Question

vic-Dibromides usually react with bases to produce more alkyne than conjugated diene. What factor(s) may be controlling this? Two exceptions to this "behavior are 1,2 -dibromo-cyclohexane and 2,3 -dichloro \(-2\), 3-dimethylbutane. How do you account for these exceptions?

Short answer

In the reaction of vic-dibromides with bases, the formation of alkynes is favored due to factors such as less steric hindrance, electronic factors favoring anti elimination, and the stability of intermediates. However, in the cases of 1,2-dibromo-cyclohexane and 2,3-dichloro-2,3-dimethylbutane, the formation of conjugated dienes is favored due to the destabilizing effects on the potential alkynes, like a highly strained cyclic alkyne in the former and increased steric hindrance in the latter.

Step-by-step solution

Understanding Vic-Dibromide Reactions

Typically when a vicinal dibromide reacts with a base, it undergoes an E2 elimination reaction to produce an alkyne rather than a conjugated diene. This is because the two bromines are on adjacent carbon atoms, making it more likely for the elimination reaction to form a triple bond between those carbons. Let's consider these factors in detail:

Discussing the Factors Controlling the Reaction Outcome

There are several factors that contribute to the formation of alkynes in the reaction of vic-dibromides with bases: 1. Steric effect: There is often less steric hindrance for the formation of an alkyne as an elimination product, as the preferred transition state requires relatively less steric force from surrounding groups. 2. Electronic factors: The formation of a conjugated diene requires the removal of two vicinal protons in a syn or anti conformation. This increases the selectivity for the anti elimination, leading to less of the conjugated diene product. 3. Stability of intermediates: The intermediates formed during the elimination reaction to produce alkynes are typically more stable than those that would result in conjugated diene formation. Now, we will analyze the exceptions given in the exercise to understand why these vic-dibromide systems do not follow the usual trend.

Analyzing the Exceptions

To account for the exceptions - 1,2-dibromo-cyclohexane and 2,3-dichloro-2,3-dimethylbutane - we need to consider the structural and electronic factors specific to these molecules. 1,2-dibromo-cyclohexane: In this case, the formation of an alkyne would result in a highly strained cyclic alkyne, which is much less stable compared to a cyclic conjugated diene. The cyclic alkyne intermediate has difficulty adopting the conformation necessary for an elimination reaction. Hence, the reaction would proceed to form the more stable conjugated diene. 2,3-dichloro-2,3-dimethylbutane: In this situation, the steric hindrance caused by the two methyl groups flanking the vicinal chlorines prevents the formation of an alkyne. The larger chlorine atoms and t-butyl group increase steric interactions making it improbable for the necessary transition state to form and pushing the reaction towards the formation of a conjugated diene. Overall, for both exceptions, it's the increased stability of the conjugated diene and the destabilizing effects on the potential alkyne that cause the reaction to follow a different trajectory from the typical vic-dibromide reactions.

Key concepts

Vicinal Dibromides

Vicinal dibromides are organic molecules where two bromine atoms are attached to adjacent carbon atoms in a carbon chain. These compounds are significant in organic synthesis as they can be transformed into other structures through elimination reactions. Their reactivity primarily stems from the presence of these halogen atoms at vicinal, or neighboring, positions.

In such structures, the bromine atoms serve as leaving groups when the molecule undergoes chemical reactions. The proximity of these bromines influences the types of reactions that can occur, such as elimination reactions where double or even triple bonds are formed in the resulting products. Understanding the positioning of substituents in vicinal dibromides helps predict the outcome of these reactions.

E2 Elimination Reaction

The E2 elimination reaction is a concerted mechanism in organic chemistry where a base abstracts a proton, leading to the simultaneous formation of a double bond while a leaving group departs. It is a single-step reaction and is typically associated with the formation of alkenes or alkynes when starting from vicinal dihalides.

In an E2 reaction involving vicinal dibromides, the two bromine atoms facilitate the removal of hydrogens from adjacent positions, allowing the formation of a new pi bond. The geometry of the bonds plays a crucial role, with the anti-periplanar requirement often dictating which isomers are primarily formed. The outcome favors alkynes due to their stabilization and the conditions that drive the reaction pathway.

• Fast and reversible under certain conditions
• Requires a strong base
• Commonly results in the most stable alkene or alkyne product

Steric Hindrance

Steric hindrance refers to the resistance to chemical reactions due to the size of groups attached to the reactive centers in a molecule. It heavily influences the pathway of reactions like E2 eliminations from vicinal dibromides.

Large, bulky groups near the reactive site can physically block or slow down the progress of the reaction. In cases where steric hindrance is high, it can shift reaction preferences. Instead of the expected alkyne formation, molecules might favor pathways that offer less sterically hindered transition states, even if these states lead to different products like conjugated dienes.

This influence of steric effects is crucial in understanding reaction exceptions and predicting untypical product formations.

Conjugated Diene

A conjugated diene is a system where two or more double bonds are separated by a single bond, allowing for electron delocalization along the chain. This structure is stabilized by resonance and contributes significantly to the reactivity of organic molecules.

In the context of reacting vicinal dibromides, conjugated dienes may form as alternative products to alkynes under specific conditions that promote or stabilize their creation. While typically less favored in standard vic-dibromide reactions, conjugated dienes can be preferentially formed if cyclic stability or steric factors disfavor alkyne production.

• Stability due to resonance
• Typically results from planar transition states
• Offers unique reactivity in further reactions

Reaction Intermediates

Reaction intermediates are transient species that appear during the steps transforming reactants to products. They determine the path and final products of an organic reaction. In the elimination reactions involving vicinal dibromides, intermediates can lead to the formation of various products based on their stability and the reaction conditions.

Stable intermediates often guide the reaction towards the most favored product. For example, in the creation of alkynes from vicinal dibromides, intermediates that favor the strong triple bond over a conjugated diene are preferred. However, unusual structural features or reaction conditions can shift this balance, exemplifying why some reactions don't follow the typical rule and result in unexpected di-structures.

• Key in determining reaction pathways
• Can be stabilized by electronic and steric factors
• Drive the reaction towards equilibrium