Inductive Effect PDF

INDUCTIVE EFFECT DEFINITION: The inductive effect refers to polarity produced in a molecule as a result of high electronegativity difference of one atom as compared to another. Or It is the process of electron shift along the chain of atoms due to the presence of...

INDUCTIVE EFFECT DEFINITION: The inductive effect refers to polarity produced in a molecule as a result of high electronegativity difference of one atom as compared to another. Or It is the process of electron shift along the chain of atoms due to the presence of polar covalent bond in the molecule.  EXPLANATION The electrons of the covalent bond formed between similar atoms lie exactly in the middle of both atoms and the separation of charge does not exists. For example: In H2, CH3-CH3 and Cl2 molecules do not have a displacement of H-H, C-C, and Cl-Cl bonded electrons. so the induction effect is not present in these molecules. As it involves the sigma electrons, the sigma electrons which form covalent bond are seldom shared equally between two different atoms. This is because different atoms have different electronegativity values i.e. different powers of attracting electrons in the bond. Consequently electrons are displaced towards the more electronegative atom. This produces a certain degree of polarity in the bond. The more electro-negative atom acquires a partial negative charge and less electro-negative atom acquires a partial positive charge. EXAMPLE 1:  Consider the carbon-chloride bond. As chloride is more electronegative, charge on it appears partial negative with respect to carbon atom.  The arrow head shows the direction in which the electrons are shifted.  EXAMPLE 2:  The C-Cl bond in butylchloride is polarized due to electronegative difference.  The electrons are withdrawn by chlorine atom.  Thus the first carbon atom gets a partial positive charge, in turn this carbon drags electron density from next carbon which also gets partial positive charge,  Thus the inductive effect is transmitted through the carbon chain  Inductive effect weakens along the chain and is not significant beyond third carbon atom. SALIENT FEATURES OF INDUCTIVE EFFECT: It is due to electronegativity difference. It is transmitted through sigma bond. The magnitude of inductive effect decreases while moving away from the group causing it. It is a permanent effect. It influences chemical and physical properties of compound. TYPES OF INDUCTIVE EFFECTS: 1. NEGATIVE INDUCTIVE EFFECT: The electron withdrawing nature of groups or atoms is called as negative inductive effect. It is indicated by -I. Following are the examples of groups in the decreasing order of their -I effect: NH3 + > NO > CN > SO3H > CHO > CO >COOH > COCl > CONH2 > F > Cl > Br > I > OH > OR > NH2 > C6H5 > H 2. POSITIVE INDUCTIVE EFFECT: It refers to the electron releasing nature of the groups or atoms and is denoted by +I. Following are the examples of groups in the decreasing order of their +I effect. C(CH3)3 > CH(CH3)2 > CH2CH3 > CH3 > H APPLICATIONS OF INDUCTIVE EFFECT 1. Stability of carbocations: The stability of carbocations increases with increase in number of alkyl groups due to their +I effect. As the alkyl groups release electrons to carbon bearing positive charge, thus stabilizes the ion. The order of stability of carbocations is : 2.Stability of carbanions: However the stability of carbanions decreases with increase in the number of alkyl groups since the electron donating alkyl groups destabilize the carbanions by increasing the electron density. Thus the order of stability of carbanions is: 3. Acidic strength of carboxylic acids: Rule: higher acidic strength means acid can easily release proton which is possible when negative charge on carboxylate ion is less/or less e- density  The electron withdrawing groups (-I) decrease the negative charge on the carboxylate ion (COO) and thus stabilizing it.  Hence the acidic strength increases when -I groups are present.  However the +I groups decrease the acidic strength. e.g. The acidic strength increases with increase in the number of electron withdrawing Fluorine atoms as shown below. CH3COOH < CH2FCOOH < CHF2COOH < CF3COOH  Formic acid is stronger acid than acetic acid since the –CH3 group destabilizes the carboxylate ion. EXERCISE: a. b. Which of the two is more acidic in nature? Why? 4. Basic strength of amines: Rule: more the electron density on nitrogen atom, more basic is the base The electron donating groups like alkyl groups increase the basic strength of amines. whereas the electron withdrawing groups like aryl groups decrease the basic nature. Therefore alkyl amines are stronger Lewis bases than ammonia, whereas aryl amines are weaker than ammonia. Thus the order of basic strength of alkyl and aryl amines with respect to ammonia is : CH3NH2 > NH3 > C6H5NH2  Exercise: CCl3NH2 a. b. Trichloromethyl amine Which of the two is more basic amine? Why?  5. Reactivity of compounds: Rule: a good substitution reaction is one which is fast. For fast reaction the group to be replaced showed be removed easily. For easy removal the removing group should be held loosely by rest of the molecule. For example if we replace chloride ion by hydroxyl group. Tertiary butyl chloride will give best reaction than secondary propyl chloride then primary ethyl chloride. This is because of highest +I effect in tertiary chloride due to the presence of 3 donating carbons to the carbocation holding the chloride ion. The donating group decreases the positive charge on carbon to maximum by donating electron to it. Thus ability of attracting negatively charged chloride is markedly decreased in it. primary- ethyl sec- propyl T- butyl chloride chloride chloride LEAST MORE MOST REACTIVE REACTIVE REACTIVE SLOWEST INTERMEDI FASTEST REACTION ATE REACTION REACTION -Cl being substituted by – OH group The +I groups increase the electron density at carbonyl carbon. Hence their reactivity towards nucleophiles decreases. Thus formaldehyde is more reactive than acetaldehyde and acetone towards nucleophilic addition reactions. Thus the order of reactivity follows: 6.Alcoholic strength: Rule: higher alcoholic strength means alcohol can easily release hydroxyl group which is possible when positive charge on carbon having OH group is less/more stable/or have less e- deficiency The strength of alcohols increases with increase in number of +I (alkyl) groups due to their +I effect. More the number of alkyl groups donating electrons to partial positive carbon bearing hydroxyl group, lesser the +ive charge and more the stability. Hence –OH attraction by the charged carbon decreases and release from molecule becomes easier. THE END