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General Concepts in Organic Chemistry

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General Concepts in Organic Chemistry General Organic Chemistry-Part I 1 General Concepts in Organic Chemistry (A) Types of Organic Reactions: There six common types of organic reactions for the purpose of a beginner. (a) Substitution (b) Addition (c) Elimination (d) Oxidation (e) Reduction (f) Molecular Rearrangement Substitution Reactions: One group or atom is substituted by another group or atom. heat/ R H ++ Br R Br HBr 2 light R Br ++ KOH R OH KBr In the first example, H atom has been substituted by Br atom, so that an alkane is converted to alkyl bromide. In the second example, a Br atomm is substituted by OH group, so that an alkyl bromide has been converted to an alcohol. Addition Reactions: A double bond or a triple bond in a molecule gets added up with another molecule(addendum). Hence the unsaturation in the molecule is vanished or reduced. Br Br CH2 CH2 + Br2 CH2 CH2 Cl CH CH + HCl CH2 CH In the first example, an alkene(ethene) adds with a Br2 molecule, each carbon atom joins with one Br atom to give 1,2-dibromoethane. Thus the unsaturation is vanished. In the second example, alkyne(ethyne) adds onto one HCl molecule, one carbon joins with H atom and the other with Cl atom to give ethenyl chloride(vinyl chloride). Thus degree of unsaturation is reduced. Note that other conditions of the reactions are not given here. It is just an introduction. Elimination Reactions: One molecule is eliminated from the organic molecule to increase the degree of unsaturation. A saturated compound gives an alkene and an alkene gives an alkyne. Two groups or atoms leave from the adjacent carbon atoms so as to produce a π bond in between them.In fact, this is called β-elimination. We shall know about other eliminations later. H Cl KOH(alcholic) CH2 CH2 CH2 CH2 + KCl + H2O H Cl NaNH CH CH 2 CH CH + NaCl + NH 3 Dr. S. S. Tripathy 2 General Organic Chemistry-Part I In the first example, ethyl chloride undergoes β-elimination in which one carbon loses H atom and the the other loses Cl atom to give ethene. In the second example, same thing happens with vinyl chloride to form a triple bond(ethyne). Oxidation Reactions: OH O O + K2Cr2O7/H O CH3 CH2 CH C H CH3 C OH O 3 Ethyl alcohol is first oxidised to ethanal(acetaldehyde) and then to ethanoic acid(acetic acid). So an organic molecule existing in the reduced state can be oxided by an oxidising agent to its oxidised state. Reduction Reactions: O OH LiAlH4 CH3 C CH3 + 2 [H] CH3 CH CH3 Propanone is reduced to propan-2-ol by a reducing agent such as LiAlH4. The organic molecule in its oxidised state can be reduced by a suitable reducing agent to its reduced state. Molecular Rearrangement: These reactions are rare and of not important for beginners like you. An organic molecule can undergo total reshuffling of bonds within itself by the action of heat or light or suitable catalyst to form a new product(isomeric). heat 3,4-dimethylhexa-1,5-diene octa-2,6-diene By mere heating 3,4-dimethylhexa-1,5-diene we get octa-2,6-diene. This is molecular rearragement. An isomeric product is formed from the reactant by intramolecular reorganisation of bonds. Types of Bond Cleavages: (a) Homolytic Cleavage(Homolysis): A single bond is broken with a one-electron shift to the respective atoms, so that each atom gets back its own electron. Such cleavage is shown by a fish-hook arrow mark( ). Such a cleavage results two free radicals(species having one one unpaired electron). XY X + Y Br Br 2 Br The homolytic cleavage of a X–Y bond in the molecule X–Y results two free radicals. A bromine molecule cleaves to two bromine free radicals. The single dot over the atom represents an unpaired electron(free radical). Heterolytic Cleavage(Heterolysis): When a covalent bond(single bond) is broken with a two-electron shift to the more electronegative atom or group, we get a positive and a negative ion. This is shown by a normal arrow mark( ). Dr. S. S. Tripathy General Organic Chemistry-Part I 3 XY X + Y The heterolytic cleavage of X–Y bond has resulted X+ and Y– ions. Here Y is taken as the more electronegative atom or group. (B) Reactants in Organic Reactions (a) Substrate: The main organic compound undergoing chemical change is called the substrate. (b) Reagent: The reactant which is responsible for the chemical change of the substrate is called the reagent. Reagent is mostly an inorganic compound. However, it can also be organic(we shall see later). CH3 CH2 Br + KCN CH3 CH2 CN + KBr Substrate Reagent Ethyl bromide is the substrate and KCN is the reagent. (C) Reactive Species from Reagents : 1. Electrophiles 2. Nucleophiles 3. Free Radicals Electrophiles: (Electron Seeking) These are themselves electron deficient species which seek electrons(–ve charge). These