Nomenclature | Alkyl Halides | Organic chemistry | lec 03 II Dr Rizwana

Functional groups drive the chemical and physical behavior of organic molecules, and attaching a halogen to an alkyl chain changes both naming and properties. In this lecture on nomenclature, the focus is on alkyl halides—organic compounds where halogens (fluorine, chlorine, bromine, iodine) replace hydrogen on an alkyl chain—and how to systematically assign names when one, two, or three halogen substituents (and possibly other groups like methyl) appear.

The naming process starts by identifying the parent chain: the longest carbon chain in the molecule. Numbering then follows the rule that the halogen substituent(s) should receive the lowest possible set of locants. If multiple numbering directions are possible, the direction that gives the halogen(s) the smaller numbers is preferred. After numbering, substituents are listed before the parent alkane name, using the halogen’s position numbers (e.g., “2-bromo” means bromine attached at carbon 2). The parent chain is named as an alkane based on the number of carbons—three carbons becomes propane, four becomes butane, and so on.

For a monohalide example, the lecture demonstrates how to name a compound with a single halogen on a three-carbon chain. With bromine attached at carbon 2, the correct format becomes “2-bromopropane.” The same logic applies to other single-halogen cases, where the halogen name is converted into the appropriate prefix: bromo, chloro, iodo, or fluoro.

For dihalides, the lecture emphasizes two additional steps: (1) assign locants to both halogens using the lowest-number rule, and (2) list substituents in alphabetical order when writing the full name. An example with two bromines on a four-carbon chain is named using “di-” for two identical substituents and locants for both positions, producing a form like “2,2-dibromobutane” (as illustrated through the idea of “di-bromo” plus the carbon count for the parent). When a different substituent such as methyl is also present, the halogens and methyl are all numbered first, then ordered alphabetically in the final name.

A more complex example includes three different halogen types plus a methyl group on a longer chain. The lecture shows that numbering aims to give the lowest locants across the substituents, then the final name lists substituents alphabetically: bromine (b) before chlorine (c) before iodine (i) before methyl (m). The parent alkane name is written last, with the chain length determined by the number of carbons in the longest chain. Overall, the lecture’s core message is that alkyl halide nomenclature is a repeatable checklist: choose the longest chain, number to minimize halogen locants, attach locants to substituents, and order substituent names alphabetically before the parent alkane.