Types of Formulas and Interconversion of Representations
- Consider that you're in a chemistry lab, analyzing a small vial of an unknown organic compound.
- You’re tasked with figuring out its molecular structure.
- Where do you begin?
- You might start by examining its formula, but which one?
- A single compound can be represented in multiple ways: as an empirical formula, molecular formula, structural formula, or skeletal formula.
Empirical Formula: The Simplest Ratio
Definition
Empirical formula
The empirical formula represents the simplest whole-number ratio of atoms of each element in a compound.
It tells you the relative proportions of atoms but not the actual number of atoms in a molecule.
Example
- Glucose has the molecular formula C₆H₁₂O₆, but its empirical formula is CH₂O.
- This means that, for every carbon atom, there are two hydrogens and one oxygen in the simplest ratio.
Key Features:
- Provides no structural information.
- Useful for understanding the relative composition of a compound.
Example
- If a compound has 40% carbon, 6.7% hydrogen, and 53.3% oxygen by mass, its empirical formula can be determined.
- Divide each percentage by the atomic mass of the element:
- Carbon:$( \frac{40}{12.01} = 3.33 $
- Hydrogen: $ \frac{6.7}{1.01} = 6.63 $
- Oxygen: $ \frac{53.3}{16.00} = 3.33 $
- Divide by the smallest value (3.33) to get the ratio: C₁H₂O₁, or simply CH₂O.
Tip
- When calculating empirical formulas, ensure all percentages add up to 100%.
- If not, account for rounding errors or missing data.
Molecular Formula: The Actual Atom Count
Definition
Molecular formula
The molecular formula specifies the exact number of atoms of each element in a molecule.
Example
For glucose, the molecular formula C₆H₁₂O₆ shows that it contains six carbons, twelve hydrogens, and six oxygens.
Key Features:
- Reflects the actual composition of a molecule.
- Can be derived from the empirical formula if the molar mass is known.
Tip
- To find the molecular formula from the empirical formula, divide the molar mass of the compound by the molar mass of the empirical formula.
- Multiply the subscripts in the empirical formula by this factor.
Common Mistake
- Students sometimes confuse the molecular formula with the structural formula.
- Remember, the molecular formula does not show how atoms are connected.
Structural Formula: Connectivity of Atoms
Definition
Structural formula
The structural formula shows how atoms are connected in a molecule.
There are three main types:
- Full structural formula: Displays every atom and bond explicitly.
- Condensed structural formula: Groups atoms together to simplify the representation (e.g., CH₃CH₂OH for ethanol).
- Skeletal formula: Represents carbon atoms as vertices and omits hydrogen atoms bonded to carbons.
Example
- Full structural formula: H–C–C–O–H (with all bonds shown explicitly)
- Condensed structural formula: CH₃CH₂OH
- Skeletal formula: A zigzag line with an "OH" group attached at the end.
Note
Skeletal formulas are especially useful for large organic molecules, as they simplify complex structures and make patterns easier to recognize.
Interconversion of Formulas
Being able to switch between molecular, structural, and skeletal formulas is a critical skill in organic chemistry. Let’s break this down with an example.
Example
- Molecular formula: C₃H₈O
- Condensed structural formula: CH₃CH(OH)CH₃
- Skeletal formula: A zigzag line with an "OH" group attached to the middle vertex.
Steps for Interconversion:
- Molecular to Structural: Use the molecular formula to determine the number of atoms and arrange them logically, considering bonding requirements (e.g., carbon forms four bonds).
- Structural to Skeletal: Remove all hydrogen atoms bonded to carbon and represent carbon atoms as vertices.
Common Mistake
- Students often forget to include functional groups like $–OH$ or double bonds in skeletal formulas.
- Always double-check that all functional groups are represented!
3D Models: Visualizing Molecular Geometry
- While 2D formulas are useful, they don’t capture the true three-dimensional shape of molecules.
- This is where 3D models are useful.
- These models represent the spatial arrangement of atoms, including bond angles and relative positions.
Key Features:
- Ball-and-stick models: Show atoms as spheres and bonds as sticks, emphasizing connectivity.
- Space-filling models: Represent the actual sizes of atoms, giving a sense of molecular volume.
- Skeletal 3D models: Simplified 3D representations, often used for large molecules
Example
In methane (CH₄):
- Bond angles: Approximately 109.5° (tetrahedral geometry).
- 3D representation helps visualize the symmetry of the molecule.
