Key Difference – Alkyl Halide vs Aryl Halide
Both alkyl halides and aryl halides are organic compounds. These are also called organic halides. Types of halogens that can be attached in order to produce this type of molecule are fluorine, chlorine, bromine and iodine. These halogen atoms are attached to a carbon atom in organic halides. The key difference between alkyl halide and aryl halide is that the halogen atom in alkyl halides is attached to a sp3 hybridized carbon atom whereas the halogen atom in aryl halides is attached to a sp2 hybridized carbon atom.
CONTENTS
1. Overview and Key Difference
2. What is Alkyl Halide
3. What is Aryl Halide
4. Side by Side Comparison – Alkyl Halide vs Aryl Halide in Tabular Form
5. Summary
What is Alkyl Halide?
Alkyl halide, as 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 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 which is attached to the halogen atom. Accordingly, primary alkyl halides, secondary alkyl halides, and tertiary alkyl halides can be observed.
Figure 01: A Primary Alkyl Halide
However, alkyl halides may sometimes be confused with aryl halides. For example, if the halogen atom is attached to a carbon atom, which is attached to a benzene ring (Cl-CH2-C6H5), one would think it is an aryl halide. But, it is an alkyl halide because the halogen atom is attached to the carbon that is sp3 hybridized.
Halogens are more electronegative than carbon. Thus, a dipole moment is observed in the carbon-halogen bond, that is, the molecule becomes a polar molecule when the bond becomes polar. The carbon atom gets a small positive charge, and the halogen gets a small negative charge. This leads to dipole-dipole interactions between alkyl halides. But the strength of this interaction is different in primary, secondary, and tertiary halides. This is because the side chains attached to the carbon atom can reduce the small positive charge on the carbon atom.
What is Aryl Halide?
An aryl halide is a molecule having a halogen atom attached to a 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 the dipole-dipole interactions. The carbon-halogen bond is stronger than that of alkyl halides due to the presence of ring electrons. This happens because the aromatic ring gives electrons to the carbon atom, reducing the positive charge. 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. That is also in the ortho, para or meta positions.
Figure 02: Difference Between Alkyl Halide and Aryl Halide
Chemical Test to Distinguish Alkyl Halide and Aryl Halide
To distinguish an alkyl halide and an aryl halide, one can use a chemical test. First, NaOH should be added followed by heating. Then the mixture is cooled, and HNO3 id is added, followed by the addition of AgNO3. Alkyl halide may give a white precipitate whereas aryl halide does not. That is because, aryl halides do not undergo nucleophilic substitution, unlike alkyl halides. The reason for not undergoing nucleophilic substitution is that the electron cloud of the aromatic ring causes a repulsion of the nucleophile.
What is the difference between Alkyl Halide and Aryl Halide?
Alkyl Halide vs Aryl Halide |
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| Alkyl halide is a compound having a halogen atom attached to a chain of carbon atoms. | Aaryl halide is a molecule having a halogen atom attached to a sp2 hybridized carbon in an aromatic ring directly. |
| Attachment of Halogen Atom | |
| The halogen atom is attached to sp3 hybridized carbon atom in alkyl halides. | The halogen atom is attached to a sp2 hybridized carbon atom in aryl halides. |
| Structure | |
| Alkyl halides have a linear or branched structure most of the times. | Aryl halides are always ringed structures. |
| Electron Density | |
| The carbon-halide bond of alkyl halides has a low density of electrons compared to aryl halides. | The carbon-halide bond of aryl halides has a high density of electrons. |
| Reactions | |
| Alkyl halides undergo nucleophilic substitution. | Aryl halides do not undergo nucleophilic substitution. |
Summary – Alkyl Halide vs Aryl Halide
Alkyl halides and aryl halides are organic halides. The key difference between alkyl and aryl halide is that the halogen atom in alkyl halides is attached to a sp3 hybridized carbon atom whereas in aryl halides it is attached to a sp2 hybridized carbon atom.
References:
1. Hunt, D., n.d. Chem.UCalgary. [Online] Available here. [Accessed 30 05 2017].
2. Clark, J., 2014. Libretexts. [Online] Available here. [Accessed 30 05 2017].
Image Courtesy:
1. “Halide-group” By Benjah-bmm27 assumed (based on copyright claims). (Public Domain) via Commons Wikimedia
2. “Organohalogen-chlorides” RicHard-59 – Üleslaadija oma töö (CC BY-SA 3.0) via Commons Wikimedia