Which of the following will be least Basic ?

Options

A.
B.
C.
D.

Answer

To determine which of the given compounds is the least basic, we need to analyze the basicity of each compound based on the availability of lone pairs of electrons and the influence of resonance or electron-withdrawing groups. Here’s a step-by-step breakdown of the reasoning:

### Step 1: Understanding Basicity
Basicity is determined by the ability of a compound to donate a lone pair of electrons. The more available the lone pair is, the more basic the compound is. Factors that can affect this availability include resonance and the presence of electron-withdrawing groups.

**Hint**: Remember that compounds with lone pairs involved in resonance or those adjacent to electronegative atoms tend to be less basic.

### Step 2: Analyzing the First Compound
The first compound has a fused aromatic ring. In this case, the lone pair of electrons on the nitrogen is involved in resonance with the aromatic system, making it less available for donation.

**Hint**: Look for resonance structures; if the lone pair is delocalized, it is less available for bonding.

### Step 3: Analyzing the Second Compound
The second compound is benzylamine (CH2-NH2). Here, the lone pair on the nitrogen is not involved in resonance, making it more available for donation. Thus, this compound is more basic.

**Hint**: Compounds without resonance stabilization of the lone pair are generally more basic.

### Step 4: Analyzing the Third Compound
The third compound has a nitrogen atom adjacent to two carbonyl groups (sp2 hybridized carbons). The carbonyl groups are electron-withdrawing and can delocalize electrons away from the nitrogen, making the lone pair less available. Therefore, this compound is less basic.

**Hint**: Identify electron-withdrawing groups; they can significantly decrease basicity.

### Step 5: Analyzing the Fourth Compound
The fourth compound is cyclohexylamine. The nitrogen is directly connected to a cyclohexane ring, which exhibits a +I effect (inductive effect). This effect increases the electron density on the nitrogen, making it more basic.

**Hint**: Inductive effects from alkyl groups can enhance basicity by increasing electron density.

### Step 6: Comparing Basicity
Now, we compare the basicity of all four compounds:
- Compound 1: Less basic due to resonance.
- Compound 2: More basic due to no resonance.
- Compound 3: Less basic due to two electron-withdrawing carbonyl groups.
- Compound 4: More basic due to +I effect from cyclohexyl group.

### Conclusion
Among the compounds analyzed, the third compound (with two electron-withdrawing groups) is the least basic due to the combined effects of resonance and the presence of multiple electron-withdrawing groups.

**Final Answer**: The compound with the least basic characteristics is **Option 3**.