Choose the correct set of reagents for the following conversion trans $\left( Ph - CH = CH - CH _3\right) \rightarrow \operatorname{cis}\left( Ph - CH = CH - CH _3\right)$
Choose the correct set of reagents for the following conversion trans $\left( Ph - CH = CH - CH _3\right) \rightarrow \operatorname{cis}\left( Ph - CH = CH - CH _3\right)$
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- $Br _2$, alc $\cdot KOH , NaNH _2, Na \left( Liq NH _3\right)$
- $Br _2$, alc $\cdot KOH , NaNH _2, H _2$ Lindlar Catalyst
- $Br _2$, aq $\cdot KOH , NaNH _2, H _2$ Lindlar Catalyst
- $Br _2, aq \cdot KOH , NaNH _2, Na \left( Liq NH _3\right)$
The Correct Option is B
Approach Solution - 1
So, the correct option is (b) Br2, alc ⋅KOH, NaNH2, H2 Lindlar Catalyst
Approach Solution -2
The given conversion involves several steps in organic chemistry, starting with the use of bromine (\( \text{Br}_2 \)) in the presence of aqueous KOH. This reaction leads to the formation of a vicinal dibromide. A vicinal dibromide is a compound where two bromine atoms are attached to adjacent carbon atoms in a molecule.
The next step involves treatment with sodium amide (\( \text{NaNH}_2 \)) in liquid ammonia. Sodium amide is a strong base and induces an elimination reaction, removing the two bromine atoms and forming a triple bond (alkyne). This is followed by hydrogenation using Lindlar's catalyst, which selectively hydrogenates alkynes to cis-alkenes (the double bond is formed in a "cis" configuration, meaning both substituents are on the same side of the double bond).
Thus, the correct set of reagents for this conversion is:
\[ \text{Br}_2, \, \text{aq KOH}, \, \text{NaNH}_2, \, \text{H}_2 \, \text{Lindlar Catalyst} \]
These reagents are crucial for carrying out this transformation in an efficient and controlled manner. The combination of bromine, aqueous KOH, and sodium amide in liquid ammonia is commonly used for the synthesis of alkynes, while Lindlar's catalyst is specifically used to achieve selective hydrogenation to form cis-alkenes. Understanding the role of each reagent and the conditions under which they operate is key to mastering this reaction.
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Concepts Used:
Alkenes
Alkenes
In organic chemistry, an alkene is a hydrocarbon containing a carbon-carbon double bond.[1]
Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.
Read More: Ozonolysis
IUPAC Nomenclature of Alkenes
- An alkene or cycloalkene is indicated by the suffix (ending) ene.
- For the root name, the longest chain must include both carbon atoms of the double bond.
- The root chain must be numbered beginning at the end closest to a double bond carbon atom. If the double bond is in the chain's centre, the nearest substituent rule is used to determine where the numbering begins.
- If the compound contains more than one double bond, it is named with a diene, triene, or equivalent prefix indicating the number of double bonds, and each double bond is assigned a locator number.
Read More: Unsaturated Hydrocarbon