5.2 Organic Chemistry and Nomenclature

Organic chemistry is the study of carbon-containing compounds. Carbon’s unique ability to form four stable covalent bonds through hybridization ((sp^3) tetrahedral, (sp^2) trigonal planar, and (sp) linear) allows for an immense diversity of molecular architectures. Understanding organic nomenclature, structures, and key reaction mechanisms is highly tested on the FE Chemical exam.

IUPAC Nomenclature and Functional Group Priority

The International Union of Pure and Applied Chemistry (IUPAC) nomenclature system provides systematic names based on the longest continuous carbon chain containing the principal functional group. Numbering begins from the end closest to the highest-priority functional group. The priority order of common functional groups, from highest to lowest, is:

1. Carboxylic acids (suffix: -oic acid)
2. Esters (suffix: -oate)
3. Amides (suffix: -amide)
4. Aldehydes (suffix: -al)
5. Ketones (suffix: -one)
6. Alcohols (suffix: -ol)
7. Amines (suffix: -amine)
8. Alkenes/Alkynes (suffixes: -ene, -yne)
9. Halogens and Alkyl groups (prefixes: halo-, alkyl-)

For example, a molecule containing both a ketone and an alcohol is named as a ketone with a 'hydroxy' prefix. Alkanes are saturated hydrocarbons containing only single carbon-carbon bonds. Alkenes contain at least one double bond, and alkynes contain a triple bond. Aromatic hydrocarbons contain one or more benzene rings, which exhibit resonance stabilization due to delocalized pi electrons.

Functional Groups and Physical Properties

Functional groups determine the physical and chemical properties of organic molecules.

• Alcohols ((R-\text{OH})) and Carboxylic Acids ((R-\text{COOH})) possess polar (\text{O-H}) bonds, allowing them to form intermolecular hydrogen bonds. Consequently, they exhibit higher boiling points and water solubility compared to hydrocarbons of similar weight.
• Ethers ((R-\text{O}-R')) and Esters ((R-\text{COOR}')) are polar but cannot act as hydrogen bond donors, having intermediate boiling points.
• Aldehydes ((R-\text{CHO})) and Ketones ((R-\text{CO}-R')) contain the carbonyl group ((\text{C=O})) with a strong dipole moment.
• Amines ((R-\text{NH}_2), (R_2\text{NH}), (R_3\text{N})) act as weak bases due to the lone pair of electrons on the nitrogen atom.
• Amides ((R-\text{CONH}_2)) contain a carbonyl linked to a nitrogen; the nitrogen lone pair is delocalized, making them non-basic but highly polar with strong hydrogen bonding.

Isomerism

Isomers are compounds with the same molecular formula but different arrangements of atoms.

• Constitutional (Structural) Isomers differ in connectivity (e.g., butane and isobutane).
• Stereoisomers have the same connectivity but differ in spatial orientation.
  • Geometric Isomers (Cis/Trans or E/Z) occur due to restricted rotation around double bonds. In the cis isomer, high-priority groups are on the same side; in the trans isomer, they are on opposite sides.
  • Enantiomers are non-superimposable mirror images of each other. They occur in molecules containing a chiral center—a carbon bonded to four distinct substituents. Enantiomers share identical physical properties (boiling point, density) but rotate plane-polarized light in opposite directions.
  • Diastereomers are stereoisomers that are not mirror images of one another.

Key Organic Reaction Mechanisms

The FE Chemical exam tests several fundamental organic reaction mechanisms.

Nucleophilic Substitution ((S_N1) vs. (S_N2))

Nucleophilic substitution involves replacement of a leaving group ((X)) with a nucleophile ((Nu^-)).

• (S_N1) (Substitution Nucleophilic Unimolecular): A two-step mechanism. First, the leaving group departs to form a flat carbocation intermediate. Second, the nucleophile attacks the carbocation. The rate-determining step is carbocation formation, making the rate law first-order: (\text{Rate} = k[R-X]). Because the intermediate is planar, nucleophilic attack occurs from either side, resulting in a racemic mixture. (S_N1) is favored by tertiary substrates (due to carbocation stabilization) and polar protic solvents (which stabilize the leaving group and carbocation).
• (S_N2) (Substitution Nucleophilic Bimolecular): A concerted, single-step mechanism. The nucleophile attacks from the backside while the leaving group departs. The rate law is second-order: (\text{Rate} = k[R-X][Nu^-]). This mechanism is highly sensitive to steric hindrance; primary and methyl substrates undergo (S_N2) rapidly, whereas tertiary substrates do not react. The reaction results in inversion of stereochemistry and is favored by polar aprotic solvents.

Elimination ((E1) vs. (E2))

Elimination reactions involve removal of a proton and leaving group to form an alkene. (E1) is a two-step process sharing the carbocation intermediate of (S_N1). (E2) is a concerted one-step process requiring a strong base. Both follow Zaitsev’s rule, which states that the major product is the most highly substituted (and thus most stable) alkene.

Electrophilic Addition

Alkenes undergo addition reactions where a pi bond is broken and two sigma bonds are formed. In the addition of asymmetric reagents (e.g., (\text{HCl}), (\text{H}_2\text{O}) with acid catalyst) to an asymmetric alkene, the reaction follows Markovnikov’s rule: the electrophile ((H^+)) adds to the double-bonded carbon with more hydrogen atoms. This is favored because it generates the more stable carbocation intermediate.

Oxidation and Reduction

The oxidation state of carbon changes when bonded to different heteroatoms. Primary alcohols can be oxidized to aldehydes (using mild oxidants like PCC) and further to carboxylic acids (using strong oxidants like (\text{KMnO}_4)). Secondary alcohols are oxidized to ketones. Tertiary alcohols resist oxidation because they lack a hydrogen atom on the carbinol carbon.

Polymerization

Polymers are macromolecules formed by linking repeating monomer units.

• Addition (Chain-Growth) Polymerization: Monomers containing double bonds (e.g., ethylene) link together without the loss of any atoms (e.g., polyethylene).
• Condensation (Step-Growth) Polymerization: Monomers with two or more functional groups react, releasing a small byproduct like water (e.g., Nylon-6,6 synthesis).