Question

An alkene 'A' on ozonolysis gives a mixture of ethanal and pentan-3one. Write structure and IUPAC name of 'A'.

Short answer

The structure of 'A' is CH₃CH=CHCH₂CH₂CH₃, and the IUPAC name is hex-3-ene.

Step-by-step solution

Understanding Ozonolysis

Ozonolysis is a reaction where alkenes react with ozone (O₃) to form ozonides, which then decompose to give carbonyl compounds such as aldehydes and ketones. In this case, the alkene 'A' upon ozonolysis yields ethanal and pentan-3-one.

Analyzing the Products

The products of ozonolysis are ethanal (CH₃CHO) and pentan-3-one (CH₃COCH₂CH₂CH₃). Ethanal is a 2-carbon aldehyde and pentan-3-one is a 5-carbon ketone, indicating cleavage occurs at the double bond of 'A'.

Deducing the Alkene Structure

For the given products, the original alkene 'A' must have had a double bond between the carbon atoms that are now carbonyl carbons in ethanal and pentan-3-one. Therefore, the structure of 'A' must join ethene and the three-carbon fragment of pentan-3-one before any cleavage.

Identifying the Alkene

The compound can be structured by joining the ends of ethanal and pentan-3-one, resulting in the alkene: CH₃CH=CHCH₂CH₂CH₃. The IUPAC name for this compound with a double bond between the second and third carbon atoms is hex-3-ene.

Key concepts

Alkenes

Alkenes are a vital class of hydrocarbons characterized by the presence of at least one carbon-carbon double bond (\(C=C\)). This double bond is the source of the rich chemistry associated with alkenes. When naming or describing alkenes, it's crucial to emphasize this bond.

• **Structure:** Alkenes have a general formula of \(C_nH_{2n}\), depicting their saturated nature with respect to carbon.
• **Reactivity:** The double bond is highly reactive due to the presence of pi electrons above and below the plane of the atoms.
• **Properties:** They are characterized by being unsaturated, which can take part in addition reactions such as hydrogenation, halogenation, and ozonolysis.

In our exercise, the alkene 'A' undergoes ozonolysis, a reaction that serves to break the double bond, leading to the formation of simpler molecules, which are carbonyl compounds.

Carbonyl Compounds

Carbonyl compounds are organic molecules that contain a carbon-oxygen double bond (\(C=O\)). These include aldehydes and ketones, which are produced when alkenes undergo ozonolysis.

• Aldehydes have the general formula \(RCHO\) where \(R\) can be hydrogen or a carbon group.
• Ketones follow the formula \(RCOR'\), where both \(R\) and \(R'\) are carbon groups.

In the provided exercise, ethanal (an aldehyde) and pentan-3-one (a ketone) are the products of ozonolysis.

• **Ethanal**: Also known as acetaldehyde, this is the simplest aldehyde with the structure \(CH_3CHO\).
• **Pentan-3-one**: A ketone with a five-carbon chain where the carbonyl group is attached to the third carbon, giving it its name.

These compounds result from the breaking of the double bond in alkene 'A,' hinting at the positions of the original double bond.

IUPAC Naming

The systematic naming of organic compounds follows the rules set by the International Union of Pure and Applied Chemistry (IUPAC). This scheme ensures uniformity and uniqueness in the names of compounds.

• **Identify the Longest Carbon Chain:** This determines the root name, such as hex- for six carbons.
• **Number the Carbon Chain:** Assign numbers starting from the end nearest to the double bond or functional group, depending on the priority.
• **Locate Double Bonds and Substituents:** Indicate the position of double bonds, substituents, and functional groups using numbers.

For instance, the compound derived from our exercise, with six carbons and a double bond between the third and fourth carbons, is hex-3-ene. This illustrates the importance of IUPAC naming in accurately describing and identifying molecules in organic chemistry.