Question

number of stereoisomers of n hexane where 2 and 4 position is substituted by D

Answer 1

4 stereoisomers

Answer 2

Solution:

We start with the structure of n‑hexane substituted by deuterium (D) at the 2‑ and 4‑positions:

$$\text{CH}_3-\text{CH(D)}-\text{CH}_2-\text{CH(D)}-\text{CH}_2-\text{CH}_3.$$

Step 1:

At carbon 2, the four substituents are:

• a methyl group ($\text{CH}_3$) from C1,
• a hydrogen (H),
• a deuterium (D), and
• the chain $\text{CH}_2-\text{CH(D)}-\text{CH}_2-\text{CH}_3$ from C3 to C6.

Since all four groups are different, C2 is a stereocenter.

Step 2:

At carbon 4, the substituents are:

• an ethyl group (from C5–C6: $\text{CH}_2-\text{CH}_3$),
• a propyl group (from C3–C1: $\text{CH}_2-\text{CH(D)}-\text{CH}_3$),
• a hydrogen (H), and
• a deuterium (D).

Thus, C4 is also a stereocenter.

Step 3:

Since there are two chiral centers, the maximum number of stereoisomers is:

$$2^2 = 4.$$

Inspection of the molecule shows that the two centers are in different environments (one has a methyl versus butyl group, while the other has an ethyl versus propyl group) so no meso (achiral) form exists.

Final Answer:

There are 4 stereoisomers.

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Explanation (minimal):

Two chiral centers (at C2 and C4) give a maximum of $2^2=4$ stereoisomers. No meso form is possible as the substituents around the chiral centers are different.