Question

What is the number of $\text{-CH}_2\text{-}$ groups present in dodecane?

• A. 12
• B. 10
• C. 15
• D. 13

Hint:

For any straight-chain alkane with $n$ carbon atoms (where $n \ge 2$), the number of internal methylene ($\text{-CH}_2\text{-}$) groups is always exactly $n - 2$.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
We need to determine the total number of methylene ($\text{-CH}_2\text{-}$) units present in the structural carbon chain of the alkane named dodecane.

Step 2: Key Formula or Approach:
The general chemical formula for a non-cyclic alkane is $\text{C}_n\text{H}_{2n+2}$.
A straight-chain alkane consists of two terminal methyl groups ($\text{-CH}_3$) at each end, with the rest of the interior chain made up entirely of repeating methylene ($\text{-CH}_2\text{-}$) groups.
Therefore, the number of $\text{-CH}_2\text{-}$ groups is simply the total number of carbon atoms minus the 2 terminal carbon atoms.

Step 3: Detailed Explanation:
The prefix "dodec-" signifies 12 carbon atoms. So, dodecane has $n = 12$.
Its molecular formula is $\text{C}_{12}\text{H}_{26}$.
Its expanded structural formula is $\text{CH}_3\text{-CH}_2\text{-CH}_2\text{-CH}_2\text{-CH}_2\text{-CH}_2\text{-CH}_2\text{-CH}_2\text{-CH}_2\text{-CH}_2\text{-CH}_2\text{-CH}_3$.
Condensing this structure gives: $\text{CH}_3\text{-(CH}_2\text{)}_{10}\text{-CH}_3$.
Subtracting the 2 terminal $\text{CH}_3$ groups from the 12 total carbons leaves exactly 10 $\text{-CH}_2\text{-}$ groups.

Step 4: Final Answer:
There are 10 $\text{-CH}_2\text{-}$ groups in dodecane, matching option (B).