Alkyl Groups
Home Up Simple Alkanes Structural Isomers IUPAC Names Alkyl Groups Naming Guidelines Interpreting Names Cyclic Compounds Comments

 

Alkyl Groups

The most common alkyl groups are discussed on this page. (They are also shown in example 11 in your workbook.) I would like to point out the similarities among these alkyl groups and the alkanes we discussed earlier.

Methyl Group

Methane has one carbon and four hydrogens. The methyl group also has one carbon but only three hydrogens. In place of the fourth hydrogen, there is a bond to something else.
methyl H
|
H-C- 
|
H
Model of methyl group.[61mod07.JPG (7131 bytes)]

Ethyl Group

Ethane has two carbons and six hydrogens. If any one of those hydrogens is removed and replaced with a bond to something else, you end up with an ethyl group.
ethyl H H
| |
H-C-C- 
| |
H H
Model of ethyl group.[61mod08.JPG (7500 bytes)]

Propyl Groups

Propane has three carbons and eight hydrogens. If one of the six end hydrogens is replaced by a bond to something else, you end up with a normal propyl group. If one of the two middle hydrogens is replaced by a bond to something else, you end up with an isopropyl group.
n-propyl
or simply
propyl
H H H
| | |
H-C-C-C- 
| | |
H H H
Model of n-propyl group.[61mod11.JPG (8646 bytes)]
isopropyl H H H
| | |
H-C-C-C-H
| | |
H   H
Model of isopropyl group.[61mod12.JPG (9688 bytes)]

 

The skeletal structural formulas shown here are intended to emphasize the fact that the n-propyl and isopropyl groups can be drawn in a variety of orientations. It is not the way that the Cs are drawn that makes the difference. It is the location of the bond to something else that distinguishes n-propyl from isopropyl. Starting from the bonding point in a normal propyl group there is a continuous string of three carbon atoms. Starting from the bond point in an isopropyl group the three carbon atoms are not continuous, instead they are branched.
n-propyl isopropyl

 

In these skeletal formulas the hydrogen atoms have been left off, just to emphasize the relationships of the carbon atoms. Keep in mind that every carbon atom has four bonds and if only one bond is shown, it is implied that the other three bonds go to three hydrogen atoms. Make sure you understand that by identifying where those seven hydrogen atoms would go.

Butyl Groups

The next set of alkyl groups contains four carbons. That is why they are called butyl groups. There are four types of butyl groups each with a different combination of carbon chain and bonding point.

The first one is a straight chain with an H missing from the end and is called a normal butyl group (or  n-butyl group).
n-butyl
or
butyl
The next one is iso-butyl (usually spelled isobutyl). It has a branched chain at the end opposite where it attaches to something else.
iso-butyl
or
isobutyl
There is also a secondary-butyl which is often referred to as sec-butyl. Its structural arrangement is shown here.
   sec-butyl
Also, tertiary-butyl (also called tert-butyl or t-butyl) has a fourth kind of arrangement as shown.
tert-butyl
or
t-butyl

These examples are shown as skeletal structural formulas. You should be able to flesh those out and make them full structural formulas by adding the hydrogen atoms. Remember that if a bond is shown, don't attach a hydrogen atom to it. If you have any questions about how many hydrogen atoms there should be and where they should go, be sure to check with the instructor when you are in the lab.

Other Alkyl Groups

There are other alkyl groups and their names are similarly based on the names of corresponding alkane molecules. Note that only normal groups (attachment point at the end of the chain) are shown here.

There are times when it is necessary to indicate the presence of an alkyl group without having to specify exactly what it is. In these cases, a capital R- is used to represent that unspecified alkyl group.

CH3(CH2)3CH2- pentyl
CH3(CH2)4CH2- hexyl
CH3(CH2)6CH2- octyl
etc.
R- alkyl

 

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