The Alkyl Groups - Video Tutorials & Practice Problems
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Alkyl Groups
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Now before learning to name more organic compounds, we must first take a look at the names of alkyl groups. Now what are alkyl groups? Well, alkyl groups, they're formed by removing an h from an alkane compound. And we're going to say we name alkyl groups by using the alkane prefix with YL suffix. So if we take a look here, we're starting out with a 5 carbon chain that's an alkane. That means its prefix is pent for 5, and since it's an alkane it ends with ane. So this is pentane. Now, here we're gonna remove an H. So remove an H from this CH3, and it becomes CH2. This bond here with the squigglies just means that this here is connected to some other thing, maybe a larger molecule, larger portion somewhere else. K. So it's acting as a branch. So because it's now removed in h, it's no longer an alkane, it becomes an alkyl group, and its ending is gonna change from ane to yl. So this becomes Pentyl, pentyl group. Now, here, our alkane prefixes are based on the number of carbons, and typically when it comes to alkyl groups, we work within the range of 1 to 6 carbons. Beyond this is very unlikely for it to exist as an alkyl group because again, they're kind of serving as branches. If you have a branched carbon that's 8 carbons long, that means your original molecule is something huge, which you're not gonna see at this level of chemistry. So here if we have one carbon, our prefix is meth, so as an alkyl group it's methyl, 2 carbons is eth, as an alkyl group it's ethyl, 3 carbons is prop propyl, 4 carbons is but, butyl, 5 carbons is pent, pentyl, and then 6 carbons is hex, hexyl. So that's how we named each one of these alkyl groups. And remember, we create an alkyl group by removing an h from the original alkane group, and then it's attached to something. In this case, we don't know what it is, we just know it's attached to something else.
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example
The Alkyl Groups Example
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So here it says provide names of the alkyl groups below. Alright. So these are alkyl groups because they're attached to something else which we don't know, so we know they're going to end with 'yl'. Now, their prefix name is based on the number of carbons present. Remember lines connect carbons together. So this one here has 1, 2, 3, 4 carbons present, the prefix would be 'but', so this would represent a butyl alkyl group. For the next one we have how many carbons? 123456. 6 carbons means that our prefix is hex, so the alkyl group is hexyl, so this is a hexyl alkyl group.
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3 and 4 Carbon Alkyls
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Now we must learn additional names for alkyl groups with 3 and 4 carbons. These molecules have structural or constitutional isomers. Remember, these are isomers that have the same molecular formula, but different connectivities. If we take a look here, if we're dealing with a 3 carbon alkyl group, so here we have 1, 2, 3 carbons, this would be a propyl. But who's to say that the connection is at this end carbon? We have a chance that that connection could be on the middle carbon instead. So here we've rearranged our bond, and the connection now is on the middle carbon. When this occurs, this alkyl group now becomes isopropyl. Okay. So when it's on the end, it's propyl, but if it's on the middle carbon, it becomes isopropyl. For 4 carbons, it comes even more complicated. So if we're looking at the first image, we have 4 carbons, 1, 2, 3, 4, we know the prefix is but. Here if it's at one of the end carbons, it's butyl. But who's to say that it has to be at that end carbon? And who's to say that the chain has to be a straight chain? If we move to the left, let's say instead of being at this end carbon or this end carbon, it happens to be on one of the middle ones. In this case, we see it here on this middle carbon. Now, this is called sec butyl. Sec because it represents a secondary carbon. This is just a carbon attached to 2 other carbons. This secondary carbon in blue is connected to 1, 2 other carbons within this alkyl group, that's why it's a secondary carbon. Now, who's to say that it's a straight chain? We could rearrange the 4 carbons in a different order. Here, we still have 4 carbons here, but it's just rearranged a little bit different. Instead of being a straight chain, we actually have a branching group here, this one here. If it looks like this, then it is called Isobutyl. So in this fashion it's isobutyl. And then finally, in this last one, maybe the 4 carbons orient themselves in this fashion, where this middle carbon here is connected to 3 of the carbons, and that middle carbon is what's attached to something else. In this case, we call this tert butyl. Why is it called tert butyl? Because that carbon in red is a tertiary carbon. Tertiary carbon means the carbon is attached to 3 other carbons. If we look, that carbon in red is connected to 1, 2, 3 other carbons. That's why it's a tertiary carbon. So again, for 3 carbon and 4 carbon alkyl groups, it can get a little bit tricky because we have different structural slash constitutional isomers that are possible. If you have propyl, you could also have isopropyl. Butyl, even more so. There's butyl, secbutyl, isobutyl, and terbutyl. So keep in mind the names that we see here, and the way the connections kind of work out when they exist as alkyl groups. This is key to naming them correctly, and key to when we start naming more organic compounds.
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example
The Alkyl Groups Example
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Here in this example question, it says provided below is a structure of an alkane. Identify names of all alkyl groups present in red. Alright. So if we take a look here, this is our chain, and these alkyl groups are the branches coming off of that chain. If we take a look at the first one, so it's branching off, this branch group here has 1, 2, 3 carbons that are not part of this main chain. 3 carbons means there's one of 2 possibilities, either propyl or isopropyl. It would be propyl if the connection to the main chain happened at 1 of the end carbons, but instead it's happening at the middle carbon, meaning that this alkyl group is isopropyl. Next, we have this Carbon branching off of the main chain. It is just a single carbon, so we know it's an alkyl group, so it's YL, and one carbon has the prefix of meth, so this would be methyl. Next, we have this chain branching off of the main chain, so this is 123 carbons long, Those 3 carbons, the bonds at one of the ends, so this would be propyl. And then finally we have this one branching off which is made up of 1, 2 carbons. We know the ending is y l. 2 carbons, its prefix would be f, so those 2 carbons would be ethyl. So our 4 alkyl groups in this alkane are isopropyl, methyl, propyl, and ethyl. So those would be our final answers.
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