Question

All of the following molecules are prochiral at the third carbon EXCEPT: A. succinate B. citrate C. \(\alpha\)-ketoglutarate D. isocitrate

Short answer

D. isocitrate

Step-by-step solution

Define prochiral molecule

A prochiral molecule is one that can be converted from an achiral molecule to a chiral product in a single step, typically through the addition or substitution of a single group.

Analyze succinate

Succinate is a four-carbon dicarboxylate that is symmetrical and achiral. It does not have a chiral center at the third carbon.

Analyze citrate

Citrate is a six-carbon tricarboxylate with a hydroxyl group at the third carbon. This carbon can be made chiral by modifying one of the attached groups.

Analyze \(\backslash\alpha\)-ketoglutarate

\(\backslash\alpha\)-ketoglutarate is a five-carbon dicarboxylate with a ketone group at the second carbon. The third carbon can become chiral if the groups attached to it are altered.

Analyze isocitrate

Isocitrate is a six-carbon tricarboxylate with a hydroxyl group at the third carbon. This carbon is already chiral as it has four different groups attached to it, therefore it is not prochiral.

Determine the correct answer

Based on the analysis, the molecule that is not prochiral at the third carbon is isocitrate.

Key concepts

chiral centers

A chiral center, often referred to as a stereocenter, is a carbon atom bonded to four different groups. This unique arrangement allows for molecules to exist in two non-superimposable mirror images, known as enantiomers. You can think of your two hands as an analogy; despite being mirror images, they cannot perfectly align if you stack one on top of the other.

To identify a chiral center, look for:

• Four distinct groups attached to a single carbon atom
• The possibility of creating two mirror-image forms

Chiral centers are crucial in organic chemistry because they play a role in the properties and reactions of molecules, particularly in biological systems, where the shape and orientation of molecules can determine their function and interaction with enzymes and receptors.

organic chemistry

Organic chemistry is the branch of chemistry focused on the study of carbon-containing compounds. It covers a vast array of substances, including hydrocarbons, proteins, DNA, and synthetic materials like plastics. Understanding organic chemistry is essential for fields ranging from medicine to environmental science.

Key concepts in organic chemistry include:

• Functional groups: Specific groups of atoms within molecules that dictate how the molecule reacts
• Stereochemistry: The study of the spatial arrangement of atoms, crucial for understanding chiral molecules and isomers
• Reaction mechanisms: Step-by-step descriptions of how reactions occur, allowing chemists to predict and manipulate chemical behavior

Organic chemistry also encompasses the study of molecular symmetry, which impacts whether a molecule is chiral or prochiral, as discussed in the step-by-step solution of the exercise.

molecular symmetry

Molecular symmetry refers to the balanced arrangement of atoms within a molecule. A molecule's symmetry can determine whether it is chiral or achiral. Achiral molecules possess symmetry, meaning they have a plane or center of symmetry, and are identical to their mirror images.

In prochiral molecules, altering one of the symmetrical groups can make them asymmetric or chiral. To determine molecular symmetry:

• Look for planes of symmetry: Imaginary dividing lines where one side mirrors the other
• Check for centers of symmetry: A point in the molecule where the coordinates of atoms on either side are equivalent
• Identify rotational symmetry: If rotating the molecule yields an indistinguishable structure

Understanding molecular symmetry is essential to classifying molecules as chiral, prochiral, or achiral, which can significantly influence their chemical behavior and interactions.