Question

Predict the structure of the major 1,2 -addition product formed by reaction of one mole of \(\mathrm{Br}_{2}\) with isoprene. Also predict the structure of the major 1,4 -addition product formed under these conditions.

Short answer

Question: Predict the structure of the major 1,2-addition product and the major 1,4-addition product formed by the reaction of one mole of bromine (Br2) with isoprene. Answer: The major 1,2-addition product has the structure CH2Br - C(CH3) - CHBr - CH2. The major 1,4-addition product has the structure CH2Br - C(CH3) - CH2 - CH2Br.

Step-by-step solution

Understanding the structure of isoprene

Isoprene is an organic compound with the molecular formula \(\mathrm{C}_{5}\mathrm{H}_{8}\). Its structure can be represented as follows: $$ \mathrm{CH}_{2} = \mathrm{C}(\mathrm{CH}_{3}) - \mathrm{CH} = \mathrm{CH}_{2} $$ It is a diene, which means it contains two double bonds in its structure. To predict the products formed by the reaction of isoprene with \(\mathrm{Br}_2\), we need to understand how the double bonds will react during the 1,2- and 1,4-addition reactions.

Addition of \(\mathrm{Br}_2\) to a diene

When a diene like isoprene reacts with bromine, there are two major addition pathways that can occur: 1,2-addition and 1,4-addition. In the 1,2-addition pathway, the bromine atom adds across one double bond, resulting in the formation of a bromonium ion. The second bromine atom then attacks the bromonium ion to form the 1,2-addition product. In the 1,4-addition pathway, the bromine atom adds across the second double bond, extending the bromonium ion and resulting in the formation of an allylic carbocation. The second bromine atom then attacks the allylic carbocation to form the 1,4-addition product.

Predicting the structure of the major 1,2-addition product

In the 1,2-addition reaction, the double bond between carbons 1 and 2 reacts with one bromine atom from \(\mathrm{Br}_2\). A new bond is formed between carbon 1 and the bromine atom. The second bromine atom then attacks the bromonium ion, resulting in a new bond between carbon 2 and the second bromine atom. This yields the following structure for the major 1,2-addition product: $$ \mathrm{CH}_{2} \mathrm{Br} - \mathrm{C}(\mathrm{CH}_{3}) - \mathrm{CH}\mathrm{Br} - \mathrm{CH}_{2} $$

Predicting the structure of the major 1,4-addition product

In the 1,4-addition reaction, one bromine atom from \(\mathrm{Br}_2\) reacts with the double bond between carbons 1 and 2, extending the bromonium ion as described in Step 2. A transient carbocation is formed at the allylic position before the second bromine atom attacks, resulting in a final structure for the major 1,4-addition product: $$ \mathrm{CH}_{2} \mathrm{Br} - \mathrm{C}(\mathrm{CH}_{3}) - \mathrm{CH}_{2} - \mathrm{CH}_{2}\mathrm{Br} $$

Key concepts

Diene Reactivity

Dienes are hydrocarbon molecules that contain two carbon-carbon double bonds. Their reactivity is central to many organic reactions, including the isoprene bromine addition reaction. Understanding how these double bonds behave is key to predicting the outcome of reactions they undergo.

Isoprene, a common diene, demonstrates versatile reactivity owing to its conjugated system - this is where the double bonds are separated by a single carbon-carbon bond. Such a system can stabilize charge through resonance when the molecule reacts, making it more reactive than isolated double bonds. In bromine additions to compounds like isoprene, two pathways are commonly observed - 1,2-addition and 1,4-addition - which yield different products based upon how the bromine molecule interacts with the double bonds.

1,2-Addition and 1,4-Addition

When it comes to diene reactions, 1,2- and 1,4-addition represent two important pathways for product formation. In 1,2-addition, bromine reacts with the double bond closest to the reactant side, forming a product with bromine atoms added across this double bond.

Conversely, 1,4-addition sees the reaction extend across the entire diene, with the addition of bromine to the first and fourth carbons in the chain. The choice between these two pathways depends on factors like the nature of the diene, the reaction conditions, and the presence of substituents that can stabilize the intermediate formed during the reaction process. The major product will typically be the one that is most stable or can be formed under milder conditions.

Bromonium Ion Formation

In the bromine addition reaction to dienes, a key intermediate that forms is the bromonium ion. This species is created when a bromine molecule approaches the double bond in a diene and forms a three-membered ring with two of the carbons.

This transient state is highly reactive due to the positive charge located on the bromine, making it a crucial step in the mechanism of both 1,2- and 1,4-additions. The bromonium ion is subsequently attacked by a nucleophile, commonly the second bromine atom, leading to the formation of the final dibrominated product. The bromonium intermediate is important because the nature of its formation can explain why certain addition patterns are favored over others.

Organic Chemistry Reaction Mechanisms

In organic chemistry, understanding reaction mechanisms allows chemists to predict and explain how and why certain products are formed. A reaction mechanism details the step-by-step sequence of elementary reactions by which overall chemical change occurs.

Each step typically involves a reconfiguration of electrons and can be represented using arrow-pushing notation to illustrate the movement of electron pairs. For diene addition reactions with bromine, the mechanisms involve the formation of bromonium ions and, depending on the reaction conditions, lead to distinct products through 1,2- or 1,4-addition pathways. Grasping these mechanisms provides insight into not only the nature of the products but also the factors affecting the reaction rate and outcomes.