Question

In which of the following new carbon-carbon bond is not formed? (1) Wurtz rcaction (2) Fricdel Crafts reaction (3) Passing benzene through hot iron tubes (4) Sabatier Scnder's reaction

Short answer

Passing benzene through hot iron tubes does not form new carbon-carbon bonds.

Step-by-step solution

Understand each type of reaction

Identify what each reaction entails and the type of product formed. This will help in determining whether a new carbon-carbon bond is formed in each case.

Wurtz Reaction

The Wurtz reaction involves the formation of a carbon-carbon bond by reacting two alkyl halides with sodium metal in dry ether.

Friedel-Crafts Reaction

The Friedel-Crafts reaction involves an alkylation or acylation of an aromatic ring, leading to the formation of a new carbon-carbon bond.

Passing Benzene through Hot Iron Tubes

When benzene is passed through hot iron tubes, it typically undergoes dehydrogenation or polymerization, not specifically forming new carbon-carbon bonds in a straightforward manner.

Sabatier-Senderens Reaction

This reaction involves the hydrogenation of alkenes and alkynes to form alkanes in the presence of a catalyst, which usually does not form new carbon-carbon bonds.

Identify the Reaction without New Carbon-Carbon Bond

Compare the reactions and identify the one where a new carbon-carbon bond is not formed.

Key concepts

Wurtz Reaction

The Wurtz reaction is an important method in organic chemistry primarily used to form carbon-carbon bonds. In this reaction, two alkyl halides react with sodium metal in a dry ether solution. Here is the general reaction:
\[ 2 R-X + 2 Na \rightarrow R-R + 2 NaX \]
Where R-X is an alkyl halide. As a result, the product includes a new carbon-carbon bond, forming an alkane. The reaction is effective for creating larger molecules from smaller ones and is particularly useful in synthesizing symmetrical alkanes.
Limitations include:

• Only effective with primary alkyl halides.
• Produces a mixture of products if dissimilar halides are used.

This reaction does not work well for tertiary halides as they tend to undergo elimination rather than substitution.

Friedel-Crafts Reaction

The Friedel-Crafts reaction involves two main types: alkylation and acylation. Both reactions occur via reactions of aromatic rings (such as benzene) with alkyl or acyl halides in the presence of a Lewis acid catalyst, usually aluminum chloride (\text{AlCl}_3).

In Friedel-Crafts alkylation:

\[ \text{C}_6\text{H}_6 + R-X \rightarrow \text{C}_6\text{H}_5-R + \text{HX} \] Here, \text{C}_6\text{H}_6 is benzene, and R-X is an alkyl halide.

In Friedel-Crafts acylation:

\[ \text{C}_6\text{H}_6 + RCOCl \rightarrow \text{C}_6\text{H}_5COR + \text{HCl} \] In this case, RCOCl is an acyl chloride.

These reactions create new carbon-carbon bonds by attaching alkyl or acyl groups to the aromatic ring. The Friedel-Crafts reactions are pivotal in synthetic organic chemistry and serve as building blocks for more complex molecules. However, one limitation is that they often face issues such as polyalkylation.

Benzene Dehydrogenation

When benzene is passed through hot iron tubes, particularly at high temperatures, it can undergo a reaction known as dehydrogenation. In this process, benzene (\text{C}_6\text{H}_6) loses hydrogen atoms to form other structures, such as naphthalene or even graphite if the conditions are extreme. No new carbon-carbon bonds are formed directly in this process.

Dehydrogenation of benzene may be represented as:

\[ \text{C}_6\text{H}_6 \rightarrow \text{C}_6\text{H}_4 + \text{H}_2 \]

This reaction typically results in the loss of hydrogen atoms and possible formation of double bonds or polymerization. Thus, passing benzene through hot iron tubes does not lead to the formation of new carbon-carbon bonds, unlike the Wurtz or Friedel-Crafts reactions. Instead, it either slightly rearranges the carbon structure or forms larger aromatic systems through a condensation process.

Sabatier-Senderens Reaction

The Sabatier-Senderens reaction focuses on the hydrogenation of alkenes or alkynes to convert them into alkanes. This reaction uses finely divided nickel as a catalyst and generally occurs at elevated temperatures and pressures:

\[ RCH=CH_2 + H_2 \rightarrow RCH_2-CH_3 \]

\[ RC≡CH + 2H_2 \rightarrow RCH_2-CH_3 \]

Here, \text{H}_2 represents hydrogen gas, and the process typically converts double bonds (alkenes) or triple bonds (alkynes) into single bonds (alkanes). The key aspect of this reaction is that it adds hydrogen atoms to the parent compound without forming any new carbon-carbon bonds.

Limitations of Sabatier-Senderens include:

• Requirement for high pressure and temperature.
• The need for a suitable catalyst like nickel.

The reaction is instrumental in producing saturated hydrocarbons from their unsaturated counterparts, making it essential in fuel production and various industrial applications.