Question

Dehydrobromination (-HBr) of the following in increasing order is (1) CCCCBr (2) CCCC(C)Br (3) CC(C)(C)Br (a) \(1<2<3\) (b) \(3<2<1\) (c) \(2=3<1\) (d) \(3<1<2\)

Short answer

The increasing order of dehydrobromination is \(1 < 2 < 3\), option (a).

Step-by-step solution

Understand Dehydrobromination

Dehydrobromination is an elimination reaction where hydrogen bromide (HBr) is removed from an alkyl bromide, resulting in an alkene formation. The ease of this reaction depends largely on the structure and stability of the formed alkene.

Analyze the Structures

Examine the molecular structure of each alkyl bromide: 1. (1) CCCCBr: a primary bromide 2. (2) CCCC(C)Br: a secondary bromide 3. (3) CC(C)(C)Br: a tertiary bromide Tertiary bromides are more reactive than secondary, which in turn are more reactive than primary in elimination reactions due to the stability of the resulting carbocation intermediate.

Compare the Reactivity Based on Structure

Tertiary alkyl bromides dehydrobrominate most easily, followed by secondary, and then primary. Therefore: - (3) has the most stable resulting cation, making it the easiest to dehydrobrominate. - (2) follows as it is secondary. - (1) is primary and will dehydrobrominate with the most difficulty.

Determine the Order of Ease

Given the ease of dehydrobromination from the previous step: 1. (3) > (2) > (1), which aligns with increasing order of difficulty as (1) < (2) < (3).

Select the Correct Option

From the determined order, the correct increasing order of dehydrobromination is (1) < (2) < (3). Therefore, the correct answer is option (a) \(1 < 2 < 3\).

Key concepts

Dehydrobromination

Dehydrobromination is a specific type of chemical reaction known as an elimination reaction that involves the removal of a hydrogen atom (H) and a bromine atom (Br) from a molecule, typically an alkyl bromide. This reaction results in the formation of an alkene, which is a hydrocarbon containing at least one carbon-carbon double bond. Dehydrobromination is important in organic synthesis because it transforms saturated molecules into unsaturated ones, providing a pathway to form double bonds.

In dehydrobromination, the presence of a strong base is usually necessary to facilitate the removal of HBr from the alkyl bromide. This reaction often takes place via an E2 mechanism, where both the hydrogen and bromine are removed in a single concerted step. This leads to the formation of an alkene as the main product. The structure of the starting alkyl bromide influences the ease of the dehydrobromination process.

Elimination Reactions

Elimination reactions are a broad class of reactions in organic chemistry that involve the removal of atoms or groups from a molecule. This creates a new double bond and results in the formation of alkenes. In the context of dehydrobromination, it specifically refers to the loss of a hydrogen and halogen atom (often HBr) from the substrate.

The two common mechanisms through which elimination reactions occur are E1 and E2.

• E1 Mechanism: This is a two-step process where a carbocation intermediate forms. The leaving group departs first, followed by the removal of a proton.
• E2 Mechanism: A one-step, bimolecular process where the base abstracts a proton while the leaving group is simultaneously expelled, leading directly to the alkene.

The choice of mechanism depends on several factors, including the structure of the substrate, the presence and strength of the base, and the reaction conditions.

Carbocation Stability

Carbocation stability is a pivotal concept when considering elimination reactions, particularly those involving E1 mechanisms. A carbocation is a positively charged species with a carbon atom bearing a positive charge. Its stability can significantly influence the reaction pathway and the ease of the reaction.

Carbocation stability depends on several factors:

• Alkyl Substitution: Tertiary carbocations (surrounded by three carbon-containing groups) are more stable than secondary, which are more stable than primary carbocations. This is due to hyperconjugation and the inductive effect, where electron-donating alkyl groups help stabilize the positive charge.
• Resonance: If the carbocation can be stabilized through resonance with a neighboring π-bond or a lone pair, its stability increases.
• Inductive Effects: Electronegative atoms or groups can withdraw electron density through sigma bonds, stabilizing or destabilizing carbocations depending on their position.

The more stable the carbocation, the more favorable the elimination reaction tends to be.

Alkyl Bromides

Alkyl bromides are important starting materials in dehydrobromination reactions. These molecules consist of an alkyl group attached to a bromine atom. The classification of alkyl bromides is based on the nature of the carbon that the bromine atom is bonded to, being primary, secondary, or tertiary.

Alkyl bromides have varying tendencies to participate in elimination reactions, largely influenced by:

• Carbon Structure: Tertiary bromides dehydrobrominate more readily than secondary, which dehydrobrominate more readily than primary. This is due to the increasing stability of the resulting carbocations as you move from primary to tertiary structures.
• Leaving Group Ability: Bromine is a good leaving group, making alkyl bromides generally reactive in substitution or elimination reactions. This is due to the relatively weak C-Br bond strength.
• Steric Effects: Bulkier alkyl bromides may face more steric hindrance, affecting the reaction mechanism and rate.

By understanding these factors, one can predict the reactivity and outcome of dehydrobromination reactions involving alkyl bromides.

Alkene Formation

Alkene formation is the primary result of dehydrobromination reactions, occurring when a hydrogen and bromine atom are removed to form a carbon-carbon double bond, characterizing the alkene. The formation of alkenes is a crucial step in many synthetic pathways in organic chemistry, as it introduces unsaturation into molecules.

The outcome of alkene formation is governed by:

• Zaitsev's Rule: This rule states that the more substituted alkene will be the major product. In simple terms, the more stable alkene, usually with more alkyl groups attached to the double-bonded carbons, is favored.
• Stereochemistry: The stereochemistry of the alkene formed can also impact the reaction, usually favoring the more stable trans-isomer over the cis-isomer.
• Reaction Conditions: Temperature, concentration of the base, and the solvent can influence which alkene is formed and in what proportions.

Understanding these principles helps chemists predict and control the outcome of alkene formations in elimination reactions.