Question

Outline all steps in the conversion of: (a) ethylbenzene into phenylacetylene; (b) trans-1-phenylpropene into cis-1-phenylpropene.

Short answer

(a) To convert ethylbenzene into phenylacetylene, first perform dehydrogenation by using a catalyst (Fe_2O_3) at 500-600 °C to obtain styrene. Then selectively hydrogenate with Lindlar's catalyst (Pd/CaCO_3) to form phenylacetylene. \( \) (b) To convert trans-1-phenylpropene into cis-1-phenylpropene, first brominate with Br2 in CH2Cl2 to form a vicinal dibromide. Next, eliminate the dihalide using NaNH2 in anhydrous NH3 to obtain 1-phenylpropyne. Finally, selectively hydrogenate with Lindlar's catalyst (Pd/CaCO_3) to form cis-1-phenylpropene.

Step-by-step solution

Convert ethylbenzene to styrene (ethylbenzene dehydrogenation)

First, we need to transform ethylbenzene into styrene by removing two hydrogen atoms from the CH3 side chain. This process is known as dehydrogenation. The reaction requires a transition metal catalyst such as Iron(III) oxide and Potassium oxide mixture at a temperature of 500-600 °C. The balanced chemical reaction can be written as follows: \[ C_6H_5CH_2CH_3 \xrightarrow[500-600\,\text{°C, KO}]{Fe_2O_3} C_6H_5CH=CH_2 + H_2 \]

Convert styrene to phenylacetylene (partial hydrogenation)

Now, we need to transform the styrene into phenylacetylene. This can be done by hydrogenating the double bond in the presence of a poisoned catalyst (Lindlar's catalyst). Lindlar's catalyst consists of Palladium on Calcium carbonate with lead acetate and quinoline as a poison to avoid full hydrogenation to ethylbenzene. This catalyst is very selective and only reduces alkynes into cis-alkenes without further reduction to alkanes. The balanced chemical reaction can be written as follows: \[ C_6H_5CH=CH_2 + H_2 \xrightarrow[\text{Pd/CaCO}_3]{\text{Lindlar's catalyst}} C_6H_5C\equiv CH \] So, our final product is phenylacetylene, as desired. \( \) (b) Conversion of trans-1-phenylpropene to cis-1-phenylpropene

Transform trans-1-phenylpropene to 1-phenylpropyne (bromination and elimination)

First, we need to brominate the trans-1-phenylpropene to form a vicinal dibromide. This can be done using Br2 in the presence of an inert solvent like dichloromethane (CH2Cl2). The balanced chemical reaction can be written as follows: \[ C_6H_5CH=CHCH_3 \xrightarrow[\text{CH}_2\text{Cl}_2]{Br_2} C_6H_5CHBrCHBrCH_3 \] Next, we need to eliminate the vicinal dihalide formed by using a strong base such as NaNH2 in anhydrous ammonia (NH3) solvent. The base will remove two hydrogens in a two-step elimination process, converting into an alkyne, which is 1-phenylpropyne. The balanced chemical reaction can be written as follows: \[ C_6H_5CHBrCHBrCH_3 + 2\, NaNH_2 \xrightarrow[NH_3]{\Delta} C_6H_5C\equiv CCH_3 + 2\, NaBr + 2\, NH_3 \]

Transform 1-phenylpropyne to cis-1-phenylpropene (partial hydrogenation)

Now, we will convert the 1-phenylpropyne into cis-1-phenylpropene using Lindlar’s catalyst. The reaction is similar to the one in first part of the exercise. The balanced chemical reaction can be written as follows: \[ C_6H_5C\equiv CCH_3 + H_2 \xrightarrow[\text{Pd/CaCO}_3]{\text{Lindlar's catalyst}} C_6H_5CH=CHCH_3 \] So, our final product is cis-1-phenylpropene, as desired.

Key concepts

Dehydrogenation

Dehydrogenation is a fundamental reaction in organic chemistry where hydrogen atoms are removed from a molecule, typically forming double bonds in the process. In the case of ethylbenzene, dehydrogenation involves removing hydrogens to create a double bond, transforming it into styrene. This is accomplished using a transition metal catalyst, like Iron(III) oxide and Potassium oxide, under high temperatures (500-600 °C).
This process is crucial in industrial chemistry for the production of important compounds. The removal of hydrogen promotes the formation of more reactive groups like alkenes, such as in this transformation where styrene is produced.

Partial Hydrogenation

Partial hydrogenation selectively reduces alkenes to alkanes or alkynes to alkenes without further over-reduction. This controlled reaction is achieved using Lindlar's catalyst, which consists of palladium deposited on calcium carbonate, modified with lead acetate and quinoline.

For converting styrene to phenylacetylene, the double-bonded carbon must be partially hydrogenated. The specific nature of Lindlar's catalyst prevents the complete reduction to an alkane, leaving us with the desired alkyne, phenylacetylene. This selectivity is crucial in synthetic processes where precise alteration of functional groups is required for the synthesis of complex organic compounds.

Bromination

Bromination is a halogenation process where bromine is added across a carbon-carbon double bond. This is often used to introduce functional groups or facilitate further chemical transformation.

In the case of trans-1-phenylpropene, bromination with Br environ-2 forms a vicinal dibromide. The reaction occurs in an inert solvent like dichloromethane, ensuring the bromine can react efficiently with the alkene. This step is essential because it prepares the molecule for subsequent elimination reactions, leading to the formation of a triple bond in an alkyne.

Alkyne Formation

Alkyne formation often involves an elimination reaction where molecules, like vicinal dibromides, are converted into triple-bond structures. Strong bases, such as NaNH ordered-2 in anhydrous ammonia, are used to facilitate this two-step elimination process.

This method is evident in the conversion of brominated trans-1-phenylpropene to 1-phenylpropyne. Initially, the base removes two hydrogens and two bromides, forming an alkyne group adjacent to the phenyl ring. This process collapses the vicinal arrangement, enabling the construction of a linear, conjugated triple bond necessary for further chemical reactions.