organic chemistry – What is the IUPAC nomenclature of alkane with large numbers of C atoms (more than 200)?

What is the IUPAC nomenclature for alkanes with large numbers of $\ce{C}$ atoms (more than 200)? For example, what is the IUPAC accepted name for $\ce{C_{205}H_{412}}$?

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:

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

So, $\ce{C_205H_412}$ is the molecular formula of pentadictane. $\ce{C_7547H_15096}$ is heptatetracontapentactaheptaliane.

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