Organic Chemistry

IUPAC Naming Rules:

1. Identify longest C chain containing the principal functional group (parent)
2. Number from end giving lowest locant to principal functional group
3. Identify substituents (prefixes) — alphabetize
4. Combine: locants–prefixes–parent–suffix
5. Use Greek prefixes (di, tri) for multiple substituents — these don't affect alphabetization

Common Functional Groups (in order of priority for suffix):

Examples:

• $CH_3CH_2CH_2OH$ → propan-1-ol
• $(CH_3)_2CHCH_2CH_3$ → 2-methylbutane
• $CH_3COOH$ → ethanoic acid
• $CH_3CH=CH_2$ → propene

Isomerism: Compounds with same molecular formula but different structure/properties.

Types of Isomerism:

Structural (Constitutional) Isomerism:

Stereoisomerism: Same connectivity; differ in 3D arrangement.

Geometrical (cis-trans / E-Z): Around C=C or ring restricted rotation.

• cis: same group on same side
• trans: groups on opposite sides
• E/Z system: Based on CIP priority — Z (zusammen, same side) and E (entgegen, opposite)

Optical Isomerism: Mirror image molecules (enantiomers) that are non-superimposable. Requires chiral center — usually a C with 4 different groups.

• Enantiomers: Non-superimposable mirror images; identical properties except rotate polarized light in opposite directions (one (+), other (–))
• Diastereomers: Stereoisomers not mirror images; different properties
• Racemic mixture: $50:50$ of enantiomers; no optical activity (rotations cancel)
• Meso compound: Has chiral centers but is superimposable on mirror image due to internal symmetry; optically inactive

For $n$ chiral centers without symmetry: $2^n$ optical isomers; with internal symmetry, fewer.

R/S Configuration (CIP): Assign priority by atomic number; for the lowest-priority pointing away: clockwise = R, counterclockwise = S.

Electron displacement effects explain charge distribution in molecules, stability of intermediates, acidity/basicity, reactivity, etc.

1. Inductive Effect (I-Effect):

• Permanent displacement of electrons along $\sigma$ bonds due to electronegativity difference
• Transmitted through sigma bonds with diminishing intensity
• Distance-dependent: decreases rapidly beyond 3 bonds

Types:

• -I (electron-withdrawing): Group pulls electrons toward itself. e.g., $-NO_2 > -CN > -F > -Cl > -Br > -I > -COOH > -OH > -NH_2$
• +I (electron-donating): Group pushes electrons away. e.g., alkyl groups: $-CH_3, -C_2H_5$, etc. Order: $tert > sec > primary > methyl$ (more substituted → more +I).

Applications:

• Acidity of carboxylic acids:
• Cl₃C-COOH > Cl₂CH-COOH > ClCH₂-COOH > CH₃COOH (more -I → more acidic)
• Basicity of amines (gas phase): $(CH_3)_3N > (CH_3)_2NH > CH_3NH_2 > NH_3$ (more +I → more basic)

2. Resonance Effect (M-Effect or R-Effect):

• Permanent delocalization of $\pi$ electrons or lone pairs
• Only operates through conjugated $\pi$ system
• More effective than I-effect

Types:

• +R (electron-donating to ring): $-OH, -OR, -NH_2, -NR_2, -X$ (donate lone pairs)
• -R (electron-withdrawing from ring): $-NO_2, -CN, -CHO, -COOH, -COR, -SO_3H$

Resonance Structures: Different Lewis structures contributing to actual structure; differ in placement of electrons (not atoms).

3. Hyperconjugation:

• Delocalization of $\sigma$ C-H electrons into adjacent $\pi$ system or empty p-orbital
• Also called "no-bond resonance"

Stability of carbocations $\propto$ number of $\alpha$-H atoms (more $\alpha$-H → more hyperconjugation → more stable):

• $(CH_3)_3C^+$ (9 α-H) > $(CH_3)_2CH^+$ (6 α-H) > $CH_3CH_2^+$ (3 α-H) > $CH_3^+$ (0 α-H)

Stability of alkenes: More alkyl substituents at C=C → more stable (hyperconjugation): $$(CH_3)_2C=C(CH_3)_2 > (CH_3)_2C=CH_2 > CH_3CH=CHCH_3 > CH_3CH=CH_2 > CH_2=CH_2$$

4. Electromeric Effect (E-effect):

• Temporary; only when an attacking reagent approaches
• $\pi$ electrons transferred entirely to one atom
• +E: electrons toward attacking reagent; -E: electrons away

Reactive Intermediates are short-lived, high-energy species formed during reactions.

