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Basic Concepts of Organic Reactions

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Basic Concepts of Organic Reactions Unit 12 Basic concepts of organic reactions Learning Objectives After learning this unit, students will be able to • understand the concept of organic reaction mechanism Otto dielsdiels andand Kurtkurt Alderalder • describe homolytic and heterolytic fission of bonds describesdescribe anan important reaction mechanism for • identify free radicals, nucleophiles and electrophiles, the reaction between a conjucated diene and a • classify organic reactions into substitution, substituted alkene. For this elimination, addition, oxidation and reduction work they were awarded • describe electron movement in organic reactions nobel prize in chemistry in 1950 DielsDiels - -Alder Alder reaction reaction is • explain the electronic effects in co-valent bonds isaa powerful powerful tool tool inin synthetic organic chemistry. 12.1 Introduction A chemical reaction can be treated as a process by which some existing bonds in the reacting molecules are broken and new bonds are formed. i.e., in a chemical reaction, a reactant is converted into a product. This conversion involves one or more steps. A In general an organic reaction can be represented as Substrate + Reagent [Intermediate state (and/or) Transition State] Product Here the substrate is an organic molecule which undergoes chemical change. The reagent which may be an organic, inorganic or any agent like heat, photons etc., that brings about the chemical change Many chemical reactions are depicted in one or more simple steps. Each step passes through an energy barrier, leading to the formation of short lived intermediates or transition states. The series of simple steps which collectively represent the chemical change, from substrate to product is called as the mechanism of the reaction. The slowest step in the 161 Unit 12 Final - jagan.indd 161 4/14/2020 6:27:13 PM mechanism determines the overall rate of of UV light in a compound containing the reaction. non polar covalent bond formed between atoms of similar electronegativity. In such 12.1.1 Fundamental concepts in organic molecules, the cleavage of bonds results reaction mechanism into free radicals. They are short lived and are highly reactive. The type of reagents that The mechanism is the theoretical promote homolytic cleavage in substrate pathway which describes the changes are called as free radical initiators. For occurring in each step during the course example Azobisisobutyronitrile (AIBN) of the chemical change. An organic and peroxides such as benzoyl peroxide reaction can be understood by following are used as free radical initiators in the direction of flow of electrons and the polymerisation reactions. type of intermediate formed during the course of the reaction. The direction of hυ flow of electron is represented by curved CO O O CO arrow. The movement of a pair of electron is represented by a double headed arrow 2 CO + which starts from the negative and ends 2 with the atom to which the electrons needs to be transferred. As a free radical with an unpaired electron is neutral and unstable, it has a 12.1.2 Fission of a covalent bond tendency to gain an electron to attain All organic molecules containstability. Organic reactions involve covalent bonds which are formed by homolytic fission of C-C bonds to form the mutual sharing of electrons between alkyl free radicals. The stability of alkyl atoms. These covalent bonds break in two free radicals is in the following order different ways, namely homolytic cleavage ˙C(CH ) > ˙CH(CH ) >˙CH CH >˙CH (symmetrical splitting) and heterolytic 3 3 3 2 2 3 3 cleavage (unsymmetrical splitting). The Heterolytic Cleavage cleavage of a bond in the substrate is influenced by the nature of the reagent Heterolytic cleavage is the (attacking agent). process in which a covalent bond breaks unsymmetrically such that one of the Homolytic Cleavage bonded atoms retains the bond pair of electrons. It results in the formation Homolytic cleavage is the processof a cation and an anion. Of the two in which a covalent bond breaks bonded atoms, the most electronegative symmetrically in such way that each of atom becomes the anion and the other the bonded atoms retains one electron. atom becomes the cation. The cleavage is It is denoted by a half headed arrow (fish denoted by a curved arrow pointing towards hook arrow). This type of cleavage occurs the more electronegative atom. under high temperature or in the presence 162 Unit 12 Final - jagan.indd 162 4/14/2020 6:27:14 PM For example, in tert-butyl bromide, Empty the C-Br bond is polar as bromine is more p orbital electronegative than carbon. The bonding Sp2–S electrons of the C-Br bond are attracted Overlap more by bromine than carbon. Hence, the H C-Br undergoes