Sky Of Chemistry

The word " organic " was introduced by Berzelius in 1780 to introduce chemicals which were obtained from living sources. Organic chemistry has long been regarded as a separate and unique branch of chemistry, organic chemistry is the body of knowledge which is concerned with the physical and chemical properties of the broad group of carbon compounds containing hydrogen and one or more additional elements like oxygen, nitrogen, sulphur  etc. However, compounds of carbon containing only carbon and oxygen like CO, CO 2 , CO 3 -2 are not studied under organic chemistry. Similarly, compounds containing carbon and nitrogen are not considered to be organic compounds. To understand organic chemistry, knowledge of valency of atoms is essential. Valency of different atoms: Atom              Valency   H                      1  O                  ...

Introduction Proteins are very big molecules made up of smaller units known as 'Amino Acids'. Amino acids are organic molecules that contain an amino group and a carboxylic acid group. all amino acids have a simple chemical backbone with an amine group (the nitrogen-containing part) at one end. At the other end is the acid part. This backbone is the same for all amino acids. The difference between them depends on a distinctive structure, the chemical side chain that is attached to the backbone. It is the nature of the side chain that gives identity and chemical nature to each amino acid. There are about 20 different naturally occurring amino acids that combine to from proteins of all living tissue. The amino acids that make up proteins differ from fats and carbohydrates in that they contain the element nitrogen. The amino acids in proteins are called alpha (α)-amino acids because the amino group is attached to the α-carbon connected to any carboxylic acid carbon is termed a...

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.

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...

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...

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. ...

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 participat...

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 ...