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Purification And Characterisation Of Organic Compounds

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Purification and Characterization of Organic Compounds PDF Notes, Important Questions and Formulas

Purification and Characterization of Organic Compounds

Organic Chemistry - Some Basic Principles and Techniques

  • Study of chemistry of hydrocarbons and their derivatives is called organic chemistry.
  • Reactivity of π- bonded compounds: The electron charge cloud of the π-bond is located above and below the plane of bonding atoms. These electrons are more exposed and therefore are easily available to the electron seeking attacking reagents. Therefore π- bonds provide the most reactive centres in the molecules containing multiple bonds.
  • Structural representation of Organic compounds

 

Examples

Lewis structure :

In this representation, the bonds between atoms are represented by pairs of dots or lines and lone pairs on atoms are represented by a pair of dots

 

Complete structural formula :  In this representation, a single bond is represented by a single dash (-), a double bond by a double dash (=) and a triple bond by a triple dash (≡). Lone

 

 

 

Condensed structural formula : In this representation, atoms are shown but bonds (dashes) between some or all atoms may be omitted and the number of identical groups attached to an atom is indicated by a subscript

 

CH3CH2CH2COOH

Or

CH3 (CH2)2COOH

Bond – line structural formula : In this representation, carbon and hydrogen atoms are not shown and the lines representing carbon-carbon bonds are drawn in a zigzag fashion. Atoms other than carbon and hydrogen are writte. The terminals denote methyl (-CH3) groups (unless otherwise indicated by a functional group), while the line juctions denote carbon atom bonded to an appropriate number of hydrogen atoms required to satisfy the valency of the carbon atoms.

 

 

  • Three-dimensional representation of organic molecules: Solid wedges are used to indicate a bond projecting out of the plane of paper towards the observer. Dashed wedges are used to depict the bond projecting out of the plane of the paper and away from the observer. Wedges are shown such that the broad ends of the wedges are towards the observer. The bonds lying in the plane of the paper are depicted by using a normal line.


  • Acyclic or open chain compounds: Compounds contain an open chain of carbon atoms in their molecules. Chains may be either branched chain or straight chain.
    Examples:


  • Closed chain or cyclic or ring compounds: Two types of ring compounds namely (i) homocyclic or carbocyclic compounds and (ii) heterocyclic compounds.

  • Homocyclic or carbocyclic compounds: In these types of compounds, the ring consists of only carbon atoms. There are two types of homocyclic compounds as follows:
  1. Alicyclic or closed chain or ring compounds: These compounds contain rings of three or more carbon atoms in them. These compounds resemble aliphatic compounds in many of their properties.
    Examples:

  2. Aromatic compounds: These compounds have a cyclic system containing at least one benzene ring. Aromatic compounds have delocalised  electron system.

 Classification of Aromatic compounds:

Benzenoid aromatic compounds

Non-benzenoid compound

These compounds contain benzene or other related ring compound and exhibits aromatically

Examples:

 

These compounds show aromatically but do not contain any benzene ring

 

Examples:

 

 

  • Heterocyclic compounds: In these compounds, rings contain one or more atoms of N, O or S in addition to carbon atoms. Atoms other than C, i.e. N, O or S is called heteroatom.
    Examples :

  • Functional group: An atom or group of atoms that determine the characteristic chemical properties of an organic compound.
  • Homologous series: A series of similarly constituted compounds in which members possess the same functional group and have similar chemical characteristics. Two  consecutive members differ in their molecular formula by the –CH2 group. Different members of the series are known as homologous.
  • Isomerism: The phenomenon of existence of two or more compounds possessing the same molecular formula but different properties is known as isomerism. Such compounds are called as isomers.
    Different types of isomerism

  1. Structural isomerism: Compounds having same molecular formula but different structures, i.e. arrangement of atoms or groups of atoms within molecules are called structural isomers and the phenomenon is called structural isomerism.
  2. Stereo isomerism: The isomers which have the same structural formula but have different relative arrangement of atoms or groups of atoms in space are called stereo isomers, and the phenomenon is called stereo isomerism.
  • Tautomerism: It is a special type of functional isomerism in which the isomers differ in the arrangement of atoms but they exist in dynamic equilibrium with each other, and this phenomenon is termed as tautomerism. Example: Acetaldehyde and vinyl alcohol are tautomers and exist in equilibrium as shown.

