Hello friends, I am Rajneesh Kumar, I am working as an assistant professor in NIT institute in greater Nevada. So, I am here to deliver or discuss the concept of structure of benzene. benzene, how the benzene looks likes, what is the structure of benzene actually is and the evidence synthetic as well as analytical evidence for the support of different structure of the benzene. So, we will talk all these topic, we will discuss all these topic in this the present lecture. So, we will start now.
So, first of all as we know that benzene is aromatic compounds, benzene is an aromatic compounds, aromatic compounds again lot of aromatic compounds are there in the organic chemistry, basically these are the parts of organic chemistry. So, organic chemistry is the chemistry the branch of chemistry which deals with the organic compounds, what are organic compound actually are, organic compounds are the compounds which contain carbon in their structure. So, all the organic compounds are all carbon in it. When we talk about it, then organic compounds are what we can categorize it.
Generally, we put it in two classifications. First aliphatic organic compounds and second aromatic organic compounds. So, aliphatic organic compounds are generally of two types. They are cyclic as well as a Acyclic means in which ring structure is present Acyclic means in which open chain structure is present Aliphatic word means fatty Where fatty comes from? All fatty acids are present So open chain structure is kept So we have inspired people with that We called open chain structure as aliphatic Second category is of aromatic compounds aromatics compounds generally the aromatic word is aromas means some pleasant smell or very good fragrance okay so okay aromatic compounds can be of two types benzenoids or non benzenoids benzenoids compounds are those compounds which contains benzene as a ring structure present okay the other type of aromatic compounds are those which are aromatic but benzene as a ring compounds in their structure is not present got it.
So, it is very simple thing we have to keep this in mind then organic compounds are aromatic compounds. So, aromatic compounds are the organic compounds which are composed of carbon and hydrogen atoms arranged in the ring structure with delocalized pi electrons such as benzene. benzene compounds of benzene key trade behave carrying a ticket in the chemical behavior benzene guitar a huga when sorry sorry aromatic compounds come cabling a hum aromatics on sorry sorry compounds come look a bullying aromatic compounds bullying a ticket aromatic important thing is that all the aromatic compounds are very stable all the organic compounds are the most stable compounds all the other compounds are the most stable compounds Now when we have started aromatic compounds, we have said that aromatic compounds are generally Benzene, Benzene is the structure of the benzene in the compounds.
So Benzene is a kind of parent molecule, parent compound. So we start reading about benzene. So the word Benzene was used by European perfumers, European pharmacists. They were known for this word from a long time.
Generally this word is derived from the word resin. What is gum benzoin? As I told you, this is resin and it was called benjamin. It is important to keep in mind the spelling. Benjamin was called at that time.
This is a 16th century story when European pharmacist and perfumer know this name as Benjamin What was this? It was an acidic material which was derived from the benzoin by the sublimation reaction And it was named as the named flowers of benzoin or benzoic acid ok so people of europe knew about this from long time after that a scientist came his name was Michael Faraday he studied 18th century in the first of its purest form in the year 1825. Where did they isolate it? They derived the oily residue from the illuminating gas. What is the illuminating gas?
It is a mixture of combustible gas primarily methane and hydrogen. And where do we get it from? It is very simple reaction.
It is obtained from coal pyrolysis. It is obtained from pyrolytic reactions. If you do simple carburetion then also we will get it.
So, because it was obtained from coal, that is why its name is called as bicarburet of hydrogen. Then, if we talk about this, then in 1834, there was a scientist, Ilhard Misch-Cherlis, he said that the formula of benzene will be C6H6, that is, he established the molecular formula of benzene. He established it. and all these things were theoretical, there was no practical involved in it, there was no practical data involved, these are all hypothesis given by the different persons, so they gave molecular formula of benzene, what they gave C6S6, and they called it benzene, keep in mind the spelling, B-E-N-Z-I-N, this benzene later became a benzene.
benzene became benzene which we still use today after that a big milestone came in 1858 when a great scientist Agastya Kekule proposed the structure of benzene, the ring structure proposed what happened actually this was a very cosmic event, so what did they have at that time, no one had any equipment of this kind that they could establish the molecular structure. so it is said that once he was sleeping at night, he had a dream, he saw in his dream that a snake is running to catch his tail, so when he was running to catch his tail, what was he doing, he was running in the circle, so it is said from this dream, he was affected by this dream and he gave the structure of benzene and gave the structure of the ring, he said that he said that the six carbon benzin and after that they make a ring form. After that when they gave this structure, after giving this structure, some things were produced, some problems were created, some demilitation was created, some questions were raised, the question was raised that the benzene has three double bonds, three double bonds are kept, so the question was raised that the benzene has three double bonds, So, when it will go for hydrogenation means it will react with hydrogen So, if it was an open chain structure Normally if we talk about it, suppose it would be something like this Suppose something like this happens.
