The Alkyl Groups - Online Tutor, Practice Problems & Exam Prep

Alkyl groups are formed by removing a hydrogen atom from an alkane, resulting in names that use the alkane prefix with a "yl" suffix. For example, a 5-carbon alkane, pentane, becomes a pentyl group. Structural isomers can occur with 3 and 4 carbon alkyl groups, leading to names like isopropyl, sec-butyl, isobutyl, and tert-butyl, depending on the carbon's position and branching. Understanding these names and structures is crucial for correctly identifying and naming organic compounds.

Alkyl Groups Concept 1

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 are formed by removing an H from an alkane compound. We name alkyl groups by using the alkane prefix with a 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 going to remove an H. Removing an H from this CH₃, it becomes CH₂. This bond here with the squiggles just means that this is connected to something else, maybe a larger molecule, a larger portion somewhere else. So, it's acting as a branch. Because it's now removed an H, it's no longer an alkane, it becomes an alkyl group, and its ending is going to change from ane to yl. So this becomes a pentyl group.

Now, 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, it is very unlikely for it to exist as an alkyl group because they are serving as branches. If you have a branched carbon that's 8 carbons long, that means your original molecule is something huge, which you are not going to see in this level of chemistry. So here, if we have one carbon, our prefix is meth, thus as an alkyl group it's methyl; 2 carbons is eth, as an alkyl group it's ethyl; 3 carbons is prop, as an alkyl group it's propyl; 4 carbons is but, as an alkyl group it's butyl; 5 carbons is pent, as an alkyl group it's pentyl; and then 6 carbons is hex, as an alkyl group it's 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.

Alkyl Groups Example 1

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? 1, 2, 3, 4, 5, 6. Six carbons means that our prefix is hex, so the alkyl group is hexyl, so this is a hexyl alkyl group.

3 and 4 Carbon Alkyls Concept 2

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 becomes 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 differently. 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. A 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/constitutional isomers that are possible. If you have propyl, you could also have isopropyl. Butyl, even more so. There's butyl, sec-butyl, isobutyl, and tert-butyl. 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.

3 and 4 Carbon Alkyls Example 2

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. Three carbons mean there's one of two possibilities, either propyl or isopropyl. It would be propyl if the connection to the main chain happened at one 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 1, 2, 3 carbons long. Those three 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. Two carbons, its prefix would be eth, so those two carbons would be ethyl. So our four alkyl groups in this alkane are isopropyl, methyl, propyl, and ethyl. So those would be our final answers.

Here’s what students ask on this topic:

What are alkyl groups and how are they formed?

Alkyl groups are formed by removing a hydrogen atom from an alkane. This process results in a group that can attach to other molecules, acting as a branch. The names of alkyl groups use the alkane prefix with a 'yl' suffix. For example, a 5-carbon alkane, pentane, becomes a pentyl group when an H is removed. The general formula for an alkyl group is C_nH_2n+1, where n is the number of carbon atoms.

How do you name alkyl groups with different numbers of carbons?

Alkyl groups are named based on the number of carbon atoms they contain, using the alkane prefix with a 'yl' suffix. For example, a 1-carbon alkyl group is called methyl, 2-carbons is ethyl, 3-carbons is propyl, 4-carbons is butyl, 5-carbons is pentyl, and 6-carbons is hexyl. The naming follows the pattern of the corresponding alkanes but with the 'ane' ending replaced by 'yl'.

What are structural isomers in the context of alkyl groups?

Structural isomers are molecules with the same molecular formula but different connectivities. In alkyl groups, this means the carbon atoms can be connected in different ways. For example, a 3-carbon alkyl group can be propyl or isopropyl, depending on whether the connection is at an end carbon or the middle carbon. For 4-carbon alkyl groups, the isomers include butyl, sec-butyl, isobutyl, and tert-butyl, each with different carbon arrangements and connections.

What is the difference between sec-butyl and tert-butyl groups?

Sec-butyl and tert-butyl are both 4-carbon alkyl groups but differ in their structure. Sec-butyl (secondary butyl) has the connection at a secondary carbon, which is a carbon attached to two other carbons. Tert-butyl (tertiary butyl) has the connection at a tertiary carbon, which is a carbon attached to three other carbons. These differences in structure affect their chemical properties and reactivity.

Why is it important to understand the naming of alkyl groups in organic chemistry?

Understanding the naming of alkyl groups is crucial in organic chemistry because it helps in correctly identifying and naming organic compounds. Alkyl groups often act as branches in larger molecules, and knowing their names and structures allows for accurate communication and understanding of molecular structures. This knowledge is essential for studying reactions, mechanisms, and properties of organic compounds.