oat hulls, Corn cobs, Rice Rice Hulls Oat Hulls Corn Cobs

Structure of Furan And Thiophene The structure of thiophene and furan are closely similar to that discussed in detail for pyrrole above, except that the NH is replaced by S and O , respectively.  A consequence is that the heteroatom in each has one lone pair as part of the aromatic sextet, as in pyrrole, but also has a second lone pair that is not involved, and is located in an sp2 hybrid orbital in the plane of the ring. Mesomeric forms exactly similar to those for pyrrole can be written for each, but the higher electronegativity of both sulfur and oxygen means that the polarised forms, with positive charges on the higher on the heteroatoms, make a smaller contribution.  The larger bonding radius of sulfur is one of the influences making thiophene more stable (more aromatic) than pyrrole or furan - the bonding angles are larger and angle strain is somewhat relieved, but in addition, a contribution to the stabilisation involving sulfur d-orbital participation may be sig

Structure of Pyrrole Before we talk about pyrrole it is necessary to remember the structure of the cyclopentadienyl anion, which is a six 𝜋-electron aromatic system produced by the removal of a proton from cyclopentadiene. This system serves to illustrate nicely the difference between aromatic stabilisation and reactivity, for it is a very reactive, fully negatively charged entity, and yet is 'resonance stabilised' - everything is relative. Cyclopentadiene, with a p K a of about 14, is much more acidic than a simple diene, just because the resulting anion is resonance stabilised. Five equivalent contributing structures, 28-32, show each carbon atom to be equivalent and hence to carry one-fifth of the negative charge. Pyrrole is isoelectronic with the cyclopentadienyl anion but is electrically neutral because of the higher nuclear charge on nitrogen. The other importance of the presence of nitrogen in the ring is the loss of radial symmetry, so that pyrrole does not

Heterocyclic Compounds Compounds classified in heterocyclic probably constitute the largest and most varied family of an organic compound. After all, every carbocyclic compound, regardless of structure and functionality, may in principle be converted into a group of heterocyclic analogs by replacing one or more of the ring carbon atom with a differ element. Even if we control our consideration to oxygen, nitrogen and sulfur, the  arrangement and combinations of such a replacement are numerous. Nomenclature The aromatic heterocycles have been grouped into those with six-membered rings and those with five-membered rings.  The name of six-membered aromatic heterocycles that having nitrogen generally end in 'ine' , though note that 'purine' is the name for a very important bicyclic system which has both a six- and a five-membered nitrogen having heterocycle. Five-membered heterocycles having nitrogen general end with 'ole'. Note the use of italic ' H &#

Hello everyone, you all are familiar with the term matter. Anything which has mass and occupies space is called matter . Everything around us has mass and they occupy space.  Also, you are aware that matter can exist in three physical states viz. Solid, Liquid, Gas.   Solid: these particles are held very close to each other in an orderly fashion and there is not much freedom of movement. Liquid: the particles are close to each other but they can move around. Gas:  the particles are far apart as compared to those present in solid or liquid states and their movement is easy and fast. Characteristics of the given matter: (i) Solids have definite volume and definite shape. (ii) Liquids have definite volume but not a definite shape. They take shape of the vessel in which they are placed. (iii) Gases have neither definite volume nor definite shape. They completely occupy the vessel in which they are placed Properties of matter: There are some changes during which no new