Animated Solution for Chemistry - Organic Chemistry: The major product in the following reaction is

Imagine you are a highly reactive, energy-packed molecule like a Grignard reagent. You are dropped into a flask containing 5-chloro-2-pentanone. As you look around, you see two very tempting targets.

On one side of the molecule, there is a carbonyl group ($C=O$). The oxygen is pulling electron density away from the carbon, leaving it partially positive and highly electrophilic.

On the other side, there is an alkyl chloride ($C-Cl$). The chlorine is also electronegative, creating another partially positive carbon center ripe for a nucleophilic attack.

This is the classic dilemma of chemoselectivity. Which site do you attack first?

Addition vs. Substitution

Grignard reagents, like methyl magnesium bromide (${\text{CH}}_3\text{MgBr}$), are essentially sources of carbanions (${\text{CH}}_3^{-}$). They are incredibly strong nucleophiles and powerful bases.

When faced with a choice between a polar $\pi$-bond (the ketone) and a polar $\sigma$-bond (the alkyl chloride), the Grignard reagent almost always prefers the $\pi$-bond.

Why? Because the $\pi$-bond is weaker, more exposed, and the resulting transition state for nucleophilic addition is generally lower in energy than the transition state for an $S_{N}2$ substitution at an unactivated primary carbon.

Therefore, the methyl group confidently attacks the carbonyl carbon.

As the ${\text{CH}}_3^{-}$ carbanion crashes into the carbonyl carbon, the carbon atom realizes it cannot have five bonds.

To accommodate the incoming methyl group, the weaker $\pi$-bond of the carbonyl group breaks. The two electrons from that bond are pushed entirely onto the electronegative oxygen atom.

This transforms the neutral ketone into a negatively charged alkoxide intermediate.

The structure is now $\text{Cl}-{\text{CH}}_2-{\text{CH}}_2-{\text{CH}}_2-\text{C}({\text{O}}^{-})({\text{CH}}_3{)}_2$.

Intramolecular $S_{N}2$

Now, something magical happens. We have created a molecule that contains both a strong nucleophile (the ${\text{O}}^{-}$) and a good leaving group (the $\text{Cl}$) within its own structure.

The molecule doesn't need to wait for another molecule to bump into it. It can react with itself! This is called an intramolecular reaction.

The flexible carbon chain wiggles and bends until the negatively charged oxygen comes into close proximity with the carbon bearing the chlorine atom.

The oxygen launches a backside attack on that carbon, initiating an intramolecular $S_{N}2$ mechanism. The carbon-chlorine bond breaks, and the chloride ion (${\text{Cl}}^{-}$) is expelled into the solution.

Before we finalize the product, we must ask a crucial question: Is this ring formation actually favorable?

In organic chemistry, the speed and success of an intramolecular reaction depend heavily on the size of the ring being formed.

Let's count the atoms involved in our newly formed ring. We start with the attacking oxygen (atom 1), then the carbon it was attached to (atom 2), then the next two methylene carbons (atoms 3 and 4), and finally the carbon that was attacked (atom 5).

We have formed a 5-membered ring.

Five-membered rings (like cyclopentane or tetrahydrofuran) are incredibly stable. They have very little angle strain and minimal torsional strain. Because of this stability, the formation of 5-membered rings is kinetically very fast and thermodynamically highly favorable.

The reaction is complete. The aqueous acid added in the second step of the reaction conditions is essentially unemployed here; the alkoxide has already reacted with the alkyl chloride before the acid could protonate it. The acid simply neutralizes the expelled chloride and any leftover Grignard reagent.

Our final product is a 5-membered cyclic ether. Specifically, it is a tetrahydrofuran ring.

Looking closely at the carbon that used to be the carbonyl carbon, we see it now holds two methyl groups—one from the original ketone, and one delivered by the Grignard reagent.

The final molecule is 2,2-dimethyltetrahydrofuran.

This perfectly matches option (D). A beautiful sequence of chemoselective addition followed by a rapid intramolecular trap!