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Some Basic Principles Of Organic Chemistry

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Some Basic Principles of Organic Chemistry PDF Notes, Important Questions and Synopsis

SYNOPSIS

  • All organic compounds are made up of carbon and hydrogen essentially.

    Type of bond

    Hybridisation

    Bond angle

    Structure

    C-C single bond

    sp3

    109°28’

    Tetrahedral

    C=C double bond

    sp2

    120°

    Triagonal planar

    C≡C triple bond

    sp

    180°

    Linear

  • The order of s-character of these hybridized orbital is sp3 > sp2 > sp.

    Characteristic of these hybridized orbital

    Increasing order

    s-character

    sp3 > sp2 > sp

    Bond length

    spspsp

    Bond energy or bond strength

    sp> sp> sp3

  • IUPAC rules for nomenclature of organic compounds:
  • Functional group: An atom or a group of atoms present in the molecules, which determines the characteristics property of the organic compounds, is called the functional group.

    Organic compound

    Functional group

    General formulae

    Secondary suffix/Prefix

    Haloalkanes

    Halide    -X

    (F,Cl,Br,I)

    R-X

    Halo

    Alcohols

    Hydroxyl    -OH

    R-OH

    -ol

    Aldehydes

    Aldehyde   -CHO

    -al

    Carboxylic acids

    Carboxyl    -COOH

    -oic acid

    Ketones

    Keto      

    -one

    Ethers

    Ethers 

    R-O-R’

     

  • 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.
  • Chain isomerism: Compounds having the same molecular formula but a different arrangement of carbon chains (skeletons) within the molecule are called chain isomers, and the phenomenon is termed as chain isomerism.
    Example: Chain isomers of begin mathsize 12px style straight C subscript straight 5 straight H subscript 12 end style

  • Position isomerism: Compounds which have the same molecular formula but differ in the position of the functional groups, carbon–carbon multiple bonds or substituent groups are called position isomers and the phenomenon is termed as position isomerism.
    Example: Position isomers of begin mathsize 12px style straight C subscript straight 3 straight H subscript straight 8 straight O end style : 

  • Functional groups isomerism: Compounds having the same molecular formula but different functional groups in the molecule are called functional group isomers, and this phenomenon is termed as functional group isomerism.
    Example: Functional groups isomers of begin mathsize 12px style straight C subscript straight 3 straight H subscript straight 6 straight O end style

  • Metamerism: Compounds having same molecular formula but different number of carbon atoms (or alkyl groups) on either side of the functional group are called metamers, and this phenomenon is called metamerism.
    Example: Metamers of begin mathsize 12px style straight C subscript straight 4 straight H subscript 10 straight O end style



  1. 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. 

Classification of stereoisomerism

  1. Geometrical isomerism: Isomerism due to the difference in spatial arrangements of groups about the doubly bonded carbon atoms is known as geometrical isomerism.
    Example: Geometrical isomers begin mathsize 12px style straight C subscript 4 straight H subscript 8 end style

  2. Optical isomerism: Stereo isomers involving the arrangement of substituents about an asymmetric carbon atom or atoms so that the various isomers differ in the way they rotate a plane of polarized light are called optical isomers, and the phenomenon is called optical 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.

  • 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).
  • Reaction intermediates: Species produced during cleavage of bonds

    Name of the species

    Order of stability of the species

    Reason for stability order

    Free radical: An atom or group of atoms having an unpaired electron.

     

     begin mathsize 12px style stack CH subscript 3 superscript • with blank on top <  1 to the power of straight o <   2 to the power of straight o <   3 to the power of straight o end style

    Larger the number of alkyl groups attached to the carbon atom carrying the odd electron, greater is the delocalization of the odd electron and hence more stable is the free radical.

    Carbocation: Group of atoms which contain positively charged carbon having only six electrons (sextet of electrons)          

     

     begin mathsize 12px style CH subscript 3 superscript plus <  1 to the power of straight o <   2 to the power of straight o <   3 to the power of straight o end style

    More the number of alkyl groups, the greater will be the dispersal of charge, and therefore carbocation will be more stable.

