Problem 112 - A Complete Resource Book in Chemistry for JEE Main

Question

Which alkene is formed from the following yield carbonyl pair? \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{PPh}_{3}+2\)-butanone (a) 1-methyl-5-methane (b) 3 -methyl-3-heptene (c) 4 -methyl-3-heptene (d) 5 -methyl-3-heptene

Step-by-Step Solution

Answer

The alkene formed is 4-methyl-3-heptene (option c).

Step 1: Understand the Reaction

The reaction involves using a Wittig reagent, of the form \( ext{RCH=PPh}_3 \), with a carbonyl compound, typically a ketone or aldehyde, to form an alkene. This process involves replacing the oxygen from the carbonyl compound with the alkylidene group (\( ext{RCH} \)) forming an alkene.

Step 2: Analyze the Wittig Reagent Provided

The Wittig reagent given is \( ext{CH}_3 ext{CH}_2 ext{CH}_2 ext{CH=PPh}_3 \). This indicates that the alkyl group attached to the phosphorus in the Wittig reagent is \( ext{CH}_3 ext{CH}_2 ext{CH}_2 ext{CH}= \).

Step 3: Identify the Carbonyl Compound

The carbonyl compound provided is 2-butanone. It has the structure \( ext{CH}_3 ext{COCH}_2 ext{CH}_3 \). The ketone functional group occurs at the second carbon of butane.

Step 4: Form the Alkene

Perform the Wittig Reaction. The carbonyl oxygen in 2-butanone is replaced by the alkylidene group from the Wittig reagent \( ext{CH}_3 ext{CH}_2 ext{CH}_2 ext{CH}= \). This substitution results in the formation of the alkene \( ext{CH}_3 ext{CH}_2 ext{CH}_2 ext{CH}= ext{C}( ext{CH}_3) ext{CH}_2 ext{CH}_3 \).

Step 5: Determine the Name of the Alkene Formed

The resulting alkene is a heptene because it contains seven carbon atoms in total. The double bond is between the third and fourth carbon atoms, counting from the end with the double bond. The molecular structure is \( 4 ext{-methyl}-3 ext{-heptene} \).

Key Concepts

Alkene Formation

Alkenes are a type of hydrocarbon characterized by at least one carbon-carbon double bond. In organic chemistry, forming alkenes often involves specific reactions, such as the Wittig reaction, which is particularly useful in converting carbonyl groups into alkenes. The Wittig reaction is a popular method for alkene formation due to its ability to form a variety of unfunctionalized alkenes with specific regiochemistry. During this reaction, a triphenylphosphonium ylide reacts with a carbonyl compound to create an alkene and a byproduct of triphenylphosphine oxide.

• In the reaction, the ylide acts as a powerful nucleophile towards the electrophilic carbonyl carbon.
• The reaction mechanism involves a four-membered ring intermediate (often referred to as an oxaphosphetane) which collapses to form the desired alkene.
• This process allows for the formation of either E or Z isomers of the alkene, depending on the starting materials and reaction conditions.

Understanding alkene formation is crucial in synthesizing complex molecules, especially in pharmaceuticals and organic electronics.

Carbonyl Compounds

Carbonyl compounds are organic molecules that contain a carbon-oxygen double bond (\( >\text{C}=\text{O} \)). These play a significant role in organic synthesis and are pivotal in reactions such as the Wittig Reaction.The carbonyl group is highly polar due to the electronegativity difference between carbon and oxygen, making the carbon atom electrophilic. This property makes carbonyl compounds highly reactive in nucleophilic addition reactions, which is the fundamental reaction type in the Wittig process.

• Aldehydes and ketones are two main types of carbonyl compounds.
• 2-butanone is an example where the carbonyl group is part of a ketone, with the carbonyl oxygen readily available for reaction.
• During the Wittig reaction, the carbonyl carbon is attacked by the ylide, leading to the formation of the alkene and replacement of the oxygen by the new alkylidene group.

Benefiting from their reactivity, carbonyl compounds are central to many synthesis pathways, including the preparation of different types of alkenes.

Alkylidene Group

An alkylidene group is introduced into a molecule via a Wittig reaction, acting as the linkage in the transformation of a carbonyl compound into an alkene. The term "alkylidene" refers specifically to a type of carbene where a carbon atom is attached to another carbon atom by a double bond.In the context of the Wittig reaction, the alkylidene group is transferred from the Wittig reagent (ylide) to replace the oxygen atom of the carbonyl group.

• In the exercise example, the alkylidene group transferred is from \( \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}= \).
• This group's formation is crucial for defining the structure and configuration of the final alkene product.
• The resulting alkene, from the exercise, is named based on the chain of carbon atoms including the position of the double bond initiated by the alkylidene substitution.

The role of the alkylidene group is central in organic synthesis, driving the conversion process and influencing the properties of the newly formed alkene.