Free Chemistry NEET Video Lectures for Hydrocarbons

00:00:01 [Music] [Applause] [Music] Uh myself Pune Morti from Department of Chemistry IIA Gojhati. Uh in this class we will study about hydrocarbons. Hydrocarbons are the compounds that contain only carbon and hydrogen atoms. Examples methane. Ethan propane and so on. You can go on butane you can have if you look at this compound it has only one carbon is bonded with four hydrogen atoms. Here you have two carbon atom and they are bonded together and also in addition to that each carbon is bonded with three hydrogen atoms. This

00:01:44 is called ethane and if you have three carbon atoms is called propane and four is called butane. You can go on like this. So they play they are important source of energy. Um uh you must be familiar with LBG, CNG, petrol, diesel, polythin bag, paint, drug and so on. If you look at LBG is abbreviated form of liquid petroleum gas we use as fuel uh in um home and CG is compressed natural gas and petrol diesel they are obtained by fractional dissolation from petroleum that is obtained from earth's crust we

00:02:44 use them as uh automo fuels polythin bag and this is a material this also ethyline is involved is hydrocarbon to make this material and we use as polyin bag and similarly hydrocarbons also find applications in drugs as well as in paints and so on. Therefore they play a key role in daily life. So hydrocarbons uh can be uh classified broadly into three types. So just we have seen fuels is a mixture of hydrocarbons and now let us they can be broadly divided into three types. Saturated hydrocarbons, unsaturated

00:04:00 hydrocarbons, aromatic hydrocarbons. Examples saturated just we have seen ethane, propane. If you look at these molecules, they have a carbonarbon single bond. The hydrocarbons that contain carbonarbon single bonds. It can be linear. It can also be cyclic. This molecule has three carbon atoms. The linear molecule here also has three carbon atom. Right? You have this has four carbon atoms. It is closed one. This called cyclopropane. This is propane. And so these are examples for saturated hydrocarbons. Compounds that contain

00:04:59 carbonarbon single bonds. Unsaturated hydrocarbons. Carbon compounds that contain carbonarbon multiple bonds. For examples, this molecule has a carbonarbon double bond. And so you can write like this is carbon is bonded with two hydrogen atom again bonded with carbon it has a carbonarbon double bond. So this molecule has carbonarbon triple bond. So these compounds are called unsaturated hydrocarbons. This also can be cyclic. For example when you have the four carbon atoms it can be cyclic. So whenever you have the carbonarbon

00:05:47 multi bonds double bond triple bonds those combo hydrocarbons are called unsaturated hydrocarbons. The difference between these two hydrocarbons is it has two carrogens less. This is C2 H6. So they have general formula for the linear one CN H2N + 2. This is a common formula for the uh saturated hydrocarbons, linear hydrocarbons. Um CN 2 carbon and hydrogen is going to be six. And if you compare these two, this compound has two hydrogen's less comparing to that they have the common formula CN H2N.

00:06:37 And this compound has four hydrogen's less comparing to the alkan. Two hydrogen's less comparing to the ethylene. This more unsaturated hydrocarbon. This example this this is called ethylene and this is called ethine. They have the common formula CN H2N minus2. So this is the name of this compound is ethane. You see the difference ethane. So in this case the a is converted into ethine right a is changed to in ethine. And in this case the A is converted E is changed to Y ethine. These are the names of these

00:07:38 compounds. You can go on like this. Um if you increase one more carbon is called this is propene. Propane. Propane. This is propane. This compound is propane. A is changed to in propene. If you have the corresponding uh this one triple bond then propine can go on. Anyway, these are the examples for unsaturated hydrocarbons. And if you look at all the compounds, they have only carbon and hydrogen atoms, aromatic hydrocarbons. So these class of compounds have a benzene ring as unit. Say for example if

00:08:46 it can be benzene or derivatives. So here what you look at this compound is a cyclic compound you have the double bond single bond double bond single bond double bond that means it has six carbon atom and is bonded with the carbon is bonded with another carbon double bond and at the same time you have the CH one single hydrogen atom. So this compound is called aromatic compound and also this is derivatives right. The one of the hydrogen is replaced by methyl group called methylbenzene and this is benzene and this you have

