Organic Formulae
Section: 11. Organic Chemistry | Syllabus: Cambridge AS Level Physics 9702
Organic Formulae Organic Chemistry The study of compounds containing carbon (except carbon dioxide, carbon monoxide, and carbonates). Most organic compounds also contain hydrogen, and many contain oxygen, nitrogen, or halogens.
Types of Chemical Formulae Organic compounds can be represented using different types of formulae, each showing different levels of structural detail: Molecular Formula Shows the actual number of atoms of each element present in one molecule of a compound.
Example: C₂H₆O (ethanol) Displayed Formula Shows all the atoms and all the bonds in a molecule. Each bond is represented by a line, and the arrangement shows how atoms are connected. Example - Displayed Formula Ethanol (C₂H₆O) displayed formula: [Diagram: Displayed formula showing H-C-C-O-H structure with all hydrogen atoms bonded to carbons, depicting the complete structural arrangement] Structural Formula Shows the arrangement of atoms in a molecule using the minimum detail necessary to indicate how atoms are grouped together.
It shows which atoms are bonded to which, but without showing every bond. Examples of Structural Formulae Ethanol: CH₃CH₂OH or C₂H₅OH Propane: CH₃CH₂CH₃ Butanoic acid: CH₃CH₂CH₂COOH or C₃H₇COOH Propene: CH₃CH=CH₂ or CH₂=CHCH₃ General Formula An algebraic formula that represents all members of a homologous series, where 'n' represents the number of carbon atoms.
Homologous Series General Formula Example (n=3) Functional Group Alkanes C n H 2n+2 C₃H₈ (propane) None (saturated hydrocarbon) Alkenes C n H 2n C₃H₆ (propene) C=C (carbon-carbon double bond) Alcohols C n H 2n+1 OH C₃H₇OH (propanol) -OH (hydroxyl group) Carboxylic Acids C n H 2n+1 COOH C₃H₇COOH (butanoic acid) -COOH (carboxyl group) Understanding General Formulae Alkanes (C n H 2n+2 ): For 3 carbons, H = 2(3) + 2 = 8, giving C₃H₈ Alkenes (C n H 2n ): For 4 carbons, H = 2(4) = 8, giving C₄H₈ Alcohols (C n H 2n+1 OH): For 2 carbons, H = 2(2) + 1 = 5, giving C₂H₅OH Carboxylic Acids (C n H 2n+1 COOH): For 1 carbon, H = 2(1) + 1 = 3, giving C₁H₃COOH (which is CH₃COOH) Functional Group An atom or group of atoms that determines the characteristic chemical properties of a compound.
Compounds with the same functional group react in similar ways. Functional Group Structure Found In Example C=C (double bond) -C=C- Alkenes Ethene (C₂H₄) Hydroxyl -OH Alcohols Methanol (CH₃OH) Carboxyl -COOH Carboxylic acids Ethanoic acid (CH₃COOH) Structural Isomers Definition: Structural isomers are compounds with the same molecular formula but different structural formulae.
The atoms are arranged in different ways, giving them different properties. Molecular Formula Isomer 1 Isomer 2 C₄H₁₀ (butane) Butane: CH₃CH₂CH₂CH₃ (straight chain) Methylpropane: CH₃CH(CH₃)CH₃ (branched chain) C₄H₈ (butene) But-1-ene: CH₂=CHCH₂CH₃ But-2-ene: CH₃CH=CHCH₃ Visualizing Isomers [Diagram: Displayed formulae of butane and methylpropane showing different carbon chain arrangements - butane as a straight 4-carbon chain, methylpropane as a 3-carbon chain with one CH₃ branch] [Diagram: Displayed formulae of but-1-ene and but-2-ene showing C=C double bond in different positions along the carbon chain] Molecular formula: Shows total number of each atom type (e.g., C₂H₆) Displayed formula: Shows all atoms and all bonds in detail Structural formula: Shows atom groupings without all bonds (e.g., CH₃CH₃) General formula: Algebraic representation of a homologous series (e.g., C n H 2n+2 ) Functional groups: Determine chemical properties (-OH, -COOH, C=C) Isomers: Same molecular formula, different structural arrangements
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