  • Substrate: In an organic reaction, the reactant which supplies carbon to the new bond is called the substrate.
  • Attacking reagent: In an organic reaction, chemical substances that attack the organic molecule (substrate) and leads to the formation of product are called attacking reagents.
  • Reaction mechanism: Reaction mechanism is a sequential account of each step describing the details of electron movement, energetics during bond cleavage and bond formation and the rates of transformation of reactants into products (kinetics).
  • Homolytic cleavage of a covalent bond: Symmetrical cleavage of covalent bonds between two atoms takes place resulting in the formation of a neutral species (atoms or groups of atoms) having unpaired electrons called free radicals.

     
  • Heterolytic cleavage of a covalent bond: Unsymmetrical cleavage of covalent bond takes place resulting in two charged particles. The species that has a sextet of carbon atoms and shared pair of electrons and carrying a negative charge is called carbanion.


    Positively charged species is carbocation
    Negatively charges species is carbanion
  • Electron displacement in covalent compounds
  1. Inductive effect: The process of electron displacement along the chain of carbon atoms due to the presence of a polar covalent bond at one end of the chain is called the inductive effect (I-effect); it is a permanent effect.
    Note: I-effect decreases on moving away from the atoms involved in the initial polar bond and becomes negligible from the fourth atom onwards.
    For comparing the relative effects, hydrogen is taken as a standard and the atoms or groups can be classified into two categories:
    • Atoms or groups of atoms having electron-attracting more than hydrogen are referred to as having –I- effect (electron withdrawing or attracting)
       Example
      NO> - CN > -COOH>-COOR>-F>-Cl>-Br>-I-OH>-OCH>-C6H5 > H
    • Atoms or groups of atoms having smaller electron attracting power than hydrogen are referred to as having +I-effect (electron donating or repelling).
      Example
      (CH3)3C->(CH3)2CH->CH3CH2->CH3-

  2. Electromeric effect: It is a temporary effect which takes place between two atoms joined by a multiple bond, i.e. a double or triple bond. This occurs at the requirements of the attacking reagent and involves instantaneous transfer of a shared pair of electrons of the multiple bonds to one of the linked atoms.
    The electromeric effect is classified as +E effect and −E effect:
    • When the π -  electrons of multiple bonds are transferred to that atom to which the reagent gets attached, it is called +E effect (positive electromeric).
      Example:


