The key difference between primary secondary and tertiary halogenoalkanes is the position of the carbon atom that carries the halogen atom. In primary halogenoalkanes, the carbon atom, which carries the halogen atom, is attached to only one alkyl group. But, in secondary halogenoalkanes, this carbon atom is attached to two alkyl groups. Whereas, in tertiary halogenoalkanes, this carbon atom is attached to three alkyl groups.
Halogenoalkanes or haloalkanes are alkanes containing halogens. Halogens are chemical elements of group 17 of the periodic table. It includes fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). There can be one or more halogens in the same haloalkane. There are many important applications of halogenoalkanes as flame retardants, fire extinguishers, refrigerants, propellants, etc. However, many haloalkanes are considered as toxic compounds and pollutants.
CONTENTS
1. Overview and Key Difference
2. What are Primary Halogenoalkanes
3. What are Secondary Halogenoalkanes
4. What are Tertiary Halogenoalkanes
5. Side by Side Comparison – Primary Secondary vs Tertiary Halogenoalkanes in Tabular Form
6. Summary
What are Primary Halogenoalkanes?
Primary halogenoalkanes are organic compounds that have a carbon atom attached to one alkyl group and one halogen atom. Therefore, the general structure of a primary halogenoalkanes is R-CH2-X; R is an alkyl group while X is a halogen. We can denote them as 10 haloalkanes. A common example is a halothane, which contains an ethyl group as the R group and a chlorine atom as the X group or halogen. However, methyl halides are an exception for these primary halogenoalkanes structures because they have three hydrogen atoms attached to the carbon atom which carries the halogen atom. This means, there are no alkyl groups attached to these compounds. But they are considered as primary haloalkanes.
Moreover, if we consider the reactivity of primary halogenoalkanes, the carbon atom, which is attached to the halogen atom, is a reactive center because the halogen is more electronegative than carbon; thus, it gives a partial positive charge to the carbon atom by attracting the bond electrons toward itself. Further, these compounds can be attacked by nucleophilic reagents which seek positive charges. So, this leads to a nucleophilic substitution reaction. And, this reaction has a high activation energy barrier. It is an SN2 type reaction, and we name it as a bimolecular reaction.
What are Secondary Halogenoalkanes?
Secondary halogenoalkanes are organic compounds that have a carbon atom attached to two alkyl groups and a halogen atom. The general structure of a secondary halogenoalkanes is R2-C(-H)-X. Here, the two alkyl groups (R group) can be similar or different groups. We can denote these compounds as 20 haloalkanes. Moreover, secondary halogenoalkanes undergo SN2 nucleophilic substitution reactions. Therefore, they are bimolecular reactions.
Figure 02: 2-Bromopropane
The reactivity of secondary haloalkane is in between the reactivities of primary and tertiary halogenoalkanes because the presence of two alkyl group lower the positive charge on the carbon atom since alkyl groups are electron-withdrawing species.
What are Tertiary Halogenoalkanes?
Tertiary halogenoalkanes are organic compounds that have a carbon atom attached to three alkyl groups (no hydrogen atoms attached directly to this carbon) and a halogen atom. The general structure for a tertiary haloalkane is R3-C-X, where three R groups (alkyl groups) can be either the same or different groups. We can denote these compounds as 30 haloalkanes. Moreover, these compounds undergo SN1 nucleophilic substitution reactions. But, this mechanism is different from the nucleophilic substitution reactions of primary and secondary halogenoalkanes.
The carbon atom which carries the halogen atom has a very low positive charge because there are three electron-withdrawing groups attached to this carbon atom. Therefore, it does not require the formation of high energy intermediates, and the nucleophile can directly attack the carbonium ion as soon as it forms. So, this is why we call it a unimolecular reaction.
What is the Difference Between Primary Secondary and Tertiary Halogenoalkanes?
Halogenoalkanes have three types depending on the structure; primary, secondary, and tertiary halogenoalkanes. In primary halogenoalkanes, the carbon atom which carries the halogen atom is attached to only one alkyl group, and in secondary halogenoalkanes, this carbon atom is attached to two alkyl groups, whereas in tertiary halogenoalkanes, this carbon atom is attached to three alkyl groups. So, this is the key difference between primary secondary and tertiary halogenoalkanes.
Following infographic summarizes the difference between primary secondary and tertiary halogenoalkanes.
Summary – Primary Secondary vs. Tertiary Halogenoalkanes
There are three types of halogenoalkanes depending on the structure; primary, secondary, and tertiary halogenoalkanes. The key difference between primary secondary and tertiary halogenoalkanes is that in primary halogenoalkanes, the carbon atom, which carries the halogen atom, is attached to only one alkyl group. And, in secondary halogenoalkanes, this carbon atom is attached to two alkyl groups. Meantime, in tertiary halogenoalkanes, this carbon atom is attached to three alkyl groups.
Reference:
1. “10.34 – Halogenoalkanes.” Organic Chemistry, Available here.
Image Courtesy:
1. “2-bromopropane-2D-flat” By Ben Mills – Own work (Public Domain) via Commons Wikimedia