Introduction and Vital Force Theory
* Carbon exhibits tetravalency.
* Carbon has the ability to successively bond to other carbon atoms to form long chain. This property of carbon is called as catenation.
*Carbon also forms covalent bonds with hydrogen, oxygen, nitrogen, sulphur and halogens. These complex carbon compounds are known as organic compounds.
* The branch of chemistry that studies the formation and properties of organic compounds is known as organic chemistry.
* Organic compounds made of only hydrogen and carbon atoms are known as hydrocarbons.
Organic Compounds: Shapes
* Carbon is tetravalent in nature.
* The formation of molecules and their shapes can be explained by considering the hybridisation of s and p orbitals respectively.
* Hybridisation influences bond length and bond strength.
* Bond strength of sp hybrid orbitals is greater than sp2 followed by sp3.
* Bond length of sp3 hybrid orbitals is greater than sp2 followed by sp.
* The Government of India and Andhra Pradesh have set up several offices and laboratories in different parts of the state.
Organic Compounds: Structural Representation
* A chemical formula which represents the arrangement of atoms in a molecule is called the Structural Formula of a molecule.
* Structures of organic compounds can be represented by the following ways:
• Lewis structural formula or dot structural formula,
• Dash structural formula,
• Condensed structural formula and
• Bond-line structural formula.
* In Lewis dot structure, a pair of electrons forming a covalent bond is represented by using a dot or cross.
* The simplified representation of Lewis dot structure is called Dash structure. In this formula, a pair of electrons forming a single covalent bond is represented by a dash.
* In a condensed structural formula some or all the dashes that represent covalent bonds are omitted and the number of identical groups attached to a carbon atom is represented by a subscript.
Organic Compounds: Classification
* Open chain or acyclic compounds are compounds in which carbon atoms are linked by straight or branched chains.
* In a cyclic or ring or closed chain compounds, the carbon atoms are joined to form a closed ring.
* Homocyclic or carbocyclic compounds are those in which the ring consists of only carbon atoms.
* Heterocyclic compounds are those in which the ring consists of carbon atoms as well the other atoms like sulphur, oxygen or nitrogen.
* Alicyclic compounds are aliphatic cyclic compounds.
* An Alicyclic compound consists of a ring which has only carbon atoms.
* Aromatic compounds are benzene and the compounds that resemble benzene in their behaviour.
* Aromatic compounds containing one or more fused or isolated benzene rings and their functional derivatives are known as benzenoid aromatic compounds.
* A non-benzenoid compound is one that does not contain a benzene ring.
* A functional group is an atom or a group of atoms present in a molecule, which largely determine its chemical properties.
* A group or a series of organic compounds having a general molecular formulae and a characteristic functional group in which the successive members differ by – CH2 unit are called homologous series.
* The successive members of a homologous series are called Homologues.
Organic Compounds: Nomenclature – General Rules
* The system of assigning a name to a compound is known as nomenclature.
* The trivial names are given on the basis of the source and certain properties of organic compounds.
* The IUPAC system is a systematic nomenclature in which the name of a compound correlates to its molecular structure.
* The IUPAC name of an organic compound consists of three parts – the word root, the suffix and the prefix.
* The word root indicates the number of carbon atoms present in the selected longest chain.
* A primary suffix is added to the end of the root word. It indicates if a compound is saturated or unsaturated, that is if a single or multiple bond is present.
* A secondary suffix indicates the nature of the functional group present in a compound.
* A primary prefix is used to differentiate acyclic and cyclic compounds.
* In the IUPAC system some groups are not considered as functional groups; they are treated as substituent groups. In such cases, the substituent acts as the secondary prefix.
Organic Compounds: Nomenclature – Alkanes
* Saturated hydrocarbons are known as alkanes.
* Alkanes can be straight chain alkanes or branched chain alkanes, based on their structure.
* Straight chain alkanes can either be acyclic or cyclic.
* The group that consists of one hydrogen atom less than an alkane is called Alkyl group.
* An alkyl group is obtained by removing one hydrogen atom from an alkane.
* The alkyl groups obtained from straight chain hydrocarbons are known as normal alkyl groups.
* Cyclic compound which consist of carbon-carbon single bond are named by prefixing cyclo to corresponding straight chain alkane.
