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How to Determine Molecular Formula from Line Structure?

Published in Organic Chemistry 4 mins read

Determining the molecular formula from a line structure involves counting the atoms of each element present, remembering that carbon and hydrogen atoms are often implied. Here's a step-by-step guide:

1. Understanding Line Structures

Line structures (also called skeletal structures or stick figures) are simplified ways of representing organic molecules. Key features include:

  • Carbon Atoms: Carbon atoms are represented at the end of each line and at the intersections of lines.
  • Hydrogen Atoms: Hydrogen atoms bonded to carbon are not explicitly drawn. The number of hydrogens attached to each carbon is inferred by assuming each carbon atom forms four bonds.
  • Other Atoms (Heteroatoms): Atoms other than carbon and hydrogen (e.g., oxygen, nitrogen, chlorine) are explicitly drawn and are called heteroatoms. Hydrogens bonded to heteroatoms are explicitly drawn.
  • Double and Triple Bonds: Double and triple bonds are represented by two or three lines between atoms, respectively.

2. Identifying and Counting Carbon Atoms

  • Locate the Carbons: Identify all the locations where carbon atoms are present in the line structure. Remember these are at the end of each line and at each intersection.
  • Count the Carbons: Count the total number of carbon atoms you've identified.

3. Determining the Number of Hydrogen Atoms

  • Determine Hydrogen Count per Carbon: For each carbon atom, determine how many hydrogen atoms are attached. Remember, carbon must have four bonds. So:
    • If a carbon has one bond to another atom, it has three hydrogens attached (CH3).
    • If a carbon has two bonds to other atoms, it has two hydrogens attached (CH2).
    • If a carbon has three bonds to other atoms, it has one hydrogen attached (CH).
    • If a carbon has four bonds to other atoms, it has no hydrogens attached (C).
  • Sum the Hydrogens: Add up the number of hydrogen atoms for each carbon atom to get the total number of hydrogen atoms in the molecule.

4. Identifying and Counting Other Atoms (Heteroatoms)

  • Locate Heteroatoms: Identify all atoms that are not carbon or hydrogen (e.g., O, N, Cl, Br). These will be explicitly shown in the structure.
  • Count the Heteroatoms: Count the number of each type of heteroatom present.

5. Writing the Molecular Formula

  • Standard Order: Write the molecular formula in the order of Carbon (C), then Hydrogen (H), then other elements in alphabetical order.
  • Subscripts: Indicate the number of each type of atom as a subscript to the right of the element symbol. If there is only one atom of an element, the subscript "1" is usually omitted.

Example

Let's say we have a line structure that looks like this (a simplified representation of ethanol):

C-C-OH

  1. Carbon Atoms: There are two carbons (at the ends of the first two lines and the intersection). So, C2.

  2. Hydrogen Atoms:

    • The first carbon has one bond, so it has three hydrogens (CH3).
    • The second carbon has two bonds, so it has two hydrogens (CH2).
    • The oxygen has one hydrogen attached (OH).
    • Total Hydrogens: 3 + 2 + 1 = 6. So, H6.

  3. Other Atoms: There is one oxygen atom. So, O.

  4. Molecular Formula: The molecular formula is C2H6O.

Table Summary

Step Description Example (Ethanol)
1. Identify Carbons Locate all carbon atoms (ends of lines, intersections). Two carbons
2. Determine Hydrogens Determine the number of hydrogens bonded to each carbon by subtracting bonds from 4. 3 on first C, 2 on second C, 1 on the oxygen
3. Identify Heteroatoms Locate and count all atoms that are not carbon or hydrogen. One Oxygen
4. Write Formula Write the formula in CH order, followed by other elements alphabetically. C2H6O

By following these steps, you can accurately determine the molecular formula from a line structure.

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