Question

Propylene oxide can be converted into propylene glycol by the action of either dilute acid or dilute base. When optically active propylene oxide is used, the glycol obtained from acidic hydrolysis has a rotation opposite to that obtained from alkaline hydrolysis. What is the most likely interpretation of these facts?

Short answer

The most likely interpretation of the observed differences in the optical rotation of propylene glycol obtained from acidic and alkaline hydrolysis of optically active propylene oxide is that these reactions proceed via different mechanisms. Acidic hydrolysis results in retention of stereochemistry, leading to the same rotation as the starting propylene oxide, while alkaline hydrolysis causes an inversion of stereochemistry, leading to an opposite rotation of the glycol compared to the starting propylene oxide.

Step-by-step solution

Understand the propylene oxide structure and its optical activity

Propylene oxide is an epoxide with a three-membered ring containing an oxygen, and it has a chiral carbon with four different groups attached to it. This chiral carbon causes the molecule to be optically active, having one enantiomer that rotates plane-polarized light to the left (S-enantiomer) and another that rotates it to the right (R-enantiomer).

Write the reaction of propylene oxide with dilute acid and dilute base

When propylene oxide reacts with dilute acid (H+) or dilute base (OH-), it undergoes ring-opening, and the oxygen in the epoxide group is protonated (acid) or deprotonated (base) to form propylene glycol. The reactions are as follows: Acidic hydrolysis: R- or S-Propylene oxide + H+ -> H-R- or H-S-propylene glycol Alkaline hydrolysis: R- or S-Propylene oxide + OH- -> HO-R- or HO-S-propylene glycol

Understand the stereochemistry of the reactions

Propylene oxide has a chiral center, and during the ring-opening, the attack of the nucleophile (H+ or OH-) can occur from either side of the chiral carbon. The attack from the same side as the leaving group will lead to the retention of the stereochemistry (retention product), whereas the attack from the opposite side will lead to the inversion of the stereochemistry (inversion product).

Analyze the rotation of products in acidic and alkaline hydrolysis

In acidic hydrolysis, the nucleophile (H+) usually attacks from the same side as the leaving group, leading to the retention of the original stereochemistry of propylene oxide. Therefore, the rotation of the glycol obtained in acidic hydrolysis is the same as the starting propylene oxide. In alkaline hydrolysis, the nucleophile (OH-) typically attacks from the opposite side of the leaving group, leading to the inversion of the original stereochemistry of propylene oxide. Therefore, the rotation of the glycol obtained in alkaline hydrolysis is opposite to the starting propylene oxide.

Provide the most likely interpretation of the given facts

Based on the analysis of the stereochemistry of the reactions and the optical rotation of the products, the most likely interpretation of the observed differences in the optical rotation of propylene glycol obtained from acidic and alkaline hydrolysis is that the reactions proceed via different mechanisms. In acidic hydrolysis, the reaction occurs with retention of stereochemistry, while in alkaline hydrolysis, the reaction occurs with inversion of stereochemistry. Hence, the glycol obtained from acidic hydrolysis has the same rotation as the starting propylene oxide, whereas the glycol obtained from alkaline hydrolysis has the opposite rotation.

Key concepts

Optical Activity in Organic Chemistry

Optical activity is a fascinating phenomenon in organic chemistry that involves the rotation of plane-polarized light by chiral molecules. A substance is considered chiral if it cannot be superimposed on its mirror image, much like left and right hands. These mirror images are referred to as enantiomers.

Each enantiomer of a chiral compound can rotate plane-polarized light, but in opposite directions. One will rotate light clockwise, termed dextrorotatory (+), while the other rotates it counterclockwise, or levorotatory (-). The extent of this rotation is measured using a polarimeter and is quantified by a specific property called optical rotation. Understanding optical activity is essential when studying reactions that produce chiral compounds, as the reaction conditions can affect the stereochemistry of the product, leading to different optical rotations.

Propylene Oxide Structure

Propylene oxide is an organic epoxide, characterized by its three-membered cyclic ether structure. This molecule contains a chiral center — a carbon atom bonded to four distinct groups, allowing for two possible enantiomers. In the context of propylene oxide, these enantiomers can rotate plane-polarized light either to the left or to the right.

The presence of this chiral center and the resulting optical activity make propylene oxide's structure a key player in the stereochemistry of its reactions, such as hydrolysis. When understanding the outcomes of reactions involving propylene oxide, knowing its 3D structure and the spatial arrangement of its substituents is crucial.

Acidic vs Alkaline Hydrolysis

Hydrolysis of epoxides like propylene oxide can occur under both acidic and alkaline conditions, with each leading to different products due to distinct mechanisms of ring-opening. During acidic hydrolysis, the reaction involves a proton (H+) as a nucleophile. In this scenario, the nucleophile commonly adds to the same face of the chiral carbon from which the leaving group departs, leading to retention of configuration. Thus, if you start with a chiral, optically active propylene oxide, the product under acidic conditions usually retains the same optical activity.

Conversely, in alkaline hydrolysis, a hydroxide ion (OH-) serves as the nucleophile. Here, the attack typically happens from the backside, causing a flip in the configuration known as an inversion of stereochemistry. This ring-opening mechanism ultimately results in a product with optical activity opposite to that of the starting material. Recognizing the differences between acidic and alkaline hydrolysis is essential when predicting the outcome of a reaction involving chiral substrates.

Ring-Opening Mechanisms

The ring-opening mechanisms of cyclic compounds like propylene oxide are influenced by the conditions under which the reaction occurs. The hydrolysis under acidic conditions tends to proceed via an SN1-like pathway, where the oxygen in the ring is first protonated, making it a better leaving group. This leads to a more stable carbocation intermediate, which can then be attacked by water, usually resulting in retention of configuration.

In contrast, the alkaline hydrolysis often follows an SN2-like pathway. The hydroxide ion attacks the electrophilic carbon from the opposite side to the leaving group, pushing the ring to open and leading to an inversion of the configuration at the chiral center. Understanding these ring-opening mechanisms is critical in predicting the outcome of the stereochemistry in the products formed from such reactions.