Write IUPAC names of the products obtained by addition reactions of HBr to hex-1-ene (i) in the absence of peroxide and (ii) in the presence of peroxide.

Question

Accepted Answer

> [!TLDR]
> The addition of HBr to hex-1-ene follows **Markovnikov’s rule** in the absence of peroxide (giving 2-bromohexane) and **anti-Markovnikov addition** in the presence of peroxide (giving 1-bromohexane). The peroxide effect reverses the regioselectivity due to a free-radical mechanism.

---

## Why This Happens: The Concept of Chain Isomerism in Addition Reactions

When HBr adds to an unsymmetrical alkene like hex-1-ene, two possible products can form — **chain isomers** (same molecular formula, different carbon skeleton arrangement of the bromine atom). The actual product depends entirely on the reaction conditions.

The key is the **mechanism**:
- **Without peroxide**: Ionic mechanism, following Markovnikov’s rule — the hydrogen attaches to the carbon with more hydrogens already.
- **With peroxide**: Free-radical mechanism, giving anti-Markovnikov addition — the bromine ends up on the less substituted carbon.

Hex-1-ene is $\ce{CH3CH2CH2CH2CH=CH2}$. Let’s name both products systematically.

---

## Step-by-Step Solution

### 1. Identify the alkene structure
Hex-1-ene has the double bond between C1 and C2:
$$\ce{CH3-CH2-CH2-CH2-CH=CH2}$$
Numbering from the double bond end: C1 (terminal) and C2 (internal).

### 2. Addition in the absence of peroxide (ionic mechanism)
HBr ionises to $\ce{H+}$ and $\ce{Br-}$. The proton attacks the double bond, forming the more stable carbocation. The secondary carbocation at C2 is more stable than the primary at C1. So $\ce{H+}$ adds to C1, giving a carbocation at C2. Then $\ce{Br-}$ attacks C2.

Product: bromine at C2, hydrogen at C1.  
IUPAC name: **2-bromohexane**.

### 3. Addition in the presence of peroxide (free-radical mechanism) …

Isomer Name	Common Name	Structure (Condensed)	Boiling Point (°C)
Pentane	n-Pentane	$CH_3(CH_2)_3CH_3$	36.1
2-Methylbutane	Isopentane	$(CH_3)_2CHCH_2CH_3$	27.7
2,2-Dimethylpropane	Neopentane	$C(CH_3)_4$	9.5

Q.Write IUPAC names of the products obtained by addition reactions of HBr to hex-1-ene

Concept understanding — Chain Isomerism

Chain Isomerism: The Skeleton of a Molecule

The Intuition

The Precise Statement

A Concrete Example: Pentane ( $C_5H_{12}$ )

When Does Chain Isomerism Occur?

Why This Happens: The Concept of Chain Isomerism in Addition Reactions

Step-by-Step Solution

1. Identify the alkene structure

2. Addition in the absence of peroxide (ionic mechanism)

3. Addition in the presence of peroxide (free-radical mechanism) …