Question

A dextrorotatory alkyl halide is hydrolysed under \(\mathrm{S}_{\mathrm{N}} 2\) conditions to form the corresponding alcohol. The resulting alcohol (a) will be levorotary (b) will be dextrorotatory (c) will be optically inactive due to racemization (d) may be dextro or laevorotatory

Short answer

Answer: Levorotary

Step-by-step solution

Recall the SN2 mechanism

The SN2 (Substitution Nucleophilic Bimolecular) reaction involves the direct attack of a nucleophile (such as water or hydroxide) on a chiral center, leading to the inversion of its stereochemistry. The nucleophile attacks from the opposite side of the leaving group (halide) and the leaving group departs, resulting in a 180-degree flip of the configuration.

Determine the stereochemistry of the initial dextrorotatory alkyl halide

A dextrorotatory compound has a positive optical rotation, meaning it rotates plane-polarized light in a clockwise direction. The initial alkyl halide has a chiral center that results in this dextrorotatory orientation.

Predict the stereochemistry of the resulting alcohol after the SN2 reaction

Since the SN2 reaction involves the attack of the nucleophile from the opposite side of the leaving group and results in the inversion of the stereochemistry, the resulting alcohol will have the opposite optical rotation from the initial alkyl halide. Since the initial alkyl halide was dextrorotatory (positive optical rotation), the resulting alcohol will be levorotatory (negative optical rotation). Answer: (a) The alcohol resulting from the hydrolysis of a dextrorotatory alkyl halide under SN2 conditions will be levorotary.

Key concepts

Optical Activity

Optical activity is a fascinating property that some compounds possess, allowing them to rotate plane-polarized light. When light passes through such a substance, it can rotate either to the right (dextrorotatory) or to the left (levorotary). This rotation is measured using an instrument called a polarimeter, and it indicates the presence of chirality in the molecule.
Not all compounds exhibit optical activity. It is a special characteristic of chiral molecules. Chiral molecules have a non-superimposable mirror image, much like our hands. This property makes them optically active and capable of influencing light in such a unique way.

• **Dextrorotatory:** Rotates light clockwise; indicated by a positive "+" sign.
• **Levorotary:** Rotates light counterclockwise; indicated by a negative "-" sign.

Understanding optical activity is crucial in stereochemistry because it directly relates to how molecules are structured and how they interact with polarized light.

Chiral Center

A chiral center, often referred to as a stereocenter, is an atom within a molecule that has four different substituents attached to it. This unique arrangement leads to two nonsuperimposable mirror images, called enantiomers. Enantiomers can have vastly different properties in chemical reactions and biological systems, making them extremely important in fields like pharmaceuticals and chemistry.
In organic molecules, especially carbon compounds, chiral centers are usually carbon atoms with four distinct groups bonded to them. The presence of a chiral center is what gives a molecule its ability to be optically active. Each enantiomer of a chiral center will rotate polarized light differently:

• If one enantiomer is dextrorotatory, its mirror image will be levorotary.
• Switching any two groups around a chiral center will invert its stereochemistry, leading to a new optical activity.

Recognizing and understanding chiral centers are essential for predicting the behavior of molecules in stereochemical reactions, especially in the context of SN2 reactions where the stereochemistry gets inverted.

Stereochemistry Inversion

Stereochemistry inversion is a key concept in SN2 reactions, where the spatial arrangement of atoms around a chiral center is flipped. This occurs during a bimolecular nucleophilic substitution reaction (SN2), where a nucleophile attacks a chiral carbon from the side opposite to the leaving group.
This inversion is often compared to an umbrella turning inside out in strong wind, representing a complete 180-degree flip. The incoming nucleophile effectively pushes out the leaving group, causing this inversion and altering the molecule's stereochemical configuration.

• **Reaction Mechanism:** The nucleophile's back-side attack leads to stereochemistry inversion.
• **Resulting Configuration:** If starting from a dextrorotatory enantiomer, the product will be levorotary.
• **Implications:** The inversion can change the optical activity from positive to negative or vice versa.

Understanding stereochemistry inversion is crucial for predicting the outcome of reactions involving chiral centers and provides insight into how molecular interactions can change optical activity, as seen when a dextrorotatory alkyl halide is converted into a levorotative alcohol in SN2 reactions.