Introduction to General Organic Chemistry

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Presentation credits:
Mr. Shivam Saha
Dr. Tathagata Deb

1. ANALYTICAL MINDS -A UNIT OF BHUBAN ENTERPRISES INTRODUCTION TO GENERAL ORGANIC CHEMISTRY

2. INTRODUCTION Organic chemistry is the chemistry discipline that is concerned with the study of compounds containing  carbon that is chemically bonded to hydrogen . Organic chemistry encompasses the synthesis, identification, modeling, and chemical reactions of such compounds. The range of chemical compounds studied in organic chemistry include  hydrocarbons (compounds containing only carbon and hydrogen), carbons containing other elements, especially oxygen, nitrogen, sulphur, phosphorus and the certainly halogens. The following is an example of an organic molecule Methane(CH 4 ) . (The line-angle structural formula shows four carbon-hydrogen single bonds (σ, in black), and the typical 3D shape of  tetrahedral molecules, with ~109° interior bond angles.)

3. CLASSIFICATION OF ORGANIC MOLECULE ORGANIC COMPOUNDS are of two types : ALIPHATIC COMPOUNDS : open chain compounds. CLOSED CHAIN COMPOUNDS: closed chain compounds. These are again classified into two types: HETEROCYCLIC COMPOUNDS : ring skeleton of compounds containing carbon atoms along with atoms of other elements. HOMOCYCLIC COMPOUNDS: ring skeleton of compounds containing only carbon atoms. These are of two types: Alicyclic compounds: these compounds have resemblance in properties with aliphatic compounds. Aromatic compounds : compound containing at least one benzene ring and having alternating double bonds within the ring. They have   agreeable  odor . These are further divided into two parts: Benezoid : carbocyclic aromatic. Non-Benezoid: heterocyclic aromatic.

4. STRUCTURAL REPRESENTATION OF ORGANIC MOLECULE ORGANIC MOLECULES can be represented in the following ways: Complete structural formula: A structural formula in which two electron covalent bond is depicted by a dash is called complete structural formula. Condensed structural formula: The structural formula obtained after deleting some or all of the covalent bonds and by indicating the number of identical groups attached to an atom by a subscript. The structural formula so obtained is called condensed structural formula. Bond line structural formula: The structural formula obtained by representing carbon-carbon bond by zigzag lines and not highlighting the carbon and hydrogen atom is known as bond line structural formula.

5. FUNCTIONAL GROUP: A Functional group may be defined as an atom or group of atoms which largely determines the properties of an organic compound. The family of  carboxylic acids contains a carboxyl (COOH) functional group. Acetic acid , shown above , is an example.

6. The above diagram is the homologous series of alkenes while the diagram below portrays the homologous series of the alkanes . HOMOLOGOUS SERIES: A homologous series may be defined as a series of similarly constituted compounds in which the members possess the same functional group and have similar chemical characteristics and the two consecutive members differ in their molecular formula by-

7. CLEAVAGE OF COVALENT BONDS O rganic reactions involve breaking of bonds in the reacting molecules and formation of new bonds to give rise to product molecules. The breaking of bonds can take place in two ways- HOMOLYTIC CLEAVAGE: When the bonding electrons are equally divided into the atoms of a bond resulting in the formation of free radicals are known as homolytic cleavage. HETEROLYTIC CLEAVAGE: When the bonding electron pair goes to the more electronegative atom of a covalent bond resulting into the formation of positively and negatively charged species are known as heterolytic cleavage.

8. REACTION INTERMEDIATES REACTION INTERMEDIATES are generally charged or neutral but electron deficient unstable species which are formed from the cleavage of bonds during chemical reactions. They are as the following: Free Radical: Have one odd electron as it is formed due to homolytic cleavage . Para-magnetic in nature due to the presence of unpaired electrons. Have both sp2 and sp3 hybridization . Mostly undergoes hyperconjugation. Stability of free radicals is as the following: Carbocation : Positively charged carbocation having 6 valance electrons due to the lacking of octet . It is sp2 hybridized and has a triangular planner structure with bond angle 120 ° .

9. iii. It has vacant p orbital which is perpendicular to the plane ,so it acts as a lewis acid . iv. Stability of the carbocation is as the following: Carbanion : These are sp3 hybridized , negatively charged trivalent carbon . Has one pair of non-bonded electrons. It has a tetrahedral shape with bond angle 109 ° 28` , so it has pyramidal shape . Stability of carbanion is as the following:

10. ATTACKING SPECIES NUCLEOPHILE : Nucleophiles are electron donating species . They are of two types: Neutral nucleophile : They have non-bonded donatable electrons. Charged nucleophile : These have less no of electrons due to vacant orbitals. ELECTROPHILE : Electrophiles are positively charged or electron deficient neutral species which can accept a pair of electrons in its vacant site. These are electron loving species .

