CHEM 261: EXAM 1

What is the key difference between stereoisomers and constitutional isomers?

Stereoisomers share the same molecular formula and constitution but possess a different spatial arrangement, whereas constitutional isomers have the same molecular formula but a different structure or connectivity

A stereocenter, or chiral center, is a carbon atom where all four attached groups are _____.

different

Molecules that are non-superimposable on their mirror image are known as _____.

chiral

What is the first step in assigning R/S configuration to a chiral center?

Prioritize the four attached groups using atomic numbers, looking at the first layer of atoms at a time.

In R/S configuration, if the priorities 1 → 2 → 3 trace a clockwise circle, the configuration is _____.

R

In R/S configuration, if the priorities 1 → 2 → 3 trace a counterclockwise circle, the configuration is _____.

S

When assigning R/S configuration, what must be done with the lowest priority group (4) before counting?

It must be arranged to face away from the viewer (on a dash).

What is the 'shortcut' for assigning R/S configuration if the lowest priority group faces towards you?

Determine the direction (clockwise/counterclockwise) and assign the opposite configuration of what you read.

What is the 'double interchange method' used for in assigning R/S configuration?

It is used to reorient the molecule by switching two groups twice when the lowest priority group is not facing away.

Only _____ compounds are capable of rotating plane-polarized light.

chiral

What is the term for a compound that rotates plane-polarized light to the right (clockwise)?

Dextrorotatory (+)

What is the term for a compound that rotates plane-polarized light to the left (counterclockwise)?

Levorotatory (-)

Is there a direct correlation between a molecule's R/S configuration and the direction (+ or -) it rotates plane-polarized light?

No, there is no correlation

Racemic mixture

A 50/50 mixture of enantiomers, resulting in a net optical rotation of 0°.

What is the formula for calculating enantiomeric excess (% ee)?

% ee = ( |observed α| / |α of pure enantiomer| ) × 100%

Stereoisomers that are non-superimposable mirror images of each other are called _____.

enantiomers

How do the physical properties of enantiomers compare?

They have identical physical properties, except for how they rotate plane-polarized light.

Stereoisomers that are NOT mirror images of each other are called _____.

diastereomers

How do the physical properties of diastereomers compare?

They have different physical properties.

What is the formula to determine the maximum number of stereoisomers for a molecule with 'n' chiral centers?

The maximum number of stereoisomers is 2n.

Meso compound

A molecule with two or more chiral centers that is achiral due to a plane of symmetry.

What is the optical rotation of a meso compound?

0°, because it is achiral.

In a Fischer projection, vertical lines represent bonds going _____ the page, and horizontal lines represent bonds coming _____ the page.

into; out of

What is the general strategy for separating a mixture of enantiomers (enantiomer resolution)?

Convert the enantiomers into diastereomers, which have different physical properties and can be separated.

What is a 'resolving agent' used for in enantiomer resolution?

It is a chiral compound that reacts with a racemic mixture to form a pair of diastereomeric salts.

What principle states that electrons fill the lowest energy orbitals first?

The Aufbau principle.

What principle states that each orbital can hold a maximum of two electrons with opposite spins?

The Pauli exclusion principle.

What rule states that electrons are placed in each degenerate orbital singly before they are paired up?

Hund’s Rule

In a bond-line structure, what do the vertices and ends of lines represent?

Carbon Atoms

According to typical bonding patterns in organic chemistry, carbon makes _  bonds, nitrogen makes _ bonds, and oxygen makes __ bonds.

4, 3, 2

A _____ bond is formed by the lateral overlap of s orbitals and is stronger than a pi bond.

sigma (σ)

A _____ bond is formed by the side-by-side alignment of p orbitals.

pi (π)

What is the hybridization and geometry of a carbon atom with four single bonds?

sp3 hybridization with tetrahedral geometry.

What is the hybridization and geometry of a carbon atom with one double bond?

sp2 hybridization with trigonal planar geometry.

What is the hybridization and geometry of a carbon atom with one triple bond or two double bonds?

sp hybridization with linear geometry.

How is the steric number of an atom calculated?

Steric number = (number of sigma bonds) + (number of lone pairs).

What is the formula for calculating formal charge on an atom?

Formal charge = (valence e⁻) - ½(bonding e⁻) - (nonbonding e⁻).

How does resonance stabilize a molecule?

It delocalizes (spreads out) the charge over multiple atoms.

What are two key rules for drawing valid resonance structures?

Do not break a sigma bond, and do not exceed the octet rule for second-row elements.

Define a Brønsted-Lowry acid and base.

An acid is a proton (H+) donor, and a base is a proton acceptor.

Define a Lewis acid and base.

An acid is an electron pair acceptor, and a base is an electron pair donor.

How is pKa related to acid strength?

A stronger acid has a lower pKa.

The stronger the acid, the more _____ its conjugate base.

stable

In an acid-base reaction, equilibrium favors the side with the _____ acid.

weaker (higher pKa)

List the three main intermolecular forces from strongest to weakest.

Hydrogen bonding, dipole-dipole, London dispersion forces.

Constitutional Isomers

Molecules with the same molecular formula but a different structure (connectivity).

In IUPAC nomenclature, what step follows selecting the parent chain?

Identify and name the substituents.

When numbering a parent chain, how do you decide which direction to number from?

Number in the way that gives the substituents the lowest possible numbers.

Which prefix is the only one considered for alphabetization when naming alkanes?

The prefix 'iso'.

How does increasing molecular weight generally affect the boiling point of alkanes?

Increasing molecular weight increases the boiling point.

How does increased branching affect the boiling point of an alkane of the same molecular weight?

Increased branching lowers the boiling point.

What type of diagram is ideal for comparing the relative stability of conformations from single bond rotation?

A Newman projection.

A _____ conformation is the lowest in energy and most stable, while an _____ conformation is the highest in energy and least stable.

staggered; eclipsed

What is the difference between torsional strain and steric strain?

Torsional strain is from electron repulsion in eclipsed bonds, while steric strain is from groups occupying the same space.

In a Newman projection of butane, what is the name for the staggered conformation where the methyl groups are 180° apart?

Anti conformation.

In a Newman projection of butane, what is the name for the staggered conformation where the methyl groups are 60° apart?

Gauche conformation.

What is the term for the energy increase associated with a bond angle that deviates from the ideal 109.5°?

Angle strain.

Which cycloalkane conformation is the most stable due to having no angle or torsional strain?

The chair conformation of cyclohexane.

In a chair conformation, a substituent pointing straight up or down is in the _____ position.

axial

In a chair conformation, a substituent pointing out to the side is in the _____ position.

equatorial

During a chair flip, an axial substituent becomes _____ and an equatorial substituent becomes _____.

equatorial; axial

Why do large substituents on a cyclohexane ring prefer to be in the equatorial position?

To avoid unfavorable 1,3-diaxial interactions (steric strain).

In disubstituted cycloalkanes, the term _____ means the substituents are on the same side of the ring.

cis

In disubstituted cycloalkanes, the term _____ means the substituents are on opposite sides of the ring.

trans

For decalin (two fused cyclohexane rings), which isomer (cis or trans) is rigid and cannot undergo a ring flip?

The trans isomer.