Question

Assign priorities to the groups in each set. (a) \(-\mathrm{H}-\mathrm{CH}_{3}-\mathrm{OH}-\mathrm{CH}_{2} \mathrm{OH}\) (b) \(-\mathrm{CH}_{2} \mathrm{CH}=\mathrm{CH}_{2}-\mathrm{CH}=\mathrm{CH}_{2}-\mathrm{CH}_{3}-\mathrm{CH}_{2} \mathrm{COOH}\) (c) \(-\mathrm{CH}_{3}-\mathrm{H}-\mathrm{COO}^{-}-\mathrm{NH}_{3}^{+}\) (d) \(-\mathrm{CH}_{3}-\mathrm{CH}_{2} \mathrm{SH}-\mathrm{NH}_{3}{ }^{+}-\mathrm{CHO}\)

Short answer

Question: Assign priorities to the following groups of atoms within each set: a) OH, CH2OH, CH3, H b) CH2COOH, CH=CH2, CH2CH=CH2, CH3 c) COO-, NH3+, CH3, H d) CH2SH, NH3+, CHO, CH3 Answer: a) 1. OH (highest priority) 2. CH2OH 3. CH3 4. H (lowest priority) b) 1. CH2COOH (highest priority) 2. CH=CH2 3. CH2CH=CH2 4. CH3 (lowest priority) c) 1. COO- (highest priority) 2. NH3+ 3. CH3 4. H (lowest priority) d) 1. CH2SH (highest priority) 2. NH3+ 3. CHO 4. CH3 (lowest priority)

Step-by-step solution

Set (a)

First, look up the atomic numbers of the atoms involved: H: 1 C: 6 O: 8 Now compare the atomic numbers to assign priorities: 1. \(\ce{OH}\) (highest priority, contains O with atomic number 8) 2. \(\ce{CH2OH}\) (contains O with atomic number 8) 3. \(\ce{CH3}\) (contains C with atomic number 6) 4. \(-\mathrm{H}\) (lowest priority, contains H with atomic number 1)

Set (b)

First, look up the atomic numbers of the atoms involved: C: 6 O: 8 H: 1 Now compare the atomic numbers to assign priorities: 1. \(\ce{CH2COOH}\) (highest priority, contains O with atomic number 8) 2. \(\ce{CH=CH2}\) (contains C with atomic number 6) 3. \(\ce{CH2CH=CH2}\) (contains C with atomic number 6) 4. \(-\mathrm{CH3}\) (lowest priority, contains C with atomic number 6)

Set (c)

First, look up the atomic numbers of the atoms involved: C: 6 H: 1 O: 8 N: 7 Now compare the atomic numbers to assign priorities: 1. \(\ce{COO^{-}}\) (highest priority, contains O with atomic number 8) 2. \(\ce{NH3^{+}}\) (contains N with atomic number 7) 3. \(-\mathrm{CH3}\) (contains C with atomic number 6) 4. \(-\mathrm{H}\) (lowest priority, contains H with atomic number 1)

Set (d)

First, look up atomic numbers of the atoms involved: C: 6 H: 1 O: 8 N: 7 S: 16 Now compare the atomic numbers to assign priorities: 1. \(\ce{CH2SH}\) (highest priority, contains S with atomic number 16) 2. \(\ce{NH3^{+}}\) (contains N with atomic number 7) 3. \(\ce{CHO}\) (contains O with atomic number 8) 4. \(-\mathrm{CH3}\) (lowest priority, contains C with atomic number 6)

Key concepts

Assigning Stereochemical Priorities

Understanding how to assign stereochemical priorities is crucial when dealing with chiral molecules in organic chemistry. Stereochemistry refers to the three-dimensional arrangement of atoms within molecules. When a molecule contains a chiral center, typically a carbon atom with four different groups attached, it can exist in two enantiomeric forms (mirror images). To distinguish between these forms, we use the Cahn-Ingold-Prelog (CIP) priority rules.

The first step in assigning stereochemical priorities is to look at the atoms directly attached to the chiral center. The atom with the highest atomic number gets the highest priority (number 1). If two atoms are the same, we proceed to look at the next set of atoms along the chain until a difference is found. In cases where double or triple bonds are involved, each double bond is considered as two single bonded atoms, and each triple bond as three single bonded atoms for priority determination purposes.

• Double and triple bonds can affect priority due to the 'ghost atom' approach.
• Tiebreakers can involve comparing atomic masses if isotopes are present.
• Multiple chiral centers in a single molecule require assigning priorities independently for each center.

For example, in an exercise where we assign priority to \(\ce{-H-CH3-OH-CH2OH}\), the hydroxyl group \(\ce{-OH}\) with the higher atomic number oxygen takes precedence over the other groups.

Atomic Number Priority Assignment

The Cahn-Ingold-Prelog priority rules dictate that the atomic number is the most significant factor when determining priority in stereochemistry. The higher the atomic number of the atom bonded to the chiral center, the higher the priority given to that group. Hydrogen, with the lowest atomic number (1), almost always has the lowest priority.

When comparing groups attached to a chiral center, you start with the first atom in each group. For instance:

• A group starting with an oxygen atom will have a higher priority than one starting with a carbon due to oxygen's higher atomic number (8 vs. 6).
• If two groups have the same first atom, we compare the next atom in the group.
• In the absence of heavier atoms, isotopes can determine priority based on atomic mass.

As we saw in the exercise where we assign priority to \(\ce{-CH_{2}SH-NH3^{+}-CHO}\), the thiol group \(\ce{CH2SH}\) gets the highest priority because it contains sulfur with a higher atomic number (16) than the other atoms in the competing groups.

Functional Groups in Organic Chemistry

Functional groups are specific groupings of atoms that impart distinctive chemical properties to the organic molecules they're part of. When determining stereochemical priorities, functional groups with atoms of higher atomic numbers take precedence. Common functional groups include hydroxyl (\(-OH\)), carbonyl (\(C=O\)), amino (\(-NH_2\)), carboxylic acid (\(-COOH\)), and many others.

Each functional group has a characteristic reactivity pattern that is useful in synthesis and reaction mechanisms:

• Alcohols (\(-OH\)) are polar and can form hydrogen bonds.
• Carboxylic acids (\(-COOH\)) are acidic and can donate a proton.
• Amines (\(-NH_2\)) are basic and can accept a proton.

In one of our exercise examples, the carboxylic acid group \(\ce{CH2COOH}\) was assigned the highest priority because it contains oxygen atoms with higher atomic numbers.

Stereochemistry

Stereochemistry is the study of the spatial arrangement of atoms in molecules and its influence on their chemical behavior. It plays a key role in determining the properties and reactivity of molecules, particularly in the pharmaceutical industry, where the 3D shape of a molecule can affect its interaction with biological targets. Key concepts in stereochemistry include chirality, enantiomers, and the importance of stereochemical configuration for molecule function.

Enantiomers are stereoisomers that are non-superimposable mirror images of each other, and they often exhibit significantly different biological activities. For example, one enantiomer of a drug may be therapeutically active, while the other is inactive or may even cause adverse effects.

Stereochemical considerations also influence the outcome of chemical reactions, especially stereoselective and stereospecific reactions where the relative configuration of the reactants dictates the configuration of the product. The assignment of stereochemical priorities using the CIP rules is therefore an essential skill for correctly predicting these outcomes.