Numbering a chain with halogen and alkyl groups, plus allylic vs benzylic vs vinylic halides, confuses me
Pooja Iyer12th Sample
15 Sept 2026 · 1 min read
Naming haloalkanes drove me slightly mad today and I need a clean rule set. Here is where I keep losing marks.
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Numbering: when the chain has a halogen and a methyl, which one decides the direction of numbering? For I wrote 4-bromo-2-methylpentane, my friend wrote 2-bromo-4-methylpentane. Both numberings give locants 2 and 4. Who is right and why?
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The terms. My teacher spoke about allylic, benzylic, vinylic and aryl halides in one breath and my brain said nope. I need one example of each and how they are named.
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Gem and vic dihalides. I know they are "same carbon" and "adjacent carbons" but I mix which is which, and the common names (ethylidene dichloride, ethylene dichloride) don't help.
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Also isopropyl chloride, tert-butyl bromide and neopentyl chloride are common names I should know for the IUPAC conversion.
Drama aside, I like this chapter, it is just that the naming is a minefield. If someone has practice examples with answers, even better!
8 answers & replies
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[+][–]Arjun Nair12thSample· 1mo ago
Rules for halogen: it is treated as a prefix substituent exactly like an alkyl group. Steps: (1) choose the longest chain, (2) number from the end that gives the lowest set of locants to all substituents together, (3) if the sets tie, the substituent that comes first alphabetically gets the lower number.
Your example: both directions give {2,4}, tie. Alphabetical: bromo before methyl, so bromine gets 2. The answer is 2-bromo-4-methylpentane, your friend is right. Another one: is 1-chloro-3-methylbutane (locants {1,3} beat {2,4}).
[+][–]Pooja Iyer12thSample· 1mo agoAuthor
so I was wrong and my friend was right, ugh. but the alphabetical tiebreak is clear now, thanks
[+][–]Harsh Gupta12thSample· 1mo ago
Simple: halogen is the boss, it always gets the lowest number. Halogen first, rest follows. Naming should be this easy 😂
[+][–]Arjun Nair12thSample· 1mo ago
Harsh, no. Halogen has no seniority over alkyl groups in naming. Both are just substituents. Lowest locant set first, alphabetical on a tie. Under your rule, 1-chloro-3-methylbutane would still work out, but in other compounds you'd get a wrong name.
[+][–]Harsh Gupta12thSample· 1mo ago
fair, I retire from teaching naming 🫡
[+][–]Kavya NaikDropperSample· 1mo ago
Types, with examples:
- Allylic: halogen on an sp³ carbon next to a C=C. , allyl chloride, 3-chloroprop-1-ene.
- Benzylic: halogen on an sp³ carbon next to a benzene ring. , benzyl chloride, (chloromethyl)benzene.
- Vinylic: halogen on the sp² carbon of a C=C. , vinyl chloride, chloroethene.
- Aryl: halogen directly on the benzene ring. , chlorobenzene.
Allylic and benzylic are reactive (stable carbocation). Vinylic and aryl are unreactive in simple substitution because the C–X bond has partial double-bond character.
[+][–]Riya Singh12thSample· 1mo ago
Gem = twins (both halogens on the same carbon). Vic = neighbours (adjacent carbons). So gem-dichloride is 1,1-dichloroethane (ethylidene chloride), vic-dichloride is 1,2-dichloroethane (ethylene dichloride).
Common names to IUPAC: isopropyl chloride is 2-chloropropane, tert-butyl bromide is 2-bromo-2-methylpropane, neopentyl chloride is 1-chloro-2,2-dimethylpropane.
[+][–]Tanvi Deshmukh12thSample· 1mo ago
Practice set with answers!!
Structure Name 1-bromopropane 1-chloro-2-methylpropane 2-bromo-3-methylbutane (chloromethyl)benzene Cover the right column and try yourself, repeat daily for a week!
