How to Write Condensed Structural Formula?

Follow the steps mentioned below to write a condensed structural formula or semi-structural formula. Let us discuss them with the help of an example. Write the condensed structural formula of 3-methyl pentane.

Step 1: Draw the 2-dimensional structural formula for the given molecule. 

Here, the given molecule is 3-methyl pentane, so draw its 2-dimensional structural formula.

Step 2: Identify the carbon skeleton or backbone and check whether the structural formula has any branches or sidechains. Also check functional groups like amine (NH₂), alcohol (OH), halogens, and others.

Step 3: From the given structural formula, identify groups of atoms along the carbon backbone. 

Here, 3-methyl pentane has five groups of atoms along its backbone, i.e., CH₃, CH₂, CH, CH₂, and CH₃. It also has a branch at the third carbon atom.

Step 4: Now, to write the condensed structure or semi-structural formula of the given molecule, redraw the given formula by replacing the identified groups of atoms in the 2-dimensional structural formula.

Step 5: From the backbone or skeleton, remove the single carbon-carbon covalent bonds that are present. (This step is not necessary)

Step 6: When the given molecule has branches or sidechains:

• First, condense the structure as for the carbon backbone

• Use parentheses (round brackets) for enclosing groups of atoms that are attached to the chain
  For 3-methyl pentane, there is one CH3 group bonded to the third carbon atom, so we use parentheses to enclose this group of atoms: (CH3)
• Now, use a subscript number to indicate the number of times this group occurs at that position. 
  Here, we have one CH3 group bonded to the third carbon atom, so write it as (CH3).
• Finally, place this group to the right of the carbon atom of the backbone at the position where it is located.
  Write (CH3) to the right of the carbon atom of the backbone at the position where it is located. The condensed structural formula of 3-methyl pentane is CH₃CH₂CH(CH₃)CH₂CH₃.

A molecular formula indicates the number of atoms of each element that are present in a molecule. A structural formula is a graphical representation of a chemical compound that shows the arrangement of atoms as well as chemical bonding around the atoms. A condensed structural formula may also be called a semi-structural formula and is used to describe organic compounds in a line of text. It is illustrated in a single line of text and specifies the atoms in the order that they are located in the molecule. It also shows the functional groups present in the molecule like amine (NH2), alcohol (OH), halogens, and others. It also shows parentheses (round brackets), which indicate the repetition of a group in the chain. For example, the condensed formula of butane is CH₃CH₂CH₂CH₃, and that of 2-chloropropane is (CH₃)₂CHCl.

The subscript number that is written to the right of the final bracket indicates the number of times a group of atoms occurs at that position in the given molecule. Parenthesis (round brackets) is used for enclosing groups attached to a chain when there is a group of atoms attached to a chain. Mostly, covalent bonds are not always shown, but for the sake of clarity, double and/or triple covalent bonds between carbon atoms are usually shown in a condensed structural formula. The same type of atoms that are bonded to one another is always grouped together. When a condensed structural formula is written in a line showing covalent bonds, then it is also called a “linear formula.” Though the condensed structural formula is a useful method for representing simple structures, it tends to be problematic in the case of cyclic compounds.