Which of the following statements are wrong?

Options

A. Isobutane and `n`- butane are chain isomers.
B. Ethyl cyanide and ethyl isocyanide are functional isomers.
C. `H-C-=N` and `H-overset(o+)N-=overset(Θ)C` are tautomers.
D. Maleic and fumaric acids are enantiomers.

Explanation

To determine which of the statements regarding isomerism is incorrect, we will analyze each statement one by one.

### Step 1: Analyze the first statement
**Statement:** Isobutane and n-butane are chain isomers.

- **Explanation:** Chain isomers differ in the arrangement of the carbon skeleton. Isobutane (C4H10) has a branched structure, while n-butane has a straight-chain structure. Therefore, this statement is correct.

### Step 2: Analyze the second statement
**Statement:** Ethyl cyanide and ethyl isocyanide are functional isomers.

- **Explanation:** Functional isomers differ in the functional groups present. Ethyl cyanide (CH3CN) has a cyanide group (–CN), while ethyl isocyanide (CH3NC) has an isocyanide group (–NC). Since they have different functional groups, this statement is also correct.

### Step 3: Analyze the third statement
**Statement:** HCN and HNC are tautomers.

- **Explanation:** Tautomers are isomers that can interconvert, typically involving the migration of a hydrogen atom. HCN (hydrogen cyanide) and HNC (hydrogen isocyanide) can interconvert through the migration of a hydrogen atom, thus they are indeed tautomers. This statement is correct.

### Step 4: Analyze the fourth statement
**Statement:** Malic and fumaric acid are enantiomers.

- **Explanation:** Enantiomers are a type of stereoisomer that are non-superimposable mirror images of each other. Malic acid (cis isomer) and fumaric acid (trans isomer) are not mirror images of each other; instead, they are geometrical isomers. Therefore, this statement is incorrect.

### Conclusion
The incorrect statement is:
**Malic and fumaric acid are enantiomers.** They are actually geometrical isomers.

### Summary of Answers
1. Isobutane and n-butane are chain isomers. **(Correct)**
2. Ethyl cyanide and ethyl isocyanide are functional isomers. **(Correct)**
3. HCN and HNC are tautomers. **(Correct)**
4. Malic and fumaric acid are enantiomers. **(Incorrect)**