OpenOChem Learn First Semester Topics Alkyl Halides and Alcohol Classes of Alcohols and Alkyl Halides

Classes of Alcohols and Alkyl Halides

The classification of alkyl halides and alcohols as primary (1°), secondary (2°), or tertiary (3°) is based on the number of carbon atoms directly bonded to the carbon atom that bears the halogen or hydroxyl (-OH) group. This classification is crucial because it significantly influences the reactivity of these compounds, particularly in reactions like nucleophilic substitution (SN1 and SN2) and elimination (E1 and E2), which will be discussed in detail in the following chapter.

Alkyl Halides

Primary (1°) Alkyl Halides: In a primary alkyl halide, the carbon atom bonded to the halogen is attached to only one other carbon atom. Often, this carbon is at the end of a carbon chain.
```
R-CH2-X   (where X = halogen)
```
- Examples:
Secondary (2°) Alkyl Halides: In a secondary alkyl halide, the carbon atom bonded to the halogen is attached to two other carbon atoms.
```
   R
   |
R-CH-X   (where X = halogen)
```
- Examples:
Tertiary (3°) Alkyl Halides: In a tertiary alkyl halide, the carbon atom bonded to the halogen is attached to three other carbon atoms.
```
   R
   |
 R-C-X   (where X = halogen)
   |
   R
```
- Examples:
Benzylic and Allylic Alkyl Halides: In a benzylic halide, the carbon atom bonded to the halogen is attached to a benzene ring and an allylic the carbon atom bonded to the halide is adjacent to a pi bond .
- Examples

Alcohols

The same classification system applies to alcohols:

Primary (1°) Alcohols: The carbon atom bonded to the hydroxyl (-OH) group is attached to only one other carbon atom.
```
R-CH2-OH
```
- Examples:
Secondary (2°) Alcohols: The carbon atom bonded to the hydroxyl group is attached to two other carbon atoms.
```
   R
   |
R-CH-OH
```
- Examples:
Tertiary (3°) Alcohols: The carbon atom bonded to the hydroxyl group is attached to three other carbon atoms.
```
   R
   |
 R-C-OH
   |
   R
```
- Examples:
Benzylic and Allylic Alcohols: The carbon atom bonded to the hydroxyl group is attached to a benzene ring or adjacent to a pi bond.

Importance of Classification:

The classification of alkyl halides and alcohols is essential for predicting their reactivity. For example, in the next chapter we will see:

SN2 Reactions: Primary alkyl halides are generally more reactive in SN2 reactions due to less steric hindrance.
SN1 Reactions: Tertiary alkyl halides are more likely to undergo SN1 reactions due to the formation of stable tertiary carbocations.
Elimination Reactions: The degree of substitution of the carbon bearing the halogen or hydroxyl group influences the mechanism and regiochemistry of elimination reactions.

Take Note

Understanding these classifications will be crucial as you explore the reactions of alkyl halides and alcohols in the following chapters. Make sure you can easily identify primary, secondary, and tertiary examples of each.