# 15.2 Representation of Organic Molecules Organic compounds often have structures that are more complex than those of inorganic compounds. Consequently, several different types of chemical formulae are used to represent them, each providing a different level of detail. --- ### 15.2.1 Molecular Formula The **molecular formula** is a representation that shows the *type* of atoms and the *actual whole number* of atoms for each element present in one molecule of a compound. It provides the exact composition of the molecule but gives no information about its structure. | Compound | Molecular Formula | Compound | Molecular Formula | | :--- | :--- | :--- | :--- | | Ethanol | $C_2H_6O$ | Ethene | $C_2H_4$ | | Ethanoic Acid | $C_2H_4O_2$ | Benzene | $C_6H_6$ | | Glucose | $C_6H_{12}O_6$ | Ethyne | $C_2H_2$ | --- ### 15.2.2 Empirical Formula The **empirical formula** represents the *simplest whole-number ratio* of atoms of the different elements in a compound. **Interesting Information:** Inorganic compounds typically have simple structures, so their empirical formula is often sufficient for representation. #### Comparison of Molecular and Empirical Formulae | Compound | Molecular Formula | Empirical Formula | | :--- | :--- | :--- | | Ethanol | $C_2H_6O$ | $C_2H_6O$ | | Ethanoic Acid | $C_2H_4O_2$ | $CH_2O$ | | Glucose | $C_6H_{12}O_6$ | $CH_2O$ | | Benzene | $C_6H_6$ | $CH$ | | Ethyne | $C_2H_2$ | $CH$ | **Note:** The empirical formula can sometimes be misleading. For example, both ethanoic acid and glucose share the same empirical formula ($CH_2O$), making it impossible to distinguish between them using only this information. ### Determination of Empirical and Molecular Formulae The empirical and molecular formulas can be determined experimentally, often through combustion analysis. This process is fundamental to stoichiometry. --- ## Worked Examples ### Example 1: Determining Empirical Formula from Percentage Composition An organic compound has the following percentage composition by mass: C = 38.4%, H = 4.8%, and Cl = 56.8%. Calculate the empirical formula of this compound. **Solution:** 1. **Assume 100 g of the substance and calculate the moles of each element.** (The percentage by mass is equivalent to the mass in grams for a 100 g sample). * Moles of Carbon (C): $$ \frac{38.4\,g}{12.0\,g\,mol^{-1}} = 3.2\,mol $$ * Moles of Hydrogen (H): $$ \frac{4.8\,g}{1.0\,g\,mol^{-1}} = 4.8\,mol $$ * Moles of Chlorine (Cl): $$ \frac{56.8\,g}{35.5\,g\,mol^{-1}} = 1.6\,mol $$ 2. **Divide the moles of each element by the smallest mole value to find the simplest ratio.** The smallest value is 1.6 mol. * Carbon (C): $$ \frac{3.2}{1.6} = 2 $$ * Hydrogen (H): $$ \frac{4.8}{1.6} = 3 $$ * Chlorine (Cl): $$ \frac{1.6}{1.6} = 1 $$ 3. **Write the empirical formula.** The ratio of atoms is C:H:Cl = 2:3:1. Therefore, the empirical formula is **$C_2H_3Cl$**. ### Example 2: Determining Molecular Formula from Empirical Formula and Molecular Mass An organic compound has the empirical formula "CH" and a molecular mass of 78 amu. Calculate its molecular formula. **Solution:** 1. **Calculate the empirical formula mass.** * Empirical formula mass of CH = $12 + 1 = 13$ amu. 2. **Find the integer multiple 'n' relating the molecular mass to the empirical formula mass.** $$ n = \frac{\text{molecular mass}}{\text{empirical formula mass}} = \frac{78}{13} = 6 $$ 3. **Calculate the molecular formula.