Naming Organic Compounds
Section: 11. Organic Chemistry | Syllabus: Cambridge AS Level Physics 9702
Naming Organic Compounds IUPAC Nomenclature Organic compounds are named systematically using rules set by the International Union of Pure and Applied Chemistry (IUPAC). The name indicates the structure of the molecule, including the number of carbons and the functional groups present.
Naming System The name of an organic compound consists of three parts: PREFIX (indicates number of carbon atoms) + ROOT (indicates type of bonds) + SUFFIX (indicates functional group). Step 1: Carbon Chain Prefixes The prefix indicates the number of carbon atoms in the longest continuous chain: Number of Carbons Prefix Example Alkane 1 Meth- Methane 2 Eth- Ethane 3 Prop- Propane 4 But- Butane 5 Pent- Pentane 6 Hex- Hexane 7 Hept- Heptane 8 Oct- Octane Step 2: Suffixes for Different Homologous Series The suffix indicates the functional group or type of compound: Homologous Series Functional Group Suffix Example Alkanes None (C-C single bonds) -ane Propane (C₃H₈) Alkenes C=C (double bond) -ene Propene (C₃H₆) Alcohols -OH (hydroxyl) -anol Propanol (C₃H₇OH) Carboxylic Acids -COOH (carboxyl) -oic acid Propanoic acid (C₂H₅COOH) Naming Alkanes Alkanes use the prefix for the number of carbons + suffix "-ane" [Diagram: Structural formulae of first 5 alkanes with their names labeled] Molecular Formula Structural Formula Name CH₄ CH₄ Methane (meth + ane) C₂H₆ CH₃CH₃ Ethane (eth + ane) C₃H₈ CH₃CH₂CH₃ Propane (prop + ane) C₄H₁₀ CH₃CH₂CH₂CH₃ Butane (but + ane) C₅H₁₂ CH₃CH₂CH₂CH₂CH₃ Pentane (pent + ane) Naming Alkenes Alkenes use the prefix for the number of carbons + suffix "-ene" [Diagram: Structural formulae of first 4 alkenes with their names and C=C bond positions labeled] Molecular Formula Structural Formula Name C₂H₄ CH₂=CH₂ Ethene (eth + ene) C₃H₆ CH₂=CHCH₃ Propene (prop + ene) C₄H₈ CH₂=CHCH₂CH₃ Butene (but + ene) C₅H₁₀ CH₂=CHCH₂CH₂CH₃ Pentene (pent + ene) Naming Alcohols Alcohols use the prefix for the number of carbons + suffix "-anol" (note: the 'e' from alkane is replaced) [Diagram: Structural formulae of first 4 alcohols with their names and -OH group positions labeled] Molecular Formula Structural Formula Name CH₃OH CH₃OH Methanol (meth + anol) C₂H₅OH CH₃CH₂OH Ethanol (eth + anol) C₃H₇OH CH₃CH₂CH₂OH Propanol (prop + anol) C₄H₉OH CH₃CH₂CH₂CH₂OH Butanol (but + anol) Naming Carboxylic Acids Carboxylic acids use the prefix for the number of carbons (including the carbon in -COOH) + suffix "-oic acid" [Diagram: Structural formulae of first 4 carboxylic acids with their names and -COOH group labeled] Molecular Formula Structural Formula Name HCOOH HCOOH Methanoic acid (meth + anoic acid) CH₃COOH CH₃COOH Ethanoic acid (eth + anoic acid) C₂H₅COOH CH₃CH₂COOH Propanoic acid (prop + anoic acid) C₃H₇COOH CH₃CH₂CH₂COOH Butanoic acid (but + anoic acid) Important Note on Carboxylic Acids When counting carbons in carboxylic acids, remember to include the carbon atom in the -COOH group: HCOOH: 1 carbon total → Methanoic acid CH₃COOH: 2 carbons total (1 in CH₃ + 1 in COOH) → Ethanoic acid CH₃CH₂COOH: 3 carbons total → Propanoic acid Naming Strategy - Step by Step To name an organic compound: Count the longest carbon chain: This determines the prefix (meth-, eth-, prop-, but-, etc.) Identify the functional group: This determines the suffix (-ane, -ene, -anol, -oic acid) Combine prefix + suffix: methane, ethene, propanol, butanoic acid Practice Examples [Interactive diagram: Structural formulae of various organic compounds with blank spaces for students to fill in the names] Structural Formula Carbon Count Functional Group Name CH₃CH₂CH₂CH₃ 4 carbons None (alkane) Butane CH₃CH=CH₂ 3 carbons C=C (alkene) Propene CH₃CH₂CH₂OH 3 carbons -OH (alcohol) Propanol HCOOH 1 carbon -COOH (carboxylic acid) Methanoic acid CH₃CH₂CH₂CH₂CH₃ 5 carbons None (alkane) Pentane CH₃OH 1 carbon -OH (alcohol) Methanol Common Naming Mistakes to Avoid Forgetting the COOH carbon: CH₃COOH is ethanoic acid (2C), not methanoic acid Confusing suffixes: Alcohols end in -anol, not -ane or -ol alone Wrong prefix: Always count the longest continuous carbon chain Carboxylic acid suffix: It's "-oic acid" (two words), not "-oicacid" Summary Table - All Four Series [Comprehensive table showing first 4 members of each homologous series with molecular formulae, structural formulae, and systematic names] Carbons Alkane Alkene Alcohol Carboxylic Acid 1 Methane (CH₄) - Methanol (CH₃OH) Methanoic acid (HCOOH) 2 Ethane (C₂H₆) Ethene (C₂H₄) Ethanol (C₂H₅OH) Ethanoic acid (CH₃COOH) 3 Propane (C₃H₈) Propene (C₃H₆) Propanol (C₃H₇OH) Propanoic acid (C₂H₅COOH) 4 Butane (C₄H₁₀) Butene (C₄H₈) Butanol (C₄H₉OH) Butanoic acid (C₃H₇COOH) Prefixes: meth- (1C), eth- (2C), prop- (3C), but- (4C), pent- (5C) Alkanes: prefix + "-ane" (e.g., methane, ethane) Alkenes: prefix + "-ene" (e.g., ethene, propene) Alcohols: prefix + "-anol" (e.g., methanol, ethanol) Carboxylic acids: prefix + "-oic acid" (e.g., methanoic acid, ethanoic acid) Remember: Count all carbons including those in functional groups (especially …
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