are called Lewis acids. These are of the following types. + + + + + + (a) All positive ions (E ): Cl , NO2 , H , (SO3H) , R ( alkyl carbocations), RCO+(acyl carbocations) etc. We shall discuss about carbocations later. (b) All electron deficient neutral molecules having incomplete octet: like BF3, AlCl3, BeCl2, carbenes(to be discussed later). (c) Neutral molecules having vacant d-orbitals such as PCl5, SF6, IF7, SiCl4 etc. (d) Acidic oxides such as SO3, CO2, P2O5 etc. Nucleophiles: (Nucleus Seeking): These carry electron pair and hence seek +ve charge. They are of the following types. – – – – – (a) All negative ions: Cl , CN , Br , OH , CH3COO etc. (b) Neutral molecules having a lone pair: H2O, NH3, PH3, ROH, R–O–R, RNH2 etc. (c) Akenes, alkynes, benzene and other compounds having multiple bonds. (d) Ambidentate Nucleophile: Such nucleophiles have two alternative donor atoms such as CN–(cyanide: donor atom is C) and NC–(isocyanide: doonor atom is N), similiarly SCN– – – (thiocyanate: donor atom is S) and NCS (isothiocyanate: donor atom is N), NO2 (nitro: donor atom is N) and ONO–(nitrito: donor atom is O) etc. Free Radicals: These are species containing unpaired electrons, formed by homolytic fission of sigma bonds(discussed before) catalysed by light(photolysis) or heat(pyrolysis) or by both. heat/ Br Br 2 Br heat light HO OH 2 OH OH + H Br H2O + Br Dr. S. S. Tripathy 4 General Organic Chemistry-Part I In the third case, a hydroxyl free radical takes out a H atom from HBr to produce bromine free radical and water molecule. Free radicals produced from reagents often bring about homolysis in organic substrates to form carbon free radicals(see later). Therefore, they are often called radical initiators. Organic peroxides like benzoyl peroxide(BPO) or di tert-butyl peroxide(DBPO) or azobisisobutyronitrile(AIBN) are excellent radical initiators. On heating, they easily produce primary free radicals which intiate the formation of carbon free radicals. O O O heat - CO Ph C OOCPh 2 Ph C O 2 Ph phenyl Benzoyl peroxide benzoyloxy radical radical Benzoyl peroxide gives both benzoyloxy and phenyl radicals, latter being a carbon free radical. CH 3 CH3 CH3 heat CH3 C NNCCH3 2 CH3 C + N2 CN CN CN isobutyronitrile radical AIBN AIBN on heating gives the isobutyronitrile radical, which is carbon free radical can can futher initiate formation of other carbon radicals(we shall see later). (D) Reactive Intermediates From Organic Substrates: Most reactions proceed via the formation of intermediates from the substrates. These intermediates are highly reactive and react further with active species from reagent to form the product. We discuss below five types of such intermediates. Carbocations: Positive ion in which charge is lying on carbon atom is called a carbocation( earlier called carboniunum ion). These are of two types. + (a) carbenium ions (trivalent: bonded to three atoms/groups) eg. CH3 (methenium ion or methyl carbocation). Carbon is sp2 hybridised(planar) with a vacant p-orbital. C (sp2 hybridised) Types of carbenium ions: 0 + + + 1 (primary) : R–CH2 (CH3 , CH3–CH2 etc.) 0 + + 2 (secondary) : R2CH [ (CH3)2CH ] 0 + + 3 (tertiary) : R3C [ (CH3)3C ] (b) carbonium ions:(penta or hexavalent: bonded to 5 or 6 atoms) eg. + + CH5 (methanium ion, also called hypervalent methenium ion), C2H7 (ethanium ion). These are very uncommon. H H H C H H Dr. S. S. Tripathy General Organic Chemistry-Part I 5 + + CH5 is the simplest carbonium ion(methanium ion) in which carbon atom is pentavalent. A CH3 structure is bonded with a H2 molecule with a 2e-3c( two electron three centred) bond. The two hydrogen atoms in the H2 molecule can continuously exchange positions with the three hydrogen + atoms in the CH3 ion. Formation of trivalent carbocations(carbenium ions): Carbocations(carbenium ions) can be formed in many ways, two of which are given below. (i) Heterolytic cleavage of C–Y bond in which is Y is more electronegative atom or group like a halogen atom(–Cl/–Br/–I). + - CY C + Y (ii) Addition of an electrophile(E+) to a C-C double or triple bond. CH2 CH2 + H CH2 CH3 nucleophile electrophile ethyl carbocation (ethenium ion) Reactions carbocations: (i) Addtion with a Nucleophile: Carbocations are electrophiles and hence react with nucleophiles to form the product. C + OH COH nucleophile H H C + O CO H H nucleophile (ii) Eliniation of proton :It can lose a proton from the adjacent carbon atom to form C=C (β-elimination) H B C C CC + BH alkene A base (B–) abstracts a proton from the adjacent carbon atom and thus a C=C is formed. (iii) Rearragement of carbocations: We shall know later that the stability of carbocations is in the order 30 > 20 > 10 Carbocations swiftly rearrage to equally stable and more stable carbocation by hydride ion or a carbanion shift from adjacent position.
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