00:09:50 one more ring is this called napthylene. So these also hydrocarbons look at it they have only carbon hydrogen atoms. So these kind of compounds is called aromatic hydrocarbons. In this class now let us focus on alkanes just we have seen saturated hydrocarbons those are called alkanes and the first member of this series is methane. If you replace one of the hydrogen with methyl group then we get the next series ethane. This is called methane. This is ethane. You can go on you replace the hydrogen

00:10:53 you'll get propane, butane or the corresponding u alkanes. Firstly let us look at the structure and bonding of alkanes. So this structure of methane. You can look at this structure is the methane. Look at it. Carbon is bonded with the structure of methane. It has tetraateral geometry. If you look at it, it has a tetraal geometry. And so the bond angle between this is 109 5°. The bond length is 1 0 09 armstrong. This bond length and this carbon involves sp3 hybrid orbital. SP3 hybrids orbital that overlaps with S

00:12:41 orbital of hydrogen. You make bond the sigma bond. The sigma bond the carbon has four CH sigma bonds. These bonds are made by overlapping of sp3 hybridized orbital of the carbon with s orbital of hydrogen. They make four sigma bonds. Now let us go for ethane. So this sp3 orital of this carbon overlaps with sp3 carbon of this carbon. You make a carbon karma sigma bond. In addition to that this one of the sp3 hybrid of this carbon overlaps with the hydrogen s orbital make another sigma bond. Similarly

00:15:02 overlapping of this sp3 orbital carbon sp3 hydra orbital with this hydrogen make another sigma bond. So this is the structure of ethane. If you look at it and you have the um carbon carbonarbon sigma bond this is formed due to the overlapping of the sp3 hybrids after this carbon with the sp3 hybrids are of this carbon to form the carbonarbon sigma bond. In addition to that there is a carbon hydrogen sigma bond. There are three hydrogen carbon carbon hydrogen sigma bonds in this carbon and which is formed due to the

00:16:10 overlapping of sp3 hybrid of this carbon with this hydrogen s orbital. Similarly this one the bond length uh between these two carbon this bond length is 1.54 arm armstrong 1 09 armstrong this one. So the carbon hydrogen bond length is 1.0 09 Armstrong and this carbon carbon bond length is 1.54. This the structure and bonding of methane and ethane. You can go on like this for other alkanes. Just we have seen the structure of methane is ethane they have tetraal geometry and uh so they involve they

00:16:56 have carbon carbon carbon hydrogen sigma bonds. Now let us see the nomure and isomeism. So methane, ethane, propane, they have only one structure. Methane and ethane propane they no problem. We can call methane ethane propane. When you go for butane or higher alkanes, butane. So there are two structures possible for this molecule. So one is the linear one. The other alternative we can also have other structure. So both have same molecular formula C4 H10 C4. So they have same molecular formula but

00:18:33 different structures. They shows different properties. The boiling point is different. So to uh when you go for uh pentane further there are three structures possible. Huh? We look at these molecules. They have C5 H12. C5 H12 C5 H12 Th. These three compounds have same molecular formula C5 H12. However, they have different properties, different structures. to name these compounds. IUPAC, International Union Pure and Applied Chemistry have introduced some rules to distinguish these compon name these compounds that is called IUPAC

00:20:09 nomenclature. So using this up nomature you can name all the compounds but when you go for um hexane you'll have more structures and heptine you'll have with seven carbon atoms and octane eight carbon atoms non nine carbon atoms take 10 carbon atoms you'll have number of structures using this nom culture and you can name all the compounds this is called Iupac nom culture and let how you will name these compounds. To name these compound, you have to follow some uh guidelines. And so first

00:20:49 what we have to do uh we have to find out the linear alkan there is no problem you can there are five carbon atoms you can call pentane when you go for branched one and so what we have to do in this case we have to find out the longest chain in this molecule and you can there are two possibilities 1 2 3 this is one chain you have three carbon atoms. The other ch other possibility is there three four. So you can name like this then one chain you have three carbon atoms. If you name like this it comes

