Of the following statements , which are true for `S_(N)2` reaction.
(a) Tertiar alkyl halides reacts faster than secondary.
(b) The absolute configuration of product is opposite to that of the reactant when an optically active substrate is used.
(c) The used shows first order kinetics.
(d) The rate of the reaction depends markedly on the nucleophilicity of the attacking reagent.
(e) The mechanism is one step.
(f) Carbocations are intermediate.
(g) Rate `prop` [Alkyl][halides]
(h) The rate of the reaction depends on the nature of the leaving group.

Answer

To determine which statements are true for the \( S_N2 \) reaction, let's analyze each statement step by step:

### Step 1: Analyze Statement (a)
**Statement**: Tertiary alkyl halides react faster than secondary.
**Analysis**: This statement is **false** for \( S_N2 \) reactions. \( S_N2 \) reactions involve a backside attack by the nucleophile, which is hindered by steric bulk. Tertiary alkyl halides are more sterically hindered than secondary alkyl halides, making them less reactive in \( S_N2 \) reactions.

### Step 2: Analyze Statement (b)
**Statement**: The absolute configuration of the product is opposite to that of the reactant when an optically active substrate is used.
**Analysis**: This statement is **true**. \( S_N2 \) reactions result in inversion of configuration at the carbon atom where the nucleophile attacks, leading to a product with opposite stereochemistry compared to the reactant.

### Step 3: Analyze Statement (c)
**Statement**: The rate of the reaction shows first-order kinetics.
**Analysis**: This statement is **false**. \( S_N2 \) reactions are second-order kinetics, as the rate depends on the concentration of both the alkyl halide and the nucleophile.

### Step 4: Analyze Statement (d)
**Statement**: The rate of the reaction depends markedly on the nucleophilicity of the attacking reagent.
**Analysis**: This statement is **false**. While nucleophilicity does play a role, the rate of \( S_N2 \) reactions depends on both the nucleophile and the alkyl halide. Therefore, it is not solely dependent on nucleophilicity.

### Step 5: Analyze Statement (e)
**Statement**: The mechanism is one step.
**Analysis**: This statement is **true**. The \( S_N2 \) mechanism is a single-step process where the nucleophile attacks and the leaving group departs simultaneously.

### Step 6: Analyze Statement (f)
**Statement**: Carbocations are intermediates.
**Analysis**: This statement is **false**. \( S_N2 \) reactions do not involve carbocation intermediates; they proceed through a transition state.

### Step 7: Analyze Statement (g)
**Statement**: Rate ∝ [Alkyl][halides].
**Analysis**: This statement is **false**. The rate of \( S_N2 \) reactions is proportional to both the concentration of the alkyl halide and the nucleophile, not just the alkyl halide.

### Step 8: Analyze Statement (h)
**Statement**: The rate of the reaction depends on the nature of the leaving group.
**Analysis**: This statement is **true**. The efficiency of the leaving group significantly affects the rate of \( S_N2 \) reactions; better leaving groups lead to faster reactions.

### Summary of True Statements
- **True Statements**: (b), (e), (h)
- **False Statements**: (a), (c), (d), (f), (g)