What is a condensed structural formula?

A condensed structural formula is a system of writing organic structures in a line of text

It shows all atoms, but omits the vertical bonds and most or all the horizontal single bonds.

The condensed structural formulas of ethane, propane, and ethanol are

CH₃CH₃, CH₃CH₂CH₃, and CH₃CH₂OH

It uses parentheses to show that polyatomic groups within a formula are attached to the nearest non-hydrogen atom on the left.

So the condensed structural formula of propan-2-ol is CH₃CH(OH)CH₃.

It also uses parentheses to show that polyatomic groups at the beginning of a formula are attached to atom on the right.

So we could also write the condensed structural formula of propan-2-ol as (CH₃)₂CHOH.

And we could write the condensed structural formula of hexane as CH₃CH₂CH₂CH₂CH₂CH₃ or CH₃(CH₂)₄CH₃.

We could also write the following condensed structural formulas:

propene: CH₃CH=CH₂

propyne: CH₃C≡CH

isopropyl methyl ether: (CH₃)₂CHOCH₃

propanone: CH₃(C=O)CH₃ or CH₃COCH₃

propanal: CH₃CH₂(C=O)H or CH₃CH₂CHO

propanoic acid: CH₃CH₂(C=O)OH or CH₃CH₂COOH

You may see the formula of 2,2,5,5-tetramethylhexane written as

Some chemists call this a structural formula. Others call it a condensed structural formula. It is a hybrid of both.

A true condensed structure would be written on one line as CH₃C(CH₃)₂CH₂CH₂C(CH₃)₂CH₃ or (CH₃)₃CCH₂CH₂C(CH₃)₃.

Here's a video on drawing condensed structures.