Organic Compounds: Nomenclature –Functional Groups
* A functional group is an atom or a group of atoms present in a molecule, which largely determine the chemical properties of the molecule.
* Organic compounds that have similar functional groups show similar chemical reactions.
* Functional groups play an important role in determining the properties of a compound, and are used to organize organic compounds in different classes.
* The steps to name compounds with functional groups are:
• Step 1: Identify the functional group of the compound.
• Step 2: Number the longest chain which includes functional group.
• Step 3: Assign a word root.
• Step 4: Assign a primary suffix.
• Step 5: Specify the position of the functional group.
* Some compounds have more than one functional group; they are known as polyfunctional compounds.
Organic Compounds: Nomenclature – Substituted Benzene Compounds
* Mono substituted benzene has only one substituent attached to the benzene ring.
* While writing the IUPAC name of the mono substituted benzene, the substituent name is placed as a prefix to the word ‘Benzene’.
* Disubstituted benzene has two substituents attached to the benzene ring.
* The three isomers of a disubstituted benzene are differentiated in the trivial system by using the names ‘ortho’, ‘para’ and ‘meta’, which are abbreviated as o, p, m respectively.
* Trisubstituted benezene has three substituents attached to the benzene ring.
* If more than two substituents are present, then the positions of the groups are indicated by numbering the carbon atoms according to the lowest locant rule. According to this rule, the sequence in which the numbering is done should have the lowest combination of numbers.
Isomerism
* The existence of two or more compounds with the same molecular formula but different properties is known as isomerism. The compounds themselves are called as isomers.
* A compound having same molecular formula but different structures i.e. the way by which the atoms are linked is called as structural isomers and the phenomena is known as structural isomerism.
* Compounds that have the same molecular formula but different carbon chain, that is straight or branched chain are known as chain isomers, and the phenomena is known as chain isomerism.
* Compounds that have the same molecular formula, same chain but which differ in the position of the substituent or functional group are known as position isomers, and the phenomenon is known as position isomerism.
* The compounds having same molecular formula but different functional group are called as functional isomers and the phenomenon is known as functional isomerism.
* The compounds having same molecular formula but different alkyl group attached to the same functional group are called as metamers and the phenomenon is known as metamerism.
* The isomers which possess the same structural formula but differ in three dimensional arrangement of atoms or groups in space within the molecule are known as stereoisomerism.
Reaction Mechanism
* The energetics during breaking and forming of bonds, movement of electrons and the rate of reaction in a sequential description are referred to as the reaction mechanism.
* In homolytic cleavage, a bond breaks in such a way that each broken part gets one of the two bonded electrons.
* In heterolysis, a bond cleaves in such a way that the shared electron pair goes to the more electronegative atom.
* An electrophile is an electron-seeking species that is electron deficient.
* An electron-rich reagent that donates electrons to the positive or electron deficient site in a compound is called a nucleophile. A nucleophile is a nucleus-loving species as it is electron-rich.
Organic Compounds: Electronic Displacements
* The factors which influence the electron density in the substrate are:
• Inductive effect
• Resonance effect
• Electromeric effect and
• Hyperconjugation
* The polarisation of a sigma bond caused by the polarisation of an adjacent sigma bond is called the inductive effect.
* The fractional electronic charges in a polar covalent bond are denoted by delta positive and delta negative, and the shift of the electron density is shown by an arrow that points from delta positive to delta negative end of the polar bond.
* Resonance refers to the phenomenon where two or more structures can be written for a molecule.
* The polarity produced in a molecule by the interaction of two pi – bonds, or between a pi – bond and lone pair of electrons present on an adjacent atom, is known as the resonance or mesomeric effect.
* If the positive resonance effect, the transfer of electrons is away from an atom or substituent group attached to the conjugated system.
* When the electron shift is towards the atom or substituent group attached to the conjugated system, it is called the negative resonance effect.
* Electromeric effect is a temporary effect and observed only in organic compounds with multiple bonds in the presence of an attacking reagent.
* Electromeric effect may be defined as the complete transfer of shared pair of pi electrons of multiple bonds to one of the atoms in presence of an attacking reagent.
* In the positive electromeric effect, the pi electrons of the multiple bond are transferred to that atom to which the attacking reagent is bonded.
* When the pi electrons of the multiple bond are transferred to the atom to which the attacking reagent does not get attached is called the negative electromeric effect.
