VISUAL SKILLS Visualize the structural formula of each of the fol...

Question

VISUAL SKILLS Visualize the structural formula of each of the following hydrocarbons. Which hydrocarbon has a double bond in its carbon skeleton?
a. C3H8
b. C2H6
c. C2H4
d. C2H2

Accepted Answer

Welcome back everyone. In this example, we need to determine which of the below hydrocarbons is not straight chained. We should first recall what a hydrocarbon actually is and recall that this is an organic compound with a series of carbon atoms bonded to hydrogen atoms. We next want to recognize that there are 2 types of categories of hydrocarbons. We have aliphatic hydrocarbons versus what we would recall as aromatic hydrocarbons. Now going to aliphatic hydrocarbons we have certain criteria. This can be either open or cyclic structural molecules where by open what we're referring to is having either branches or just being a straight chain. The second criteria to be an aliphatic hydrocarbon is to either be unsaturated or saturated. And by unsaturated we are referring to our hydrocarbon structure having pi bonds. So recall that pi bonds are double bonds and that would correspond to being an alkene or we can have triple bonds so we have three lines which would correspond to being an alkyne Whereas on the converse we have saturated hydrocarbons which only consist of sigma bonds only Recall that sigma bonds are single bonds Now, we have to consider the criteria for aromatic hydrocarbons which are always going to be unsaturated meaning they will normally contain either, pi bonds or triple bonds whereas they also should have benzene like properties and we will get to that in one moment but what that means here is that it would either be molecules that are sp2 hybridized which we should recall means we have 3 bonding regions with 0 lone pairs. Another benzene like property would be to have planar geometry because of this sp2 hybridization specifically molecular geometry. Going back to our examples, we can begin with example a which gives us ethane so we want to pay attention to our prefix and our suffix. Looking at our prefix, we can see that eth, we recall, means we have a 2 carbon structure where the suffix ane tells us that this is an alkane meaning we should only have sigma bonds in our structure and so drawing this out we would have 2 carbon atoms next to each other where each carbon atom to fulfill its octet, because we recall carbon has a bonding preference of having 4 bonds, should have a total of 3 bonds to hydrogen all singly bonded because, again, this is an alkane. And as we can see, we have an open system here because this is just a straight chain of 2 carbon atoms and so this specifically is an aliphatic saturated hydrocarbon because it is an open system with sigma or single bonds making up the structure and so this would be, because it is straight chained, an incorrect answer choice so we can rule choice A out. So let's label that as straight chained. Moving on to choice B we have but which we should recognize this prefix tells us we have a 4 carbon chain and again the ane ending means we have an alkane. So drawing out our carbons we have 1, 2, 3, 4 carbon atoms bonded to one another where the outer carbons have 3 bonds to hydrogen all singly bonded because again this is an alkane so it should only have sigma bonds and the inner carbon atom should have just 2 bonds to hydrogen because that's all it needs to complete its octet. Yet again, we have an open structure since this is just a straight chain here where this is specifically an aliphatic saturated hydrocarbon yet again because it is just single bonds making up our structure. Again, because it is straight chained, we would rule choice B out and let's move on to choice D since we can clearly see that it is yet again another alkane with the prefix prop- which tells us that we have 3 carbon atoms bonded to one another where each of the outer carbon atoms have 3 bonds to hydrogen to complete their octet the middle one has just 2 bonds to hydrogen to complete its octet, and the outer carbon has 3 single bonds to hydrogen to complete its octet since this is an alkane. Yet again, we have another aliphatic saturated hydrocarbon in an open chain system where this is again a straight chain so we would rule out choice D. This leaves us with considering choice C as benzene so what we should recognize when it comes to benzene, we can't tell how many carbon atoms it has from the prefix or anything here so we want to recall from lecture that benzene is an aromatic functional group Recall that Benzene follows the formula C6H6. Now, we also want to recognize that Benzene is also characterized by having a fully sorry, that says fully conjugated ring system. And so what this means is that when we draw our structure for benzene, we want to recall that each corner represents a carbon atom so we would have 2 carbon atoms here, a third here, a 4th carbon atom here, and sorry let's just make this a bit more even. So we have 1, 2, 3, 4, 5 and then connecting these ones here we have 6. Now when we normally draw benzene in a structure we have the hydrogen atoms be implied so recall that each of these corners here or carbon atoms have 2 bonds to hydrogen to complete their octet and these are implied hydrogens so we don't have to draw them in. Now the next characterization as we stated is to have a fully conjugated ring system meaning we must have 3 pi bonds within our structure of benzene. Now, you can place them in any location and that is important because it doesn't matter technically because this is a fully conjugated ring system so this bond can move to this location, this bond can move to this location, or this bond can be conjugated to this location. Recall that this is described as delocalization of these pi bonds in our structure. Because of these pi bonds and because of the suffix that we see in our name, ene, this tells us that we have an alkyne which makes sense as to why we have these double bonds or these pi bonds characterizing our benzene structure. So as we can see, benzene is a ring or cyclic structure. This would correspond to being not straight chained and that means that c is our correct answer choice to complete this example as an example of a hydrocarbon that is not straight chained. I hope everything I reviewed was clear. If you have any questions, leave them down below and I'll see everyone in the next practice video.

VISUAL SKILLS Visualize the structural formula of each of the following hydrocarbons. Which hydrocarbon has a double bond in its carbon skeleton? a. C3H8 b. C2H6 c. C2H4 d. C2H2

주요 개념

Hydrocarbons

Structural Formula

Double Bonds

다음 시청