What is the Difference Between 1 2 Addition and 1 4 Addition?
🆚 Go to Comparative Table 🆚The main difference between 1,2-addition and 1,4-addition lies in the position of the carbon atoms in the conjugated diene that are involved in the addition reaction. In a 1,2-addition, the reactant adds to the first and second carbon atoms of the diene, resulting in a product where the added atoms or groups are adjacent to each other. In a 1,4-addition, the reactant adds to the first and fourth carbon atoms of the diene, resulting in a product where the added atoms or groups are separated by two carbon atoms.
Here are some key points about 1,2- and 1,4-additions:
- The 1,2-addition has a smaller activation energy than the 1,4-addition, making it occur faster.
- At low temperatures, the 1,2-addition product is favored, while at higher temperatures, the 1,4-addition product becomes more dominant.
- The 1,4-addition product is more thermodynamically stable than the 1,2-addition product due to its more substituted double bond.
For example, when HBr is added to butadiene, the 1,2-addition product is formed when HBr bonds to the first carbon atom and the Br bonds to the second carbon atom, resulting in 1-bromo-2-butene. The 1,4-addition product is formed when HBr bonds to the first carbon atom and the Br bonds to the fourth carbon atom, resulting in 2-bromo-1-butene.
Comparative Table: 1 2 Addition vs 1 4 Addition
The difference between 1,2-addition and 1,4-addition lies in the position at which the new attacking molecule bonds to the double bond of a conjugated diene. Here is a summary of the two types of addition:
Addition Type | Description | Product Formation |
---|---|---|
1,2-Addition | In this addition, the attacking molecule (e.g., HBr) bonds to the adjacent carbons of a double bond, resulting in the formation of a new C-H single bond and a C-Br single bond. This product is formed more rapidly than the 1,4-addition product at higher temperatures. | The product is a mixture of 3,4-dibromo-1-butene and 1,4-dibromo-2-butene, with the 1,2-addition being the expected product. |
1,4-Addition | In this addition, the attacking molecule bonds to two carbon atoms separated in the carbon chain by two intervening atoms, forming a new C-H single bond on one end of the diene and a C-Br single bond on the other end. This product is more stable than the 1,2-addition product. | The resultant product is primarily formed as the (E) isomer. |
At low temperatures, the products are formed irreversibly and reflect the relative rates of the two competing reactions. As the temperature increases, the amount of 1,4 product increases.
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