Hydrocarbons, or Don’t Count Your Carbons Before they Catonate

See Carbon Catenate. Catenate, Carbon, Catenate.

Following our cheat sheets for common polyatomic ions, acids, and bases, here is one for organic chem: a study sheet for hydrocarbons, in particular the first 10 straight-chained alkanes.

Alkanes, the Simplest of Hydrocarbons

In addition to the image or PDF below, you can study the same material in our memorization web app, Study Putty, for absolutely free. Study Putty is in public beta. Try it out here. 

Study the table below, or download as PDF. Then continue reading for a tip.

Hydrocarbons (Alkanes)“The Trick” for Alkanes

You can predict the formula from the name for pentane on up, if you know what number the Greek root in the name represents (pent- means five carbons) and that the hydrogens will always be two more than twice the carbons.

Pentane=5-ane, 5×2+2=12, therefore: C5H12

pent- =5
hex-
=6
hept-
=7
oct-
=8
enne- 
sounds lame so let’s arbitrarily switch just this once to Latin: non- =9
dec-
=10

For the first four alkanes, you can’t get the number from the root, but the other part of the rule still works, the carbon times two plus two part. To remember the order of the first four, you can use this mnemonic device: ME Play Ball. (Methane, Ethane, Propane, and Butane.)

Wonder why carbon times two plus two works? Of course not. That’s not on the test. But for the curious, consider a molecular diagram:

Butane

Diagram by Wikipedia user NEUROtiker. From [link].

This is butane, but the pattern is true for all of ’em. All of the carbons will be bonded to two hydrogens, except for the outermost, which are able to accept one more hydrogen each. Therefore: carbon times two plus two.

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About Bret Norwood

Bret Norwood is a staff blogger for Learning Laboratory in addition to other roles, including UI design and content development for Study Putty, our free memorization tool for chemistry and many other course topics. He is also a published writer of literary fiction--see BretNorwood.com
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