Question

When 4 -chlorobutan-1-ol is treated with a strong base such as sodium hydride, NaH, tetrahydrofuran is produced. Suggest a mechanism.

Short answer

The mechanism involves deprotonation of the alcohol to form an alkoxide ion, followed by intramolecular nucleophilic substitution to form tetrahydrofuran.

Step-by-step solution

Identify the functional groups and structure

4-Chlorobutan-1-ol has a chlorine atom attached to the fourth carbon and an alcohol group (\(-OH\)) attached to the first carbon of the butane chain. Sodium hydride \((NaH)\) acts as a strong base, capable of deprotonating alcohols.

Deprotonation of the alcohol

When 4-Chlorobutan-1-ol reacts with sodium hydride, the alcohol's hydrogen \( -OH \) is removed by \( NaH \), forming an alkoxide ion and hydrogen gas: \( C_4H_8ClO + NaH \rightarrow C_4H_8ClO^- + Na^+ + H_2 \).

Intramolecular nucleophilic substitution

The alkoxide ion (a strong nucleophile) attacks the electrophilic carbon that is bonded to chlorine in an intramolecular substitution reaction. This process is an \( S_N2\) reaction which involves the alkoxide ion forming a five-membered ring by displacing the chlorine atom, resulting in the formation of tetrahydrofuran.

Formation of tetrahydrofuran

The displacement of the chlorine atom by the alkoxide ion within the same molecule completes the ring formation, leading to tetrahydrofuran: \[ C_4H_8ClO^- \rightarrow C_4H_8O + Cl^- \]. This results in the cyclization and formation of a stable five-membered ether ring known as tetrahydrofuran.

Key concepts

Understanding Organic Chemistry Mechanisms

In organic chemistry, mechanisms describe the step-by-step process that leads to the transformation of reactants into products. These mechanisms follow a specific sequence of events that include bond formation and bond breaking.
Organic reactions often involve intermediates and transitions, where electron movement determines the structure of the resulting compound. For students learning organic chemistry, visualizing these processes is crucial.
Consider the example of turning 4-chlorobutan-1-ol into tetrahydrofuran. This transformation helps illustrate the significance of intramolecular nucleophilic substitution.

• Step 1 involves identifying the functional groups: the chlorine atom and the hydroxyl group ( - OH).
• Step 2 requires using sodium hydride as a strong base to deprotonate the alcohol group, forming an alkoxide ion.
• Finally, recognizing the reaction type, an S_N2 reaction, is integral to understanding the formation of the product, tetrahydrofuran.

Each step emphasizes the role of different functional groups and intermediates, showcasing the dynamic nature of organic reactions.

Delving into S_N2 Reactions

The term "S_N2 reaction" stands for "substitution nucleophilic bimolecular reaction." This type of reaction is characterized by a single concerted step where a nucleophile attacks an electrophile from the opposite side, leading to the displacement of a leaving group. The notable feature of S_N2 reactions is the inversion of stereochemistry at the carbon center. This flip is often likened to an 'umbrella inversion'.
In the context of our example, when the alkoxide ion acts as a nucleophile, its attack on the electrophilic carbon happens in one swift motion. This results in the displacement of the chlorine atom, which serves as the leaving group.

• The initial nucleophile is the alkoxide ion, which carries a negative charge, making it reactive.
• The electrophilic site is the carbon atom bonded to chlorine, which is partially positive.
• The leaving group is the chlorine atom, which exits with the lone pair electrons.

This simultaneous bond forming and breaking during S_N2 reactions make them distinct, offering a clear path from reactants to products.

Tetrahydrofuran Synthesis Explained

Tetrahydrofuran (THF) is an essential compound in organic synthesis and is obtained through a cyclization process involving intramolecular reactions. In this process, a molecule rearranges itself forming a cyclic structure. It's like the molecule folding in on itself to close a loop.
In our example, the formation of THF from 4-chlorobutan-1-ol highlights how the alkoxide ion, acting as a nucleophile, reaches back to attack an internal electrophilic carbon. This results in the creation of a five-membered ring.

• The alkoxide ion's nucleophilic attack forms a new carbon-oxygen bond.
• The ring closure is facilitated by the favorable geometric arrangement of the atoms involved.
• Once the chlorine atom departs as the leaving group, the cycle is complete.

This transformation into THF exemplifies the elegance of intramolecular nucleophilic substitution. These reactions not only efficiently create cyclic ethers but also demonstrate how specific conditions favor cyclization over linear reactions. Understanding these can empower students to synthesize more complex organic compounds.