Which of the following statements is true for a pair of diastereomers ?

Options

A. they will have identical physical properties.
B. they will have specific rotations of opposite sign
C. they will have identical chemical properties (e.g. reactivity)
D. they will have different physical properties.

Answer

To solve the question regarding which statement is true for a pair of diastereomers, we need to analyze the properties of diastereomers and evaluate the given statements. Here’s a step-by-step solution:

### Step 1: Understand the Definition of Diastereomers
Diastereomers are stereoisomers that are not mirror images of each other. They typically have multiple chiral centers and differ at one or more of those centers. Unlike enantiomers, which are mirror images and have identical physical properties, diastereomers can have different physical and chemical properties.

**Hint:** Remember that diastereomers are not mirror images, and this is key to understanding their properties.

### Step 2: Evaluate the Physical Properties of Diastereomers
One of the main characteristics of diastereomers is that they often have different physical properties (such as boiling points, melting points, solubility, etc.). This difference arises due to the spatial arrangement of atoms and groups in the molecule, which affects how they interact with other substances.

**Hint:** Consider how the arrangement of atoms in diastereomers can lead to differences in their physical properties.

### Step 3: Analyze the Chemical Properties of Diastereomers
While diastereomers can have different physical properties, they may also exhibit different chemical reactivities. However, they are not guaranteed to have identical chemical properties. Thus, it is incorrect to say that they have identical chemical properties.

**Hint:** Think about how the structure of a molecule influences its reactivity and interactions with other chemicals.

### Step 4: Specific Rotation of Diastereomers
The specific rotation of diastereomers does not have a defined relationship like enantiomers do. Enantiomers have opposite specific rotations, while diastereomers can have different specific rotations that are not necessarily related in a predictable way.

**Hint:** Recall that enantiomers have specific rotations that are equal in magnitude but opposite in sign, while diastereomers do not share this property.

### Step 5: Conclusion
Based on the analysis:
- Diastereomers do **not** have identical physical properties (false).
- Diastereomers do **not** have identical chemical properties (false).
- Diastereomers do **not** have specific rotations of opposite sign (false).
- Diastereomers do have **different physical properties** (true).

Thus, the correct statement regarding diastereomers is that they will have different physical properties.

**Final Answer:** The true statement for a pair of diastereomers is: **They will have different physical properties.**