The stereochemical formula of diastereomer `'Y'` of optically active compound `'X'` is,
`(` If `X=2,3-` Dihydroxybutanedioic acid. ).

Explanation

To solve the problem of finding the stereochemical formula of diastereomer 'Y' of optically active compound 'X', which is 2,3-dihydroxybutanedioic acid, we will follow these steps:

### Step 1: Understand the Structure of Compound X
2,3-Dihydroxybutanedioic acid has two chiral centers. We need to draw its structure to identify the stereochemistry.

- The structure can be represented as:
```
HOOC-CHOH-CHOH-COOH
```

### Step 2: Identify the Chiral Centers
In the structure of 2,3-dihydroxybutanedioic acid, the chiral centers are at carbon atoms 2 and 3.

### Step 3: Assign R/S Configuration to the Chiral Centers
To assign the R or S configuration to the chiral centers, we will use the Cahn-Ingold-Prelog priority rules.

1. For Carbon 2:
- Assign priorities based on the groups attached:
- COOH (1)
- OH (2)
- H (3)
- CH (4)
- The order of priority is 1 > 2 > 3 > 4.
- The arrangement is anti-clockwise, but since hydrogen is on the horizontal line, we reverse it to clockwise. Thus, C2 is R.

2. For Carbon 3:
- The groups are the same as for Carbon 2.
- Following the same priority rules, we find that C3 is also R.

Thus, the configuration for compound X is (R,R).

### Step 4: Determine the Diastereomer
A diastereomer differs at one or more chiral centers but not all. Since compound X has the configuration (R,R), the diastereomer 'Y' can be (R,S) or (S,R).

- The diastereomer we are interested in is (R,S).

### Step 5: Write the Structure of Diastereomer Y
The structure of diastereomer 'Y' can be represented as:
```
HOOC-CHOH-CHOH-COOH
```
Where:
- C2 is R
- C3 is S

### Conclusion
The stereochemical formula of diastereomer 'Y' of optically active compound 'X' (2,3-dihydroxybutanedioic acid) is (R,S).

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