Last updated: March 21st, 2021 | Show Recall that there are at least 4 major ways of representing molecules that you’re introduced to in the first week of ochem.
Condensed FormulaeA 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. What’s The Purpose Of Using Brackets?Brackets help in two ways. They can 1) reduce the amount of work, and 2) remove ambiguity from a structure. 1) Saving Time 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. 2) Reducing Ambiguity 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 (but less efficient) way to write the structure would be CH3C(CH3)3. 3) Carbonyls 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. 4) Branching 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. Some Additional Tricks To KnowThere 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) Some Representative ExamplesHere’s a table with some representative examples. As more come up, I’ll add them. Let me know if I’ve missed anything important!
Skills to Develop We use several kinds of formulas to describe organic compounds. A molecular formula shows only the kinds and numbers of atoms in a molecule. For example, the molecular formula C4H10 tells us there are 4 carbon atoms and 10 hydrogen atoms in a molecule, but it doesn’t distinguish between butane and isobutane. A structural formula shows all the carbon and hydrogen atoms and the bonds attaching them. Thus, structural formulas identify the specific isomers by showing the order of attachment of the various atoms. Unfortunately, structural formulas are difficult to type/write and take up a lot of space. Chemists often use condensed structural formulas to alleviate these problems. The condensed formulas show hydrogen atoms right next to the carbon atoms to which they are attached, as illustrated for butane: Even more abbreviated is a line-angle formula, also called a skeletal structure, in which carbon atoms are implied at the corners and ends of lines, and each carbon atom is understood to be attached to enough hydrogen atoms to give each carbon atom four bonds. All other types of atoms are shown and hydrogens bonded to atoms other than carbon are shown. For example, we can represent pentane (CH3CH2CH2CH2CH3) and isopentane [(CH3)2CHCH2CH3] as follows:
Parentheses in condensed structural formulas indicate that the enclosed grouping of atoms is attached to the adjacent carbon atom. Below is an example of a more complicated molecule. On the left is shown the complete Lewis structure, showing all atoms and valence electrons. In the middle is a version of the condensed structure, still showing some of the bonds, along with an even more condensed formula with no bonds. Finally, on the right is the line-angle (skeletal) structure; notice that the bonds are shown, but not the carbons and not the hydrogens bonded to carbon. The following are suggested steps for drawing a line-angle structure (also known as a skeletal structure).
Key Takeaways
1. Write the condensed formula for each Lewis structure. A. B. 2. Draw a line-angle structure for the compound CH3CH2CH(CH3)CH2CH2CH3. 3. Give the condensed formula for the compound represented by this line-angle structure: 4. Draw the Lewis structure of the molecule below, showing all atoms and all valence electrons (bonds and lone pairs). 5. Draw the Line-Angle structure for the molecule below.
2 3. (CH3)2CHCH2CH(CH3)CH2CH3 4. 5. |