AIEEE and Earlier JEE Main Questions: Polyhalogen Compounds

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Heat chloroform with silver powder, and you get ethyne, with a triple bond between its two carbons. Now replace the hydrogen of chloroform with a methyl group. That gives one, one, one trichloroethane. What will silver powder make from this one? Ethyne again, or something new? Hold that thought. Today we solve older A I triple E and JEE Main questions on polyhalogen compounds, and these ideas do come up in the exam. So, let's begin.

0:35Recap: dihalides with aqueous alkali (KOH)

So, first, a quick recap from this chapter. Here, X stands for any halogen. In a geminal dihalide, both X atoms sit on one carbon; in a vicinal one, they sit on adjacent carbons. Okay? Now heat a vicinal dihalide with aqueous alkali, such as the KOH in our heading. Cl C H two, dash, C H two Cl gives HOCH two, dash, C H two O H, a diol. The small numbers written low count the atoms, the dash is a bond, and the arrow means gives. Right? Now a geminal dihalide. Here, both O H groups land on the same carbon. So what happens to that carbon? It loses a molecule of water at once, yes, exactly, because two O H groups on one carbon are usually unstable. Clear? So which product do we get? At the end of the chain, CHX two, with the dash joining it to the rest, is terminal and gives an aldehyde. Inside the chain, the angle sign before C X two shows two bonds to other carbons, and that non-terminal carbon gives a ketone. With me? And three halogens on one carbon, C X three, give a carboxylic acid, present as its salt in the alkali. Got it?

1:56Recap: chloroform with silver powder

Now our second tool, from the chloroform lesson. Heat chloroform, C H Cl three, with A g, silver powder. Two molecules and six silver atoms give H C, triple bond, C H, which is ethyne, plus six A g Cl, silver chloride. The three short lines between the carbons show the triple bond. Okay? Now tell me this. How many silver atoms does one molecule of chloroform use up? It uses three, correct, one silver atom for each chlorine atom, and each pair leaves as silver chloride. Right? And the carbons? Each one has lost all its chlorine, and the two join by a triple bond, giving ethyne. Keep this picture in mind, because it is the key to our puzzle. Clear?

2:45DDT: how it is made, and why it is banned

Now our third tool is DDT, the third compound in this figure, after chloroform and freon twelve, and before BHC, the compound whose gamma isomer is gammaxene. In DDT, two benzene rings, each with a chlorine at the far end, hang on one C H carbon, which carries a C Cl three group. Okay? Its full name is p p prime dichlorodiphenyltrichloroethane. The small dash after the second p is called a prime, and p means para: each ring's chlorine sits opposite the bond to the C H carbon. Right? How is it made? Two molecules of chlorobenzene, C six H five Cl, join one molecule of chloral, C Cl three CHO, and water, H two O, comes out. The brackets with a small two outside show the two Cl C six H four groups. With me? The reagent is concentrated sulphuric acid, H two S O four, and the short form c o n c means concentrated. Clear? DDT was the first chlorinated organic insecticide, used against the mosquitoes that spread malaria and the lice that carry typhus. Got it? So here is a question for you. Such a useful insecticide, and yet many countries have banned it. Why would they do that? Because it is not biodegradable, exactly! It is not broken down quickly in the body or the environment. It is fat-soluble, so it is stored in fatty tissue and builds up along the food chain. Okay? It is also very toxic to fish, and for these reasons it is banned in many countries. Two past questions on this page test exactly these facts. Right?

4:27Example 1: which dichloride gives acetaldehyde (AIEEE 2009)

Now Example one, from A I triple E two thousand and nine. Which of these four gives acetaldehyde on heating with aqueous KOH: C H three C O Cl, C H three C H two Cl, C H two Cl C H two Cl, or C H three C H Cl two? Pause the video and try it first. Done? Okay, let's check. Many students see two chlorines in C H two Cl C H two Cl and pick it at once. Two chlorines, so an aldehyde, right? Careful, because that is exactly the trap! Its chlorines are on adjacent carbons, so it is vicinal, and it gives a diol, HOCH two C H two O H, not an aldehyde. With me? Next, C H three C H two Cl, chloroethane, has only one chlorine, so we get an alcohol, C H three C H two O H, which is ethanol. Okay? Then C H three C O Cl, acetyl chloride, is an acid chloride. Water turns it into the acid, C H three COOH, which stays as its salt in the alkali, so again there is no aldehyde. Right? So, the last one, C H three C H Cl two. Both chlorines are on the end carbon, so it is geminal and terminal. We get C H three C H, O H in brackets, two, with two O H groups on one carbon. It loses water at once and gives C H three CHO, which is acetaldehyde! Clear? So the answer is C H three C H Cl two, one, one dichloroethane. Remember, from a dihalide, an aldehyde needs both halogens on the end carbon. Got it?

6:22Example 2: CH3CCl3 with silver powder gives but-2-yne (2014)

Now Example two, from JEE Main two thousand and fourteen, and it is our puzzle from the start. What is the major organic compound formed when one, one, one trichloroethane reacts with silver powder? Pause the video and try it first. Done? Okay, let's check. One, one, one means all three chlorines sit on carbon number one, so the compound is C H three, dash, C Cl three. Right? Now, silver powder gave ethyne from chloroform. So this one gives ethyne again, doesn't it? No, it does not, so look at the chloroform equation again. Each carbon lost all three chlorines, but its hydrogen stayed on and ended up in ethyne, H C triple bond C H, as you see. With me? In our compound, a methyl group, C H three, sits where that hydrogen was. Silver removes only the chlorine, so the methyl group stays on. Clear? So, two molecules and six silver atoms, as before, give C H three C, triple bond, CCH three, and six A g Cl. Count the atoms: four carbons, six hydrogens and six chlorines on each side. Okay? So the product has four carbons, with the triple bond between carbons two and three, and that is but two yne. Puzzle solved: the methyl groups ride along, so we get but two yne, not ethyne. Got it?

7:52Quick revision: dihalides with KOH, silver powder and DDT

So, quick revision. With aqueous alkali, a vicinal dihalide gives a diol, and a terminal geminal one gives an aldehyde. Silver powder removes all the chlorine, and two C Cl three carbons join by a triple bond. And DDT comes from chlorobenzene and chloral with concentrated sulphuric acid, and it is not biodegradable. Two more past questions, both on DDT, are solved on this page. Try them yourself before you read the answers. All the best!