Constitutional isomers of C6H14 | Alkanes – Dr K
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Constitutional isomers of C6H14 | Alkanes – Dr K

The question is asking to draw out all constitutional isomers for C6H14. Since this this chemical formula fits in the
general formula of CnH2n+2, well that means C6H14 is an alkane. We’re going to use a systematic way of drawing
the isomers. That way we won’t accidentally leave out any
structures. So, do make sure you stay until the end to
pick up on this systematic way of drawing. Let’s start by drawing a 6 carbon chain, meaning
6 carbons connected straight in a row. Now, if I were to draw 5 carbons straight
in a row, and connect a carbon at the end or at the front, are they isomers? Or are they the same compound? Well, let’s count. Looks like we have 6 carbons straight in a
row in all three of the structures. So, that means, they are all the same. So, to avoid confusion, I’ll just keep the
structure with 6 carbons straight in a row like that. Since there’s no other way to draw 6 carbons
straight in a row, let’s move on to a 5 carbon chain. So, we draw 5 carbon straight like that, and
we stick the remaining carbon on the second carbon. What if I stick the extra carbon on the second
last carbon like this? Are these two the same? When we count the carbon chain for the alkane,
we always start counting from the end that is closest to the substituent. Substituent is the stuff hanging, like you
see that extra carbon hanging on carbon number 2? That is substituent. So, for the furthest right structure, if we
were to count, we would have to count from the right hand side, so we would have a substituent
on carbon number 2. Looks like these two 5 carbon chain are the
same. Continue on with the 5 carbon chain, but now
we place that extra carbon in the middle. There’s no other way we can place the extra
carbon on a 5 carbon chain, so we have exhausted all possibilities for 5 carbon chain. So, we go from 6 carbon then we go to 5 carbon
and we try to place the substituent in different places and then once we exhausted that, then
we move on to a 4 carbon chain. With 4 carbon straight in a row, we have 2
remaining carbons and we place them on carbon number 2. Let’s try moving 1 of that carbon, the substituent
to carbon number 3. That will actually give us another isomer. Look, we have on the left hand side, the 2
substituents is on carbon 2. On the right hand structure, the 2 substituents,
one is on carbon 2 one is on carbon 3. So, these 2 are actually isomers, so, we’ll
keep them. Is there any other way we can place the 2
remaining carbon in a 4 carbon chain? Looks like no. If we stick the extra carbon at the end, it
will just lengthen the chain making it a 5 carbon chain. What if we move to 3 carbon chain, like that? So, we have 3 extra carbons, right?So, we
place in 2 of the carbons on carbon 2 and 1 of the carbon on carbon 3. Do you see something? If you look closely, it’s actually a 4 carbon
chain and it’s the same as this guy right here. So, that’s it. We’ve drawn out all possible isomers for C6H14. To complete the structures, I’ve added in
the hydrogens. Just keep in mind that each carbon makes 4
bonds, so subtract the number of bonds it’s making with neighboring carbons and fill in
with the correct number of hydrogens. To recap, the systematic way that we used
was we started off with 6 carbon straight in a row, so 6 carbon chain, and then that’s
it. There’s no other way to draw other 6 carbon
chain, so, we move on 5 carbon chain. So because we have 5 carbon straight in a
row, we have 1 extra carbon, so we try sticking it from carbon number 2 and then we move it
to carbon number 3. And then we’ve exhausted all possibilities,
then we move on to 4 carbon chain. So, with 4 carbon straight in a row, you have
2 extra carbons, right? So, we start by placing 2 carbons on carbon
number 2, and then we move 1 of the carbon to carbon number 3, and then we tried 3 carbon
chain, and it looks like it’s a repeat of a 4 carbon chain. So, then we know that we have exhausted all
possibilities. And that’s what I mean by a systematic way
of drawing isomers. I hope you find the video helpful. Do subsribe. Thanks for watching and I’ll see you in the
next video.


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