What is the IUPAC nomenclature of alkane with higher C atoms (more than 200)?

Question

What is the IUPAC nomenclature of alkane with higher C atoms (more than 200)? Example, what is the IUPAC nomenclature of $C_{205}H_{412}$?

Answer 1

The definitive answer is, of course, in the rules established by the IUPAC’s “Commission on nomenclature in organic chemistry”. The reference you are looking for is:

“Extension of Rules A-1.1 and A-2.5 Concerning Numerical Terms Used in Organic Chemical Nomenclature”, Pure Appl. Chem., 1986, 58, 1693-1696

which can be found here as a PDF and here as an HTML version. In particular:

NT-1.1 - The fundamental numerical terms for use in hydrocarbon names or as multiplying prefixes for simple features are given in the following list:

1 mono- or hen-* 10  deca-      100 hecta-      1000  kilia-
2   di- or do-*  20 icosa-**    200 dicta-      2000    dilia-
3   tri-         30 triaconta-  300 tricta-     3000    trilia-
4   tetra-       40 tetraconta- 400 tetracta-   4000    tetralia-
5   penta-       50 pentaconta- 500 pentacta-   5000    pentalia-
6   hexa-        60 hexaconta-  600 hexacta-    6000    hexalia-
7   hepta-       70 heptaconta- 700 heptacta-   7000    heptalia-
8   octa-        80 octaconta-  800 octacta-    8000    octalia-
9   nona-        90 nonaconta-  900 nonacta-    9000    nonalia-

So, C₂₀₅H₄₁₂ is the molecular formula of pentadictane. C₇₅₄₇H₁₅₀₉₆ is heptatetracontapentactaheptaliane.

Answer 2

Presuming it is linear, then it should follow the trend of the higher alkanes.

For $\ce{C10}: \ce{CH3(CH2)8CH3}$ - the name is decane (deca means ten).

For $\ce{C20}: \ce{CH3(CH2)18CH3}$ - the name is icosane (icos means twenty)

For $\ce{C25}: \ce{CH3(CH2)23CH3}$ - the name is pentacosane

After icosane, the higher alkanes follow the following:

For $\ce{C30}: \ce{CH3(CH2)28CH3}$ - the name is triacontane

For $\ce{C40}: \ce{CH3(CH2)38CH3}$ - the name is tetracontane

For $\ce{C50}: \ce{CH3(CH2)48CH3}$ - the name is pentacontane

So...

For $\ce{C100}: \ce{CH3(CH2)98CH3}$ - the name would be decacontane

For $\ce{C200}: \ce{CH3(CH2)198CH3}$ - the name would probably be icosacontane

And for $\ce{C205}: \ce{CH3(CH2)203CH3}$ - the name might be pentacosacontane