Carbon chains

Introduction aside, almost every organic chemistry textbook begins with alkanes, that is, acyclic hydrocarbons with the general formula C_nH_2n+2. Maybe because of that, chemists tend to think of their naming as something too basic and thus boring. I myself thought so until coming across the book by Edward Godly [1] who, in a stroke of genius, put the chapter on silicon chains [1, pp. 19—21] before the chapter on hydrocarbon chains [1, pp. 25—28]. It prompted me to compare the naming of the two classes side-by-side.

Let’s start with unbranched alkanes, also known as linear alkanes, straight-chain alkanes, or n-alkanes (n for “normal”). Now that we know what chains are, we can see that, from propane up, n-alkanes can be defined as saturated carbon chains. (According to our definition, methane and ethane are not chains, carbon or otherwise.) Univalent groups derived from alkanes are called alkyl groups, and alkyl groups derived by removal of a hydrogen atom from a terminal carbon atom of n-alkanes are called, somewhat unsurprisingly, n-alkyl groups. I put the names of n-alkanes, corresponding n-alkyl groups and their silicon analogues in the table below.

Order	Formula	Name	Group	Name	Formula	Name	Group	Name
1	CH4	methane	–CH3	methyl	SiH4	silane	–SiH3	silyl
2	H3C–CH3	ethane	–CH2–CH3	ethyl	H3Si–SiH3	disilane	–SiH2–SiH3	disilanyl
3	H3C–CH2–CH3	propane	–[CH2]2–CH3	propyl	H3Si–SiH2–SiH3	trisilane	–[SiH2]2–SiH3	trisilanyl
4	H3C–[CH2]2–CH3	butane	–[CH2]3–CH3	butyl	H3Si–[SiH2]2–SiH3	tetrasilane	–[SiH2]3–SiH3	tetrasilanyl
5	H3C–[CH2]3–CH3	pentane	–[CH2]4–CH3	pentyl	H3Si–[SiH2]3–SiH3	pentasilane	–[SiH2]4–SiH3	pentasilanyl
6	H3C–[CH2]4–CH3	hexane	–[CH2]5–CH3	hexyl	H3Si–[SiH2]4–SiH3	hexasilane	–[SiH2]5–SiH3	hexasilanyl
7	H3C–[CH2]5–CH3	heptane	–[CH2]6–CH3	heptyl	H3Si–[SiH2]5–SiH3	heptasilane	–[SiH2]6–SiH3	heptasilanyl
8	H3C–[CH2]6–CH3	octane	–[CH2]7–CH3	octyl	H3Si–[SiH2]6–SiH3	octasilane	–[SiH2]7–SiH3	octasilanyl
9	H3C–[CH2]7–CH3	nonane	–[CH2]8–CH3	nonyl	H3Si–[SiH2]7–SiH3	nonasilane	–[SiH2]8–SiH3	nonasilanyl
10	H3C–[CH2]8–CH3	decane	–[CH2]9–CH3	decyl	H3Si–[SiH2]8–SiH3	decasilane	–[SiH2]9–SiH3	decasilanyl
11	H3C–[CH2]9–CH3	undecane	–[CH2]10–CH3	undecyl	H3Si–[SiH2]9–SiH3	undecasilane	–[SiH2]10–SiH3	undecasilanyl
12	H3C–[CH2]10–CH3	dodecane	–[CH2]11–CH3	dodecyl	H3Si–[SiH2]10–SiH3	dodecasilane	–[SiH2]11–SiH3	dodecasilanyl
13	H3C–[CH2]11–CH3	tridecane	–[CH2]12–CH3	tridecyl	H3Si–[SiH2]11–SiH3	tridecasilane	–[SiH2]12–SiH3	tridecasilanyl
14	H3C–[CH2]12–CH3	tetradecane	–[CH2]13–CH3	tetradecyl	H3Si–[SiH2]12–SiH3	tetradecasilane	–[SiH2]13–SiH3	tetradecasilanyl
15	H3C–[CH2]13–CH3	pentadecane	–[CH2]14–CH3	pentadecyl	H3Si–[SiH2]13–SiH3	pentadecasilane	–[SiH2]14–SiH3	pentadecasilanyl
16	H3C–[CH2]14–CH3	hexadecane	–[CH2]15–CH3	hexadecyl	H3Si–[SiH2]14–SiH3	hexadecasilane	–[SiH2]15–SiH3	hexadecasilanyl
17	H3C–[CH2]15–CH3	heptadecane	–[CH2]16–CH3	heptadecyl	H3Si–[SiH2]15–SiH3	heptadecasilane	–[SiH2]16–SiH3	heptadecasilanyl
18	H3C–[CH2]16–CH3	octadecane	–[CH2]17–CH3	octadecyl	H3Si–[SiH2]16–SiH3	octadecasilane	–[SiH2]17–SiH3	octadecasilanyl
19	H3C–[CH2]17–CH3	nonadecane	–[CH2]18–CH3	nonadecyl	H3Si–[SiH2]17–SiH3	nonadecasilane	–[SiH2]18–SiH3	nonadecasilanyl
20	H3C–[CH2]18–CH3	icosane	–[CH2]19–CH3	icosyl	H3Si–[SiH2]18–SiH3	icosasilane	–[SiH2]19–SiH3	icosasilanyl
21	H3C–[CH2]19–CH3	henicosane	–[CH2]20–CH3	henicosyl	H3Si–[SiH2]19–SiH3	henicosasilane	–[SiH2]20–SiH3	henicosasilanyl
22	H3C–[CH2]20–CH3	docosane	–[CH2]21–CH3	docosyl	H3Si–[SiH2]20–SiH3	docosasilane	–[SiH2]21–SiH3	docosasilanyl