00:21:31 four carbon atoms. So you have to find out the longest chain. You have to start numbering the chain. You have to other words you have to find out the longest chain in the molecule. So in this molecule this is the longest chain right? Not this one. You have only three carbon atoms. If you go in this way, you have the four carbon atoms. You have to find out the longest chain in the molecule. Once you find the longest chain, now you have to look at it. You have to start numbering the compound. So already I have done before the numbering

00:22:02 but first you have to find out the longest chain then you start numbering the longest chain. How will you? There are two ways you can start numbering from here or there. You can start numbering from here. This is the right one. If you start numbering here this is wrong one. But what you have to do you have to start numbering where the substitution is present closer to the uh end. So if here if you look at it this one the substitution is present uh closer to this carbon. This end and you have substitution is present here.

00:22:34 Therefore you have to start numbering from this carbon not from this side. So once then numbering is done. Now you have to uh combine you have to find out the substitution position and combine this one and with that one and in this case you have the methyl group. So just we have seen when you have four hydrogen atoms we call it as methane. So when you replace one of the hydrogen you write then we call methyl. So this is methane. When you remove one of the hydrogen you have the C CH3 then we call it as methyl

00:23:20 the A has been replaced. Similarly this is ethane the A end can be ref for example and so this is ethane right. So when you have this one substituent what happens in this case one of the hydrogen has been replaced. So this called ethyl all right ethane ethyl that a has been replaced by oil. Similarly when you go for propane then when you have the CH3 CH2 CH2 substituent is called propile butile pentile so on. So now what we have to you have to bring the substitution as prefix and this in this case and position is two two methyl

00:24:15 butane it has four carbon atoms. So the substitutions present on second carbon atom the present substitution is methyl group. Therefore two methylb butane the name of the compound is 2methylb butane. So now let us look at this molecule and this is a you have to find out the longest chain anyway in this both way same there is no problem then you have to start numbering the chain has three carbon atoms the substitution is present and second carbon atoms. So you have to write 2 comma 2 dimethyl propane.

00:25:07 So if you have one carbon atom we call methane, two carbon atoms of ethane, three propane, four butane and pentane, five carbon atoms, six hexane, seven heptine so on. And in this case you have the chain has three carbon atoms and the second carbon atom you have the two methyl group and so so 2a 2 dimethyl propane the name of the compound is 2 comma 2 dimethyl propane here 2methyl butane this is pentane. So in this way once if you understand this one you can name all the compounds. So now let us look at this one and in

00:25:53 the case of butane and one is a linear structure another one is branch structure. uh this called butane there is no problem and name of this compound is the now you have to start numbering this compound here also no problem you have either way you can start numbering you have three carbon atoms and now you have to find out the sub presence of substitution of the substitution at present and second carbon atom so you have to write Twomethyl propane. Two methyl propane. Now let us uh name this compound.

00:27:18 So this how will we name this compound? First now we have to find out the longest chain in this molecule. The longest chain is this one. Right? So longest chain has six carbon atoms. This is the longest chain. Once you find after finding the longest chain then you have to find out where the substitution is. Now this is a two terminal is present in this this side and that side. You have to find out which carbon which end is very close to the carbon that contains a substitution in this case here. So you have to start numbering

00:28:00 from here. So numbering is done. Once numbering is done you have to find out the position of the substitution here the present on second carbon atoms and in this case present four carbon atoms. So now you have to write 1 comma 4 dimethyl. That means sorry 2 comma 4 dimethyl. The methyl group present at second carbon atoms and four carbon atom dimethyl hexane. this IUPAC name of this compound. So next the primary, secondary, tertiary carbon atoms. Let us look at this molecule. or methane. Methane or that carbon contains bonded

00:29:29 with only one carbon is called primary carbon atom. So this carbon is bonded with otherwise you can call terminal carbon. It is bonded with one carbon atom is called primary carbon atom. And in this case the carbon is bonded with two carbon atoms. Right? One with the tertiary another one is the primary. This is called secondary carbon atom. When the carbon is bonded with this three carbon atoms, four carbon atom, this is called quartinary carbon atom. This is called quatinary is bonded with four carbon atoms. The