Was this possible? This was possible, this would have been like this, so we are seeing what is present in it, four double bonds are present, four double bonds are present, which means it is alkene, right? If we suppose that the calculation that the person said that the ring structure is not this, then if open So, this is the structure of the hydrogen. So, it is a 4 double bond, which means it should have consumed 8 hydrogen atoms Here some question arises that really this happens So we saw in it that sir The bonds in benzene By the way, we are showing three double bonds But the bonds in benzene are actually all of them have the same length Bond length is same How much is around 1.39 angstrom Right or not Around 1.39 angstrom While we know that single bond has the same length percent how much is the bond length 1.54 angstrom double bond is 1.34 around ok so the double bond present in benzene their bond length lies somewhere between double bond and single length ok before starting further we see that structure of benzene is represented in three The first one is about the calculus structure, the second one is about the chemical structure, and the third one is about the resonance.
Now, so all the carbon to carbon bonds and benzene are equivalent just like we told you all three are double bonds and three are single bonds. carbon equivalent. Second, how is it happening now? That is why we said that the bond length of single bond is 1.54, double bond is 1.34, the carbon-carbon bond of benzene is 1.54, length is 1.39 and it is same in all carbon it is same the molecule is unusually stable ok so this point is very important the molecule of benzene is very stable and generally we show it in the form of hexagon in which a circle is made like this you can see we represent it like this ok the circle of aner is saying that it is a very stable molecule and it that is saying that the three double bond electrons are there, pi electrons are there, they are being shared through 6 to 6 carbon, what does it mean, they are delocalizing electrons, they are spread out over all the carbon atoms. So, you can see here resonance occur, these double bond can move from here to here, this can move there and this can move there.
So, delocalization of the pi bonds or double bonds occur in the benzene and we are getting the hybrid structure of the benzene. Now if we talk about it, we go a little further, then we see that when we showed the chemical structure, then we showed that the carbon is attached in hexagon with each carbon, there is one hydrogen attached, then total six carbon is there which is bonded to the six hydrogen atom, means each carbon atom has a carbon atom. has single bondly bonded with the hydrogen atom ok any other atom or group of atom are substituted for the hydrogen atom must be shown bonded to the particular corner of the hexagon if we talk about any reaction of benzene what does benzene do further we will talk about it but we tell here that it gives electrophilic substitution reaction preferred reactions are electrophilic substitution reactions ok so if we want to substitute then we remove hydrogen atom and substitute another group of atom The six bond lengths are identical and they are one and half bond and their length is one point three one point three nine angstrom which is just lying between the length.
of single bond and the double bond. So, I have told you that the bond length of single bond is 1.54 angstrom and double bond is 1.39 angstrom. So, the bond present in benzene, whose bond length is 1.34 angstrom. This is double bond 1.34 and benzene present in the bond present carbon carbon bond present in them 1.39 angstrom.
So, this lies between single bond and double bond. Now, Now, we will talk about it in more detail, so what is Benzene in reality, Benzene is a planar molecule, it is a planar molecule, why is it a planar? Because all the six carbon atoms which is Benzene having, what is this? These are sp2 hybridized, sp2 hybridized means, you must have studied hybridization in intermediate first, so I will tell you again, I will try to tell you, suppose, suppose what do you do?
electron, if we see the electron configuration then how we will tell, we will tell 1s2 2s2 2p2, if we see in the form of orbital then it comes in this way. This is how it should have come, as per the Aufbau rule, which goes in the course of increasing energy. According to the Aufbau rule, if after that Mr. Hund came, Mr. Hund said that sir, until the pairing is not done, until all the orbitals do not come in single electrons, then according to Mr. Hund.