And so on and so forth, you get the picture. Apart from the first four hydrocarbons, the rest appears to be named in a pretty regular fashion: a name consists of a multiplier corresponding to the number of carbon atoms in a chain followed by ‘ane’. If you don’t look at the right part of the table, that is.

First you might have noticed the absence of “carbon” from the hydrocarbon names. By comparison, silicon hydrides all have the combining form ‘sil’ (which is a truncated form of the root ‘silic’) in them. Thus, the name ‘hexasilane’ can be easily analysed in terms of combining forms ‘hex(a)’ = six, ‘sil’ = silicon and ‘an’ = hydride, i.e. “hydride containing six silicon atoms”. On the other hand, ‘hexane’ means just “hydride containing six non-hydrogen atoms”. This sounds natural to organic chemists because for them carbon is a default element and saying ‘hexacarbane’ is too much a bother.

Next, note that n-alkyl group names, just like the word ‘alkyl’ (not alkanyl), all uniformly lack the ‘an’ bit: methane → methyl (not methanyl), ethane → ethyl (not ethanyl), decane → decyl (not decanyl)... In contrast, their silicon counterparts keep ‘an’ as in x-silane → x-silanyl, with the singular exception of silyl group^*.

Not all alkyl group names lose ‘an’ though. Starting with propane, there is more than one way to form a group. So if the n-alkyl group –CH₂–CH₃ is called ‘propyl’ (no need to name it ‘propan-1-yl’), the group ‒CH(CH₃)₂ derived from propane by removal of a hydrogen from the carbon-2 atom is named systematically ‘propan-2-yl’ (acceptable alternative name ‘isopropyl’).

Let us now move on to alkenes and alkynes. Their names are derived from the names of corresponding alkanes by replacing the ‘an’ with ‘en’ for a double carbon–carbon bond and with ‘yn’ for a triple carbon–carbon bond. In my humble opinion, the resulting names are not subtractive in spite of some publications, including the IUPAC guides, referring to ‘ene’ and ‘yne’ as “subtractive suffixes”. We can also think of ‘an’ → ‘en’ operation as a substitution of two implicit hydrogen atoms with one π-bond as if this bond were a divalent substituent –π–, and similarly ‘an’ → ‘yn’ as a substitution of four implicit hydrogens with two π-bonds as if they were a tetravalent substituent =π=. Yet these operations do not create your typical substitutive names either. I believe the ‘en’ and ‘yn’ operations are truly in a class of their own. The linguistic analogue is a vowel mutation like umlaut (Modern English examples include fall/fell, foot/feet and man/men). I have to think more about this.

In chains longer than three carbons, the positions of unsaturated bonds should be indicated by locants immediately preceding the ‘en’ or ‘yn’. Only one, the lower, locant is cited for each pair. Compare that with genuine subtractive names where ‘didehydro’, ‘tetradehydro’, etc. are always preceded by pairs of locants indicating the positions of atoms that lose hydrogens, as in ‘1,2-didehydro’.

Since we need at least one carbon–carbon bond to start with, the simplest unsaturated hydrocarbons can be derived from ethane (a). Accordingly, the structure (b) is systematically named ethene and (c) ethyne. So far so good.

(a)	(b)	(c)

H3C–CH3 ethane H2C=CH2 ethene ethylene (trivial) HC≡CH ethyne acetylene (trivial)

The next in the series is propane (d). Replacing one of its C–C bonds with C=C results in the structure (e) called prop-1-ene or, shorter, propene^†. Makes sense?