00:30:19 other hand if it has this is called tertiary carbon atom. So this this means this carbon is bonded with three carbon atoms. Right? And if it bonded with another carbon we call it isinary carbon atom. This is followed the nomenclature. Now let us look at the isomearism. So butane it can have two structures butane and two methyl propane and they have the same molecular formula C4 H10 but different structures right so these they have different properties Now this boiling point is different these two compounds. So the

00:31:44 molecules have same molecular formula but different structures different property. They are called structural isomers. So this the relationship between these two compounds are structal isomers. molecules have same molecular formula but different uh structure they call isomers. The relationship between these two is structural isomers. Right? For pendant we can have three structures. They have different physical properties and these are called structural isomers. They have same molecular formula C5 H12.

00:33:08 Right? All have C5 H12 but they have different structures. The relationship between them is structural isomers. So far we have seen um class classification of hydrocarbons structure and bonding of methane and ethane. They have detected geometry both bond angle 109.5° and they are non-planer molecule. Then we have seen the IUPAC nomenclature and isomearism of alkanes. Now let us see the preparation of alkanes. Petroleum, natural gas are the main source of uh hydrocarbons. Um natural gas contains 80%

00:34:28 methane, 10% Ethane 10% higher alkanes, propane, but so on. This is a mixture of natural gas and petroleum contains hydrocarbons up to C40 is a mixture of hydrocarbons. They are found near earth's crust together and um so we will see them in detail later. Now we will see the other methods that we use in the laboratory to prepare alkanes. So one of the common reaction we use in the laboratory to prepare alkanes from alkenes and alkyes. Hydrogenation of unsaturated hydrocarbons. The beginning we have seen

00:35:52 carbon al ethyline and aeline as unsaturated hydrocarbons. And when you react for example propene or propane this unsaturated hydrocarbons with hydrogen. Here you need two molecule of hydrogen. And when you treat this alken with hydrogen, the presence of catalyst like paladium charcoal or platinum or nickel. The alken can react with hydrogen. Basically it under go addition you get alkan. Similarly, alkan can be transformed to it's a stereious specific reaction. And what happens in these reactions?

00:37:13 You have your catalyst for example palladium charcoal the palium charcoal when you treat exposed with hydrogen. Hydrogen absorbed from the surface you form activate the hydrogen you form this kind of intermediate. Once if you have this intermediate the hydrogen is absorbed on the surface of the catalyst your alken when you treat this with your alken the alken also can bind with your paladium by interaction by complex formation you out. For example, you have this kind of intermediate. Once you form this

00:38:31 intermediate, the hydrogen can transfer intramolecularly. So now you have the hydrogen is transferred. You have already carbon. You'll have this kind of intermediate. So you have another hydrogen here. This can transfer again. You have the reaction medium has a lot of hydrogen. The hydrogen can absorb here. It can transfer to another carbon. So you'll generate the reduced alkan Okay. And your catalyst is regenerated. So now again it can react with hydrogen gas. It can go on like this. So whenever use

00:39:48 less than 10 more% of this uh metal um with respect to a substrate we call it as catalyst. And it can go on like this with another hydrogenate catalyze like this reaction. So this commonly used anyway this very simple alken but when you have a larger alken for example actine noninine then very easy to reduce in the laboratory and we can get very pure reduced alkan. So in the case of alkine what happens a similar way first what happens the alkine is reduced to alkan depends upon the catalyst what you use

00:40:26 the catalyst active the alken can be further reduced to alkan so it involves it requires two molecule of hydrogen and it requires one molecule of hydrogen and in this reaction what happens alken when we study the alken and alkeny you see the structure the alken is a planer molecule alkan is a non-planer molecule Right? Retrital geometry. The alken is a planer molecule. You have the top face or bottom face and your hydrogen is on the plane. All right. And the alken approaches the catalyst like this. The

