Organic Chemistry Study Guide: Stereochemistry, Acids/Bases, Reactions, and Mass Spectrometry

Vocabulary flashcards covering core organic chemistry topics including stereochemistry, acid-base theories, carbocation stability, substitution reaction mechanisms (SN1 and SN2), and mass spectrometry fundamentals.

Constitutional Isomers

Molecules that share the same molecular formula but have different atom connectivity (e.g., butane or isobutane).

Stereoisomers

Isomers that have the same molecular formula and connectivity, but differ in their 3D arrangement in space; divided into enantiomers and diastereomers.

Chirality

The property of a molecule that cannot be superimposed on its mirror image.

Chiral Center

An $sp^3$ hybridized carbon atom that is bonded to four different groups.

Stereoisomer Counting Formula

$2^n$, where $n$ is the number of stereocenters; often overestimates stereoisomers if meso compounds exist.

Enantiomers

Non-superimposable mirror images where every stereocenter is opposite; they have identical melting points, boiling points, and density, but opposite optical rotation.

Diastereomers

Stereoisomers that are not mirror images and differ at some (but not all) stereocenters; they display different physical properties and different reactivities.

Meso Compounds

Molecules that contain stereocenters and an internal plane of symmetry, making them achiral with no optical activity.

R/S Configuration

A stereochemical naming system where substituents are prioritized by atomic number and group $4$ is placed in back: a clockwise $1 \rightarrow 2 \rightarrow 3$ sequence designates $R$, while counterclockwise designates $S$.

Brønsted-Lowry Acids and Bases

A classification system where an acid donates an $H^+$ and a base accepts an $H^+$.

Lewis Acids and Bases

A classification system where a Lewis acid accepts an electron pair and a Lewis base donates an electron pair.

Conjugate Acid and Base

A conjugate acid is the base after gaining an $H^+$, and a conjugate base is the acid after losing an $H^+$.

pKa

A metric of acid strength: lower $pK_a$ corresponds to a stronger acid (yielding a weak conjugate base), and higher $pK_a$ corresponds to a weaker acid (yielding a strong conjugate base).

ARIO Rules

Guidelines used to assess acidity by evaluating conjugate base stability: Atom (electronegativity across a row, size down a column), Resonance, Induction, and Orbital ($sp$ hybridization being most stable).

Nucleophile

An electron donor that carries a negative charge, lone pair, or pi bond and is attracted to positive charges (e.g., $OH^-$, $CN^-$, $NH_3$, $H_2O$, $RO^-$).

Electrophile

An electron acceptor, such as carbocations, a carbon bonded to $Br$, or $H^+$.

Carbocation

An $sp^2$ hybridized, trigonal planar intermediate with an empty $p$ orbital; stability follows tertiary > secondary > primary > methyl.

Hydride Shift

A carbocation rearrangement in which a hydrogen atom migrates along with its bonding electrons to form a more stable carbocation.

Methyl Shift

A carbocation rearrangement in which an entire methyl group moves to produce a more stable carbocation.

SN2 Reaction

A concerted substitution mechanism occurring in one step where nucleophilic backside attack and leaving group departure take place simultaneously, causing Walden inversion.

SN2 Rate Law

$\text{Rate} = k[\text{RX}][\text{Nu}]$; a second-order reaction whose rate depends on the concentrations of both the substrate and the nucleophile.

Polar Aprotic Solvents

Solvents such as acetone, DMSO, and DMF that enhance nucleophilicity and are favored in $S_N2$ reactions.

SN1 Reaction

A stepwise substitution mechanism where the leaving group departs first to form a planar carbocation intermediate before nucleophilic attack occurs, resulting in racemization.

SN1 Rate Law

$\text{Rate} = k[\text{RX}]$; a first-order reaction whose rate depends solely on the substrate concentration.

Polar Protic Solvents

Solvents such as water and alcohols that stabilize intermediate ions and are favored in $S_N1$ reactions.

Leaving Group

A departing group whose suitability is determined by base strength: weak bases (such as $I^-$, $Br^-$, $Cl^-$, and $H_2O$) make good leaving groups, whereas strong bases (such as $OH^-$, $OR^-$, and $NH_2^-$) make bad leaving groups.

Molecular Ion (M+)

The peak in mass spectrometry representing the intact ionized molecule; it usually corresponds to the highest significant mass and provides the molecular weight.

Fragmentation

The process in mass spectrometry where bonds break to generate stable carbocations, with the most stable fragments typically producing the largest peaks.