An unknown alcohol is treated with the 'Lucas reagent' to determine whether the alcohol is primary, secondary or tertairy. Which alcohol reacts faster and by what mechanism?

Options

A. Tertiary alcohol by `S_N 2`
B. Secondary alcohol by `S_N 1`
C. Tertiary alcohol by `S_N 1`
D. Secondary alcohol by `S_N 2`

Answer

To determine which alcohol reacts faster with Lucas reagent and by what mechanism, we can follow these steps:

### Step 1: Understand Lucas Reagent
Lucas reagent is a mixture of zinc chloride (ZnCl2) and hydrochloric acid (HCl). It is used to differentiate between primary, secondary, and tertiary alcohols based on their reactivity.

**Hint:** Remember that Lucas reagent facilitates the conversion of alcohols to alkyl halides.

### Step 2: Reaction Mechanism
When an alcohol (ROH) is treated with Lucas reagent, the hydroxyl group (OH) is replaced by a chloride ion (Cl). This reaction proceeds through the formation of a carbocation intermediate.

**Hint:** Focus on the role of the carbocation in determining the reaction rate.

### Step 3: Carbocation Stability
The rate of reaction depends on the stability of the carbocation formed during the reaction:
- **Tertiary alcohols (3°)** form the most stable carbocations.
- **Secondary alcohols (2°)** form moderately stable carbocations.
- **Primary alcohols (1°)** form the least stable carbocations.

**Hint:** Recall that stability of carbocations increases with the number of alkyl groups attached to the positively charged carbon.

### Step 4: Rate of Reaction
Since the reaction rate is directly proportional to the stability of the carbocation, tertiary alcohols will react the fastest, followed by secondary, and then primary alcohols.

**Hint:** Think about the order of reactivity: tertiary > secondary > primary.

### Step 5: Identify the Mechanism
The reaction mechanism for the conversion of alcohols to alkyl halides in the presence of Lucas reagent is an SN1 mechanism. This is because the rate-determining step involves the formation of a carbocation.

**Hint:** Remember that SN1 reactions involve a two-step mechanism: formation of a carbocation followed by nucleophilic attack.

### Conclusion
In summary, tertiary alcohols react faster with Lucas reagent compared to secondary and primary alcohols, and the reaction proceeds via an SN1 mechanism.

**Final Answer:** Tertiary alcohol reacts faster by SN1 mechanism.