Classification

Transcript

0:00Introduction

Hello everyone! Here is a small puzzle to start. Take two small chlorides. In the first, the chlorine sits right on a carbon of a double bond. In the second, we slip just one C H two group in between the double bond and the chlorine. Yet towards nucleophilic substitution, the second one is very reactive, and the first is very unreactive. How could you predict that, just by looking? The answer is classification: where exactly the halogen sits. It comes up often in JEE, so let's begin!

0:33Haloalkanes and haloarenes

First, two big families. In a haloalkane, or alkyl halide, the halogen sits on an s p three carbon of an alkyl group, with the three written small and raised. We write it R X, where R stands for the alkyl group, the dash for the bond, and X for the halogen. Okay? Now, in the second family, the halogen is on a benzene ring. So tell me, what is the hybridisation of a ring carbon in benzene: s p three, or s p two? It is s p two, yes, exactly! So in a haloarene, or aryl halide, the halogen is directly on an s p two carbon of an aromatic ring. We write it A r X, with A r for an aryl group. Right? And X can be fluorine, chlorine, bromine or iodine, written F, C l, B r and I on screen. Clear?

1:23Counting the halogens

The first way to classify is to count the halogen atoms. One gives a monohalogen compound, two a dihalogen, three a trihalogen, and more, a polyhalogen compound. Simple, right? With two halogens, there are two cases. If both halogens sit on the same carbon, the dihalide is geminal; its other name, in brackets, is an alkylidene dihalide. Okay? And if they sit on adjacent carbons, that is, neighbours, it is vicinal, also called an alkylene dihalide. So geminal means the same carbon; vicinal, neighbouring carbons. Got it?

1:56Halogen on an sp³ carbon

Now the most important way: look at the carbon that carries the halogen. First case, X on an s p three carbon. For a plain alkyl halide, count the carbons attached to the carbon bearing X, okay? Look at the first molecule in the figure: C H three, C H two, X, where the small numbers written low count the hydrogens. How many carbons are attached to the carbon that carries X? Just one, the C H three, correct! So it is primary, written as the number one with a small raised circle, the degree sign. Two attached carbons make it secondary, and three, tertiary. And a common trap: count only on the carbon bearing X, never on its crowded neighbour. Clear? Next, the second molecule: C H two double bond C H, C H two X; the two short lines are the double bond. X sits on the s p three carbon next to it, so it is an allylic halide. Okay? And in the third molecule, X is on the C H two, an s p three carbon, which is attached to a benzene ring. That makes it a benzylic halide. Right?

3:05Halogen on an sp² carbon

Now the second case. Look at the fourth molecule, C H two double bond CHX, and compare it with the allylic one. Is X next to the double bond now, or on it? It is right on a carbon of the double bond, yes! Those carbons are s p two, as the heading says. So X on a carbon of the double bond gives a vinylic halide. Got it? And the last molecule has X directly on a carbon of the benzene ring, which is also s p two. That is an aryl halide, our haloarene. With me? Here is why these classes matter. They tell us the reactivity towards nucleophilic substitution, the reaction from our puzzle. Okay? Allylic and benzylic halides are very reactive. Right? But vinylic and aryl halides are very unreactive. Same halogen, yet the carbon it sits on changes everything. We will see the reasons later in this unit. Clear?

4:03Topper's trick: two questions

Now here is a topper's trick: look only at the carbon that carries X, and ask two questions. Question number one is this: is that carbon s p three, or s p two? Okay? Question number two: what is that carbon attached to? How many carbons, a double bond, or a benzene ring? Those two answers give you the class. Got it?

4:25Example 1: vinylic, allylic, benzylic or aryl chloride

Time for the first example. Classify each as alkyl, allylic, benzylic, vinylic or aryl. Part A is C H two double bond CHC l, and part B, C H two double bond C H, C H two C l. In part C, the phenyl group, C six H five, a benzene ring, carries C H two C l; part D is C six H five C l. Pause the video and try it first. Done? Okay, let's check, with our two questions. In part A, the chlorine sits on a carbon of the double bond, an s p two carbon. So it is vinylic. Right? In part B, the chlorine is on the C H two, an s p three carbon right next to the double bond. So it is allylic. Okay? Now, part C has a benzene ring in it. So it must be an aryl halide, right? No, no, careful, because the chlorine is not on the ring! It sits on the C H two, an s p three carbon attached to the ring. So it is benzylic. Clear? And in part D, the chlorine is directly on a ring carbon, an s p two carbon. So that one is aryl. With me? Now, remember our puzzle? Part B is allylic, so very reactive, and part A is vinylic, so very unreactive. One extra C H two lifts the chlorine off the double bond, and that solves our puzzle!

5:49Example 2: spotting benzylic halides and their degree

Now a harder one, mixing both ideas. Classify part A, C six H five, CHC l, C H three, and part B, C six H five, C H two, C H two C l. In part C, C six H five sits in brackets with a small three after it: three phenyl groups on one carbon, which also carries the chlorine. Pause the video and try it first. Done? Okay, let's check part A first. The carbon bearing chlorine carries a C H three, and the benzene ring. Now, does the ring count as an attached carbon? Yes, it does, because the ring is joined through one of its carbons. So two carbons are attached, which makes it secondary. And that carbon is attached to a benzene ring, so it is secondary benzylic. Okay? Now look at part B, which has a benzene ring and a C H two next to it. So it is benzylic again, right? No, no, wait, look at where the chlorine is! It sits on the end C H two, attached to only one carbon, the other C H two; the ring is one carbon further away. So it is a primary alkyl halide, not benzylic. That is the trap. Clear? And in part C, the carbon bearing chlorine carries three phenyl rings, so three carbons are attached. That makes it tertiary benzylic. See, always look at the carbon bearing X, and nothing else. Got it?

7:19Quick revision: sp3 and sp2 classes and their reactivity

So, quick revision. In a haloalkane, X is on an s p three carbon; in a haloarene, on an s p two carbon of an aromatic ring. Two halogens on one carbon are geminal; on adjacent carbons, vicinal. On s p three: primary, secondary, tertiary, allylic and benzylic; on s p two: vinylic and aryl. And towards nucleophilic substitution, allylic and benzylic halides are very reactive; vinylic and aryl, very unreactive. Now go and try the practice problems on this page. All the best!