** * Molecular Formula = $n \times (\text{Empirical Formula})$ * Molecular Formula = $6 \times (CH) = C_6H_6$ --- ### 15.2.3 Structural Formulae Since molecular and empirical formulae do not show how atoms are connected, **structural formulae** are used to illustrate the arrangement of atoms and identify functional groups within a molecule. #### 1. Condensed Structural Formula This formula shows the relative positions of all atoms without explicitly drawing the single covalent bonds. Each carbon atom is written with the hydrogen atoms attached to it. Branches and functional groups are often shown in brackets. * **Example:** Acetic acid ($CH_3COOH$) shows three hydrogens on the first carbon, which is bonded to a carboxyl group ($COOH$). * **Key Information:** Side chains or functional groups are written in brackets next to the carbon they are attached to. | Alkane | Condensed Structural Formula | | :--- | :--- | | Methane | $CH_4$ | | Ethane | $CH_3CH_3$ | | Propane | $CH_3CH_2CH_3$ | | Butane | $CH_3(CH_2)_2CH_3$ | | Pentane | $CH_3(CH_2)_3CH_3$ | | Hexane | $CH_3(CH_2)_4CH_3$ | #### 2. Full Structural Formula (2D Displayed Formula) This formula is a two-dimensional representation that indicates *all* atoms and *all* the covalent bonds between them. * **Key Information:** A 2D formula is ideal for showing the connectivity in planar molecules, representing atoms in the x and y planes. **Ethane ($C_2H_6$)** **Ethanol ($C_2H_5OH$)** #### 3. Skeletal Formula For large and complex organic molecules, the **skeletal formula** is the easiest and quickest representation to draw. * **Key Information:** * Carbon and hydrogen atoms are generally not shown. * Each end of a line and each vertex (corner) represents a carbon atom. * Each carbon atom is assumed to be bonded to enough hydrogen atoms to complete its valency of four. * Double and triple bonds are shown with double or triple lines. * Functional groups and atoms other than carbon and hydrogen (heteroatoms) are explicitly shown. **Butane** **Butan-2-ol** #### 4. Stereochemical Formulae (3D Formulae) While 2D formulae show connectivity, they do not represent the three-dimensional arrangement of atoms in space. A **stereochemical formula** (or 3D formula) is used to illustrate the spatial structure of a molecule, showing atoms in the x, y, and z planes. This is closely related to the study of . **Conventions for 3D Representation:** * **Solid Line (-):** Bond lies in the plane of the paper. * **Solid Wedge (◄):** Bond is coming out of the plane, towards the observer. * **Dashed Line (····):** Bond is going into the plane, away from the observer. **3D Formula of Methane ($CH_4$)** --- ### Summary of Formulae Types | Name | Molecular Formula | Condensed Structural Formula | Displayed Formula | Skeletal Formula | | :--- | :--- | :--- | :--- | :--- | | **Propane** | $C_3H_8$ | $CH_3CH_2CH_3$ | CCCC | CCC | | **Propene** | $C_3H_6$ | $CH_3CH=CH_2$ | C=CC=C | C=CC | | **Propan-2-ol** | $C_3H_8O$ | $CH_3CH(OH)CH_3$ | CC(C)C | CC(C)O | | **Propanal** | $C_3H_6O$ | $CH_3CH_2CHO$ | CCCC=O | CCC[OH+] | | **Propanone** | $C_3H_6O$ | $CH_3COCH_3$ | CC(C)=O | CCC(C)=O | --- ## Possible Questions and Answers **Q:** What is the difference between the molecular and empirical formula for glucose? **A:** The molecular formula, $C_6H_{12}O_6$, shows the actual number of atoms in a glucose molecule. The empirical formula, $CH_2O$, shows the simplest whole-number ratio of these atoms (1:2:1). **Q:** Why are structural formulas necessary for organic compounds? **A:** Structural formulas are necessary because isomers exist: compounds with the same molecular formula but different arrangements of atoms. A structural formula shows the unique connectivity of atoms, which determines the compound's physical and chemical properties.