Hyperconjugation
* A general stabilising interaction which involves the delocalisation of sigma electrons of a C – H bond of an alkyl group directly attached to an unsaturated system or to a species with an unshared p orbital such as carbocations or free radicals is known as hyperconjugation.
* Hyperconjugation stabilises a carbocation.
* Hyperconjugation is a permanent effect.
* Greater the number of alkyl groups attached to a positively charged carbon atom, the greater is the
* Hyperconjugation interaction and stabilisation f the cation.
* Propene is more stable than ethene because in propene there are three H – C hyperconjugated bonds which lead to greater delocalisation of positive charge and hence the stability.
* Hyperconjugation is also known as no-bond resonance.
Organic Compounds: Methods of Purification-I
* Organic compounds are purified by five different methods: Sublimation, Crystallisation, Distillation, Differential Extraction and Chromatography.
* Sublimation is used to separate sublimable compounds from non-sublimable impurities.
* Crystallisation is a process of purification of solid organic compounds. It is based on the difference in solubilities of the compound and the impurities in a suitable solvent.
* Distillation helps to separate liquids from impurities or other liquids.
* Simple distillation is commonly used for liquids that are sufficiently stable at their boiling points and contain non-volatile impurities.
* Fractional distillation is used to separate liquids with a very small difference in their boiling points.
* Liquids that decompose at or below their boiling points and liquids with very high boiling points are separated using distillation under reduced pressure.
* Steam distillation is used to separate substances that are steam volatile and water insoluble.
Organic Compounds: Methods of Purification-II
* Differential extraction is a purification technique that is used for separating an organic compound present in an aqueous solution with the help of an organic solvent in which that compound is more soluble than water.
* Differential extraction is carried out in a separating funnel.
* The name Chromatography is derived from the Greek word ‘chroma’ which means colour and ‘graphein’ which means to write.
* Chromatography is used for identifying, separating and purifying the compounds of a mixture.
* Based on the principle involved, Chromatography is divided into two types – Adsorption Chromatography and Partition Chromatography.
* Adsorption chromatography is based mainly on the differences between the adsorption affinities of the different compounds in a mixture on the commonly used adsorbents like silica gel and alumina.
* The relative adsorption of each component in the mixture is expressed in terms of its retardation factor, Rf. The value of the retardation factor is determined by dividing the distance moved by the substance from the base line by the distance moved by the solvent from the base line.
* Partition chromatography is based on continuous differential partitioning of solutes between the mobile and stationary phases.
Organic Compounds: Qualitative Analysis – Introduction
* The presence of carbon and hydrogen is known when an organic compound is heated with cupric oxide.
* The presence of nitrogen, sulphur and halogens can be detected by Lassaigne’s test.
* In Lassaigne’s test, the organic compound is heated with a small amount of sodium metal in an ignition tube.
* For the detection of nitrogen, a small amount of sodium fusion extract is boiled with ferrous sulphate and then acidified with concentrated sulphuric acid.
* Sulphur can be detected in an organic compound by the lead acetate test or by the sodium nitroprusside test.
* To detect the presence of phosphorus, sodium extract is heated with sodium peroxide and then extracted with water.
Organic Compounds: Quantitative Analysis (C, H, N, X)
* The proportions of carbon and hydrogen in an organic compound can be detected by burning the compound in the presence of excess oxygen and cupric oxide.
* For estimating the proportion of nitrogen, two methods are used – the Dumas method and Kjeldahl’s method.
* In Dumas method, a known mass of an organic compound is heated strongly with coarse cupric oxide in an atmosphere of carbon dioxide.
* In Kjeldahl’s method, the organic compound is heated with concentrated sulphuric acid.
* Kjeldahl’s method is not applicable when nitrogen is present in the ring and also in nitro and azo compounds.
* For the estimation of halogen, a known mass of organic compound should be heated with fuming nitric acid in the presence of silver nitrate in a Carius tube.
Organic Compounds: Quantitative Analysis (S, P, O)
* To detect the presence of sulphur, a known mass of an organic compound is taken in a carius tube and heated with sodium peroxide or fuming nitric acid.
* The quantitative analysis of phosphorus in the given organic compound is done by heating a known mass mass of an organic compound with fuming nitric acid in a Carius tube.
* For the quantitative analysis of oxygen in an organic compound, the compound is heated in a stream of nitrogen gas.