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

Understanding alkyl groups is crucial for naming organic compounds. Alkyl groups are derived from alkanes by removing a hydrogen atom, which alters the name from an "ane" to a "yl" suffix. For example, removing a hydrogen from pentane results in a pentyl group. Common alkyl groups range from one to six carbons, known respectively as methyl, ethyl, propyl, butyl, pentyl, and hexyl.

Structural isomers add complexity to naming, particularly with propyl and butyl groups. A propyl group with a middle carbon connection is termed isopropyl. Butyl groups have more variations: butyl (straight-chain), sec-butyl (secondary carbon connection), isobutyl (branched at the end), and tert-butyl (tertiary carbon connection). Recognizing these structures and their corresponding names is essential for accurate organic compound nomenclature.

Alkyl Groups

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 it's prefix is pent for five, and since it's an alkane, it ends with ane. So this is pentane. Now here we're going to remove an H, 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. OK, so it's acting as a branch.

So 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 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 one to six 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 branch carbon that's eight carbons long, that means your original molecule is something huge. What you're not going to 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 it's 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 six carbons is hex, hexyl.

So that's how we name 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.

The Alkyl Groups Example

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 1234 carbons present. The prefix would be, but, so this will represent a butyl alkyl group. For the next one we have How many carbons? 123-456 carbons means that our prefix is hex, so the alkyl group is hexel, so this is a hexo alkyl group.

3 and 4 Carbon Alkyls

Now we must learn additional names for our kill groups with three and four 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 rearranged our bond and the connection now is on the middle carbon. When this occurs, this group, this alkyl group, now becomes ISO propyl. OK, so when it's on the end, it's propyl, but if it's on the middle carbon, it becomes isolated. For four 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 two 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 four 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 ISO butyl, so in this fashion it's isobutyl. And then finally, in this last one, maybe the four carbons orient themselves in this fashion where this middle carbon here is connected to three 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 three 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 three carbon and four 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, 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.

The Alkyl Groups Example

Here in this example question it says provided below is a structure of an alkane. Identify names of alkyl groups present in red. All right, 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 one 2-3 carbons that are not part of this main chain. 3 carbons means 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 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 U of 1-2 carbons. We know the ending is yl 2 carbons, its prefix would be F, 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 does "iso-" mean in naming alkanes?

"Iso-" in the context of naming alkanes refers to an isomer of a straight-chain alkane that contains a branching at the end of the molecule. The term "iso-" stands for "isomer," indicating that the compound has the same molecular formula as a straight-chain alkane but with a different structural arrangement. In an isoparaffin (another name for iso-alkanes), the branching occurs when a methyl group (CH₃) is attached to the second-to-last carbon atom in the chain, creating a penultimate branching. This branching changes the physical properties of the compound compared to its straight-chain counterpart. For example, isobutane (C₄H₁₀) is an isomer of butane; it has the same molecular formula but instead of a straight chain of four carbon atoms, it has three carbons in a row with a methyl group attached to the middle carbon.

How to name alkyl groups?

Naming alkyl groups follows a systematic approach based on the number of carbon atoms in the longest continuous chain of the group and the presence of any branching or substituents. Here's a basic guide:

1. Identify the longest continuous chain of carbon atoms in the alkyl group. This will determine the base name.
2. The base name is derived from the number of carbons in the chain:
   • 1 carbon: Methyl
   • 2 carbons: Ethyl
   • 3 carbons: Propyl
   • 4 carbons: Butyl
   • 5 carbons: Pentyl
   • 6 carbons: Hexyl
   • And so on, following the standard alkane nomenclature.
3. If the alkyl group is branched, identify the carbon atom in the group that will attach to the rest of the molecule (the parent chain). Number the carbons in the alkyl group starting from the point of attachment, ensuring the lowest possible numbers for any branches or substituents.
4. Name any branches or substituents off the main chain of the alkyl group using the same rules as step 2, and indicate their position with a number.
5. If there are multiple identical branches, use prefixes like di-, tri-, tetra-, etc., to indicate the number.

What are alkyl groups?

Alkyl groups are types of hydrocarbon groups that are derived from alkanes by removing one hydrogen atom. They are essentially alkanes with one less hydrogen, which gives them an open bonding site. This allows them to attach to other atoms or molecules, acting as substituents. Alkyl groups are represented by the general formula:

$C_n{H}_{2n+1}$

where 'n' is the number of carbon atoms in the group.

For example, if you take methane (CH₄) and remove one hydrogen atom, you get a methyl group (CH₃-). Similarly, removing one hydrogen from ethane (C₂H₆) results in an ethyl group (C₂H₅-). These groups are very common in organic chemistry and are often used to modify the properties of molecules. Their presence can affect the physical and chemical behavior of the molecules they are attached to, such as solubility, boiling point, and reactivity.

How can alkyl groups be identified?

Alkyl groups are branches or substituents that stem from the main carbon chain in organic molecules. They are composed solely of carbon and hydrogen atoms arranged in a chain. To identify an alkyl group, look for a group of carbon atoms attached to the rest of a molecule by a single bond. The simplest alkyl group is the methyl group (CH3-), which consists of one carbon atom bonded to three hydrogen atoms.

When identifying alkyl groups, you'll notice they are named based on the number of carbon atoms they contain, with prefixes like "meth-" for one carbon, "eth-" for two carbons, "prop-" for three carbons, and so on, followed by the suffix "-yl" to indicate that they are alkyl groups. For example, an ethyl group (C2H5- ) has two carbon atoms.

Remember that alkyl groups can be primary (1°), secondary (2°), or tertiary (3°), depending on the number of carbon atoms directly attached to the carbon atom that's connected to the rest of the molecule. Identifying the structure and branching of these groups can help determine their reactivity and role in chemical reactions.