Question

Starting with acetylene as the only source of carbon, together with any inorganic reagents required, devise a method to synthesize acetaldehyde.

Short answer

Acetylene can be converted to acetaldehyde by first reacting it with water in the presence of a base such as NaOH to achieve tautomerization. The reaction gives the enol form of acetaldehyde. In the next step, this enol form is converted into the keto form i.e., acetaldehyde itself. This is achieved by a process called Keto-Enol Tautomerization, catalyzed by addition of a small amount of acid like HCl.

Step-by-step solution

Tautomerization of Acetylene

Firstly, acetylene (C2H2) is tautomerized to acetaldehyde (CH3CHO). The tautomerization process involves reaction with water in the presence of a base. Take acetylene and react it with water in the presence of a base like NaOH. The reaction will give the enol form of acetaldehyde.

Keto-Enol Tautomerization

In the second step, the enol form of acetaldehyde formed in step 1 is converted into the keto form of acetaldehyde i.e., into acetaldehyde itself. This is done by a process called Keto-Enol Tautomerization. This process is catalyzed by adding a small amount of acid such as HCl. By the end of this step, the base-acidity sequence is completed resulting in the acetaldehyde molecule.

Key concepts

Acetylene Tautomerization

The transformation of acetylene to acetaldehyde begins with a crucial process known as acetylene tautomerization. In organic chemistry, tautomerization is a chemical reaction that results in the relocation of a hydrogen atom accompanied by a shift of a double bond within a molecule. Acetylene (C2H2), a simple alkyne with a triple bond, can undergo this type of reaction to become an intermediate aldehyde. The specific conditions for acetylene tautomerization involve reacting acetylene with water in the presence of a suitable base, like sodium hydroxide (NaOH).

This reaction under the right conditions produces an enol, which is a compound with a double bond (alkene) and an alcohol (-OH) group. Although enols are usually less stable than their carbonyl counterparts, they serve as pivotal intermediates in producing stable carbonyl compounds such as aldehydes and ketones. The enol version of acetaldehyde contains a double bond between the carbon atoms and a hydroxyl group attached, preparing it for the next critical step in the synthesis process.

Keto-Enol Tautomerization

Once the enol form of acetaldehyde is obtained through acetylene tautomerization, it must be converted to the more stable keto form, which is the desired acetaldehyde. This step is a classic example of keto-enol tautomerization, an equilibrium process where enols (compounds with C=C and C-OH groups) rapidly convert to their keto analogs (compounds with C=O groups).

The reaction typically requires a catalyst to proceed efficiently, and in this case, a small amount of an acid, like hydrochloric acid (HCl), is utilized. After the addition of the acid, the hydroxyl group of the enol is protonated, and subsequently, a shift in the double bond occurs along with the loss of a water molecule to form the carbonyl group seen in the aldehyde. The final product of this reaction is acetaldehyde (CH3CHO), a substance with a wide range of applications, especially as a precursor in the synthesis of other organic compounds.

Organic Synthesis Reactions

The journey from acetylene to acetaldehyde exemplifies the type of organic synthesis reactions integral to producing complex molecules from simpler precursors. Organic synthesis involves building up complex organic molecules through a series of chemical reactions. These reactions often include making and breaking bonds, adding functional groups, and strategically manipulating molecular structures to achieve a target compound.

Organic synthesis can be performed through various reaction mechanisms and requires a thorough understanding of chemical reactivity and selectivity. The synthesis of acetaldehyde from acetylene encompasses a two-step process involving tautomerization and highlights how a sequential approach can be used to transform basic building blocks into more complicated organic entities. By mastering organic synthesis, chemists can craft everything from pharmaceuticals to polymers, making it a cornerstone of chemical manufacturing and research.

Inorganic Reagents in Organic Synthesis

In the context of synthesizing organic compounds, inorganic reagents often play an essential role as catalysts or reactants. In the conversion of acetylene to acetaldehyde, for instance, both sodium hydroxide (NaOH) and hydrochloric acid (HCl) are inorganic substances that are crucial to the reaction process.

Sodium hydroxide acts as a base that helps activate the acetylene molecule for reaction with water, while hydrochloric acid serves as a catalyst that facilitates the keto-enol tautomerization. These inorganic reagents are indispensable because they affect the rate and direction of the chemical reactions, allowing chemists to carry out transformations with precision and control. Incorporating inorganic reagents in organic synthesis is a common strategy to promote and steer chemical reactions toward the formation of desired organic products efficiently.