Recall that there are at least 4 major ways of representing molecules that you’re introduced to in the first week of ochem.
The highest level of detail is the Lewis dot structure, which shows where all the electrons are with dots.
The second highest level of detail is the structural formula, which replaces those dots with lines.
The third level of detail is the line diagram, where carbon-carbon bonds are represented with lines, and hydrogens are omitted entirely.
Occupying a somewhat intermediate place within these formulae is what we call the “condensed formula”. It’s a way of depicting molecules completely in text form. In the days before word processors and graphics programs made it a cinch to include pictures, condensed formulae were the method of choice when you wanted to convey the structure of something without actually having to draw it.
It’s easy for simple hydrocarbons like propane: CH3CH2CH3. It’s pretty much impossible to draw a useful condensed formula for something like morphine.
In between those two extremes, there are a few tricky things to keep track of. Brackets are one of them.
Brackets help in two ways. They can 1) reduce the amount of work, and 2) remove ambiguity
For example, consider the difference between writing
CH3CH2CH2CH2CH2CH2CH2CH3 and CH3(CH2)6CH3. Much less work, right? Chemists gravitate towards solutions that involve doing less work. Using brackets is a no brainer.
A second use of brackets is to reduce ambiguity. Think back to math: there’s a difference between 4 + 2 * 3 and (4+2)*3. In organic chemistry, we use brackets in exactly the same way.
Consider a case where you have four CH3 groups attached to a carbon. You wouldn’t write it CH3CH3CH3CH3C : writing it like that implies a chain, and each of those CH3 groups can only be attached to one thing. C CH3 4 is a little better but having those numbers next to each other is confusing (it looks like CCH34). So put the CH3 groups in brackets and write C(CH3)4. This has no ambiguity. An equivalent way to write the structure would be CH3C(CH3)3.
Carbonyl oxygens (that’s C=O) can also be dealt with by putting them in brackets. So CH3C(O)CH3 implies that the second carbon is double-bonded to an oxygen.
Brackets can also be used to show branching. For example CH3CH(CH3)CH2CH3 depicts a 4-carbon chain where the CH3 in brackets is directly attached to the carbon before it. That helps to highlight a useful rule of thumb: look to the left of the bracket to see which atom it’s attached to.
There are some additional tricks with structural formulas that don’t involve brackets but are still important to know.
Aldehydes are represented by CHO
Carboxylic acids are represented by CO2H (or COOH)
Esters are represented by CO2R (or COOR)
Here’s a table with some representative examples. As more come up, I’ll add them. Let me know if I’ve missed anything important!