Arrange the following in the increasing order of expected enol content.
i. `CH_(3)COCH_(2)CHO`
ii. `CH_(3)COCH_(3)`
iii. `CH_(3)CHO`
iv. `CH_(3)COCH_(2)COCH_(3)`

Options

A. `iii lt ii lt iv`
B. `iii lt ii lt i lt iv`
C. `i lt iv lt ii lt iii`
D. `iv lt i lt ii lt iii`

Explanation

To solve the problem of arranging the compounds in increasing order of expected enol content, we will analyze each compound based on the number of alpha hydrogens and the type of functional groups present.

### Step-by-Step Solution:

1. **Identify the Compounds:**
- i. `CH₃COCH₂CHO` (3-hydroxybutanal)
- ii. `CH₃COCH₃` (Acetone)
- iii. `CH₃CHO` (Acetaldehyde)
- iv. `CH₃COCH₂COCH₃` (3-oxobutan-2-one)

2. **Count the Alpha Hydrogens:**
- **For i. `CH₃COCH₂CHO`:**
- The alpha carbons are the ones adjacent to the carbonyl group (C=O).
- There are 2 alpha hydrogens from `CH₃` and 3 from `CH₂`, totaling **5 alpha hydrogens**.

- **For ii. `CH₃COCH₃`:**
- The alpha carbon is the one adjacent to the carbonyl.
- There are 3 alpha hydrogens from `CH₃` and 3 from the other `CH₃`, totaling **6 alpha hydrogens**.

- **For iii. `CH₃CHO`:**
- The only alpha carbon is from `CH₃`, which has **3 alpha hydrogens**.

- **For iv. `CH₃COCH₂COCH₃`:**
- The alpha carbons are adjacent to both carbonyl groups.
- There are 3 alpha hydrogens from `CH₃`, 2 from `CH₂`, and 3 from the other `CH₃`, totaling **8 alpha hydrogens**.

3. **Determine the Enol Content:**
- Enol content is generally higher in compounds with more alpha hydrogens and is also influenced by the type of functional groups (ketones vs aldehydes).
- Ketones generally have higher enol content than aldehydes due to stability factors.
- Diketones (compounds with two carbonyl groups) can stabilize the enol form through intramolecular hydrogen bonding.

4. **Rank the Compounds Based on Enol Content:**
- **i. `CH₃COCH₂CHO`** (5 alpha hydrogens) - Moderate enol content.
- **ii. `CH₃COCH₃`** (6 alpha hydrogens) - Higher enol content than i.
- **iii. `CH₃CHO`** (3 alpha hydrogens) - Lowest enol content.
- **iv. `CH₃COCH₂COCH₃`** (8 alpha hydrogens) - Highest enol content due to diketone structure.

5. **Final Arrangement:**
- In increasing order of expected enol content:
- iii. `CH₃CHO` (3 alpha hydrogens) <
- ii. `CH₃COCH₃` (6 alpha hydrogens) <
- i. `CH₃COCH₂CHO` (5 alpha hydrogens) <
- iv. `CH₃COCH₂COCH₃` (8 alpha hydrogens)

### Conclusion:
The increasing order of expected enol content is:
**iii < ii < i < iv**