 We have been talking a lot about hydrocarbons, basically compounds made up of only hydrogen and carbon atoms. And we saw that there are three main families, alkenes, alkenes and alkynes. There are other families, but we will talk about them in a separate video. Over here, we are going to look at more members of each of these families. And with the help of that, we will be arriving at a general molecular formula for each of these families. Well, what is that? Well, a general molecular formula gives a relationship between the number of carbon atoms and the hydrogen atoms in a particular family. For example, in alkene family, for every carbon atom, you have twice the number of hydrogen atoms. Okay, that's the relationship. Now if you are wondering why is this important, how is this helpful? So just imagine if in future, you discover a new organic compound, which has around 50 carbon atoms and 100 hydrogen atoms. And just by knowing the general molecular formula, you will be able to categorize this compound in any one of these families. And with that, you will be able to predict its chemical properties pretty fast. Because you know what? All the members of a certain family will have similar chemical properties. So isn't that very convenient? So that's why we are going to find out the general molecular formula of each of these families. Let's begin. Now let's start with the alkene family. See by definition, they are acyclic hydrocarbons. Acyclic meaning that the carbon atoms are not arranged in a circular fashion. See, this is a cyclic hydrocarbon. Here the carbon atoms are arranged in a circular manner, right? In alkanes, they won't be. They will be in an open chain structure. Okay. And the second part is that they only have single bonds between the carbon atoms. All the carbon atoms will be connected with a single bond. No double or triple bond will be present between them. So probably the simplest alkene is going to be one with one single carbon. One single carbon, methane. And this carbon is going to have four valence electrons, which it will be sharing with four hydrogen atoms to fulfill its octate and be stable. So this is the first member of the alkene family. But that's not it. I can keep on replacing a hydrogen atom with another carbon atom and keep on elongating this chain. Something like this. Here I replace a hydrogen atom with another carbon atom. So see here, this carbon is sharing one electron with this carbon and it will be sharing the rest of the three electrons with hydrogen atoms. So this is also a valid member of the alkene family. See this is acyclic and all the carbon atoms have single bonds with each other. There are no double or triple bonds in this compound, right? So this is another alkene. Now similarly, I can keep on elongating this chain. I can take a copy of this compound and replace a hydrogen with another carbon and fulfill its valencies with hydrogens. So this is another member of the alkene family. There are three carbon atoms in this particular chain. I can keep on doing this again. This has four carbon atoms. So you can imagine that similarly I can keep on forming newer and newer alkene members. I can have five carbon chain or 10 carbon chain or 20 carbon chain. So here we saw how to find out the members of the alkene family. Now let's try to find out their general molecular formula. For this, let's try to write down the molecular formula for each of these compounds. So methane here has one carbon and one, two, three, four, four hydrogens. So its formula is going to be C1C and H4 because there are four hydrogen atoms. Similarly this compound, this has two carbon atoms. So C2 and there are one, two, three, four, five, six, six hydrogen atoms. So C2H6. Similarly can you pause the video and try to find out the molecular formula of these two molecules? Now that you have tried it, let's see. So this one has three carbon atoms. So C3 and it has three, four, five, six, seven, eight, eight hydrogen atoms. So C3H8. This one has four carbon atoms. So C4 and it has three, two, five, six, seven, eight, nine, ten. Ten hydrogen atoms. Oops. Let me change the color. C4H10. This is going to be its molecular formula. Now let's see what is the general molecular formula of alkene family, meaning what is the common relationship between the number of carbon atoms and the hydrogen atoms in every alkene. So basically what we are trying to find out is if given an alkene which has n carbon atoms, can we somehow tell how many hydrogen atoms are going to be present in this alkene? Okay. So let's see. So over here I see that for two carbon atoms, there are six hydrogen atoms. Here for three, there are eight. Here for four carbon, there are ten hydrogen atoms. Hmm. What is the relationship between the numbers of carbon and hydrogen atoms? If you can find that, we can arrive at a general molecular formula. So can you pause the video over here and try to figure out