Dr E. Ramanathan PhD
Classroom Discussion
Bond line Formula
IUPAC Names Presented by a Saitech Student
IUPAC Nomenclature Drill in the Classroom
Notes
The IUPAC nomenclature of hydrocarbons is a systematic method of naming organic chemical compounds, developed by the International Union of Pure and Applied Chemistry (IUPAC). Hydrocarbons are compounds made up of only hydrogen and carbon atoms, and they can be classified mainly into alkanes, alkenes, and alkynes.
Here’s a step-by-step guide to IUPAC naming of hydrocarbons:
1. Identify the Longest Carbon Chain (Parent Chain)
- Choose the longest continuous chain of carbon atoms.
- The base name of the compound comes from this chain.
| No. of C atoms | Root Name |
|---|---|
| 1 | Meth |
| 2 | Eth |
| 3 | Prop |
| 4 | But |
| 5 | Pent |
| 6 | Hex |
| … | … |
2. Identify the Type of Bond
- Alkane: only single bonds → use suffix –ane
- Alkene: one or more double bonds → use suffix –ene
- Alkyne: one or more triple bonds → use suffix –yne
3. Number the Carbon Chain
- Number the chain such that the multiple bond or substituent gets the lowest possible number.
4. Name the Substituents
- Substituents (like –CH₃, –C₂H₅) are alkyl groups and are named as methyl, ethyl, etc.
- Use prefixes like di-, tri-, tetra- for multiple identical substituents.
5. Write the Name in Correct Order
- Start with substituents in alphabetical order with their position numbers.
- Then, write the root name (longest chain) + bond type suffix.
Example 1:
CH₃–CH₂–CH=CH₂
- Longest chain: 4 carbons → But
- Bond type: double bond at position 1 → But-1-ene
- Final name: But-1-ene
Example 2:
CH₃–CH(CH₃)–CH₂–CH₃
- Longest chain: 4 carbons → Butane
- Substituent: one methyl on C-2
- Name: 2-Methylbutane
Here is the same list with molecular formulas explicitly showing the functional group for each compound class:
1. Alkenes (C=C double bond)
| IUPAC Name | Molecular Formula (with functional group) |
|---|---|
| Ethene | CH₂=CH₂ |
| Propene | CH₂=CH–CH₃ |
| Butene | CH₂=CH–CH₂–CH₃ |
| Pentene | CH₂=CH–CH₂–CH₂–CH₃ |
| Hexene | CH₂=CH–(CH₂)₃–CH₃ |
| Heptene | CH₂=CH–(CH₂)₄–CH₃ |
2. Alkynes (C≡C triple bond)
| IUPAC Name | Molecular Formula (with functional group) |
|---|---|
| Ethyne | HC≡CH |
| Propyne | CH≡C–CH₃ |
| Butyne | CH≡C–CH₂–CH₃ |
| Pentyne | CH≡C–(CH₂)₂–CH₃ |
| Hexyne | CH≡C–(CH₂)₃–CH₃ |
| Heptyne | CH≡C–(CH₂)₄–CH₃ |
3. Cycloalkanes (Ring of CH₂ units)
| IUPAC Name | Molecular Formula (ring form) |
|---|---|
| Cyclopropane | (CH₂)₃ |
| Cyclobutane | (CH₂)₄ |
| Cyclopentane | (CH₂)₅ |
| Cyclohexane | (CH₂)₆ |
| Cycloheptane | (CH₂)₇ |
| Cyclooctane | (CH₂)₈ |
4. Haloalkanes (C–X, X = Cl, Br)
| IUPAC Name | Molecular Formula (with functional group) |
|---|---|
| Chloromethane | CH₃–Cl |
| Bromomethane | CH₃–Br |
| Chloroethane | CH₃–CH₂–Cl |
| Bromoethane | CH₃–CH₂–Br |
| 1-Chloropropane | CH₃–CH₂–CH₂–Cl |
| 1-Bromopropane | CH₃–CH₂–CH₂–Br |
5. Alcohols (–OH group)
| IUPAC Name | Molecular Formula (with –OH) |
|---|---|
| Methanol | CH₃–OH |
| Ethanol | CH₃–CH₂–OH |
| Propanol | CH₃–CH₂–CH₂–OH |
| Butanol | CH₃–(CH₂)₃–OH |
| Pentanol | CH₃–(CH₂)₄–OH |
| Hexanol | CH₃–(CH₂)₅–OH |
6. Aldehydes (–CHO group)
| IUPAC Name | Molecular Formula (with –CHO) |
|---|---|
| Methanal | H–CHO |
| Ethanal | CH₃–CHO |
| Propanal | CH₃–CH₂–CHO |
| Butanal | CH₃–CH₂–CH₂–CHO |
| Pentanal | CH₃–(CH₂)₃–CHO |
| Hexanal | CH₃–(CH₂)₄–CHO |
7. Ketones (C=O in middle)
| IUPAC Name | Molecular Formula (with –CO–) |
|---|---|
| Propanone | CH₃–CO–CH₃ |
