Dihalides and Trihalides

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Take two compounds with exactly the same formula, C two H four Cl two. Same atoms, same numbers, okay? Now treat each one with aqueous potassium hydroxide. One of them gives an aldehyde. The other gives a diol, with two O H groups, and no aldehyde at all. So how can one formula give two completely different products? The answer is simply where the two chlorine atoms sit. This idea is useful for JEE, so let's begin.

0:35Geminal or vicinal?

So, first, two words you must know. Here, X stands for any halogen atom, like chlorine or bromine. When both halogens sit on the same carbon, the dihalide is called geminal; it is also called an alkylidene dihalide. Think of gem as Gemini, the twins, sitting together. Okay? For example, C H three, dash, C H Cl two. The dash is a single bond, and the small numbers are written low. Both chlorines are on the same carbon, carbon one, so this is one, one dichloroethane, a geminal dihalide. Right? Now the second compound from our puzzle: Cl C H two, dash, C H two Cl, which is one, two dichloroethane. Same formula, C two H four Cl two. But now each carbon carries one chlorine. So tell me, is this one geminal too? No, it is not, very good! The two chlorines sit on adjacent carbons, neighbours, so this is a vicinal dihalide. It is also called an alkylene dihalide. Vicinal, like vicinity, the neighbourhood. Clear?

1:44Hydrolysis with aqueous alkali, such as KOH

Now the main reaction: hydrolysis with an aqueous alkali, such as aqueous KOH, potassium hydroxide. Start with the vicinal one. Each chlorine is replaced by an O H group, on its own carbon. The arrow means gives, so we get HOCH two, dash, C H two O H, a diol, with two O H groups on two different carbons. With me? Now the geminal case. Both halogens are on one carbon, so both O H groups land on that same carbon. So the product is a diol with two O H groups on one carbon, and we stop there, right? No, no, careful, that is exactly the trap, because this diol, with two O H groups on one carbon, loses water at once. So that diol is never the final product. Got it? So what do we get? If the two halogens are on a terminal carbon, at the end of the chain, written dash CHX two, we get an aldehyde, the dash CHO group. Now, what if that carbon is in the middle of the chain? Then we get a ketone, yes, exactly! On screen, the angle sign before C X two shows that this carbon has two bonds to two other carbons, so it is non-terminal. It becomes the ketone group, C double bond O; the equals sign is the double bond. Right? Now one step further. What if three halogens sit on the same end carbon, written dash C X three? We get a carboxylic acid, correct! And since the solution is alkaline, the acid is present as its salt. Clear?

3:28Alkynes and alkenes from dihalides

Dihalides can do more than hydrolysis, okay? Heat either kind of dihalide with alcoholic KOH, and then with sodamide, written N a N H two, with the two written low. Look, alcoholic KOH here, not aqueous. Both geminal and vicinal dihalides give alkynes, with a carbon to carbon triple bond. Right? And with zinc dust, a vicinal dihalide gives an alkene, with a carbon to carbon double bond. So remember the pair: sodamide route for the alkyne, zinc dust for the alkene. Got it?

4:02Dichloromethane, CH₂Cl₂

Now one dihalide you meet in daily life: C H two Cl two, dichloromethane, also called methylene chloride. It has just one carbon, carrying both chlorines. Okay? It is used as a solvent, a paint remover, an aerosol propellant, and a metal-cleaning agent. Clear? But it is not harmless. It harms the central nervous system, and if it gets into the eyes, it can burn the cornea. Questions on uses and hazards can be very direct, so keep these in mind.

4:33Example 1: hydrolysis of 1,1- and 1,2-dichloroethane

With me? Now our first example, straight from the puzzle. Name the products of hydrolysis with aqueous KOH of part a, one, one dichloroethane, and part b, one, two dichloroethane. Pause the video and try it first. Done? Okay, let's check. One, one dichloroethane is C H three C H Cl two, a geminal dihalide with both chlorines on the end carbon. Hydrolysis puts two O H groups on that carbon; the brackets around O H, with a small two after them, mean two O H groups. That carbon loses water at once, so we get C H three CHO, ethanal, an aldehyde. If you wrote ethane one, one diol, remember the trap. Okay? Part b is the vicinal one, exactly the line we saw earlier. Each chlorine becomes an O H group on its own carbon, giving HOCH two C H two O H, ethane one, two diol. And there is our puzzle solved: same formula, but the geminal one gives an aldehyde, and the vicinal one gives a diol. Right?

5:44Example 2: 2,3-dibromobutane with zinc dust and sodamide

Now a harder one. What is formed when two, three dibromobutane is, in part a, heated with zinc dust, and in part b, heated with alcoholic KOH and then sodamide? Pause the video and try it first. Done? Okay, let's check. First, write the structure. Butane has four carbons, and the bromine atoms, written B r, sit on carbons two and three; C dash two and C dash three on screen mean carbon two and carbon three. They are neighbours, so this is a vicinal dihalide. Clear? Part a, zinc dust, and a vicinal dihalide, so we get an alkene. But tell me, where will the double bond form? Between carbons two and three, yes, exactly, the two carbons that carried the bromines! The equals sign between them is the double bond. So the product is but two ene. Got it? Part b, alcoholic KOH, and then sodamide. Now, alkene or alkyne this time? Alkyne, correct, because alcoholic KOH followed by sodamide turns a dihalide into an alkyne. The three lines between carbons two and three are the triple bond. So the product is but two yne. Same compound, two different reagents, two different products. With me?

7:11Quick revision: dihalide hydrolysis, alkynes and alkenes

So, quick revision. Geminal means both halogens on one carbon, and vicinal means on adjacent carbons. With aqueous alkali, a vicinal dihalide gives a diol, a geminal one gives an aldehyde or a ketone, and dash C X three gives an acid. Alcoholic KOH, then sodamide, gives an alkyne, and zinc dust turns a vicinal dihalide into an alkene. Now go and try the practice problems on this page. All the best!