the relationship between these two numbers? Pause and try by yourself first. Now if you have tried it, let's see. So here maybe if I multiply two with three, then I get six. So maybe into three is the relationship. Let's try that out. Three into three. This is going to be giving me nine and not eight. So multiplying by three is not the right relationship. How about addition? Maybe if I add four over here, four plus two, six. Okay. Let's add four over here. No, I'm not getting eight over here, right? So adding four is also not going to be the relationship. How about I do both? I multiply and then I add some number. Like for example, I multiply two with two, two to the four plus two, okay? Then I am getting six. Let's try the same over here. Three into two and plus two. Maybe this is the relationship into two and plus two. So three to the six plus two is eight. Oh, wow. I'm getting this number. Again, let's try it one more time. Into two and plus two, four to the eight plus two is 10. So yes, this is the relationship for alkene family. Now if you take the carbon atom and multiply it by two and add two to it, then you will get the number of hydrogen atoms, okay? So over here, our general formula is going to be if the number of carbon atoms are n, then I'm going to multiply this by two first. So that will give me two n and then I'm going to add two to it. This will be the number of hydrogen atoms present. So this is the general molecular formula for the alkene family. All the alkanes will have carbon and hydrogen according to this ratio only. If tomorrow you come across any alkene, which has let's say 25 carbons, then the number of hydrogen's will be twice of 25, which is 50 plus two, 52. So C 25 H 52 will be the molecular formula. Isn't that very convenient? Now let's move on to alkenes. So alkenes also are acyclic hydrocarbons, meaning the carbon atoms are not arranged in a circular pattern and they have at least one pair of carbon atoms with a double bond between them. Okay? There can be more, but at least one pair of carbon atoms. And this is an example of alkene. Now can you pause the video and think about whether there can be an alkene with just one single carbon atom? Pause and think. So no, right? That's not possible because we know that alkenes need at least one pair of carbon atom with a double bond in between them. So one pair means at least we need two carbons, right? So an alkene with a single carbon atom is not possible. So here I have two carbons with a double bond in between them. Okay? Now that this carbon is sharing two electrons with this carbon, there are two more valencies, two more electrons pending, right? These will be satisfied by hydrogen, hydrogen, similarly over here, hydrogen and hydrogen. Okay? Now this is just the first member. Just like how in alkenes, we took a copy of the first alkene and we replaced hydrogen with more and more carbon atoms and elongated the chain. We can do similar things over here also. Let's do that. Now this carbon is sharing one electron with this carbon. Now the three pending electrons will be shared with hydrogen atoms. So this is another member of the alkene family. See this is acyclic in nature and it has got one pair of carbon atom with a double bond in between them. See there can be more than one, okay? There can be a double bond over here also between these two carbon. But in this video, we are talking about simple alkenes, meaning alkenes which have only one pair of carbon atom with a double bond in between them and we will be trying to write the general molecular formula of simple alkenes. Okay? Now before doing that, let's write the next member of this family. Let's take a copy of this and let's replace the hydrogen atom with another carbon. So this is another member of the simple alkene family, okay? Now let's try to find out their molecular formulas. So over here, I can see that there are two carbons, so C2 and then there are four hydrogens. So C2H4, this will be its molecular formula. This has three carbons. So C3 and 1, 2, 3, 4, 5, 6, 6 hydrogens. This has 1, 2, 3, 4 carbons, C4, 1, 2, 3, 4, 5, 6, 7, 8, 8 hydrogens. So over here, you can clearly see that the number of hydrogen atoms, they are double the number of carbon atoms. See over here, 4, 2, 6, 3, 8 and 4. That means the general molecular formula can be Cn if there are n carbons, then the number of hydrogens have to be twice the number, twice the number, 2n. So this is the general molecular formula of alkenes. Now let's talk about alkenes. The alkenes are also acyclic hydrocarbons, but they have at least one pair of carbon atoms with a triple bond in between them, a triple bond, okay? And for similar reasons like alkenes, we cannot have an alkyne with just one carbon atom. We will at least need two carbon atoms with a triple bond in between them. So the simplest alkyne will have at least two carbons in it, okay? Two carbons and there will be a