The Inductive Effect

Transcript

0:00Introduction

Hello everyone! Let us begin with a small puzzle. Take a chain of three carbons, with a chlorine atom on the first one. The chlorine touches only that first carbon, right? So why do the second and third carbons also become slightly positive? That is strange, isn't it? And why does this pull almost die out after the third carbon? The answer is the inductive effect. It comes up often in JEE, so let's begin.

0:30The inductive effect: a pull passed along σ bonds

So, the basic idea first. When two atoms of different electronegativity are joined by a sigma bond, the shared pair is pulled towards the more electronegative atom. Sigma is the Greek letter on screen, and every single bond is a sigma bond. Okay? Here is the chain from our puzzle: C H three, C H two, C H two, then chlorine, and the small numbers written low are subscripts. The little arrow points the way the electrons are pulled, towards chlorine. Above the chlorine is delta minus, the Greek letter delta with a minus sign, which means a small, partial negative charge. With me? Now look at the carbons. Next to chlorine is delta plus, then delta delta plus, then delta delta delta plus at the end. So the end carbon, with three deltas, is the most positive, right? No, no, careful, it is the other way round! Each extra delta means a smaller charge. The pull is passed on along the sigma bonds, getting weaker at each step, so the carbon bonded to chlorine is the most positive. That solves the first part of our puzzle. Clear? Now, two more facts about this effect. It is permanent, present in the molecule all the time. And it is almost negligible after the third carbon, which solves the second part of our puzzle. Got it?

1:50-I and +I groups: hydrogen is the reference

Now, we compare every group with hydrogen, the reference. Minus I, a minus sign and a capital I for inductive, means the group withdraws electrons more than hydrogen does. Plus I means it releases electrons more than hydrogen does. Clear? So let me ask you this. Fluorine is the most electronegative element of all, so it must have the strongest minus I effect, right? That is a common mistake, so be careful! Here is the course's order, and each greater than sign means stronger than. The short dash in front of each group is the bond that joins it to the chain. At the top is N R three, where R is an alkyl group and the small plus shows a positive charge on the nitrogen. Then nitro, N O two, cyano, C N, and the acid group, COOH, and only then fluorine, followed by chlorine, bromine and iodine. Okay? The list goes on with the alkoxy group, O R, then O H, and last of all phenyl, C six H five. They all still pull harder than hydrogen, so all of them are minus I groups. With me? Now come the plus I groups, which release electrons. At the top is O minus, an oxygen with a negative charge, shown by the small minus sign written high. Then the carboxylate ion, COO minus. Then tert butyl, isopropyl, ethyl and methyl; the groups in brackets are branches, and each extra alkyl group adds to the electron release. Then D for deuterium, and finally hydrogen. Clear? Now wait, look closely. O H is a minus I group, but O minus is the strongest plus I group, with the same oxygen! So tell me, what changed here? Yes, exactly, the charge! So, a positive charge adds to the pull on electrons, and a negative charge reverses it. That is why N R three plus tops the minus I list, while O minus and COO minus top the plus I list. Got it?

3:51Carbon's s-character, and adding up the effects

Now, even a carbon can pull harder, depending on its hybridisation. The more s character a carbon has, the more electronegative it is, and the double arrow means implies. Okay? The small three and two written high in these names are superscripts. So which carbon is the most electronegative: s p three, s p two, or s p? It is the s p carbon, very good! An s p three carbon has twenty five percent s character, s p two has thirty three percent, and s p has fifty percent. Right? So ethynyl, C triple bond C H, with its s p carbon, has the strongest minus I effect here; the three short lines are a triple bond. Then comes phenyl, C six H five, then vinyl, C H double bond C H two. Clear? Now, two last rules. When a molecule has several such groups, their effects add up. With me? And the effect falls off quickly with the number of bonds in between, just like in our chain. Keep this in mind for the second example. Okay?

4:55Example 1: classify NO2, CH3, Cl, COO-, OH as +I or -I

Alright, example one, a warm up. Classify as plus I or minus I: nitro, N O two; methyl, C H three; chlorine; carboxylate, COO minus; hydroxyl, O H; and tert butyl, C, then C H three in brackets, three times. Pause the video and try it first. Done? Okay, let's check. Nitro and chlorine are on the minus I list, easy. And O H is minus I as well, since it pulls harder than hydrogen. Clear? Now the tricky one, COO minus. It has two oxygens, and COOH was high on the minus I list. So it must be minus I too, right? No, no, wait, look at the charge! Remember, a negative charge reverses the pull, so COO minus is plus I, second only to O minus. Methyl and tert butyl are alkyl groups, so they are plus I as well. So the minus I groups are nitro, chlorine and hydroxyl, and the plus I groups are methyl, carboxylate and tert butyl. Did you get all six?

6:04Example 2: strongest -I effect among chlorobutanoic acids

Now example two, where our puzzle comes back. In which acid is the minus I effect of chlorine on the COOH group the strongest: two chloro, three chloro, or four chloro butanoic acid? Pause the video and try it first. Done? Okay, let's check. Butanoic acid is a chain of four carbons, and the small numbers above the carbons are their positions. Remember from naming: the COOH carbon is carbon one. Okay? In two chloro butanoic acid, chlorine sits on carbon two, right next to the COOH carbon, and the brackets show it is a branch. Clear? In the three chloro acid, it sits on carbon three, one carbon further away. Right? And in the four chloro acid, it is at the far end of the chain, on carbon four. With me? So now tell me, in which acid does chlorine pull the hardest on the COOH group? Yes, think of our chain! The effect weakens with each bond between the chlorine and the group, so fewer bonds in between means a stronger pull. Got it? So the answer is two chloro butanoic acid. In the four chloro acid, the pull travels down the whole chain and is very weak by the time it arrives, just like the end carbon in our puzzle. See how simple that was?

7:22Quick revision: -I and +I orders, charge, s-character

So, quick revision. The inductive effect is a polarisation passed along sigma bonds; it is permanent, and it fades fast. Minus I groups pull electrons and plus I groups release them, compared with hydrogen, and a charge changes the effect. More s character pulls harder, the effects of several groups add up, and fewer bonds in between means a stronger effect. Now go and try the practice problems on this page. You will find them easy. All the best!