11. ELECTRONIC EFFECTS Effects in organic chemistry : Displacement of electrons : Temporary effect - Hyperconjugation Permanent effect - Inductive effect & Resonance INDUCTIVE EFFECT : Only seen in sigma bonds . It depends upon distance. Seen in ground state of a molecule. It is of two types : +I effect -I effect

12. RESONANCE : It occurs in conjugate system i.e. sigma & pi system . Here electron release occurs via the pi bond . Here we see double electron transfer . It is of two types : -R effect : Eg: resonance of nitrobenzene +R effect : Eg: resonance of aniline

13. HYPERCONJUGATION : It occurs in sigma-sigma-pi system . It is known as no bond resonance . Stability depends on the availability of the no of alpha hydrogen (more alpha hydrogen, more stability)

14. HYBRIDIZATION OF ORGANIC COMPOUNDS The phenomenon of intermixing of orbitals of the same atom having slightly different energies to form new orbitals which have identical shapes and equivalent energies is known as hybridization . Hybridization is classified into six types : sp : Two equivalent orbitals formed due to intermixing of one s & one p orbital are called sp hybrid orbitals and the phenomenon is known as sp-hybridization. Eg: Ethyne s p2 : Three equivalent orbitals formed due to intermixing of one s & two p orbital are called sp2 hybrid orbitals and the phenomenon is known as sp2-hybridization . Eg: Ethene

15. sp3 : F our equivalent orbitals formed due to intermixing of one s & three p orbital are called sp3 hybrid orbitals and the phenomenon is known as sp3-hybridization . Eg: Ethane Sp3d : Five equivalent orbitals formed due to intermixing of one s , three p orbital and one d orbital are called sp3d hybrid orbitals and the phenomenon is known as sp3d-hybridization. Eg: PCl 5 Sp3d2 : Six equivalent orbitals formed due to intermixing of one s , three p orbital and two d orbital are called sp3d2 hybrid orbitals and the phenomenon is known as sp3d2-hybridization . Eg: SF 6 Sp3d3 :Seven equivalent orbitals formed due to intermixing of one s ,three p orbital and three d orbitals are called sp3d3 hybrid orbitals and the phenomenon is known as sp3d3-hybridization . Eg: IF 7

16. ISOMERISM IN ORAGANIC MOLECULES ISOMERISM in organic chemistry is of two types: Stereoisomerism - phenomenon where organic compounds have same molecular formula but have difference in spatial arrangement of atoms . They are of two types again: Geometrical isomerism : isomerism arising due to restricted rotation of molecules. Optical isomerism : Optical isomers are two compounds which contain the same number and kinds of atoms, and bonds (i.e., the connectivity between atoms is the same), and different spatial arrangements of the atoms, but which have  non-superimposable mirror images . Each non-superimposable mirror image structure is called an  enantiomer.

17. STRUCTURAL ISOMERISM: The isomers having different molecular structures due to the different arrangement of atoms in their molecules . These are of six types: Chain isomerism : In this type of isomerism, the isomers have different skeletons of carbon atoms . Eg: butane and 2-methylbutane. Position isomerism : In this type of isomerism, the isomers have different position of functional group or multiple bond. Eg: propane and 2-methylpropane. Functional isomerism : In this type of isomerism, the isomers have different functional group and thus belong to different families . Eg: ethanol and methoxymethane. Metamerism : In this type of isomerism, the isomers differs in structure due to difference in distribution of carbon atoms about the functional group . Eg: diethyl ketone and methyl propyl ketone.

18. Tautomerism: This is a special type of functional isomerism where isomers exists simultaneously in dynamic equilibrium with each other. It arises due to 1,3 migration of hydrogen atom from one polyvalent atom to other within the same molecule. Ring-chain isomerism: In this type of isomerism ,one isomer possess an open chain structure while the other has a cyclic structure.

19. ORGANIC REACTIONS in general can be classified into the following types: SUBSTITUTION REACTION: When an atom or group of molecules is replaced by another atom or group of molecules without changing the structure of the resting part of the molecule is known as substitution reaction. It is of three types: Free radical substitution reaction: here the attacking species is a free radical. For example let us consider the reaction involving halogenation of alkane in presence of uv rays. The mechanism: The mechanism involves a  chain reaction .   The over-all process is known as free radical substitution, or as a free radical chain reaction . It consists of three steps. Initiation : The chain is initiated (started) by UV light breaking a chlorine molecule into free radicals . P ropagation : These are the reactions which keep the chain going- Termination : These are reactions which remove free radicals from the system without replacing them by new ones.

20. Electrophilic substitution reaction: This reaction involves the attack of an electrophile. It generally happens in reactions of compounds containing benzene rings. Suppose the electrophile is a positive ion X+. The general mechanism: First stage- Two of the electrons in the delocalized system are attracted towards the X+ and form a bond with it. This has the effect of breaking the delocalization, although not completely The ion formed in this step isn't the final product. It immediately goes on to react with something else. It is just an intermediate. There is still delocalisation in the intermediate formed, but it only covers part of the ion. Second stage- A lone pair of electrons on Y -   forms a bond with the hydrogen atom at the top of the ring. That means that the pair of electrons joining the hydrogen onto the ring aren't needed any more . These then move down to plug the gap in the delocalized electrons, so restoring the delocalized ring of electrons which originally gave the benzene its special stability . Other types of electrophilic substitution reaction: HALOGENATION NITRATION SULPHONATION FRIEDEL CRAFT ALKYLATION AND ACYLATION.

