Alkanes have the formula CnH2n+2 but if they bond to another atom, they are ·CnH2n+2 "alkyl radicals." Note the missing H where the bond will go. So, for example, an alkyl bromide would be CnH2n+1Br.
Alkanes are also often called saturated hydrocarbons since they are indeed "saturated" with hydrogen. Any "unsaturation" implies a carbon-carbon multiple bond since only there is the valence satisfied with fewer than two hydrogens per (interior) carbon. Without multiple bonds, each carbon requires 2 (or 3, if an end atom) hydrogens to complete its Group IV heritage of valence 4.
Alkanes get rational names beginning with the chain where n=5; it is called pentane from the Greek word for 5 (remember pentagon: 5-sided polygons which, among other things, bend buckyball?). So if you know Greek, you'll guess the n=6 to be hexane and on and on ... hept-, oct-, non-, dec-, undec-, duodec-, etc. So the only ones requiring memorization (if you're Greek) are n=1 through n=4:
Alkane Formula Name Derivation and uses
n=1 CH4 Methane methy "intoxicating" (wood alcohol also
toxic as well), major component of natural gas.
n=2 H3CCH3 Ethane aether "upper air" refined (distilled)
fuel and refrigerant
n=3 H3CCH2CH3 Propane prop- "before" or "in front of"
fuel
n=4 H3C(CH2)2CH3 Butane butyrum (Latin) "butter" which will
deteriorate (go rancid) making butyric acid.
(Hey, the Nepalese consider it a delicacy.)
Butane holds another distinction (pun intended) as the smallest alkane which has a structural isomer. Instead of the straight chain butane, one can produce a branched chain of the same formula but slightly different properties:
H H H
\ | /
H - C - C - C - H isobutane or methylpropane.
/ | \
H C H
/|\
H H H
It also offers us our first opportunity to show off the power of the IUPAC nomenclature for (especially branched) hydrocarbons. Isobutane is its old name; methylpropane is its new one. The latter begins by finding the longest chain of C - C links; in this case 3, so names it a propane. Next it locates side branches by name and location off the main chain; in this case, a methyl branch off carbon 2. So isobutane should be called 2-methylpropane, but since there isn't any other place to branch from except that middle carbon #2, the "2-" is dropped. With n > 3 there will be at least 2 interior carbons to support branches; so the positional designations become necessary.So the following molecule would be 2,5,5,6-tetramethyl-3-ethyl-octane
H H H H H H
\|/ \|/
H C H H C H H
\ | | | | \ /
H - C1 - C2 - C3 - C4 - C5 - C - H
/ | | | |
H H C H C6 H H
/|\ /|\ / /
H H C - H H | C7 - 8C - H
|\ C \ \
H H /|\ H H
H H H
where the numbered carbons are those of the longest chain. (You'd not normally see the numbering scheme in a structure like this since it would be confused with repetition or charges.) The numbers start not (necessarily) at the left but rather such that the 1st branch occurs at the earliest number; that would be the 1st of 4 methyl's starting at carbon #2. Since there're four (Greek tetra) methyl side "chains", it's tetramethyl-. Well, if methyl doesn't qualify as a chain, perhaps a "pendant?" (I jest.) But ethyl really does qualify as a side chain.A moment's reflection will lead you conclude that side chains might be long enough to support their own side chains! Right on. And the implied heirarchy is represented with nested parentheses.
You might imagine that the longer a chain gets, the better the chance that it would run into itself as it contorts through many conformations. Quite right. And it is possible to persuade it to condense upon itself to form a ring or cycle. It then becomes a cycloalkane.
It should not be difficult to persuade you that if an alkane "eats its tail," as it were, to form a cycloalkane, it will lose a great deal of flexibility. And that is true for the three smallest cycloalkanes:
The smallest cycloalkane is cyclopropane at left. With 3 carbons, it has no choice but to be planar; note we're not counting what the hydrogens do...they actually lie 3 above and 3 below the carbon plane.) This diagram shows (shudder) bent bonds, and I suppose if ever there was a case for them it would be these whimpering C - C - C bonds which, by their obvious "tetrahedral" hybridization, want to be 109.5 degrees but are being constrained to 60 degrees instead! Chemists would say that there is enormous "strain energy" in cyclopropane, rendering it far less stable than propane! The same is true to a lesser extent for the strained 90 degree angles of cyclobutane.
But by the time n=5, the interior angles of cyclopentane are 108 degrees, nearly perfect 109.5, and the strain energy has fallen to a piddling 27 kJ/mol...only about 6% of the extra C-C bond cyclopropane sports. (See Chapter 9 for bond energies.) So cyclopentane may well be as flat as its predecessors, but it is quite stable. As n increases, strain vanishes almost altogether, and the higher cycloalkanes are very stable creatures. And more flexible into the bargain!
In fact, n=6, cyclohexane, comes in two semi-stable configurations, both of which render the C - C - C bond angles very close to the magic 109.5 degrees. Neither is planar since that would require 120 internal degrees for a perfect hexagon...which would be trigonal hybridization! (More about that later.)
One configuration is called "boat" because it looks like a child's toy ark, and the other is called "chair" because of the "seat" jutting out from the "back." Chemists bother to name them because they are stable conformations each with (somewhat) distinguishable properties. (Note that the hydrogens aren't displayed in the confomers at left, just the carbons, and they're shown transparent to better see the bonding shape.)
Return to the CHM 1341 Lecture Notes or Go To Next or Previous Lectures.
Chris Parr University of Texas at Dallas Programs in Chemistry, Room BE3.506 P.O. Box 830688 M/S BE2.6 (for snailmail) Richardson, TX 75083-0688
Voice: (214) 883-2485 Fax: (214) 883-2925 BBS: (214) 883-2168 (HST) or -2932 (V.32bis) Internet: parr@utdallas.edu (Click on that address to send Chris e-mail.)
Last modified 15 July 1995.