Question
Name the compound with the molecular formula using IUPAC nomenclature. Also identify the functional groups present.
Solution — Step by Step
The compound has 3 carbon atoms in a chain: C1–C2–C3.
Written explicitly:
Carbon count: C1 (), C2 (), C3 ()
Longest chain = 3 carbons → prop- (prefix for 3-carbon chain)
There are two functional groups:
- C=C double bond (alkene) → suffix “-ene”
- -Br (bromine/halo substituent) → prefix “bromo-”
In IUPAC nomenclature, when both a double bond and a halogen are present, the double bond is the principal characteristic group (higher priority than halogens). So the suffix comes from the double bond: -ene.
Halogens are treated as prefixes.
We must number the chain to give the lowest possible locants to the double bond.
Numbering from left (CH₂= end): double bond is at C1-C2, Br is at C3.
Numbering from right (CH₂Br end): double bond is at C2-C3, Br is at C1.
Comparing: {1, 3} from the left vs {1, 2} from the right.
Wait — we give priority to the double bond for lowest locant. From left: double bond at C1; from right: double bond at C2. Therefore, number from the left (where double bond starts at C1).
Chain numbers:
Double bond position: between C1 and C2 → prop-1-ene (or just prop-1-ene)
Br is at C3 → 3-bromo
Prefix + parent chain + suffix: 3-bromoprop-1-ene
This is commonly known as allyl bromide (trivial name), but the IUPAC name is 3-bromoprop-1-ene.
Why This Works
IUPAC nomenclature follows a clear priority system:
- Find the longest continuous carbon chain containing the principal functional group
- Number the chain to give the lowest locant to the principal group (double/triple bond > single functional groups > substituents)
- Name substituents as prefixes, name the principal group as a suffix
- Arrange prefixes alphabetically
The compound has both a halogen substituent (Br) and a π bond (C=C). Since C=C is the principal characteristic group, the chain is named as a propene (not propane) and Br is a prefix. If there were no double bond but a Br, it would be 3-bromo-prop-… but this would be named differently.
Alternative Method
An alternative check using the structure:
The allyl group () is a commonly referenced group in organic chemistry. When a halide is attached to the allyl group, it’s an allylic halide. Allylic halides are more reactive in SN1 reactions than ordinary alkyl halides (due to resonance stabilization of the allylic carbocation).
For CBSE Class 11 and JEE organic nomenclature, always check: (1) Is there a C=C or C≡C bond? These get lower locants than substituents. (2) Is there a halogen? It’s a prefix (bromo, chloro, fluoro, iodo). (3) Arrange multiple substituents alphabetically in the name. For this compound: the answer is 3-bromoprop-1-ene. The “1” indicates the double bond starts at C1 — in prop-1-ene, the double bond is between C1 and C2.
Common Mistake
Students often number the chain from the bromine end (from the right), giving the name “1-bromoprop-2-ene.” This is incorrect because IUPAC rules state that the double bond gets the lower locant (not the substituent/halogen). When double bond can start at C1 (from the left) vs C2 (from the right), C1 is lower → number from left → double bond is prop-1-ene → bromine is at C3 → 3-bromoprop-1-ene. Always prioritize double/triple bond position over halogen position in the numbering.