This is how it is done Now what is happening In normal way it is like this But with this electronic configuration We cannot explain the valency of carbon How much is the valency of carbon? It is 4 When carbon is going to make a bond carbon can form and it only two bonds. So, we do what? A new hypothesis is there, we said that when carbon is going to form a bond, then it is excited.
in the excited state. What happens in that? In that, one electron is taken from 2 s 2 and the vacant p orbital is taken from that.
And all the spins are same. All electrons in 2 s and 2 p orbitals are having same spin. So, now you can see that there are 4 unpaired electrons in the outermost orbital of the benzene sorry of the carbon.
So, it can form total 4 bond. Now, when these four electrons are unpaired, then we are saying that the carbon which is in the benzene, the carbon which is six carbon, all of them are sp2 hybridized. Sp2 hybridized carbon means that one orbital will do hybridization with two p orbitals. So, here you can see again writing 1s2 here 2s1 has happened after the excited state and this is the stress.
1 s 2 2 s 1 is done and 2 p 3 is done after hybridization now we are saying that sp2 means 1 s 2 p will do hybridization and we will get 3 equal energy same shape of sp2 hybridized orbital will be found. Is it clear or not? So, the carbon in the benzene is sp2 hybridized carbon and we know what we know, we know if we know the hybridization of something, if we know the hybridization of someone, then we can predict its shape.
So, wherever sp2 hybridization will be there, the shape of that molecule will be trigonal point is the every molecule which is having sp2 hybridized carbon or any other molecule which is sp2 hybridized is always in the state of trigonal planer. carbon is same and this is sp2 hybridized carbon so sp2 hybridized carbon means its benzene shape will be like trigonal means not benzene but this carbon shape will be like trigonal plane will be shown like this one sp2 hybridized orbital second sp2 hybridized orbital or This is the third sp2 amylase. This is one carbon. We will make the second carbon in the same way. Here, one, two, three.
Similarly, one, two. 3 If 3 will come, then it will be formed in the form of a ring ok or not And these three are sp2 1 sp2 is the second third 1 2 And the third is going out Then 1 carbon 2 carbon 3 carbon 4 carbon 1 is our fifth and here our sixth little shape is increased so 1, 2 and 3 this is how ring is formed and these outer ones are now see you there were 3 sp2 hybridized orbital near this carbon 1, 2, 3 from this 2 sp2 hybridized orbital the edge carbon, they are overlapping with sp2 hybridized orbital. If they are overlapping, then which bond will be made?
Linear overlapping is happening. Linear overlapping means that they will make sigma bond. Two will be utilized, how many are left? one sp2 hybridized orbital is left what it does it overlaps with s orbital of the hydrogen and it makes a sigma bond so total how many bonds are formed 6 carbon carbon bonds are formed and 6 carbon and hydrogen bonds are formed got it ok the bond angle of the carbon carbon bond is increasing because it is sp2 hybridization so we know we have read it in the intermediate how much bond angle 120 degree and the ch bond angle is 120 degree and the bond sigma bond became in the same plane ok now we forgot one thing one p orbital was left there one s orbital hybridization with two p orbital but one p orbital was left that p orbital will come where that p orbital is this and what is this is perpendicular to the plane of benzene ok this is important to keep in mind there is one electron in it now So, we will go that the structure of benzene is like this, there are three double bonds, they are in conjugation or they are in delocalization and sp2 hybridization, there are six carbon-carbon bonds and six carbon-hydrogen bonds.
So, how to prove this? Because Kekulé sir had a hypothesis at that time, there was nothing practical, so we had to collect some evidence. So, we put the evidence in two, which was, first thing we put the evidence in the form we have to see the analytical evidence the first thing we go for analytical evidence ok so what we did was the simplest elemental analysis we can do in our organic chemistry lab so we did elemental analysis in elemental analysis we found that carbon and hydrogen are in equal ratio and molecular formula is C6H6 of benzene this thing happened then we have to do the analysis of the and then we got techniques like mass spectra with the help of which we could determine the molecular weight so we did molecular weight determination and we found the molecular weight of benzene is 78 ok or not now when we talk about molecular formula C6 S6 it means carbon to hydrogen ratio is very less if we talk about its corresponding alkynes Corresponding alkane and alkyne means that which contains 6 carbons.
If benzene contains 6 carbons, then we can talk about alkane and alkyne. Which contains 6 carbons. So you will see we have taken the example of N-hexane.