    Carbanion: Group of atoms which contain negatively charged carbon carrying negative charge.

     

     begin mathsize 12px style CH subscript 3 superscript minus >  1 to the power of straight o >   2 to the power of straight o >   3 to the power of straight o end style

    Larger the number of alkyl groups attached to the negatively charged carbon atom, greater will be the electron density on the carbon atom and lower will be its stability.

  • Different types of attacking reagent

    Attacking Reagent

    Examples

    Free radical: An atom or group of atoms having an unpaired electron.

     begin mathsize 12px style CH subscript straight 3 superscript straight • ,  straight C subscript straight 2 straight H subscript straight 5 superscript straight • ,  Cl to the power of straight • comma CH subscript straight 3 superscript straight • etc. end style

    Electrophile: Positively charged or neutral species which are electron deficient and can accept a pair of electrons, i.e. an electrophile takes away an electron pair.

    Positively charged:  

    begin mathsize 12px style straight H to the power of straight plus ,  straight H subscript straight 3 straight O to the power of straight plus comma CH subscript straight 3 to the power of straight plus comma NO subscript straight 2 to the power of straight plus end styleetc.

    Neutral:  

    begin mathsize 12px style AlCl subscript straight 3 ,  BF subscript straight 3 ,  SO subscript straight 3 end styleetc.

    Nucleophile: Nucleophile is a species that is electron rich and looks for electron deficient sites, i.e. nucleus loving or nucleus seeking (Nu), i.e.  a nucleophile brings an electron pair with it.

    Negatively charged:  

    begin mathsize 12px style straight X to the power of minus comma OH to the power of minus comma CN to the power of minus comma text     end text RCOO to the power of minus end styleetc.

    Neutral:

    begin mathsize 12px style straight N with • • on top straight H subscript 3 comma straight N with • • on top straight R subscript 3 comma straight H subscript 2 straight O with • • below and • • on top comma straight R straight O with • • below and • • on top straight H comma text     end text straight R straight O with • • below and • • on top straight R end style

     

  • 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 begin mathsize 12px style negative NO subscript straight 2 > - CN   > - COOH > - COOR > - straight F > - Cl > - Br  > - straight I  > - OH  > - OCH subscript straight 3 > - straight C subscript straight 6 straight H subscript straight 5 > - straight H end style
    • Atoms or groups of atoms having smaller electron attracting power than hydrogen are referred to as having +I-effect (electron donating or repelling). 
      Example: 

  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 begin mathsize 12px style straight pi minus end style 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 begin mathsize 12px style straight pi minus end style electrons of the multiple bonds are transferred to that atom to which the attacking reagent does not get 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



    Resonance effect is classified as +R effect and −R effect

    • If a substituent has tendency to donate electrons to double bond or conjugated system, then the effect is called the positive resonance effect or +R effect. Groups such as  begin mathsize 12px style negative OH , - OR , - NH subscript straight 2 , - NHR , - NR subscript straight 2 , - Cl space and  - Br end style 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 begin mathsize 12px style greater than straight C equals straight O , - CHO , - CN , - NO subscript straight 2 space and  - COOR end style  show −R effect.

  4. Hyperconjugation: It involves delocalisation of (s) 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 (s) 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 p systems (multiple bonds) and adjacent  bonds (single H–C bonds) of the substituent groups in organic compounds is called hypercojugation. It is a permanent effect.

    Example: Hypercojugation in propene



    Because there is no bond between the -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 -cloud, and hence it is not free to move. 
    Order of hyperconjugation: begin mathsize 12px style CH subscript straight 3 -  >   CH subscript straight 3 CH subscript straight 2 -> ( CH subscript straight 3 straight right parenthesis subscript straight 2 CH -   >( CH subscript straight 3 straight right parenthesis subscript straight 3 straight C minus end style

  • Types of organic reactions:

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