Understanding the Reaction: Ozonolysis of Alkenes

The given image shows a starting material, which is an alkene, undergoing ozonolysis. Ozonolysis is a reaction where an alkene is cleaved by ozone (O $3$ ) to form carbonyl compounds (aldehydes or ketones). To predict the products, we must first identify the structure of the alkene and then determine the cleavage fragments.

Step 1: Analyze the Starting Alkene

The starting compound is 3,4-Dimethylhex-3-ene. Let's break down its structure:

The parent chain is hexene, meaning a 6-carbon chain with a double bond.
The numbers "3,4-dimethyl" indicate methyl groups ( ${CH}_{3} -$ ) are attached to carbons 3 and 4.
The double bond is between carbons 3 and 4.

Therefore, the structure is: CH $3$ -CH $2$ - $C ({CH}_{3}) = C ({CH}_{3})$ -CH $2$ -CH $3$ .

Step 2: Apply Ozonolysis Cleavage

In ozonolysis, the carbon-carbon double bond is broken. Each carbon that was part of the double bond becomes a carbon in a new carbonyl group ( $C = O$ ).

If the carbon had two hydrogen atoms attached, it becomes formaldehyde (HCHO).
If the carbon had one hydrogen and one alkyl group, it becomes an aldehyde (RCHO).
If the carbon had two alkyl groups, it becomes a ketone (RCOR').

Let's apply this to our alkene. The double bond is between two carbons, each of which has one methyl group and one alkyl chain attached.

Left-side carbon of the double bond (C3): Attached groups are CH $3$ -CH $2$ - (ethyl group) and CH $3$ - (methyl group). This carbon will form a ketone: CH $3$ -CH $2$ -C(O)-CH $3$ (2-Pentanone).
Right-side carbon of the double bond (C4): Attached groups are CH $3$ -CH $2$ - (ethyl group) and CH $3$ - (methyl group). This carbon will also form a ketone: CH $3$ -CH $2$ -C(O)-CH $3$ (2-Pentanone).

Since both fragments are identical, the product is two molecules of 2-pentanone.

Step 3: Identify the Correct Product Structure

We need to find the image that represents 2-Pentanone (CH $3$ CH $2$ C(O)CH $3$ ). Its structure is a 5-carbon chain where the carbonyl group is on carbon 2.

The correct option is the one that shows this structure: 2-Pentanone

Final Answer

The product of the ozonolysis of 3,4-dimethylhex-3-ene is two molecules of 2-pentanone.

Related Topics & Formulae

Key Theory: Ozonolysis

Ozonolysis is an organic reaction used to cleave alkenes or alkynes using ozone. For alkenes, the general reaction is:

$R 2 C = C R_{2}' \to O 3, t h e n Z n / H_{2} O \to R 2 C = O + O = C R_{2}'$

The "workup" step (e.g., with Zn) reduces the initial ozonide intermediate to the final carbonyl compounds.

Rules for Predicting Products

Each carbon of the original double bond becomes a carbonyl carbon.
The groups attached to each of these carbons remain unchanged.
The type of carbonyl compound formed depends on the substituents:
- H $2$ C= → HCHO (Formaldehyde)
- RHC= → RCHO (Aldehyde)
- R $2$ C= → R $2$ C=O (Ketone)

Related Reactions

Oxidative Cleavage with KMnO4: Another method to cleave alkenes, but it gives carboxylic acids if the alkene carbon has a hydrogen atom.
Hydroboration-Oxidation: Anti-Markovnikov addition of water to alkenes.
Catalytic Hydrogenation: Addition of H $2$ across the double bond.