21. Nucleophilic substitution reaction: The replacement of one nucleophile by other nucleophile is known as nucleophilic substitution reaction. It is of two types: Nucleophilic substitution unimolecular reaction ( S N 1) - it has two steps a) Formation of carbocation- b) Formation of product- Important facts about   S N 1 reaction : The rate of reaction increases for more substituted substrate. Substitution of a nucleophile attached to a chiral center leads to the formation of a racemic mixture. Ionization through S N 1 path is accelerated in the presence of polar solvents. Reactivity of a nucleophile has no effect on the S N 1 reaction . The solvent is the nucleophile in many S N 1 reactions . This is also called solvolysis reaction.

22. Nucleophilic substitution bimolecular reaction ( S N 2): it is a one step process. The S N 2 reaction is a one step process with single transition state but without any intermediate. The rate of the reaction depends upon the concentration of the substrate and the concerned nucleophile. The nucleophiles attacks from the back side of the leaving group. Mechanism : Important facts about   S N 2 reaction : When the carbon bearing the nucleophile becomes more substituted ,the rate of reaction decreases. Generally non-polar or less polar solvents are suitable for S N 2 reaction. The rate of reaction increases with the increase in nucleophilicity of the attacking nucleophile.

23. ADDITION REACTION: In this reaction ,the reagent and the substrate combine with each other to form a single product without loosing any part of the reagent and the substrate. It is of three types: Electrophilic addition reaction : An electrophilic addition reaction is an addition reaction where a molecule is attacked by an electrophile. The molecule has a region of high electron density which is attacked by something carrying some degree of positive charge . Mechanism: Nucleophilic addition reaction : This reaction happens in two distinct stages. The first involves an addition reaction where the initial attack is by a nucleophile , which is followed by an elimination reaction .So the mechanism is also known as nucleophilic addition reaction. Stage 1: b) Stage 2: Finally the chloride ion plucks the hydrogen off the original nucleophile. It removes it as a hydrogen ion, leaving the pair of electrons behind on the oxygen or nitrogen atom in that nucleophile. That cancels the positive charge.

24. Free radical addition reaction: The most common example is the HBr addition to unsymmetrical alkene in presence of organic peroxide. The free radical addition to alkene consists of the following steps- Propagation can occur in two steps: Termination:

25. REARRANGEMENT REACTION: The term rearrangement refers to the migration of a group from one atom to another within the same molecule. E g : The rearrangement of ammonium cyanate NH 4 NCO to urea (NH 2 ) 2 CO in aqueous solution at 50  o C ELIMINATION REACTION : Reaction in which two groups or atoms are removed from a molecule to form an unsaturated linkage or center are known as elimination reaction.

26. E1 mechanism: E1 stands for  unimolecular elimination  and has the following specificities. It is a two-step process of elimination:  ionization and deprotonation . Ionization: the carbon-halogen bond breaks to give a carbocation intermediate. Deprotonation of the carbocation. E1 typically takes place with  tertiary alkyl halides , but is possible with some secondary alkyl halides. The reaction rate is influenced only by the concentration of the alkyl halide because carbocation formation is the slowest step, ie. the rate-determining step. Therefore,  first-order kinetics apply (unimolecular). The reaction usually occurs in the complete absence of a base or the presence of only a weak base (acidic conditions and high temperature). E1 reactions are in competition with S N 1 reactions because they share a common carbocation intermediate . An example  reaction of tert-butylbromide with potassium ethoxide in ethanol

27. E2 mechanism : E2 stands for  bimolecular elimination.  The reaction involves a one-step mechanism in which  carbon-hydrogen  and  carbon-halogen  bonds break to form a double bond ( C=C  Pi bond ). The specifics of the reaction are as follows: E2 is a single step elimination, with a single  transition state . It is typically undergone by tertiary substituted alkyl halides , but is possible with some secondary alkyl halides and other compounds. The  reaction rate  is  second order , because it's influenced by both the alkyl halide and the base (bimolecular ). Because the E2 mechanism results in the formation of a pi bond, the two leaving groups (often a hydrogen and a  halogen ) need to be  antiperiplanar . An  antiperiplanar   transition state  has  staggered  conformation with lower energy than a  synperiplanar  transition state which is in   eclipsed conformation with higher energy. The reaction mechanism involving staggered conformation is more favorable for E2 reactions (unlike E1 reactions). E2 typically uses a strong base. It must be strong enough to remove a weakly acidic hydrogen. In order for the pi bond to be created, the  hybridization  of carbons needs to be lowered from  sp 3  to sp 2 . The C-H bond is weakened in the  rate determining step An example is the reaction of  isobutyl bromide   with potassium with potassium ethoxide in   ethanol . The reaction products are isobutylene, ethanol and  potassium potassium bromide .

28. PRESENTATION CREDITS DR . TATHAGATA DEB MR. SHIVAM SAHA ANALYTICAL MINDS -A UNIT OF BHUBAN ENTERPRISES