1. Carbocations (Carbenium ions):

• C with $+$ charge, 6 valence electrons (sextet)
• $sp^2$ hybridized, planar, vacant p-orbital
• Electrophilic

Stability Order: $(CH_3)_3C^+ > (CH_3)_2CH^+ > CH_3CH_2^+ > CH_3^+$ (more $\alpha$-H → more hyperconjugation, more +I from alkyl)

Allylic and Benzylic carbocations are stabilized by resonance, often more stable than tertiary: $(C_6H_5)CH_2^+ \approx CH_2=CHCH_2^+ \approx$ very stable

Tertiary > secondary > primary > methyl is the basic order; resonance can override this.

Rearrangement: Carbocations can rearrange to more stable forms via:

• 1,2-Hydride shift
• 1,2-Methyl shift (or other alkyl shift)

2. Carbanions:

• C with $-$ charge, 8 valence electrons (lone pair on C)
• $sp^3$ hybridized, pyramidal (like NH₃)
• Nucleophilic

Stability Order: $CH_3^- > CH_3CH_2^- > (CH_3)_2CH^- > (CH_3)_3C^-$ (alkyl groups destabilize negative charge via +I)

Stabilized carbanions (by -R groups via resonance):

• Benzylic: $C_6H_5CH_2^-$ (more stable than methyl due to resonance)
• Allylic: $CH_2=CHCH_2^-$
• Especially: $\alpha$-carbon of carbonyl (enolate ions), nitro compounds

Hybridization Effect: More s-character → more stable carbanion. So: $sp$ (alkyne, acetylide $RC\equiv C^-$) > $sp^2$ > $sp^3$.

3. Free Radicals:

• C with 7 valence electrons (unpaired electron)
• $sp^2$ hybridized, planar (some texts say pyramidal)
• Neutral; very reactive

Stability Order: Similar to carbocations: $3° > 2° > 1° > methyl$

• Allylic, benzylic > 3° > 2° > 1° > methyl (resonance stabilization)

Generation: Photolysis ($h\nu$), peroxides ($RO-OR \to 2RO\cdot$), thermal, halogenation.

Detection: EPR (electron paramagnetic resonance).

4. Carbenes:

• C with 6 valence electrons; two non-bonding electrons
• Singlet carbene: paired electrons in one orbital
• Triplet carbene: two unpaired electrons (more stable for simple carbene)

Generation: $CHCl_3 + KOH \to :CCl_2 + KCl + H_2O$ (dichlorocarbene) Reaction: Insert into C-H bonds; add to C=C (cyclopropanation).

5. Nitrenes:

• N analog of carbenes (sextet on N)

Electrophiles (E⁺): Electron-poor; "electron lovers". Accept electron pair. Examples: $H^+, NO_2^+, Cl^+, Br^+, R^+, AlCl_3$ (Lewis acid), $SO_3$, carbonyl C.

Nucleophiles (Nu⁻): Electron-rich; "nucleus lovers". Donate electron pair.

• Negatively charged: $OH^-, CN^-, Cl^-, RO^-, RS^-, NH_2^-$
• Neutral with lone pair: $H_2O, NH_3, ROH$
• Strong nucleophiles: $OH^-, RO^-, RS^-, NH_2^-, CN^-, RC \equiv C^-$
• Weak nucleophiles: $H_2O, ROH, Cl^-, HSO_4^-$

Reaction Types in Organic Chemistry:

Subdivisions:

• Nucleophilic (N): nucleophile attacks; in saturated C: $S_N1$, $S_N2$; in unsaturated C (carbonyl): $A_N$
• Electrophilic (E): electrophile attacks; in aromatic: $S_E$ (also called $E_{Ar}$); in alkenes: $A_E$
• Radical (R): radicals involved; $S_R$, $A_R$

Common Reaction Patterns:

• Free radical substitution: alkane + halogen (light)
• Electrophilic addition: alkene + HX, X₂, H₂O (acid), etc.
• Electrophilic aromatic substitution: benzene + Friedel-Crafts, nitration, halogenation, etc.
• Nucleophilic substitution: RX + Nu⁻
• Nucleophilic addition: R₂C=O + Nu⁻ → R₂C(OH)(Nu)
• Nucleophilic acyl substitution: RCOOR' + Nu⁻ → RCONu + R'O⁻

Methods of Purification:

Chromatography Types:

• Adsorption: TLC (Thin Layer), Column
• Partition: Paper, HPLC (High Performance)
• Gas chromatography (GC): Gaseous sample
• GLC, GSC