Alkane Questions in English

(D) The reaction of a mixture of alkyl halides with sodium in dry ether is known as the Wurtz reaction. When a mixture of $CH_3Br$ and $C_2H_5Br$ is used, the following coupling reactions occur:
$1$. $CH_3Br + 2Na + CH_3Br \rightarrow CH_3-CH_3 (\text{Ethane}) + 2NaBr$
$2$. $C_2H_5Br + 2Na + C_2H_5Br \rightarrow C_2H_5-C_2H_5 (n\text{-Butane}) + 2NaBr$
$3$. $CH_3Br + 2Na + C_2H_5Br \rightarrow CH_3-C_2H_5 (\text{Propane}) + 2NaBr$
Since $2$-methyl propane (isobutane) is a branched alkane, it cannot be formed by the simple coupling of these linear alkyl halides. Thus, it is not obtained.

(B) Step $1$: The given molecular formula is $C_5H_{12}$, which follows the general formula $C_nH_{2n+2}$, indicating it is an alkane.
Step $2$: The chain isomers are structural isomers that differ in the arrangement of the carbon chain.
Step $3$: The possible isomers for $C_5H_{12}$ are:
$(i)$ $CH_3-CH_2-CH_2-CH_2-CH_3$ (n-pentane)
(ii) $CH_3-CH(CH_3)-CH_2-CH_3$ (isopentane or $2-$methylbutane)
(iii) $CH_3-C(CH_3)_2-CH_3$ (neopentane or $2,2-$dimethylpropane)
Step $4$: There are $3$ possible chain isomers.

(C) The general formula for an alkane is $C_nH_{2n+2}$.
Given the molecular formula $C_{20}H_{42}$, we have $n = 20$.
Checking the hydrogen count: $2(20) + 2 = 40 + 2 = 42$.
Since it fits the general formula, it is an alkane with $20$ carbon atoms.
The $IUPAC$ name for an alkane with $20$ carbon atoms is $Eicosane$.