Organic Chemistry Exam 3

Mass Spectrometry, Sn1, Sn2, E1, E2, Stereochemistry, Organohalides, Oxidation.

Meso Compound

A molecule with stereocenters that is achiral overall due to an internal plane of symmetry, making it superimposable on its mirror image

Optically Active

Describes a compound that rotates plane-polarized light

Dextrorotatory

Rotates plane-polarized light clockwise; designated as (+)

Levorotatory

Rotates plane-polarized light counterclockwise; designated as (−)

Racemic Mixture

A 50

Enantiomers

Two non-superimposable mirror images of a molecule that differ in configuration at every stereocenter

Diastereomers

Stereoisomers that are NOT mirror images; same connectivity but differ in configuration at some but not all stereocenters

Chirality

The property of a molecule that makes it non-superimposable on its mirror image; the molecule lacks an internal plane of symmetry

Chiral Center

An sp3 carbon bonded to four different substituents; also called a stereocenter

R Configuration

When the three substituents ranked 1→2→3 by CIP priority rotate clockwise with the lowest priority group pointing away from you

S Configuration

When the three substituents ranked 1→2→3 by CIP priority, rotate counterclockwise with the lowest priority group pointing away from you

E Configuration

In alkenes, the two higher-priority groups on each double-bond carbon are on opposite sides of the double bond

Z Configuration

In alkenes, the two higher-priority groups on each double-bond carbon are on the same side of the double bond (zame-zide-z)

Cis Configuration

Substituents on the same side of a ring or double bond

Trans Configuration

Substituents on opposite sides of a ring or double bond

Oxidation

A reaction resulting in loss of hydrogen, gain of oxygen, or an increase in carbon-heteroatom bonds; increases the oxidation state of carbon

Reduction

A reaction resulting in gain of hydrogen, loss of oxygen, or a decrease in carbon-heteroatom bonds; decreases the oxidation state of carbon

Oxidation Level

A measure of how oxidized a carbon is based on its bonds to electronegative atoms vs. hydrogen; ranges from alkane (most reduced) to CO₂ (most oxidized)

Organohalide Nomenclature

Halogens are named as prefixes (fluoro-, chloro-, bromo-, iodo-) with a locant number indicating position on the parent chain (e.g., 2-bromopropane)

pKa

The negative log of the acid dissociation constant (−log Ka); the lower the value, the stronger the acid

Acid

A proton donor (Brønsted-Lowry) or electron pair acceptor (Lewis)

Base

A proton acceptor (Brønsted-Lowry) or electron pair donor (Lewis)

Conjugate Base

The species remaining after an acid donates its proton; has one fewer proton than the original acid

Conjugate Acid

The species formed when a base accepts a proton; has one more proton than the original base

SN1

A two-step substitution mechanism that forms a carbocation intermediate, followed by nucleophile attack; favored by 3° substrates, weak nucleophiles, and polar protic solvents; produces a racemic mixture

SN2

A concerted one-step substitution mechanism involving backside attack that causes inversion of configuration; favored by 1° substrates, strong nucleophiles, and polar aprotic solvents

E1

A two-step elimination mechanism forming a carbocation intermediate followed by proton loss to yield an alkene; favored by 3° substrates, weak bases, and polar protic solvents; often competes with SN1

E2

A concerted one-step elimination mechanism where a strong base removes a proton simultaneously as the leaving group departs; requires anti-periplanar geometry; favored by strong/bulky bases

Primary Carbon

A carbon bonded to only one other carbon

Secondary Carbon

A carbon bonded to exactly two other carbons

Tertiary Carbon

A carbon bonded to exactly three other carbons

Quaternary Carbon

A carbon bonded to four other carbons with no hydrogens attached

Solvent

The medium in which a reaction takes place; the choice heavily influences the substitution or elimination pathway taken

Polar Aprotic Solvent

A polar solvent that lacks O−H or N−H bonds; does not hydrogen bond to nucleophiles, leaving them free and reactive; favors SN2; examples

Polar Protic Solvent

A polar solvent that contains O−H or N−H bonds; stabilizes ions through hydrogen bonding and favors carbocation formation; favors SN1/E1; examples

Nucleophile

An electron-pair donor that attacks an electrophilic carbon; can be negatively charged or a neutral species with lone pairs

Rate Law — SN1

Rate = k[substrate]; first order; only substrate concentration affects rate

Rate Law — SN2

Rate = k[substrate][nucleophile]; second order; both substrate and nucleophile concentration affect rate

Rate Law — E1

Rate = k[substrate]; first order; only substrate concentration affects rate

Rate Law — E2

Rate = k[substrate][base]; second order; both substrate and base concentration affect rate

Mass Spectrometry

An analytical technique that ionizes compounds and separates them by mass-to-charge ratio (m/z) to determine molecular weight and identify structural fragments

Parent Ion (M⁺)

The molecular ion peak represents the intact molecule after losing one electron; its m/z value equals the molecular weight of the compound

Base Peak

The tallest and most abundant peak in a spectrum, assigned 100% relative abundance; represents the most stable or most easily formed fragment

Bromine Isotope Pattern

Produces a ~1

Chlorine Isotope Pattern

Produces a ~3

Nitrogen Rule

A molecule containing an odd number of nitrogen atoms will have an odd m/z value for its molecular ion; zero or an even number of nitrogens gives an even m/z

Interpreting Mass Spectra

Identify molecular weight from the molecular ion peak, use mass differences between fragments to deduce structure, spot halogens via isotope patterns, apply the nitrogen rule for nitrogen-containing compounds, and use the most abundant fragment peak to identify the most stable ion

Carbon Heteroatom Bond

A bond between a carbon atom and any atom that is not carbon or hydrogen, such as oxygen, nitrogen, sulfur, or a halogen; more of these bonds on a carbon indicates a higher oxidation state

A Nucleophile

Electron-rich species in a reaction; it will have a negative charge, lone pairs, or both, and it attacks the electrophilic carbon bearing the leaving group rather than departing from it; common examples include OH⁻, CN⁻, I⁻, NH₃, and H₂O

If a Compound Is Dex or Lev

Cannot be determined from structure alone; must be measured experimentally using a polarimeter, which detects the direction plane-polarized light rotates when passed through a sample; there is no reliable correlation between R/S configuration and the direction of rotation

Calculating Oxidation Level

Assign +1 for every bond to a heteroatom (O, N, halogen) counting sigma and pi bonds separately, and −1 for every bond to hydrogen

Zaitsev's Rule

In an elimination reaction, the major product is the more substituted alkene; the proton is removed from the carbon bearing fewer hydrogens, producing the most stable, most substituted double bond