6 carbons and along with 14. hydrogen and hexane which is which has the molecular formula C6H14 clear or not so if we compare with that we will see benzene is a highly unsaturated compound highly highly unsaturated compounds molecular formula of the benzene does not show any resemblance with the corresponding alkane alkenes or the alkynes. Now so those were the analytical evidence fair we can also prove the structure of benzene with the help of certain chemical reaction they say. Alkene ki baat karen alkene ki so alkene kya hoti double bonds hoti hai double bonds rakte hai volok because alkene are having double bonds so they are easily give the addition reaction.
reaction. So, benzene can double bond like alkene it should also give the addition reaction, but dramatically benzene will never go for the addition reaction in normal conditions. So, as you can see that benzene acid presence may water molecule addition again benzene bromine water reaction. So, this is the case. yellowish brown colour if benzene reacts with it then the product that is made here is not made of yellowish brown colour so when we see that the product that is made is yellowish brown colour again it means that it did not react with bromine then what is benzene it does not react with potassium permanganate ok or not it does not react with cold potassium permanganate solution how do we know we can see like this whether the reaction is done or not how is the solution of potassium permanganate it is of purple colour if it reacts with benzene then it is purple colour.
the reaction should disappear from the mixture which does not happen ok so as like the alkene benzene never give these reactions with ok so we can say that sir it is not behaving like alkene it is not behaving like alkene it means it is not a straight chain molecule is it clear now if it is not a straight chain molecule then how is it its cyclic structure is it not cyclic means it is in the form of ring we will discuss that in the next class how we know what we did we went for substitution reaction we will go for the substitution reaction in the benzene ring so we did mono substitution first mono substitution means any one substitute we have substituted on benzene ring ok now because carbon is all equivalent of benzene right so benzene reacted with the bromine in presence of FeBr3 means the bromine without any catalyst reaction was not able to connect with benzene now in the presence of F P B R 3 which is acting as a Lewis acid so in the presence of Lewis acid it can easily form the mono bromo benzene ok mono bromo benzene is obtained now mono substituted benzene is obtained but it will be obtained in the same way it is not like that it came on this carbon then it came on the other carbon ok so it is getting the same type of mono substituted benzene it means that 6 to 6 hydrogen is identical ok it could be possible only if the benzene has the cyclic structure of the cis carbon and each carbon was attached with the one hydrogen now benzene now we have gone for disubstitution in the next step benzene is three isomeric disubstituted product point to be noted okay for disubstitution we can get three types of isomers of disubstituted product we can get dibromobenzene we can get three dichlorobenzene or three chloronitrobenzene it is up to us which electrophile we have to add which substitution we have to do So, which is the three types of isomer? See, one two dibromobenzene is formed, which we also call orthodibromobenzene, one three dibromobenzene is formed, which we also call metadibromobenzene, and one four dibromobenzene is formed, which we also call paradibromobenzene. So, three types of isomers are possible.
This fact again works to establish the structure given by Kekuli. Meaning, the structure of benzene will be cyclic and the molecular formula will be C6H6. However, in these also the isomer of ortho is 1, 2 isomer can be changed a little bit inside it.
How can it be changed? One isomer is more possible, which will depend on the presence of double bond. So as you can see here we have applied monosubstituted benzene ok, so it is applied here, double bond is here ok. What is the second isomer?
It is on the ortho position, but it is on the other carbon, but where double bond is not present here, it is clear, but there are two types of here. We can get isomer all which again prove the cyclic structure of benzene and establishes it. Kekuli sir without any technical facilities thankfully he gave this structure unthinkably anticipated and without any help he told that yes brother there can be delocalized electrons and he drew this picture.
What were the drawbacks? Even after this explanation Kekuli was failed to explain about the unusual stability of the benzene ring. This character of benzene the reason is that third we call it resonating structure 1 third we call it resonating structure 2 and third we call it resonating hybrid ok so again stability of benzene can be understood through heat of hydrogenation we call it resonance energy or heat of combustion so if we talk about cyclohexane then theoretically its heat of hydrogenation is 28.6 kcal which is doubled in cyclohexadiene which is 55.4 kcal so according to this benzene which is is a triene it should not be 3, it should be 85.8 kilo calorie but it does not happen the actual heat of hydrogenation of benzene is 36 kilo calorie less 49.8 kilo calorie so again it will prove that benzene is a very stable molecule so till now we have studied till here next lecture we will study molecular orbital picture so thank you thank you all