Organic Chemistry: Structure, Stereochemistry, and Reactivity Study Guide

Conformational Analysis of Alkanes

Newman Projections and Conformations of n-Butane

Newman projections are a way to visualize the spatial arrangement of atoms around a particular carbon-carbon bond. In n-butane, examining the C2—C3 bond reveals different conformations due to rotation about this bond.

• Eclipsed Conformation: The front and back substituents align, leading to torsional strain and higher energy.

• Staggered Conformation: The front and back substituents are offset, minimizing repulsion and resulting in lower energy.

• Anti and Gauche: In staggered conformations, anti (methyl groups 180° apart) is lowest in energy, while gauche (methyl groups 60° apart) is higher due to steric strain.

Example: Drawing the Newman projection for n-butane down the C2—C3 bond shows the anti, gauche, and eclipsed conformations.

Potential Energy Diagram for C2—C3 Bond Rotation in n-Butane

The potential energy diagram plots the energy changes as the C2—C3 bond rotates. Maxima correspond to eclipsed conformations, while minima correspond to staggered conformations.

• Maxima: Eclipsed conformations (highest energy, methyl groups aligned).

• Minima: Staggered conformations (lowest energy, methyl groups anti).

Equation:

$E_{torsional} = V_0 (1 - \cos 3\theta)$

Additional info: The diagram typically shows three maxima and three minima per 360° rotation.

Reaction Mechanisms

Free Radical Halogenation of Alkanes

Halogenation of alkanes with Br2 under heat or light proceeds via a free radical mechanism, resulting in the substitution of a hydrogen atom with a bromine atom.

• Initiation: Homolytic cleavage of Br2 to form two Br• radicals.

• Propagation: Br• abstracts a hydrogen atom from the alkane, forming an alkyl radical, which then reacts with Br2 to form the alkyl bromide and another Br•.

• Termination: Combination of two radicals to form a stable molecule.

Example: Isobutane reacts with Br2 and light to form tert-butyl bromide and HBr.

Organic Reactions and Mechanisms

Common Organic Reactions

Several fundamental organic reactions involve nucleophilic substitution, elimination, and addition mechanisms. Recognizing reactants and products is key to understanding these transformations.

• Acid-base reactions: Transfer of a proton between molecules.

• Nucleophilic substitution: A nucleophile replaces a leaving group on a carbon atom.

• Addition reactions: Atoms are added to a double or triple bond.

Example: CH3CO2H + HCN → CH3CO2H + CN-

Stereochemistry

Assigning R/S Configuration and Fischer Projections

Chirality centers are assigned R or S configuration based on the Cahn-Ingold-Prelog priority rules. Fischer projections are a two-dimensional representation of three-dimensional molecules, useful for carbohydrates and amino acids.

• Assign priorities to substituents based on atomic number.

• Orient the molecule so the lowest priority group is in the back.

• Determine the order (clockwise = R, counterclockwise = S).

Example: Assign R/S to a chiral center and draw the Fischer projection.

Stereoisomeric Relationships: Enantiomers vs. Diastereomers

Stereoisomers are compounds with the same connectivity but different spatial arrangements. Enantiomers are non-superimposable mirror images, while diastereomers are not mirror images.

• Enantiomers: All chiral centers are opposite.

• Diastereomers: At least one, but not all, chiral centers are opposite.

Functional Groups and Properties

Identification of Functional Groups

Functional groups are specific groups of atoms within molecules that determine the chemical reactivity and properties of those molecules.

• Alcohol: Contains an -OH group attached to a saturated carbon.

• Amine: Contains a nitrogen atom bonded to carbon and/or hydrogen atoms.

Water Solubility of Organic Compounds

Water solubility depends on the ability of a compound to form hydrogen bonds with water. Compounds with more polar functional groups or hydrogen bonding capability are generally more soluble.

• Amines and alcohols are more soluble than hydrocarbons.

• Branching can increase solubility by reducing surface area.

Hybridization of Carbocations

The hybridization of a carbocation is determined by the number of groups attached to the positively charged carbon. Most carbocations are sp2 hybridized, resulting in a planar structure.

• sp2 hybridization: Trigonal planar geometry, 120° bond angles.

Chirality and Meso Compounds

Chiral molecules have non-superimposable mirror images, while meso compounds contain chiral centers but are achiral due to an internal plane of symmetry.

• Chiral: No plane of symmetry, optically active.

• Meso: Plane of symmetry, optically inactive.

Electronegativity and Acid-Base Strength

Electronegativity Trends

Electronegativity is the tendency of an atom to attract electrons in a chemical bond. It increases across a period and decreases down a group.

Element	Electronegativity
F	4.0
O	3.5
N	3.0
C	2.5
H	2.1

Relative Strength of Acids and Bases

The strength of acids and bases is measured by their ability to donate or accept protons. Strong acids have weak conjugate bases and vice versa.

Acid	Conjugate Base	pKa
HCl	Cl-	-7
H2SO4	HSO4-	-3
CH3CO2H	CH3CO2-	4.8
H2O	OH-	15.7

The Periodic Table

Organization and Use in Organic Chemistry

The periodic table organizes elements by increasing atomic number and groups elements with similar chemical properties. In organic chemistry, the main group elements (especially C, H, O, N, S, P, and halogens) are most relevant.

• Groups: Vertical columns, similar valence electron configurations.

• Periods: Horizontal rows, increasing atomic number.

Example: Carbon is in group 14, period 2, and forms four covalent bonds.