Question

Predict the product of the reaction of \((\mathrm{R})-2\) -butanol with triphenylphosphorus dibromide.

Short answer

The product of the reaction between \((\mathrm{R})-2\)-butanol and triphenylphosphorus dibromide is \((\mathrm{R})-2\)-bromobutane. This reaction occurs via the Appel reaction mechanism, which converts the hydroxyl group of the alcohol to an alkyl bromide, maintaining the R-configuration at the chiral carbon. The structure of \((\mathrm{R})-2\)-bromobutane is: ``` H H H Br | | | | H-C-C-C-C | H ```

Step-by-step solution

Identify the functional groups of the reactants

In this reaction, we have two reactants: 1. \((\mathrm{R})-2\)-butanol: It is an alcohol with the hydroxyl group (-OH) attached to the second carbon atom of a four-carbon chain. The structure is given by: ``` H H H H | | | | H-C-C-C-C-OH | H ``` 2. Triphenylphosphorus dibromide: It is a phosphorus-containing compound with three phenyl groups attached to a central phosphorus atom, as well as two bromide ions. The structure is given by: ``` Phosphorus with three phenyl groups and two bromide ions (Ph)3PBr2 ```

Determine the type of reaction

Given that we have an alcohol reactant and a phosphorus-containing compound, this reaction is an example of Appel reaction. The Appel reaction is used to convert alcohols into alkyl halides, in this case, the alkyl bromides. The mechanism involves the transfer of a halogen atom from the triphenylphosphorus compound to the alcohol, producing an alkyl bromide.

Apply the reaction mechanism

In the case of \((\mathrm{R})-2\)-butanol reacting with triphenylphosphorus dibromide, the reaction leads to the substitution of the hydroxyl group with a bromine atom in the chiral center of the molecule. The mechanism is as follows: 1. Nucleophilic attack of the phosphorus atom by the oxygen atom of the alcohol, resulting in the formation of an intermediate: \[ (\mathrm{R})-2\text{-butanol }+ (\text{Ph})_3\text{PBr}_2 \rightarrow [(\mathrm{R})-2\text{-butyl} \text{-O} - (\text{Ph})_3\text{P}\text{Br}]^{+}\text{Br}^{-} \] 2. Intramolecular halogen transfer from the phosphorus to the oxygen atom, resulting in the formation of the alkyl bromide and triphenylphosphine oxide: \[ [(\mathrm{R})-2\text{-butyl} \text{-O} - (\text{Ph})_3\text{P}\text{Br}]^{+}\text{Br}^{-} \rightarrow (\mathrm{R})-2\text{-bromo} \text{-butane} + (\text{Ph})_3\text{P=O} \] The stereochemistry of the product maintains the R-configuration at the chiral carbon.

Identify the final product

The product of this reaction is the alkyl bromide \((\mathrm{R})-2\)-bromobutane, with the bromine atom replacing the hydroxyl group. The structure of \((\mathrm{R})-2\)-bromobutane is given by: ``` H H H Br | | | | H-C-C-C-C | H ```

Key concepts

(R)-2-butanol

In chemistry, (R)-2-butanol is an alcohol that features a chiral center, which means it has a specific 3D arrangement allowing for different stereoisomers. The designation "R" refers to the absolute configuration around its chiral center, following the R/S nomenclature system which is derived from the terms "rectus" (right) and "sinister" (left). The molecule has a total of four carbon atoms, with the hydroxyl group (-OH) attached to the second carbon. This configuration signifies that it's a primary alcohol, common in many chemical reactions.
It's important to recognize that (R)-2-butanol's chirality plays a crucial role in its reactivity and interactions with other chemical species. Chirality affects how (R)-2-butanol reacts, as it determines the spatial positioning of the substituents attached to the chiral carbon, hence influencing the product's stereochemistry during a reaction.

Triphenylphosphorus dibromide

Triphenylphosphorus dibromide is a reagent frequently encountered in organic chemistry, especially in halogenation reactions. This compound has a central phosphorus atom bonded to three phenyl groups (rings made of carbon atoms) and two bromide ions.
Due to its unique structure, triphenylphosphorus dibromide is commonly used in the Appel reaction, where it facilitates the conversion of alcohol into an alkyl halide, like an alkyl bromide.
The action of triphenylphosphorus dibromide can be understood through its ability to form a stable ionic intermediate with alcohols, allowing the bromination of the alcohol's site while forming triphenylphosphine oxide as a byproduct.

Alkyl bromide

Alkyl bromides are a type of organic compound characterized by the presence of a bromine atom bonded to a carbon atom. They are commonly produced through the halogenation of alcohols using various reagents like triphenylphosphorus dibromide. In the context of the Appel reaction, the alkyl bromide such as (R)-2-bromobutane is the primary product.
These compounds play a significant role in organic synthesis. Some of their applications include serving as intermediates in the formation of more complex molecules, acting in substitution reactions, or being involved in the creation of pharmaceutical compounds.
The transformation from an alcohol to an alkyl bromide usually proceeds with the inversion or retention of configuration at a chiral center depending on the reaction pathway and conditions.

Stereochemistry

Stereochemistry is a branch of chemistry that focuses on the spatial arrangement of atoms within molecules. This is particularly significant for molecules like (R)-2-butanol, which contains a chiral center. The term "stereochemistry" reflects how a molecule's three-dimensional shape can influence its chemical reactions and properties.
In reactions involving chiral molecules, like the conversion of (R)-2-butanol to (R)-2-bromobutane in an Appel reaction, the stereochemistry is crucial in determining the product's properties and biological activity. Maintaining or inverting the configuration at the chiral carbon can lead to vastly different outcomes in terms of chemical behavior and interactions.
Understanding stereochemistry helps chemists predict reaction results and design substances with desired attributes, especially when developing pharmaceuticals.

Reaction mechanism

A reaction mechanism provides a detailed sequence of steps by which reactants transform into products. In the case of the Appel reaction, (R)-2-butanol reacts with triphenylphosphorus dibromide to form (R)-2-bromobutane. This mechanism highlights nucleophilic attacks and intramolecular halogen transfers.
Initially, the oxygen on (R)-2-butanol attacks the phosphorus in triphenylphosphorus dibromide. This creates an intermediate more amenable to bromine transfer. Subsequently, bromine from the triphenylphosphorus dibromide bonds to the former alcohol's site, substituting for the hydroxyl group.
Through this process, triphenylphosphine oxide is also generated as a byproduct. Understanding this mechanism allows chemists to manipulate conditions to optimize yields and selectivity for desired alkyl halide products.