Problem 40 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

\(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{C} \equiv \mathrm{CH}\) on reaction with 1 mole of DBr gives (a) \(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CBr}=\mathrm{CHD}\) (b) \(\mathrm{CH}_{2}(\mathrm{Br})-\mathrm{CHDC}=\mathrm{CH}\) (c) \(\mathrm{DCH}_{2}-\mathrm{CHBrC}=\mathrm{CH}\) (d) \(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CD}=\mathrm{CHBr}\)

Step-by-Step Solution

Answer

Option (a): \( \text{CH}_2 = \text{CH} -\text{CBr} = \text{CHD} \) is the correct answer.

Step 1: Identify the Reactant and Reagent

The reactant in the exercise is 1-butyne, \( \text{CH}_2=\text{CH}-\text{C} \equiv \text{CH} \), and the reagent is 1 mole of DBr, which contains deuterium (D), a hydrogen isotope.

Step 2: Understand the Reaction Mechanism

1-butyne can react with 1 mole of DBr across the triple bond first, following Markovnikov's rule, which states that the proton (or its isotope D) attaches to the carbon with more hydrogen atoms. This typically leads to anti-Markovnikov addition when using DBr due to the presence of a transition-metal catalyst or other special conditions.

Step 3: Determine the Major Product

For the scenario given, the D (deuterium) will attach to the carbon at the end of the triple bond (producing a vinylic deuteride), and the Br will attach to the adjacent carbon. The resulting structure is \( \text{CH}_2 = \text{CH} -\text{CBr} = \text{CHD} \).

Step 4: Find the Correct Option

Comparing the structure from Step 3, it's evident that option (a) matches the product formed: \( \text{CH}_2 = \text{CH} -\text{CBr} = \text{CHD} \).

Key Concepts

Markovnikov's Rule

Markovnikov's Rule is a fundamental guideline in organic chemistry, especially when working with addition reactions involving alkenes and alkynes. It helps predict the final structure of the product by indicating where the hydrogen moiety, or its isotope like deuterium (D), will add in a chemical reaction. In simple terms, Markovnikov's Rule states that in the addition of a protic acid, such as HBr or DBr, to an unsymmetrical alkene or alkyne, the hydrogen (or deuterium) atom bonds with the carbon that already has more hydrogen atoms connected to it. This is primarily due to the formation of the more stable carbocation intermediate in the reaction mechanism.

For example, in the context of 1-butyne reacting with DBr, the deuterium would add to the terminal carbon—where more hydrogen atoms pre-exist—guided by Markovnikov's principle. Consequently, the bromine atom would add to the adjacent carbon. By applying Markovnikov's Rule, we are able to determine the molecular arrangement in the resulting compound, making this rule indispensable in predicting organic reaction outcomes.

Anti-Markovnikov Addition

In contrast to Markovnikov's Rule, Anti-Markovnikov Addition details situations where the proton or deuterium from a reagent, like DBr, adds to the less-substituted carbon atom. This approach is less common and often requires specific catalysts—frequently transition metals— or reagents to occur. Anti-Markovnikov Addition is valuable when the target is to control the location of the addition and alter the product stereochemistry to achieve specific configurational outcomes.

An example of when Anti-Markovnikov Addition takes place is with hydroboration-oxidation where boron adds to one carbon and subsequently is replaced by an OH group, emphasizing regioselectivity toward the less substituted carbon. However, in the given reaction of 1-butyne with DBr, without such a catalyst or specific conditions fostering anti-Markovnikov behavior, Markovnikov's outcome is favored.

• While identifying the context where these additions could swap is crucial, keeping in mind that special conditions usually drive the non-Markovnikov outcome helps in predicting unlikely scenarios.

1-butyne Reaction

When examining the reaction involving 1-butyne (\( ext{CH}_2= ext{CH}- ext{C} \ ext{equiv} ext{CH} \)) and a reagent like DBr, careful consideration of reaction conditions is key. In this particular exercise, the reaction takes place in the absence of special catalysts, leading to a Markovnikov addition across the triple bond of 1-butyne. This entails the deuterium from DBr attaching itself to the terminal, more hydrogen-rich carbon of the alkyne, while bromine attaches to the adjacent carbon atom.

The result is a transformation of the triple bond into a double bond after the first mole of DBr reacts, forming a structure with \( ext{CH}_2= ext{CH}- ext{CBr}= ext{CHD} \). This indicates the typical Markovnikov product arising from this kind of chemical reaction that aligns with known chemical properties of such unsaturated hydrocarbons when treated with halogen acids.

• This reaction accentuates the necessity of understanding both the rule guiding formation and the structural changes occurring in organic transformations.