Benzene: Structure and Preparation

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Shake cyclohexene, a ring with one double bond, with bromine water, and the bromine colour vanishes at once. Now take benzene, which looks as if it has three double bonds. So the colour should vanish even faster, right? But it does not; the colour simply stays. So why does benzene not react here? Questions on the structure of benzene come up often in JEE, so let's find out.

0:31Inside the benzene ring

So, first the shape. Benzene is C six H six; the small numbers below the letters count the atoms. The six carbons form a flat, regular hexagon, so we say the molecule is planar. Okay? Alright, a question for you. In this flat ring, what is the hybridisation of each carbon? Yes, exactly, it is s p two; the small two is raised, like a power. Each carbon forms three sigma bonds, and that Greek letter is sigma. They are one hundred and twenty degrees apart; the small circle is the degree sign. Right? Look, after those three sigma bonds, each carbon still has one p orbital left over, six in all. And these six leftover p orbitals overlap all round the ring. Clear? Each of them brings one electron, so we get six pi electrons; this Greek letter is pi. They are not stuck between two carbons; they are delocalised, spread over all six. Remember this word; the whole lesson comes from it. Got it?

1:34Alternate single and double bonds?

Now let us test an idea. Suppose benzene really had alternate single and double bonds. C dash C, a single bond, is one hundred and fifty four picometres long; p m means picometre. And C double bond C, with the equals sign, is shorter, one hundred and thirty four picometres. With me? So tell me, how many different bond lengths would we then measure in the ring? Yes, two lengths, of course, three long and three short. But experiment says no. Every carbon carbon bond in benzene is one hundred and thirty nine picometres, all six equal, between a single and a double bond. That is delocalisation, showing up in a measurement. Okay? Here is a second fact. The label one, two disubstituted means two groups on neighbouring carbons. With alternate bonds, the two groups could sit across a single or a double bond, giving two compounds. But only one exists, since all six bonds are the same. Right?

2:36Why benzene prefers substitution

So, why does delocalisation matter so much? Because it makes benzene about one hundred and fifty kilojoules per mole more stable than a ring with three ordinary double bonds. k J mol to the minus one is read as kilojoules per mole. This extra stability is the resonance energy. Clear? Now think like benzene. It could react by addition, or by substitution. Which one lets it keep all that extra stability? Yes, exactly, substitution! Addition would destroy the delocalised system, and with it the resonance energy. So benzene reacts by substitution. Write this down, okay? And this solves our puzzle. Decolourising bromine water, or Baeyer's reagent, needs an addition across a double bond. Cyclohexene decolourises them at once, but benzene will not add, so they stay coloured. That is also how you tell benzene from cyclohexene. Right?

3:36Four ways to make benzene

Now, how do we make benzene? Here is the course's map, with benzene, C six H six, in the centre. The four blue-outlined boxes on the left are the ways to make it, and the orange-outlined boxes on the right are its reactions, for the next lessons. With me? First, from ethyne, H C triple bond C H; the three short lines are the triple bond. The three in front means three molecules, and the arrow means gives. Passed through a red-hot iron tube at eight hundred and seventy three kelvin, capital K being kelvin, they join into one benzene ring. This is called cyclic polymerisation. Okay? Second, decarboxylation: heat sodium benzoate with sodium hydroxide and calcium oxide, the second box on the left. Sodium benzoate has seven carbons, so the product is toluene, with seven carbons too, right? No, no, careful, the product is benzene! C six H five COON a plus N a O H, with C a O and heat over the arrow, gives C six H six plus N a two C O three, sodium carbonate. N a O H with C a O is called soda lime. The seventh carbon leaves in the sodium carbonate. So here is a topper's trick: the COON a group is replaced by H, and you lose one carbon. Clear? Third, reduction of phenol. Phenol vapour, C six H five O H, is passed over heated zinc dust, Z n; heat is written over the arrow. Zinc takes away the oxygen as Z n O, zinc oxide, and leaves benzene. Got it? And fourth, aromatisation, from chapter one. Hexane is passed over C r two O three on A l two O three, alumina; the slash means on. At seven hundred and seventy three kelvin, the chain closes into a ring and loses hydrogen, giving benzene.

5:33Example 1: mass of benzene from 14.4 g sodium benzoate

Right? Time for the first example. What mass of benzene is obtained by heating fourteen point four grams of sodium benzoate with soda lime? Small g is grams, and capital M, one hundred and forty four, is the molar mass of the salt. Pause the video and try it first. Done? Okay, let's check. Moles of salt, n, equal the mass over the molar mass. Fourteen point four over one hundred and forty four gives zero point one mole; m o l is mole. Right? See, each COON a group is replaced by one hydrogen. So one mole of sodium benzoate gives how many moles of benzene? Yes, one mole gives one mole. So we get zero point one mole of benzene. Clear? Next, capital M of C six H six, the molar mass of benzene. Six carbons at twelve each, plus six hydrogens at one each, gives seventy eight; a number before a bracket means times. g mol to the minus one is read as grams per mole. Okay? So the mass is zero point one times seventy eight, which gives seven point eight grams; the cross means times. Notice, the benzene weighs less than the salt, because the COON a group was replaced by just one hydrogen.

6:52Example 2: percentage of carbon and benzene's sooty flame

Got it? Now a slightly harder one. Benzene burns with a sooty flame. Find its percentage of carbon, and explain the soot. Pause the video and try it first. Done? Okay, let's check. Benzene has six carbons and six hydrogens, so carbon is six out of twelve atoms. That makes fifty percent carbon, right? No, no, careful, that counts atoms, not mass! Percentage here is by mass. Six carbons, six times twelve, give seventy two grams of carbon in seventy eight grams of benzene. With me? The percentage of carbon, written with the percent sign, is seventy two over seventy eight, times one hundred. That gives ninety two point three percent. Okay? Now the explanation. That is a huge share of carbon, so much that it is not burnt completely in air. Right? The unburnt carbon appears as soot, and that makes the flame sooty. So the high carbon content explains the soot. Clear?

7:57Quick revision: 139 pm bonds, resonance energy, 4 methods

So, quick revision. Benzene is a planar hexagon of s p two carbons, with six delocalised pi electrons. All six carbon carbon bonds are one hundred and thirty nine picometres, and the resonance energy is about one hundred and fifty kilojoules per mole. So benzene reacts by substitution, not addition. And we make it from ethyne, sodium benzoate, phenol or hexane. Now go and try the practice problems on this page. All the best!