Preparation of Alkynes

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Make a solid from limestone and coke, then drop it into plain water. Out bubbles a hydrocarbon gas, with a carbon carbon triple bond. Water and a solid giving a hydrocarbon? Strange, right? Where did the carbon and the hydrogen come from? Let's find out, and learn three ways to make alkynes. Conversion questions on alkynes come up often in JEE, so let's begin.

0:29The map: ways to make an alkyne

Here is the course map. In the centre is the triple bond: three short lines between two carbons, and a dash on each side for the other bond of each carbon, okay? The boxes on the left, with dark blue borders, make alkynes, and the orange boxes on the right are reactions, for later. The first three boxes on the left are today's methods, and the last one, CHX three with silver powder, also gives ethyne. With me?

0:57Method one: from calcium carbide

Method one starts from limestone, calcium carbonate, C a C O three; the small three written low is a subscript, okay? The arrow means gives, and heat written over it is the condition. So limestone, on heating, gives C a O, calcium oxide. Right? Next, the C written over the arrow is carbon, which is the coke. Calcium oxide is heated with it at two thousand two hundred and seventy three kelvin; the capital K is kelvin. That gives C a C two, calcium carbide. Clear? Now comes the moment from our puzzle. We add water to calcium carbide. Tell me, what comes out? Yes, exactly, out comes ethyne! C a C two plus two H two O gives calcium hydroxide, written C a, then O H in brackets with a small two, meaning two O H groups. Along with it comes C two H two. Got it? So C two H two is ethyne, and this is the industrial method. And our puzzle? The two carbons were already in calcium carbide, from the coke, and the two hydrogens of ethyne came from the water.

2:06Method two: double dehydrohalogenation

Okay? Method two starts from a dihalide, with two halogen atoms; the capital X stands for the halogen. In a vicinal dihalide they sit on adjacent carbons, and in a geminal dihalide, on the same carbon. Clear? Now treat it with alcoholic KOH, potassium hydroxide in alcohol. It removes one H X, a hydrogen halide, and a double bond forms. The halogen left behind sits on a double bond carbon, so we call this a vinylic halide. With me? But we want a triple bond, so one more H X must go. Easy, we just use alcoholic KOH again, right? No, no, wait, that is the trap here! Alcoholic KOH takes us only as far as the vinylic halide. The second H X needs the much stronger base N a N H two, sodium amide, or simply sodamide. Clear? Here is the course example. B r C H two C H two B r is one, two dibromoethane, a vicinal dihalide. Over the first arrow, alc is short for alcoholic, and alcoholic KOH gives C H two double bond CHB r; the equals sign is the double bond. Then sodamide gives H C triple bond C H, ethyne. Got it?

3:25Alkene to alkyne in two steps

Now a neat use of method two. Suppose you have an alkene, with a double bond, and you need an alkyne. How would you do it? First, add bromine, B r two, across the double bond. The two bromines land on adjacent carbons, so we get a vicinal dibromide. With me? Then remove two molecules of H B r, as in method two: alcoholic KOH, and then sodamide. Add bromine, then take away two H B r, really simple. Got it?

3:55Method three: alkylation of acetylides

Method three builds bigger alkynes. R C triple bond C minus, N a plus, is a sodium acetylide, where R stands for an alkyl group; the small minus up high is the negative charge on carbon, and the small plus is the sodium ion. It reacts with R prime X, an alkyl halide; the little mark, a prime, means a second alkyl group. The products are R C triple bond C R prime, and N a X, okay? So the R prime group joins the triple bond carbon, and the carbon chain grows longer; that is alkylation. Right? Now let me ask you this. Can we use any alkyl halide here, even a tertiary one, to build a branched alkyne? No, and this is a classic trap! It works only with methyl and primary halides. The small circle up high is read as degree, so one degree means primary. Clear? Secondary and tertiary halides, two degree and three degree, undergo elimination instead, so you get an alkene, not a longer alkyne. Got it?

5:00Example 1: litres of ethyne from 6.4 g calcium carbide

Alright, Example one, and it goes back to our puzzle. What volume of ethyne at STP, standard temperature and pressure, do we get from six point four grams of calcium carbide? The small g means grams, and capital M equals sixty four is the molar mass. Pause the video and try it first. Done? Okay, let's check. First the equation: C a C two plus two H two O gives calcium hydroxide plus C two H two. There is a two in front of the water. So one mole of carbide gives two moles of ethyne, right? No, no, careful, that two belongs to the water only! Ethyne has no number in front of it, so one mole of calcium carbide gives one mole of ethyne; mol is short for mole. Clear? Now the moles of calcium carbide. The amount, n, equals six point four over sixty four, which gives zero point one mole. With me? So we get zero point one mole of ethyne, and one mole of gas at STP takes up twenty two point four litres; capital L is litres. So the volume, V, is zero point one times twenty two point four, which gives two point two four litres. Six point four grams of a solid, plus water, gives two point two four litres of a hydrocarbon gas.

6:24Example 2: two acetylide routes to hept-2-yne

Okay? Now a harder one. Which alkyne and which bromoalkane would you use to make hept two yne by acetylide alkylation? Give both possible pairs. Pause the video and try it first. Done? Okay, let's check. Hept means seven carbons, and two yne means the triple bond starts at carbon two. So, C H three, then C triple bond C, then C H two, C H two, C H two, and C H three; each dash is a single bond. Right? Now think backwards. In method three, which bond does the alkylation actually make? Yes, exactly, the bond between a triple bond carbon and the R prime group. Here the triple bond has a single bond on each side, so we can cut on either side. With me? First, cut on the right. The left piece, C H three C triple bond C H, is propyne, and the four carbon chain comes from one bromobutane. Three plus four makes seven. Okay? Now cut on the left. The right piece, a six carbon chain with the triple bond at its end, is hex one yne, and the lone C H three comes from bromomethane. Six plus one makes seven again. Clear? And notice, one bromobutane is primary and bromomethane is a methyl halide, so neither pair runs into elimination; both pairs work. Got it?

7:52Quick revision: carbide, dihalide with NaNH2, acetylides

So, quick revision. Calcium carbide plus water gives ethyne, the industrial method. From a dihalide, alcoholic KOH and then sodamide; from an alkene, add bromine first. And an acetylide with a methyl or primary halide gives a longer alkyne, while secondary and tertiary halides give elimination. Now go and try the practice problems on this page. All the best!