heterolytic cleavage to H C+ form a tert-butyl cation during hydrolysis. H CH3 CH3 δ δ H O + H C C Br 2 H3C C Br 3 CH3 CH3 CH3 Fig 12.1(a) Shape of Carbocation Thecarbanions are generally Let us consider the cleavage in a pyramidal in shape and the lone pair carbon-hydrogen (C-H) bond of aldehydes occupies one of the sp3 hybridised orbitals. or ketones We know that the carbon is more An alkyl free radical may be either pyramidal electronegative than hydrogen and hence or planar. the heterolytic cleavage of C-H bonds results in the formation of carbanion (carbon Lone Pair in bears a negative charge). For example in sp3 Orbital aldol condensation the OH- ion abstracts a α-hydrogen from the aldehyde, which leads Sp3–S to the formation of the below mentioned Overlap carbanion. C– H H OH (aq) H CH2 C H CH2 C H + H2O H –: O O CH3 Fig 12.1(b) Shape of Carbanion Hybridisation of carbon in carbocation: Unpaired electron In a carbocation, the carbon bearing positive in p orbital charge is sp2 hybridised and hence it has a Sp2–S planar structure. In the reaction involving Overlap H such a carbocation, the attack of a negatively charged species (nucleophiles) take place H C on either side of the carbocation as shown H below. CH3 Fig 12.1(c) Shape of Carbon radical 163 Unit 12 Final - jagan.indd 163 4/14/2020 6:27:14 PM The relative stability of the alkyl carbocations 12.1.3 Nucleophiles and elctrophiles and carbanions are given below. Nucleophiles are reagents that has Relative stability carbocations. high affinity for electro positive centers. +C(CH ) > +CH(CH ) > +CH CH > +CH They possess an atom has an unshared pair 3 3 3 2 2 3 3 of electrons, and hence it is in search for an relative stability of carbanions electro positive centre where it can have an opportunity to share its electrons to form a –C(CH ) < –CH(CH ) < –CH CH < –CH 3 3 3 2 2 3 3 covalent bond, and gets stabilised. They are The energy required to bring aboutusually negatively charged ions or electron homolytic splitting is greater than that of rich neutral molecules (contains one or heterolytic splitting. more lone pair of electrons). All Lewis bases act as nucleophiles. Electron Types Examples rich site Ammonia (NH3) and amines (RNH2) N: Neutral molecules having Water (H2O), alcohols (ROH) and ethers (R-O-R) :O: unshared pair of electron Hydrogen sulphide (H2S) and thiols (RSH) :S: Chlorides (Cl–), bromides (Br–) and iodides (I–) X- Negatively charged Hydroxide (HO– ), alkoxide (RO-) and Carboxlate ions O- nucleophiles (RCOO–) Cyanide (CN–) N- Electrophiles are reagents that are attracted towards negative charge or electron rich center. They are either positively charged ions or electron deficient neutral molecules. All Lewis acids act as electrophiles. Neutral molecules like SnCl4 can also act as an electrophile, as it has vacant d-orbitals which can accommodate the electrons from others. Electron deficient Types Examples entity Carbon dioxide (CO ), dichlorocarbene (:CCl ) 2 2 C Neutral electrophiles Aluminium chloride (AlCl ), boron trifluoride 3 Metal (M) (BF3) and ferric chloride (FeCl3) Carbocations (R+) C+ Proton (H+) H+ Positively charged Alkyl halides (RX) X+ electrophiles Nitrosonium ion (NO+) O+ + + Nitronium ion ( NO2) N 164 Unit 12 Final - jagan.indd 164 4/14/2020 6:27:15 PM Human body produces Type 1: A lone pair to a bonding pair free radicals when it is exposed to x-rays, ·· ·· ·· H H O H cigarette smoke, H O·· ·· industrial chemicals H and air pollutants. Free radicals ·· ·· ·· CH O C can disrupt cell membranes, O·· 2 ·· increase the risk of many forms H of cancer, damage the interior lining of blood vessels and lead Type 2: A bonding pair to a lone pair to a high risk of heart disease and stroke. Body uses vitamins and ·· H O H H O ·· H minerals to counter the effects ·· ·· ·· of free radicals. Fruits contains H antioxidants which decrease the ·· ·· ·· C O CH2 effects of free radicals. O·· ·· H 12.1.4 Electron movement in organic reactions Type 3: A bonding pair to an another bonding pair All organic reactions can be understood by following the electron H movements, i.e. the electron redistribution H BH BH3 HH during the reaction. The electron movement 3 depends on the nature of the substrate, reagent and the prevailing conditions. The CH2 H2C flow of electrons is represented by curved CH2 arrows which show how electrons move as shown in the figure. These electron H H movements result in breaking or formation H2CCH2 of a bond (sigma or pi bond). The movement H2CCH2 of single electron is indicated by a half H -headed curved arrows. H There are three types of electron movement H2CCH H2CCH2 viz., H • lone pair becomes a bonding pair. 12.1.5 Electron displacement effects in • bonding pair becomes a lone pair co-valent bonds • a bond breaks and becomes another Some of the properties of organic bond.
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