(a) Give the products expected when (+)@glyceraldehyde reacts with HCN.(b) What is the relationship between the products? How might they be separated?(c) Are the products optically active? Explain.

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(a) Identify the functional groups in (+)@glyceraldehyde, which is an aldehyde with a chiral center.

(a) React the aldehyde group with HCN to form a cyanohydrin, adding a CN group to the carbonyl carbon.

(b) Recognize that the reaction creates a new chiral center, resulting in two stereoisomers (diastereomers).

(b) Discuss separation methods such as chromatography or recrystallization, which exploit differences in physical properties.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Nucleophilic Addition

Nucleophilic addition is a fundamental reaction in organic chemistry where a nucleophile attacks an electrophilic carbon atom, leading to the formation of a new bond. In the case of glyceraldehyde reacting with HCN, the cyanide ion (CN-) acts as the nucleophile, attacking the carbonyl carbon of the aldehyde. This reaction results in the formation of a cyanohydrin, which is a key product in this context.

Stereochemistry and Isomerism

Stereochemistry refers to the spatial arrangement of atoms in molecules and how this affects their chemical properties. When glyceraldehyde reacts with HCN, the resulting cyanohydrin can exist as two stereoisomers due to the chiral center formed at the carbon that was originally part of the carbonyl group. Understanding the relationship between these isomers, including their potential to be enantiomers, is crucial for discussing their separation and optical activity.

Optical Activity

Optical activity is the ability of a chiral compound to rotate plane-polarized light, a property that arises from the presence of chiral centers in the molecule. The products of the reaction between glyceraldehyde and HCN are expected to be optically active if they are enantiomers, as each enantiomer will rotate light in opposite directions. This concept is essential for determining the optical properties of the products and their implications in separation techniques.

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Textbook Question

Which of the following compounds are chiral? Draw each compound in its most symmetric conformation, star (*) any asymmetric carbon atoms, and draw any mirror planes. Label any meso compounds. You may use Fischer projections if you prefer.

(e) (R,R)-2,3-dibromobutane

(f)

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Textbook Question

(d) (2R,3S)-2,3-dibromobutane

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Textbook Question

For each pair, give the relationship between the two compounds. Making models will be helpful.

(h)

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Textbook Question

Which of the following pairs of compounds could be separated by recrystallization or distillation?