00:40:59 bottom side the one hydrogen transfer then right the another catalyst and hydrogen is another hydrogen comes. Basically both hydrogen are transferred the same phase of the alken you get the alkan. The reaction is the is addition is in addition the reaction still specific and same thing happens in the case of alkane also right the alken approaches like this the four both hydrogen's are transferred the same phase of the alkine then you get the alken that alkan again further react and you get the this one

00:41:36 this reaction also in addition reaction stay specific this is one of the common reaction we perform in the laboratory to make alkanes from alkanes alkanes very easy reaction. The second example is reduction using nasen hydrogen. We have alkalhalide like broomethane or brooma octane or any except it can be chlorine or it can be iodine no problem but if you have florine doesn't react when you react when you treat broomethane with zinc HCl so it can be reduced into methane and HBr. So this is a reduction reaction. If you

00:42:36 have alkalide like brmo, chloro iodides, you can reduce them to the corresponding alkan. You generate the hydrogen helide as a byproduct. This another reaction used to prepare alkan. The third example is What's coupling alkalhalide? For example, broomethane can be cross coupled together. If you have take two molecule of this react with two equivalent of sodium you can generate ethane symmetrical alkan with two molecule of sodium bromide. This called coupling. And this reaction what happens? This

00:43:54 broome methane reacts with sodium. You generate. Once you form this intermediate, this can react with another molecule of broomethane. You get ethane and sodium bromide. So you generate two molecule of sodium bromide and ethane. This is good to make a symmetrical alkanes. But if you take a mixture of alkalhalide for example methyl bromide and ethyl bromide will not be good. You'll end up with mixture of compounds three compounds. For example instead of methyl bromide you take a mixture of methyl and ethyl bromide. When you react

00:45:05 these two bromides with two equivalent of sodium, you can you'll end up with mixture of compounds, you'll have ethane. Uh this is this can be formed from the reaction of this methyl bromide. In addition to that you'll also will have uh these two molecules can react then you generate butane and if they couple together you'll get propane. Basically you'll end up with mixture of three compounds ethane propane butane is it will be very difficult to separate these hydrocarbons. Therefore the woods

00:45:59 coupling is very good to make symmetrical alkanes. So far we have seen two methods only hydrogenation of satur unsaturated hydrocarbons like ethylene and alkine to alkanes. Then we have seen the coupling of the reduction of alkalhalide to alkan using zinc HCl and where you generate nas hydrogen that reduces alkalhalide to alkan then we have seen woods coupling and it will be good to make symmetrical alkanes. The uh next reaction is decarboxilation reactions. If you have for example sodium acetate,

00:46:57 sodium caroxilate, when you treat sodium caroxilate with the soda lime when you heat and uh so you generate alkan in. So the byproduct is sodium carbonate in this and this one of the method used to make alkane. This whenever you treat sodium caroxilate with the sodal lime and heat it you can generate alkan and the byproduct is sodium carbonate. This called decaroxilation reaction. You lose one carbon in this. Yeah. Instead of two carbons, you'll get only one carbon less hydrocarbon alkanes.

00:47:56 The last example is gulp synthesis. You can also if you have the sodium potassium copperate for example sodium acetate when you do electrolysis it can be converted into symmetrical alkan ethane and you will generate Sodium hydroxide. When you do electrolysis in water, so you generate sodium hydroxide and carbon dioxide, hydrogen and sodium hydroxide. This will be the byproduct. Uh this also very common method to make alkanes. how the reaction takes place. Anode, it can lose two molecule of if you take

00:50:03 sodium acetate, it can lose two electrons and you generate the radical. This radical can release carbon dioxide. It generate the methyl radical. Two methyl radical. This two methyl radical combined together you generate the ethane. This what happens at anode at cathode. water you have two electrons then you generate O minus plus H dot. So it can be now this O minus can two O minus already have in the case the an already have two sodium plus they can react together. This you take two molecule of this take

00:51:36 two and then you generate and this two can combine together you generate H2. Yes. So cathode it takes this two electron reduces you it the water is converted into hydrogen gas and sodium hydroxide you generate the alkan um carbon dioxide so with this I conclude today's lecture and we will see um um alkenes alkyes the next lectures thank you very much.