Problem 102 - A Complete Resource Book in Chemistry for JEE Main

Question

One mole of an unsaturated hydrocarbon on ozonolysis gives one mole each of \(\mathrm{CH}_{3} \mathrm{CHO}, \mathrm{HCHO}\) and OHC.CHO. The hydrocarbon is (a) \(\mathrm{CH}_{3} \cdot \mathrm{CH}_{2} \mathrm{C}=\mathrm{C} . \mathrm{CH}_{3}\) (b) \(\mathrm{CH}=\mathrm{C} \cdot \mathrm{CH}_{2} \cdot \mathrm{CH}_{2} \mathrm{CH}_{3}\) (c) \(\mathrm{CH}_{3} . \mathrm{CH}=\mathrm{CH} . \mathrm{CH}=\mathrm{CH}_{2}\) (d) \(\mathrm{CH}_{2}=\mathrm{CH} . \mathrm{CH}_{2} \cdot \mathrm{CH}=\mathrm{CH}_{2}\)

Step-by-Step Solution

Answer

The correct answer is (c) CH_3 CH=CHcdotCH=CH_2.

Step 1: Understand Ozonolysis

Ozonolysis is a reaction where unsaturated hydrocarbons break down at the double bonds when treated with ozone, usually forming two carbonyl compounds (aldehydes or ketones). The products help us identify the positions of the double bonds in the original hydrocarbon.

Step 2: Analyze Given Products

The products given are acetaldehyde (CH_3CHO), formaldehyde (HCHO), and glyoxal (OHC.CHO). These indicate that the original hydrocarbon had double bonds at positions that formed these aldehydes upon cleavage.

Step 3: Determine Product Formation

CH_3CHO comes from a break between a methyl group and a remaining alkene, HCHO from a terminal alkene, and OHC.CHO indicates adjacent carbons both sharing a bond which forms aldehyde groups upon cleavage.

Step 4: Evaluate Each Option

Examine each option: (a) CH_3 dot CH_2C=CCH_3 doesn't explain glyoxal. (b) CH=CHcdotCH_2cdotCH_2CH_3 breaks into products that do not match. (c) CH_3CH=CHcdot CH=CH_2 explains the formation of all three aldehydes. (d) CH_2=CHcdot CH_2cdot CH=CH_2 doesn't form the aldehydes as needed.

Step 5: Confirm the Correct Answer

Upon comparing the structure of option (c) CH_3CH=CHcdotCH=CH_2, the ozonolysis can clearly lead to the formation of all given aldehydes: CH_3CHO, HCHO, and OHC.CHO. Therefore, option (c) is the correct structure of the hydrocarbon.

Key Concepts

Unsaturated Hydrocarbons

Unsaturated hydrocarbons are fascinating chemical molecules that contain at least one double or triple bond between carbon atoms. These bonds offer unique chemical properties and reactivity. Depending on the type of unsaturation, these compounds can be categorized into:

• Alkenes: with one or more double bonds.
• Alkynes: containing one or more triple bonds.

The presence of these bonds makes unsaturated hydrocarbons more reactive in chemical processes like addition reactions and, notably, ozonolysis. The ability to break these bonds during chemical reactions allows us to deduce structural information about the molecule. This capacity is particularly significant in organic synthesis and analysis, where the unsaturated hydrocarbon can be methodically broken down to study its composition.

Ozonolysis Reaction Mechanism

Ozonolysis is a vital tool in organic chemistry, predominantly used to study the structure of unsaturated hydrocarbons. During this reaction, the unsaturated compound reacts with ozone ( O_3 ) to form ozonides, which typically undergo further reactions to form more stable carbonyl compounds.

• The initial step involves the addition of O_3 across the carbon-carbon double bond, creating an unstable ozonide.
• This intermediate is usually treated with a reducing agent or decomposed in another step to yield aldehydes or ketones.

This mechanism allows chemists to pinpoint the locations of double bonds. By analyzing the reaction products, like aldehydes, chemists can determine the precise structure of the original unsaturated compound.

Aldehyde Formation

In the ozonolysis of unsaturated hydrocarbons, one of the significant outcomes is the formation of aldehydes. These are organic compounds characterized by the presence of a carbonyl group ( C=O ) bonded to a hydrogen atom. This functional group notably impacts their chemical characteristics.

• Aldehydes are generally formed when a hydrocarbon chain ends in an alkene and cleaves during ozonolysis.
• In our exercise, the formation of formaldehyde ( HCHO ) indicates a terminal alkene, and other aldehyde products like acetaldehyde ( CH_3CHO ) are indicative of the specific positioning of double bonds.

This information proves instrumental in determining the precursor hydrocarbon's structure because knowing which aldehydes formed can reveal where the associated double bonds are located.

Double Bond Cleavage

One of the key highlights of ozonolysis in unsaturated hydrocarbons is the cleavage of double bonds. This reaction is invaluable due to the information it uncovers about the hydrocarbon chain.

• Cleavage occurs at the double bond, resulting in two separate molecules, each with a carbonyl group where the double bond used to be.
• For example, in our case, the cleavage of the hydrocarbon results in the creation of acetaldehyde and glyoxal, among others, showing that it had strategically placed double bonds.

Understanding where these cleavages take place enables chemists to reconstruct the original framework of the compound. This makes double bond cleavage an essential reaction for finding hidden structural details of unsaturated hydrocarbons.