It kind of does. But what about the second carbon–carbon bond which stays single? Shouldn’t the modified structure be named prop-1-en-2-ane (or prop-1-an-2-ene)? This, however, would result in much longer names.

Likewise, replacing one of C–C bonds in (d) with C≡C we get (f) called prop-1-yne, or simply propyne^†. So the systematic names of alkenes and alkynes imply that all carbon–carbon bonds are single unless otherwise stated — another disappearing act of ‘an’.

(d)	(e)	(f)

H3C–CH2–CH3 propane H2C=CH–CH3 propene prop-1-ene HC≡C–CH3 propyne prop-1-yne

Consider undecane (g) that has ten carbon–carbon bonds. In case of undec-1-ene (h), only one carbon–carbon bond is double; the remaining bonds are still single. I think it is a bit unfair to call undec-1-ene alkene when it is 90% alkane; ditto undec-1-yne (i).

(g)	(h)	(i)

H3C–[CH2]9–CH3 undecane H2C=CH–[CH2]8–CH3 undec-1-ene HC≡C–[CH2]8–CH3 undec-1-yne

Here’s a summary of groups derived from structures (a)—(f):

Alkane	Alkyl group	Alkene	Alkenyl group	Alkyne	Alkynyl group
H3C–CH3 ethane	–CH2–CH3 ethyl	H2C=CH2 ethene	–CH=CH2 ethenyl vinyl‡	HC≡CH acetylene ethyne	–C≡CH ethynyl
H3C–CH2–CH3 propane	–CH2–CH3 propyl	H2C=CH–CH3 prop-1-ene	–CH=CH–CH3 prop-1-en-1-yl	HC≡C–CH3 prop-1-yne	–C≡C–CH3 prop-1-yn-1-yl
			–CH2–CH=CH2 prop-2-en-1-yl allyl‡
	‒CH(CH3)2 propan-2-yl isopropyl‡				–CH2–C≡CH prop-2-yn-1-yl
			–C(CH3)=CH2 prop-1-en-2-yl isopropenyl‡

From four-carbon chain up, there are more than one isomer of n-alkene and n-alkyne and even more alkenyl and alkynyl groups.

If a structure has more than one double or triple bond, the multipliers ‘di-’, ‘tri-’, etc. are employed. Corresponding acyclic hydrocarbons are known as alkadienes, alkatrienes, alkadiynes, alkatriynes, etc. Hydrocarbons containing both double and triple bonds are called enynes, dienynes, enediynes and so on.

(j)	(k)	(l)

H2C=C=C=CH2 buta-1,2,3-triene butatriene HC≡C–[CH2]6–C≡CH deca-1,9-diyne H3C–C=C–C≡C–C≡C–CH=CH–CH=CH–CH2–CH3 aethusin (trivial) (2E,8E,10E)-trideca-2,8,10-triene-4,6-diyne

To sum up:

• In the systematic names of hydrocarbons neither hydrogen nor carbon are explicitly named.
• The names of n-alkanes with more than five carbon atoms consist of a multiplier followed by the combining form ‘an’ and ending ‘e’.
• The names of n-alkyl groups lose the combining form ‘an’; other groups derived from n-alkanes keep it.
• The names of hydrocarbons derived from alkanes by replacing at least one carbon–carbon bond with double or triple bond (alkenes, alkynes etc.) also lose the ‘an’ which is being replaced by ‘en’ or ‘yn’, respectively.
• The positions of unsaturated bonds are indicated by locants immediately preceding the ‘en’ or ‘yn’. Only one (the lower) locant is cited for every unsaturated bond.

*	According to IUPAC recommendations [2], the form ‘silyl’ is preferred to ‘silanyl’ in order to distinguish between two or more silyl (–SiH3) groups, i.e. ‘disilyl’, ‘trisilyl’, ‘tetrasilyl’, etc. and one disilanyl, trisilanyl, tetrasilanyl, etc. group, without using the multipliers ‘bis-’, ‘tris-’, ‘tetrakis-’, etc.
†	Here, locant can be omitted since there are only two bonds in the structure so there is no difference between, say, prop-1-ene and prop-2-ene. This is not the case for longer unsaturated carbon chains and therefore it is necessary to use locants.
‡	Acceptable alternative (non-systematic) name.

References

1. Godly, E.W. Naming Organic Compounds: A Systematic Instruction Manual, 2^nd Ed. Ellis Horwood, Hemel Hempstead, 1995.
2. Panico, R., Powell, W.H. and Richer, J.-C. A Guide to IUPAC Nomenclature of Organic Compounds (Recommendations 1993). Blackwell Science, 1993.