Alkanes are one of the simplest of Organic Compounds. They consist of single bonded Carbon and Hydrogen atoms and align themselves linearly unless they’re cyclic or ringed structures. I’m mostly going to be talking about simple linear alkanes in this post.
They’re saturated Hydrocarbons, meaning that they’re saturated or filled with Carbon and Hydrogen atoms. Alkanes can be represented formulaically as CnH2n+2; that is for every n number of carbon atoms there are twice as many hydrogen atoms plus two hydrogen atoms in the molecule. Each carbon atom can contain 3 hydrogen atoms unless its at the start or end of the chain. This simple formula easily allows you to know how many Carbon and Hydrogen atoms in an alkane, granted you know its name or the number of carbons. For example Heptane above has 7 Carbons: Plugging this into the formula we get: C(7)(7*2)+2(H) = 7 C and 16 Hydrogens.
Properties and nomenclature
Alkanes are non-polar molecules and are therefore soluble in non-polar solvents (like attracts like!). The more non-polar a solvent is the greater solubility of an alkane in it. They have low boiling points when compared to polar compounds because they only exhibit weak london dispersion forces between molecules; polar molecules exhibit much stronger dipole-dipole forces or hydrogen bonding. An example of the difference in boiling points between non-polar and polar molecules can be done by comparing Methane (CH4) to Water (H2O); both molecules have similar molecular weights (16 vs 18), but their boiling points are vastly different; Methane boils at -162 degrees C while Water boils at 100 degrees C. That is the power of Dipole-Dipole forces and Hydrogen bonds! In fact in order for an alkane to approach water’s boiling point it needs many carbons in its chain, 7 in fact. Heptane has a boiling point of 98 Degrees C and it has 7 Carbons and many Hydrogen atoms. Low carbon alkanes (Methane - Butane) are gaseous at room temperature; Medium Carbon chains are liquid at room temperature (Pentane to Hexadecane); Heavy Carbon chains are solid at room temperature ( Heptadecane and higher).
Continuous chain alkanes which are linear are named by adding a prefix followed by -ane. These are determined by the amount of Carbon atoms in the molecule. There are unique non-numerical prefixes used from C1 to C4 which are Meth-, Eth-, Prop-, and But-; when you hit 5 carbons it gets easier, all prefixes correspond to greek numerical roots which most people already know; Pent- for 5, Hex- for 6, Hep- for 7, etc. It changes a bit so I’ll list a table below:
Number of Carbons in molecule: Prefix used:
Sources: CHEM 2409 (Organic Chemistry I) notes from BCIT.