Question

Treating 1,3 -butadiene with 1 mole of HBr gives a mixture of two isomeric products.

Short answer

Answer: The isomeric products formed from the reaction between 1,3-butadiene and 1 mole of HBr are 3-bromobut-1-ene (H2C-CH(Br)-CH=CH2) and 1-bromobut-2-ene (H2C=CH-CH2-CH2Br).

Step-by-step solution

Draw the structure of the reactant (1,3-butadiene) and recognize the nucleophile and electrophile

First, draw the reactant's structure, 1,3-butadiene, which is a conjugated diene. Identify the π-bond electrons as the nucleophile and HBr as the electrophile in the reaction, which will undergo an electrophilic addition reaction. Butadiene: H2C=CH-CH=CH2 Nucleophile: π-bond electrons (double bond) Electrophile: HBr

Understand the two possible modes of electrophilic addition

There are two ways that electrophilic addition can occur in a conjugated diene system: 1,2-addition reaction (or direct addition) and 1,4-addition reaction (or conjugate addition). To predict the isomeric products, we will evaluate both possible modes of electrophilic addition. 1,2-addition: The electrophile (HBr) reacts directly with the nucleophile (double bond) at positions 1 and 2 of the diene. 1,4-addition: The electrophile reacts with the nucleophile at positions 1 and 4 of the diene in a conjugate addition.

Determine the products formed in each addition mode

Based on the two possible modes of addition, predict the isomeric products formed. 1,2-addition product: The addition of HBr to the diene at positions 1 and 2 will result in a product with a bromine atom at the second carbon, and an additional hydrogen at the first carbon. Product 1: H2C-CH(Br)-CH=CH2 1,4-addition product: The addition of HBr to the diene at positions 1 and 4 will result in a product with a bromine atom at the fourth carbon, and an additional hydrogen at the first carbon. Product 2: H2C=CH-CH2-CH2Br

Compare the isomeric products

Let's compare the two isomeric products formed in the 1,2-addition and 1,4-addition reactions. Both products are alkyl halides, but their atomic arrangement differs. Product 1: 3-bromobut-1-ene Product 2: 1-bromobut-2-ene

Write the final answer

When 1,3-butadiene reacts with 1 mole of HBr, it forms a mixture of two isomeric products depending upon the mode of addition. The isomeric products are: 1,2-addition product: 3-bromobut-1-ene (H2C-CH(Br)-CH=CH2) 1,4-addition product: 1-bromobut-2-ene (H2C=CH-CH2-CH2Br)

Key concepts

1,3-butadiene

Understanding the characteristics and behavior of 1,3-butadiene is fundamental when studying organic chemistry. It is a simple conjugated diene with the formula C₄H₆, consisting of four carbon atoms and two double bonds that are separated by a single bond. This arrangement allows the electrons in the double bonds to be more delocalized than in isolated double bonds, which contributes to the unique reactivity of conjugated dienes like 1,3-butadiene.

When introduced to an electrophile, 1,3-butadiene can undergo an addition reaction. This reaction is not just a simple addition; the conjugated system leads to multiple potential outcomes, making it a remarkable example when studying electrophilic addition reactions in conjugated systems. The interplay between the molecular structure of 1,3-butadiene and the possible outcomes of these reactions showcases the complexity and the beauty of organic synthesis.

Nucleophile and Electrophile

A clear understanding of nucleophiles and electrophiles is crucial in organic reaction mechanisms. Electrophiles are electron-seeking species, often carrying a positive charge or a significant positive character, making them ready to accept a pair of electrons. On the other hand, nucleophiles are nucleus-seeking, typically rich in electrons and often carry a negative charge or partial negative character, allowing them to donate electrons.

During the electrophilic addition to conjugated dienes, the electrons from the π-bond act as a nucleophile, whereas a molecule like hydrobromic acid (HBr) serves as the electrophile. This dichotomy is essential because it determines the nature of the chemical bond formation. In the case of 1,3-butadiene, the π-bond electrons are attracted to the electrophile (HBr), leading to a set of possible reactions where the interplay between the nucleophile and electrophile is displayed in full swing.

1,2-addition and 1,4-addition Reactions

The 1,2-addition and 1,4-addition reactions are pivotal to the understanding of how conjugated dienes like 1,3-butadiene react with electrophiles. In a 1,2-addition, the electrophile attacks the first carbon of the diene, and a nucleophilic carbon from the first double bond bonds to the electrophile, leading to a product where the addition takes place at the first and second carbon atoms.

In a 1,4-addition, the electrophile still initially forms a bond with the first carbon; however, due to the conjugation, the positive charge can resonate to the fourth carbon, allowing a nucleophilic attack at that position, resulting in the addition occurring across the width of the 1,3-butadiene.

These reactions not only yield different products but also elucidate the concept of regioselectivity, as the position of the added groups can vary based on the type of addition. It's crucial to understand these differences because they form the basis for predicting and controlling the outcome of reactions involving conjugated dienes, thereby underlining the significant role of molecular structure and electron distribution in chemical reactivity.