triple bond between them, okay? Now since this carbon is sharing its three electrons with this carbon, it has one more electron left in its valence shell, which it will share with hydrogen atom. Similarly, this will share with hydrogen atom. So this is ethane, this is your simplest alkyne. And now just like how we did in case of alkenes and alkenes, here also we can increase this, we can elongate this carbon chain and form many other members of the alkyne family by replacing hydrogen with more and more carbon atoms. Let's do that. If you copy paste and replace this hydrogen with a carbon atom, then we will get something like this. See, here we have a chain of three carbon atoms and one pair of carbon atoms have a triple bond between them. So this is also a simple alkyne. Now let's elongate the chain by replacing this hydrogen with another carbon. Then we will get something like this. Here we have a chain of four carbon atoms and one pair of carbon atoms have a double, have a triple bond in between them. So this is also a simple alkyne. See over here, we can have more than one pair of carbon atoms with triple bond, like we can have a triple bond over here also. But see, those are complex structures. In this video, we are not talking about them. Here we are only talking about simple alkynes, meaning alkynes, which have only one pair of carbon atoms with a triple bond in between them. Now let's try to figure out the general molecular formula of simple alkynes. Now before I do that, can you pause the video and try it by yourself? See, you are trying to find out if there are N carbons in a simple alkyne, then how many hydrogens will it have? Okay, pause and try it by yourself first. Now if you have tried it, let's see. So over here, the molecular formula of this compound is C2, there are two carbons and two hydrogens. So C2H2. The molecular formula of this is three carbons, so C3 and H1234, H4. This one has four carbons, so C4 and H123456, H6. Okay, so if you see in general, then let me prepare a column over here, C and H. I see for two carbons, there are two hydrogens. For three carbons, there are four hydrogens. For four carbons, there are six hydrogen atoms. Okay, now what's the relationship over here? The relationship is two into one. Into one is the relationship. Okay, if I tried over here, and this does not give me the answer, three into one is not four, but three. So multiply by one is not going to be the answer. What about, okay, over here if we see, then four plus two is six. So plus two may be the function, plus two may be the relationship. But if I do plus two over here, I should get five, not four. So plus two can't be the answer. What if I multiply this by two, four twos are eight, and then eight minus two is six. So into two and minus two, this should be the relation. Let's try it. Into two, minus two. So three twos are six, minus two is four. Yes, I'm getting it. Once again, two into two and minus two. So two twos are four, minus two is two. So yes, this is the relationship, into two, minus two. That means if you have N carbon in a simple alkyne, then the number of hydrogen atoms will be twice the number of carbon atoms to N and minus two. This is going to be the general formula, general molecular formula of simple alkynes. So in this video, we'll look at the members of the alkene family, the simple alkenes and simple alkynes family. And we also looked at their general molecular formulas. Well, if you are very keen, you would notice that the only difference between alkenes and simple alkenes is that there is a double bond. And rest of the things are exactly the same. So the difference between their molecular formulas could only be because of this double bond. If I had to put a double bond over here, I would need an electron from this carbon and this carbon. So if this carbon lets go of this hydrogen, then it can have an extra electron. Similarly, this carbon can also let go of the hydrogen and get an extra electron. Now these electrons can bond and form a double bond. Okay. So to get a double bond over here, I had to let go of two hydrogens. And that's the difference between an alkene and alkyne. See, this has H2N plus two, whereas this one has H2N, the differences of two hydrogens. Similarly, you can note the difference between alkenes and alkynes is that there are triple bond over here, two extra bonds. Now to get two extra bonds, this carbon needs to let go of two hydrogen. Similarly, this one needs to let go of two hydrogens. That means four hydrogens. And that's the difference between alkenes and alkynes general formula of four hydrogen. Now with this, we have looked at the general molecular formula of all the three families. And by the way, you don't need to mug them up. Whenever you are in need of these, I would suggest you to take some time and try to derive them on your own. Okay. With more and more practice, you'll get faster at this. Don't worry.