Question

Starting with acetylene as the only source of carbon, together with any inorganic reagents required, devise a method to synthesize 1,1,2,2 -tetrabromoethane.

Short answer

The 1,1,2,2-tetrabromoethane can be synthesized from acetylene via a two-step process. First, 1,2-Dibromoethene is synthesized by reacting acetylene with two equivalents of Bromine in a suitable solvent. Then, 1,2-Dibromoethene is further reacted with two more equivalents of Bromine to finally obtain 1,1,2,2-tetrabromoethane.

Step-by-step solution

Formation of 1,2-Dibromoethene

React acetylene with two equivalents of Bromine (Br2) in the presence of a suitable solvent such as carbon tetrachloride (CCl4). This will result in the addition of Bromine across the triple bond, forming 1,2-dibromoethene.

Formation of 1,1,2,2-tetrabromoethane

Further, react 1,2-dibromoethene with two more equivalents of Bromine, again, in the presence of a solvent. This reaction will put Bromine across the double bond to form 1,1,2,2-tetrabromoethane.

Key concepts

Acetylene Reactions

In organic chemistry, acetylene (C_2H_2), also known as ethyne, is a hydrocarbon and the simplest alkyne, recognized by its characteristic triple bonds. Reactions involving acetylene are pivotal because they can lead to a variety of complex organic structures. Acetylene's triple bond is highly reactive, providing a pathway to synthesize alkenes and alkynes through addition reactions. For instance, the addition of halogens to acetylene initiates a stepwise halogenation; each halogen atom adds across the triple bond, eventually transforming it first into a double bond (alkene) and subsequently into a single bond (alkane) if excess halogen is used. This characteristic makes acetylene a versatile starting material in synthesis problems like the formation of 1,1,2,2-tetrabromoethane.

This high reactivity is due to the significant amount of energy stored within the triple bond, which, when broken, can release energy to form new bonds with incoming atoms or groups, facilitating a wide range of chemical transformations.

Bromination of Alkenes

The bromination of alkenes demonstrates a fundamental type of chemical reaction where bromine (Br_2) specifically and selectively reacts with alkenes to form dibromides. This process is an example of a halogen addition reaction, a staple in alkenes reactivity. For the reaction to proceed, the bromine molecule approaches the double bond and the electrons from the double bond attack one of the bromine atoms, leading to the formation of a cyclic bromonium ion. The other bromine atom then attacks this intermediate ion, resulting in the dibromide.

Such reactions typically occur with anti-addition, where the two bromine atoms attach on opposite sides of the plane of the double bond, due to the nature of the bromonium ion intermediate. This detail is crucial for understanding stereoisomerism in organic products. This reaction principle is vital in the synthesis method to create 1,1,2,2-tetrabromoethane where the intermediate, 1,2-dibromoethene, produced after the first bromination of acetylene, is further brominated.

Organic Synthesis Methods

Organic synthesis involves constructing organic molecules through a series of chemical reactions, starting from simpler substances. It's akin to constructing a building from bricks; each reaction adds a new 'brick' to the growing structure. The method of adding halogens to an unsaturated hydrocarbon is one such 'brick-laying' strategy used in organic synthesis. The synthesis of 1,1,2,2-tetrabromoethane from acetylene showcases the step-by-step methodology often employed.

In this case, the process starts by forming 1,2-dibromoethene from acetylene through halogen addition. This intermediate then undergoes a further addition of bromine to yield the final product, 1,1,2,2-tetrabromoethane. Synthesis methods not only concern the addition of reactants but also the conditions under which reactions are carried out, such as the use of solvents like carbon tetrachloride (CCl_4) to facilitate reactions or control reaction rate and product formation.

Inorganic Reagents in Organic Chemistry

Inorganic reagents play an essential role in facilitating reactions within the realm of organic chemistry. These reagents can include simple diatomic molecules, like bromine (Br_2), as well as a variety of acids, bases, oxidizing agents, and others. They often act as catalysts or reactants in synthesis reactions.

In the synthesis of 1,1,2,2-tetrabromoethane, bromine is used as an inorganic reagent, which reacts with the organic molecules (acetylene and 1,2-dibromoethene) to add bromine atoms to the hydrocarbon skeleton. The use of a solvent like carbon tetrachloride is also noteworthy. It doesn't participate directly in the reaction but provides a medium that can dissolve both the reactants and the bromine, enhancing the efficiency of the halogen addition reactions. Understanding the role of inorganic reagents and solvents is crucial for conducting controlled and successful organic synthesis.