Organic Chemistry Condensed Formulas

Condensed Structural Formula shows the arrangement of those atoms. It tells you exactly which atoms are bonded to which carbon. The condensed formula of propan-1-ol is `\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}`. This formula shows that the alcohol functional group is connected to the first carbon atom.

How to Write Condensed Formulas

Step 1: Identify the Main Carbon Chain

Start at one end of the molecule (usually the left) and move along the longest continuous carbon chain. Treat each carbon atom as a "station" that holds a group of other atoms.

Step 2: Group the Hydrogen Atoms

For each carbon atom, count how many hydrogen atoms are directly bonded to it.

• A terminal carbon (at the end) usually has three hydrogens: `-\text{CH}_3`

• A carbon in the middle of a single-bonded chain usually has two hydrogens: `-\text{CH}_2-`

• A carbon with a double bond and one other bond usually has one hydrogen: `-\text{CH}-`

Step 3: Handle Branches with Parentheses

If a carbon atom is bonded to a branch (like a methyl group or a halogen) instead of just continuing the straight chain, place that branch in parentheses immediately after the carbon it is attached to.

Example (2-methylpropane): The second carbon has a $`\backslash text\{CH\}\_3`$ group connected to it. The condensed formula is `\text{CH}_3\text{CH}(\text{CH}_3)\text{CH}_3`.

Step 4: Use Parentheses for Repeating Units

If you have a very long alkane chain, you don't need to write $`\backslash text\{CH\}\_2`{}_{}$over and over. You can group the repeating units.

Example (Octane): instead of `\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{CH}_2\text{CH}_2\text{CH}_2\text{CH}_3`, the more convenient condensed formula can be written as `\text{CH}_3(\text{CH}_2)_6\text{CH}_3`.

Step 5: Incorporate Functional Groups

Functional groups have specific "shorthand" codes that must be written in a particular order to avoid being mistaken for other groups:

• Double/Triple Bonds: Usually written as $=$ or $\equiv$ between the carbons.

  • But-1-ene: `\text{CH}_2=\text{CHCH}_2\text{CH}_3`

• Alcohols: Always end the group with $-OH$.

  • Propan-1-ol: `\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}`

• Aldehydes: Always use $-CHO$.

  • Propanal: `\text{CH}_3\text{CH}_2\text{CHO}`

• Carboxylic Acids: Use $-COOH$.

  • Ethanoic acid: `\text{CH}_3\text{COOH}`

• Esters are slightly more complex because they contain two oxygen atoms: one double-bonded to a carbon (C=O) and one single-bonded between two carbons (−O−). This entire functional group is condensed as −COO−. 

  • Ethyl ethanoate: `\text{CH}_3\text{COOCH}_2\text{CH}_3`

When is it Useful to Use Condensed Formula?

There are three primary reasons why condensed formulas are the "go-to" for chemists:

1. Efficiency and Speed: Drawing out every single C−H bond for a long-chain hydrocarbon like octane (`\text{C}_8\text{H}_18`) is time-consuming. Writing the condensed formula `\text{CH}_3(\text{CH}_2)_8\text{CH}_3` conveys the same information in seconds.

2. Identifying Isomers: Molecular formulas can be ambiguous. For example, `\text{C}_3\text{H}_8\text{O}` could be ethanol or dimethyl ether. A condensed formula removes this mystery immediately: