Main Difference – Alkyl vs Aryl
Alkyl and aryl are two chemical terms used to name functional groups of organic compounds. Functional groups are parts of organic molecules having the responsibility for the characteristic properties of a certain molecule. They are moieties of large molecules. An alkyl group is a functional group that can be found in organic molecules. It is an alkane having a vacant point that is formed due to the loss of a hydrogen atom. An aryl group always contains an aromatic ring. Aryl group is a simple aromatic compound where one hydrogen atom is removed from the ring, allowing it to get attached to a carbon chain. The main difference between alkyl and aryl is that alkyl group has no aromatic ring whereas aryl group has an aromatic ring.
Key Areas Covered
1. What is Alkyl
– Definition, Alkyl Group, Alkyl Halides
2. What is Aryl
– Definition, Aryl Group, Aryl Halides
3. What is the Difference Between Alkyl and Aryl
– Comparison of Key Differences
Key Terms: Alkane, Alkyl, Alkyl Halide, Aromatic, Aryl, Aryl Group, Functional Groups
What is Alkyl
An alkyl group is a functional group that can be found in organic molecules. It is an alkane having a vacant point that is formed due to the loss of a hydrogen atom. This point can be attached to a carbon atom of a carbon chain. This alkyl group can be a simple, branched or cyclic, but it does not have aromatic rings. Alkyl groups have only carbon and hydrogen atoms in their structure.
The general formula for an alkyl group can be given as CnH2n+1 which is different from the formula of an alkane, CnH2n+2 with the loss of a hydrogen atom. Thus, alkyl groups are derived from alkanes. The smallest alkyl group is methyl group which can be given as –CH3. It is derived from the alkane methane (CH4). People sometimes confuse cycloalkyl groups with aromatic groups. But there is a big difference between them. Cycloalkanes are saturated and have no double bonds, but aromatic rings are unsaturated and have double bonds in their structure. The word saturated indicates that it has the maximum number of hydrogen atoms that it can bond with. Even in the morphology, cycloalkanes are 3D structures whereas aromatic compounds are planar structures. Hence, all alkyl groups are saturated because alkyl groups are derived from alkanes. The following examples show different alkyl groups.
Figure 1: Some Examples of Alkyl Groups
Alkyl halide, as it is expressed by its name, is a compound having a halogen atom attached to a chain of carbon atoms. Here, one hydrogen atom of the carbon chain is replaced by a halogen atom. According to the type of halogen that has been attached and structure of the carbon chain, properties of organic halides will differ from each other. Alkyl halides can be categorized depending on how many carbon atoms are attached to the carbon atom to which the halogen is attached. According to that, primary alkyl halides, secondary alkyl halides and tertiary alkyl halides can be observed.
What is Aryl
An aryl group always contains an aromatic ring. Aryl group is a simple aromatic compound where one hydrogen atom is removed from the ring, allowing it to get attached to a carbon chain. The most common aromatic ring is benzene. All aryl groups are derived from benzene structures.
Some examples of aryl groups are phenyl group derived from benzene and naphthyl group derived from naphthalene. These aryl groups can have substitutions in their aromatic structure. For example, tolyl group is derived from toluene; toluene is a benzene ring with the substitution of a methyl group. All aryl groups are unsaturated. This means the structure of aryl groups are composed of double bonds. But benzene is not the only type of aromatic ring that aryl groups can have. For example, an indolyl group is an aryl group attached to the common amino acid tryptophan. The following image shows the phenyl group which is derived from a benzene ring.
Figure 2: Benzyl Group is an Aryl Group
An aryl halide is a molecule having a halogen atom attached to an sp2 hybridized carbon in an aromatic ring directly. This is an unsaturated structure due to the presence of double bonds in the aromatic ring. Aryl halides also show dipole-dipole interactions. The carbon-halogen bond is stronger than that of alkyl halides due to the presence of ring electrons. That is because the aromatic ring gives electrons to the carbon atom, so its positive charge is somewhat reduced. Aryl halides can undergo electrophilic substitution and can get alkyl groups attached to the ortho, para or meta positions of the aromatic ring. One or two halogens can also get attached to the aromatic ring in the ortho, para or meta positions.
Difference Between Alkyl and Aryl
Definition
Alkyl: The term alkyl is used to name a functional group derived from alkanes by removal of one hydrogen atom or to name the chemical compounds containing this functional group.
Aryl: The term aryl is used to name a functional group derived from aromatic rings by removal of one hydrogen atom or to name the chemical compounds containing this functional group.
Structure
Alkyl: Alkyl groups are aliphatic.
Aryl: Aryl groups are aromatic.
Morphology
Alkyl: Alkyl groups can be linear, branched or cyclic structures.
Aryl: Aryl groups are essentially cyclic structures.
Origin
Alkyl: Alkyl groups are derived from alkanes.
Aryl: Aryl groups are derived from aromatic rings.
Saturation
Alkyl: Alkyl groups are saturated functional groups.
Aryl: Aryl groups are unsaturated functional groups.
Stability
Alkyl: Compounds containing alkyl groups are less stable compared to compounds containing aryl groups.
Aryl: Compounds containing aryl groups are more stable compared to compounds containing alkyl groups.
Conclusion
Organic compounds can be linear, branched or cyclic and may have functional groups attached to it. Alkyl groups and aryl groups are two examples of functional groups. The main difference between alkyl and aryl is that alkyl group has no aromatic ring where aryl group has an aromatic ring.
Reference:
1. “3.3 Alkyl Groups.” Chemistry LibreTexts, Libretexts, 2 Aug. 2017, Available here.
2. Helmenstine, Anne Marie, Ph.D. “Aryl Group Definition in Chemistry.” ThoughtCo, Jun. 8, 2017, Available here.
Image Courtesy:
1. “Various Presentations of an Alkyl Group V.1” By Jü – Own work (Public Domain) via Commons Wikimedia
2. “Benzyl group V.9” By Jü – Own work (Public Domain) via Commons Wikimedia
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