    • When the π -  electrons of multiple bonds are transferred to that atom to which the attacking reagent does not gets attached, it is called -E (negative electromeric) effect.
      Example:
  3. Resonance or mesomeric effect: If a molecule is assigned two or more Lewis structures, none of which is capable of describing all the known properties of the compound, then the actual structure is intermediate or resonance hybrid of these structures. This phenomenon is called resonance. The various structures written are called resonating structures.
    Example – Resonating structures of CO2 are shown below
    begin mathsize 12px style table attributes columnalign left end attributes row cell text O=C=O end text left right arrow stack text O end text with text-end text on top text -C end text identical to text O end text to the power of text + end text end exponent left right arrow stack text O end text with text + end text on top identical to text C-O end text to the power of text-end text end exponent end cell row cell text     (i)                    (ii)                     (iii) end text end cell end table end style
    Resonance effect is classified +R effect on –R effect
    • If a substituent has tendency to donate electrons to double bond or conjuagated system, then the effect is called the positive resonance effect or +R effect. Groups such as –OH, -OR,-NH2,-NHR,-NR2,-Cl and –Br show+R effect
    • If a substituent has tendency to withdraw electrons from a double bond or conjugated system towards itself, then the effect is called the negative resonance effect or –R effect. Groups such as >C=O,-CHO,-CN,-NO2 and –COOR Show –R effect
  4. Hyperconjugation: It involves delocalisation of (σ) electrons of C–H bond of an alkyl group directly attached to an atom of an unsaturated system or to an atom with an unshared p-orbital. The (σ) electrons of C–H bond of the alkyl group enter into partial conjugation with the attached unsaturated system or with the unshared p-orbital. The interaction between the electrons of π systems (multiple bonds) and adjacent σ bonds (single H–C bonds) of the substituent groups in organic compounds is called hypercojugation. It is a permanent effect.
    begin mathsize 12px style table row blank blank straight H blank blank blank blank blank blank blank cell space straight H to the power of plus end cell blank blank blank blank blank blank blank straight H blank blank blank blank blank cell text       end text straight H end cell blank blank blank blank blank row blank blank vertical line blank blank blank blank blank blank blank vertical line blank blank blank blank blank blank blank vertical line blank blank blank blank blank cell text       |  end text end cell blank blank blank blank blank row straight H minus straight C minus CH equals cell CH subscript 2 end cell left right arrow straight H minus straight C equals CH minus cell straight C with.. to the power of minus on top straight H subscript 2 end cell left right arrow straight H minus straight C equals CH minus cell straight C with.. to the power of minus on top straight H subscript 2 end cell left right arrow cell straight H to the power of plus straight C end cell equals CH minus cell straight C with.. to the power of minus on top straight H subscript 2 end cell blank row blank blank vertical line blank blank blank blank blank blank blank vertical line blank blank blank blank blank blank blank vertical line blank blank blank blank blank cell text       end text vertical line end cell blank blank blank blank blank row blank blank straight H blank blank blank blank blank blank blank straight H blank blank blank blank blank blank blank cell space straight H to the power of plus end cell blank blank blank blank blank cell text       end text straight H end cell blank blank blank blank blank row blank blank straight I blank blank blank blank blank blank blank II blank blank blank blank blank blank blank III blank blank blank blank blank cell text      end text IV end cell blank blank blank blank blank end table end style

    Because there is no bond between the α – carbon atom and one of the hydrogen atoms, hyperconjugation is also called no-bond resonance. Although a free proton has been shown in the above structures, it is still bound quite firmly to the π-cloud, and hence it is not free to move.
    Order of hyperconjugation : CH3 - > CH3CH2  -> (CH3)2 CH- > (CH3)3 C –
  • Filtration: It is the technique used to separate an insoluble solid component of the mixture from a soluble component in a given solvent.
  • Recrystallisation: This method is based on the differences in the solubility of the organic compound and its impurities in a suitable solvent.
  • Simple distillation: This method is used for the purification of liquids which boils without decomposition and contain non-volatile impurities. The simple distillation involves the heating of the liquid to its boiling point so that it is converted into vapours. On cooling the vapours, pure liquid is obtained and collected separately.
  • Fractional distillation: Method is used to separate a mixture of two or more miscible liquids which have boiling points close to each other. Distillation is carried out by using fractionating columns. Fractionating columns provide many surfaces for heat exchange between the ascending vapours and the descending condensed liquid.
  • Distillation under reduced pressure or vacuum distillation: Certain liquids have a tendency to decompose at a temperature below their boiling points. Such liquids cannot be purified by ordinary distillation. Under reduced pressure, the liquid will boil at a low temperature and the temperature of decomposition will not be reached.
  • Steam distillation: This method is used for the separation and purification of a liquid which is appreciably volatile in steam from non-volatile components of a mixture.
  • Differential extraction: This method is used to separate a given organic compound present in aqueous solutions by shaking with a suitable organic solvent in which the compound is more soluble than water. The basic requirement of the organic solvent is that it should be immiscible with water so that the organic and water layers can be easily separated.
  • Chromatography: The technique of chromatography is based on the difference in the rates at which the components of a mixture move through a porous medium (called stationary phase) under the influence of some solvent or gas (called moving or mobile phase).
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