| Butanone | CH₃–CO–CH₂–CH₃ |
| Pentanone | CH₃–CO–(CH₂)₂–CH₃ |
| Hexanone | CH₃–CO–(CH₂)₃–CH₃ |
| Heptanone | CH₃–CO–(CH₂)₄–CH₃ |
| Octanone | CH₃–CO–(CH₂)₅–CH₃ |
8. Carboxylic Acids (–COOH group)
| IUPAC Name | Molecular Formula (with –COOH) |
|---|---|
| Methanoic acid | H–COOH |
| Ethanoic acid | CH₃–COOH |
| Propanoic acid | CH₃–CH₂–COOH |
| Butanoic acid | CH₃–CH₂–CH₂–COOH |
| Pentanoic acid | CH₃–(CH₂)₃–COOH |
| Hexanoic acid | CH₃–(CH₂)₄–COOH |
9. Carboxylate Anions (–COO⁻ group)
| IUPAC Name | Molecular Formula (anion form) |
|---|---|
| Formate | H–COO⁻ |
| Acetate | CH₃–COO⁻ |
| Propanoate | CH₃–CH₂–COO⁻ |
| Butanoate | CH₃–CH₂–CH₂–COO⁻ |
| Pentanoate | CH₃–(CH₂)₃–COO⁻ |
| Hexanoate | CH₃–(CH₂)₄–COO⁻ |
10. Amines (–NH₂ group)
| IUPAC Name | Molecular Formula (with –NH₂) |
|---|---|
| Methylamine | CH₃–NH₂ |
| Ethylamine | CH₃–CH₂–NH₂ |
| Propylamine | CH₃–CH₂–CH₂–NH₂ |
| Butylamine | CH₃–(CH₂)₃–NH₂ |
| Pentylamine | CH₃–(CH₂)₄–NH₂ |
| Hexylamine | CH₃–(CH₂)₅–NH₂ |
IUPAC RULES
Here are the important IUPAC rules used for naming organic compounds, especially useful for NEET, JEE, and higher secondary chemistry:
🔹 1. Longest Chain Rule
- Choose the longest continuous carbon chain as the parent chain.
- It must include:
- The functional group, if present
- Multiple bonds (double/triple), if any
📌 Example:CH3–CH=CH–CH2–CH3 → 5-carbon chain → Pent-2-ene
🔹 2. Numbering the Chain
- Number the chain from the end nearer to:
- Functional group > multiple bond > substituent
📌 Example:CH3–CH2–CH(OH)–CH3 → Number from left to get 2-butanol
🔹 3. Naming the Substituents
- Identify side chains (alkyl groups like methyl, ethyl).
- Name them with prefixes: methyl-, ethyl-, propyl-, etc.
📌 Example:CH3–CH(CH3)–CH2–CH3 → 2-methylbutane
🔹 4. Alphabetical Order
- Name substituents in alphabetical order, regardless of position numbers.
📌 Example:3-ethyl-2-methylpentane
(E comes before M)
🔹 5. Locants (Position Numbers)
- Use lowest set of locants for multiple substituents or bonds.
📌 Example:CH3–CH=CH–CH=CH2 → Penta-1,3-diene (not 2,4)
🔹 6. Multiple Identical Substituents
- Use prefixes: di-, tri-, tetra- (not counted in alphabetical order).
📌 Example:CH3–CH(CH3)–CH(CH3)–CH3 → 2,3-dimethylbutane
🔹 7. Functional Group Priority Order
If more than one functional group is present, use the highest priority group as suffix. Others become prefixes.
Priority order (high → low):
- Carboxylic acid (–COOH) → –oic acid
- Sulfonic acid (–SO3H) → –sulfonic acid
- Ester (–COOR) → –oate
- Acid halide (–COCl) → –oyl halide
- Amide (–CONH2) → –amide
- Nitrile (–CN) → –nitrile
- Aldehyde (–CHO) → –al
- Ketone (–CO–) → –one
- Alcohol (–OH) → –ol
- Amine (–NH2) → –amine
- Alkene (C=C) → –ene
- Alkyne (C≡C) → –yne
- Alkane (–CH3) → –ane
- Halogen (–X) → prefix (chloro-, bromo-…)
📌 Example:CH3–CH(OH)–COOH → 2-hydroxypropanoic acid (–COOH is suffix)
🔹 8. Cyclic Compounds
- Use “cyclo” prefix: e.g., cyclohexane, cyclopentene
🔹 9. Aromatic Compounds
- Use benzene as the base:
e.g., methylbenzene, 2-chlorotoluene
🔹 10. Bridged and Bicyclic Compounds
- Use bicyclo[x.y.z]alkane format
(Advanced level – useful in JEE)
✅ Summary:
| Feature | Rule |
|---|---|
| Chain selection | Longest with max. priority group |
| Numbering direction | Nearest to FG > bond > substituent |
| Substituent order | Alphabetical |
| Multiple substituents | di-, tri-, etc. |
| Functional priority | Use suffix for highest, rest as prefix |
Worksheets
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