Question

How many $\beta$-hydrogen is/are present in 2-methyl-3-phenyl-pentan-1-al?

• A. 4
• B. 1
• C. 3
• D. 2

Hint:

Always draw out the structural formula fully when counting $\alpha$ or $\beta$ hydrogens.
Do not forget that branches on the $\alpha$-carbon (like a methyl group) are physically positioned at the $\beta$-distance from the functional group and must be counted.

Answer 1

The Correct Option is A

Step 1 : Understanding the Question:
The question asks us to find the total number of $\beta$-hydrogens present in the organic molecule 2-methyl-3-phenyl-pentan-1-al.
Step 2 : Key Formulas and Approach:
The approach involves drawing the IUPAC chemical structure of the molecule and identifying the carbons in the chain based on their position relative to the carbonyl group ($-CHO$).
The carbon directly attached to the carbonyl carbon is the $\alpha$-carbon.
Any carbon directly attached to the $\alpha$-carbon is a $\beta$-carbon, and the hydrogens attached to these $\beta$-carbons are $\beta$-hydrogens.
Step 3 : Detailed Explanation:
Let us write down the skeletal structure of 2-methyl-3-phenylpentanal:
The main chain has 5 carbon atoms (pentanal) with the functional group at position 1:
\[ \overset{5}{\text{CH}_3} - \overset{4}{\text{CH}_2} - \overset{3}{\text{CH}}(\text{C}_6\text{H}_5) - \overset{2}{\text{CH}}(\text{CH}_3) - \overset{1}{\text{CHO}} \]
Let us identify the positions of the carbons relative to the $-CHO$ group:

Carbonyl carbon (C1): $-\text{CHO}$

Alpha ($\alpha$) carbon (C2): $-\text{CH}(\text{CH}_3)-$

Beta ($\beta$) carbons: Any carbon directly attached to the $\alpha$-carbon (C2).
Looking at C2, it is connected to two carbon atoms:
Carbon C3 of the main chain: $-\text{CH}(\text{C}_6\text{H}_5)-$

The carbon of the methyl group attached at C2: $-\text{CH}_3$
Therefore, both C3 and the methyl carbon are classified as $\beta$-carbons.

Counting $\beta$-hydrogens:
The $\beta$-carbon C3 is bonded to one phenyl group and one hydrogen atom. Thus, it contains 1 $\beta$-hydrogen.

The $\beta$-carbon of the C2 methyl group is bonded to three hydrogen atoms ($-\text{CH}_3$). Thus, it contains 3 $\beta$-hydrogens.

The phenyl ring is attached to C3 (which is a $\beta$-position), so the ring carbons themselves are at $\gamma$ or higher positions and do not contain $\beta$-hydrogens.

Total number of $\beta$-hydrogens = $1 \text{ (from C3)} + 3 \text{ (from methyl)} = 4$.

Step 4 : Final Answer:
There are 4 $\beta$-hydrogens in the given compound.
This matches Option (A).