Preparation of Alkanes

Transcript

0:00Introduction

Hello everyone! Let us begin with a small puzzle. The Wurtz reaction and Kolbe's electrolysis are two famous ways to make alkanes. Yet neither of them can ever give you methane, C H four, the simplest alkane of all. Can you guess why? We will answer it today. Questions on these methods come up often in JEE, so let's go.

0:24Six ways to make an alkane

So, here is the course's map. In the centre, R dash H is any alkane: R is an alkyl group, the carbon part, and the dash is a single bond, okay? The six grey boxes on the left are the six ways to make it. The orange boxes on the right, its reactions, come in later lessons. With me?

0:45Method one: hydrogenation

Let us start with hydrogenation. Take an alkene or an alkyne and add hydrogen, H two; the small two written low is a subscript, two atoms in each molecule. The arrow means gives, so we get an alkane. Okay? Now look at the catalyst. Finely divided nickel, N i, at five hundred and twenty three to five hundred and seventy three kelvin; capital K is kelvin. Or platinum, P t, or palladium, P d, even at room temperature. So tell me, when an alkene takes up hydrogen, does its carbon skeleton change? No, it does not, and that is the key point. Hydrogen is only added across the double or triple bond, so the alkane has exactly the same carbons as the alkene. Clear?

1:32Reduction and Grignard reagents

Next, reduction of an alkyl halide, R dash X, where X is a halogen, okay? Zinc and hydrochloric acid, written over the arrow, supply two H in square brackets, the reducing hydrogen. We get R dash H, plus H X, the hydrogen halide. And the round brackets say it is not for fluorides. Got it? Now a Grignard reagent, R M g X, with magnesium between the alkyl group and the halogen. Add water, H two O, and R picks up a hydrogen: we get R dash H, plus M g, O H in round brackets, X, okay? And students often miss this point. Any compound with an acidic hydrogen does the same: water, alcohols, ammonia, even terminal alkynes. Right?

2:20Wurtz reaction: two R groups join

Now the famous Wurtz reaction. Two R X plus two sodium, N a, in dry ether, the condition over the arrow, gives R dash R, plus two N a X, the sodium halide. Look, two alkyl groups have joined! With me? So the chain is doubled, and that is why it is useful only for symmetrical alkanes with an even number of carbons. Now take two different halides, R X and R prime X, okay? R prime, the R with a small tick mark, is a different alkyl group. So what comes out? You get three alkanes, not one! R joins R, R joins R prime, and R prime joins R prime. That mixture is hard to separate, so the Wurtz reaction is a poor way to make an unsymmetrical alkane. That is a classic trap. Clear? And one more warning: with tertiary halides, sodium mainly gives alkenes, by elimination. Got it?

3:16Kolbe's electrolysis

Next, Kolbe's electrolysis of a concentrated aqueous solution of a sodium or potassium carboxylate. RCOON a is the sodium salt, and RCOOK the potassium salt; here capital K means potassium, not kelvin. Okay? Overall, two RCOON a plus two H two O gives R dash R, two C O two, H two, and two N a O H, right? See, again two R groups join, just like in Wurtz. Now tell me, at which electrode does the alkane form? At the anode, yes! The alkane and carbon dioxide form at the anode, and hydrogen at the cathode. We will need this in Example two. Got it?

4:03Decarboxylation: one carbon less

Now decarboxylation. Heat a sodium carboxylate with sodium hydroxide, N a O H, and calcium oxide, C a O, written over the arrow with heat. We get R dash H and sodium carbonate, N a two C O three. Okay? Sodium hydroxide with calcium oxide is called soda lime. Now count with me: the salt has R plus one more carbon, in the COO group, and the alkane has only the R part. So how many carbons are lost? Yes, exactly one carbon, and it leaves as sodium carbonate. So the alkane has one carbon less than the acid. Example one tests exactly this. Clear?

4:47Topper's trick: count the carbons

Now look at the map again; here is a trick toppers use. For each method, ask what happens to the number of carbons. Hydrogenation, zinc with hydrochloric acid, and the Grignard reagent with water keep the same carbons. Right? Wurtz and Kolbe join two R groups, so the chain is doubled, okay? And decarboxylation gives one carbon less than the acid. Now back to our puzzle from the start. Why can Wurtz and Kolbe never give methane? Yes, exactly, because both of them join two alkyl groups! The double arrow on screen means implies: two R groups give at least two carbons. The smallest product is ethane, while methane has just one carbon. So our puzzle is solved. Got it?

5:34Example 1: sodium propanoate heated with soda lime

Alright, Example one. What is formed when sodium propanoate is heated with soda lime? Pause the video and try it first. Done? Okay, let's check. Sodium propanoate is C H three, C H two, COON a, with three carbons. It is propanoate, so the product is propane, right? No, no, wait, this is exactly the trap! Soda lime removes the COO part as sodium carbonate. So the salt plus N a O H gives C H three C H three, plus N a two C O three. With me? So the answer is ethane, with two carbons, one less than the three in the salt. The name says propane, but the product is ethane. Clear?

6:24Example 2: gas volume at the anode in Kolbe electrolysis

Now a harder one. Zero point two moles of sodium ethanoate is completely electrolysed in aqueous solution; m o l on screen stands for moles. Find the total volume of gas collected at the anode at STP, standard temperature and pressure. Pause the video and try it first. Done? Okay, let's check. This is Kolbe's electrolysis, so look only at the anode. Two ethanoate ions, C H three COO minus, where the small minus sign up high is the negative charge, give C H three dash C H three, ethane, plus two C O two, plus two electrons, the e with a minus sign. Okay? So zero point two moles of ethanoate gives zero point one moles of ethane. Ethane is a gas, so zero point one times twenty two point four litres, the molar volume at STP, gives two point two four litres, with capital L for litres, right? Oh no, careful, we forgot a gas! Carbon dioxide also forms at the anode, two moles for each mole of ethane, so zero point two moles. Zero point one plus zero point two gives zero point three moles at the anode. Hydrogen, H two, goes to the cathode, so we do not count it. Clear? So V, the volume at STP, is zero point three times twenty two point four, which gives six point seven two litres. With me?

7:55Quick revision: count the carbons in each method

So, quick revision. Hydrogen addition, zinc with hydrochloric acid, and the Grignard reagent with water keep the same carbons. Wurtz and Kolbe join two R groups, so they never give methane. And decarboxylation with soda lime gives one carbon less than the acid. Now go and try the practice problems on this page. Count the carbons every time. All the best!