Explain why each of the following alkenes is stable or unstable.
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Question

Explain why each of the following alkenes is stable or unstable.
a. 1,2-dimethylcyclopentene
b. trans-1,2-dimethylcyclopentene
c. trans-3,4-dimethylcyclopentene
d. trans-1,2-dimethylcyclodecene

Accepted Answer

Hey everyone and welcome back in today's video, we're looking at a problem which asks us which of the following Elkins are stable and which ones are unstable. Support your answer with a suitable reason we have four different structures. Let's draw them out one by one. Starting with a we have trans 34 dye bromo cycle plant one in. So we are ready to draw a cycle pence in ring. Let's draw a five member dring with a double bond at any position. Now this is position one, this is position two. We have 34 dye bromo compounds. So if we count our carbon atoms that we 1234 we have two blooming atoms to these two positions in trans arrangement meaning one of them will be pointing up another will be pointing down. We don't really worry regarding which one is an actual wedge which is a dash. It's perfectly fine to choose it as you wish. As long as you're sure that you're distributing them in opposite directions with respect to the ring. You're good. So this is our compound. Now I feel like that'd be we have one comma two dye bromo cycle plant one in. So again we're dealing with a psycho Pantene ring, we may draw a five member ring a double bond but now we have positions one and two substituted with two bromo groups. So we are going to add two bromo groups at these positions. We're good to go see trans cyclo octane now because it's a trans version of cyclo octane and eight numbered ring. We're not going to draw it as a standard ring instead, we are going to change its actual shape. And we are going to draw an eight numbered ring as follows. Essentially we are going to add the first carbon atoms in that arrangement and then we are going to add the remaining carbon atoms. So that would be cyclops fencing. My apology cycle octane and now we need to make sure that it's a psycho oxen. So there must be a double bond at some position. But because it's a trans one, we want to add a double bond at either this position or that position. Of course you may choose these two as well. Because notice that if I add a double bond, let's suppose that this position that the al kiel groups would be on opposite sides of the double bond. And that means we indeed have a trans compound. Perfect. So finally, let's look at the in these days that we have trans one comment to dye bromo cyclops antoine in. And because it's trans cycle fencing, that's what the name suggests, we are going to try and draw cyclone fencing. In trans arrangements, we are going to start with a double bond, we will make sure that the al ko substations are pointing in opposite directions. If we count the number of carbon atoms we have for a total. And that means we need one more. Let's suppose that's where it is. And we're simply connecting carbon five to carbon one cycle. Pantene implies that we have five carbon atoms in our ring and we noticed that we have one common to dye bromo compound. So this is position one this position to its trans arrangements. So one of the bromo groups will be pointing up, that would be one bromo and another one would be pointing down. So that would be our second Romo substitution. Now we have our structures, we are ready to discuss the stability of each structure. Now we're looking at a we can first of all say that it is stable and the reason why it is stable is very simple. If you look at the ring, the double bond exists in a ring. The trans here essentially implies the relative position of rooming with respect to another booming trans means that the two bromo substations are on opposite sides of the plane. It has nothing to do with the trans arrangement of the ring here. So we are simply seeing a double bond in a five member ring and recall that a double bond can exist in a five member ring. So this structure is stable, it's perfectly fine, even though there's a bit of a ring strain here. Now, similarly, if we look at B, we may conclude the same, this structure would be stable because a double bond can exist in a five member ring. Notice that once again we have a double bond and a five member ring, so that's perfectly fine for us. If we look at, see we have a double bond in an eight member ID ring and that's perfectly fine as well. We may conclude that this is stable because a trans double bond can exist in an eight member drink, recall that if you have a bigger ring, an eight member ring or a bigger ring, it's perfectly fine to have a trans arrangement in it, so we can conclude that trans cycle oxen is stable. Now. Using that reason and that approach, we are essentially concluding that this structure D. Is highly unstable, remembering what we discussed and see a trans double bond can exist in a ring that is eight members or bigger. Now this is a five member ring and we have a double bond making a trans arrangement. So that's highly unstable. A trans double bond cannot exist in a five member ring. So we may say that this is highly unstable. We may now conclude that structures A B and C are perfectly stable, even though there's a bit of a ring strain in A and B, however D. Is the only structure that is highly unstable. The correct answer is this multiple choice question is B. And if you have any questions, feel free to leave them down below in the comments section and thank you for watching

Explain why each of the following alkenes is stable or unstable. a. 1,2-dimethylcyclopentene b. trans-1,2-dimethylcyclopentene c. trans-3,4-dimethylcyclopentene d. trans-1,2-dimethylcyclodecene

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