Unit 5: Fundamentals of Organic Chemistry

Electronic Effects (Permanent)

These are effects that are inherent to the molecule's ground state and cause a permanent polarization of electrons.

Inductive Effect (I Effect)

Definition: The permanent partial displacement of sigma (σ) electrons along a carbon chain towards a more electronegative atom or group.

• It's a permanent effect and weakens rapidly with distance (negligible after 3 bonds).
• -I Effect (Electron-withdrawing): Shown by groups more electronegative than carbon. This pulls electron density *away* from the carbon chain.
  Order of -I effect: -NO₂ > -CN > -COOH > -F > -Cl > -Br > -I > -OH
• +I Effect (Electron-donating): Shown by groups less electronegative than carbon. This pushes electron density *towards* the carbon chain.
  Order of +I effect: -O⁻ > -COO⁻ > -C(CH₃)₃ > -CH(CH₃)₂ > -CH₂CH₃ > -CH₃

Resonance and Mesomeric Effect (R or M Effect)

Definition: The permanent delocalization (spreading out) of pi (π) electrons in a conjugated system (a system of alternating single and multiple bonds, or lone pairs next to double bonds).

The true structure is a "resonance hybrid" of all possible contributing "canonical" structures.

• +R or +M Effect (Electron-donating): The group donates electrons to the conjugated system, increasing electron density.
  Example Groups: -OH, -OR, -NH₂, -Cl (Groups with lone pairs).
  Example: In phenol (C₆H₅OH), the -OH group donates its lone pair to the benzene ring.
• -R or -M Effect (Electron-withdrawing): The group withdraws electrons from the conjugated system.
  Example Groups: -NO₂, -CN, -COOH, -CHO, -C=O (Groups with multiple bonds).
  Example: In nitrobenzene (C₆H₅NO₂), the -NO₂ group pulls π-electrons from the ring.

Hyperconjugation (No-Bond Resonance)

Definition: The delocalization of sigma (σ) electrons of a C-H bond (or C-C bond) into an adjacent empty p-orbital or a π-orbital.

• It is also called the Baker-Nathan effect.
• Condition: Requires at least one α-hydrogen (a hydrogen on the carbon *next* to the sp² or sp hybridized carbon, or a carbocation).
• Applications:
  1. Stability of Carbocations: More alkyl groups = more α-hydrogens = more hyperconjugation structures = more stability.
     Order: 3° carbocation > 2° > 1° > Methyl.
  2. Stability of Alkenes: More substituted alkenes are more stable (Saytzeff's Rule).
     Order: Tetrasubstituted > Trisubstituted > Disubstituted > Monosubstituted.

Electronic Effects (Temporary)

Electromeric Effect (E Effect)

Definition: The complete transfer of a shared pair of pi (π) electrons to one of the atoms joined by a multiple bond, in the presence of an attacking reagent.

• It's a temporary effect. The molecule reverts to its original state if the reagent is removed.
• +E Effect: The π-electrons move towards the atom to which the attacking reagent (an electrophile) gets attached. (e.g., Addition of H⁺ to ethene).
• -E Effect: The π-electrons move away from the atom to which the attacking reagent (a nucleophile) gets attached. (e.g., Addition of CN⁻ to propanone).

Cleavage of Bonds (Bond Fission)

A covalent bond can break in two ways:

1. Homolytic Fission (Homolysis)

• What it is: The bond breaks symmetrically. Each atom takes one of the shared electrons.
• Mechanism: Represented by "fishhook" or half-headed arrows (⇀).
• Products: Generates free radicals (neutral species with an unpaired electron).
• Favored by: High temperature, UV light, or non-polar peroxides. (Mnemonic: H-E-L-P = Heat, Electricity, Light, Peroxide).

  Cl-Cl --(UV light)--> Cl• + Cl•

2. Heterolytic Fission (Heterolysis)

• What it is: The bond breaks asymmetrically. One atom takes *both* shared electrons.
• Mechanism: Represented by a full-headed arrow (→).
• Products: Generates ions (a cation and an anion).
• Favored by: Polar solvents and polar bonds.

  (CH₃)₃C-Br --> (CH₃)₃C⁺ (a carbocation) + Br⁻ (bromide ion)

Reaction Intermediates

These are short-lived, highly reactive species formed during a reaction, which quickly convert into products. Their stability determines the major pathway a reaction will take.

Organic Reagents: Electrophiles and Nucleophiles

Organic reactions typically involve an electron-rich species attacking an electron-poor species.

Electrophiles ("electron-loving")

• Definition: Reagents that are electron-deficient. They are "seeking" an electron pair.
• Function: They attack areas of high electron density (like C=C bonds or lone pairs).
• Types:
  • Positive electrophiles (E⁺): H⁺, Cl⁺, Br⁺, NO₂⁺ (nitronium ion), R⁺ (carbocations).
  • Neutral electrophiles (incomplete octet): BF₃, AlCl₃ (Lewis acids), SO₃.

Nucleophiles ("nucleus-loving")

• Definition: Reagents that are electron-rich. They have a lone pair or π-bond to "donate".
• Function: They attack areas of low electron density (like the δ+ carbon in a C=O bond or a carbocation).
• Types:
  • Negative nucleophiles (Nu⁻): OH⁻ (hydroxide), CN⁻ (cyanide), Br⁻, H⁻, R-O⁻ (alkoxide), R⁻ (carbanions).
  • Neutral nucleophiles (lone pairs): H₂O, NH₃, R-OH (alcohols), R-NH₂ (amines).
  • π-bond nucleophiles: Alkenes (e.g., C=C), Benzene ring.

Acidity and Basicity (Organic)

This section uses the electronic effects (I, R) to explain the relative strengths of organic acids and bases.

Acidity

Strength of an acid (HA) depends on the stability of its conjugate base (A⁻).

HA ⇌ H⁺ + A⁻
More stable conjugate base (A⁻) = Stronger acid (HA)

• Electron-withdrawing groups (-I, -R) *stabilize* the conjugate base (A⁻) by delocalizing the negative charge. This increases acidity.
  Example: Formic acid vs. Acetic acid.
  CH₃→COOH. The -CH₃ group has a +I effect, which *destabilizes* the acetate anion (CH₃COO⁻) by pushing more electrons onto it. Therefore, acetic acid is a *weaker* acid than formic acid (HCOOH).
  Example: Chloroacetic acid (Cl←CH₂COOH) is much stronger than acetic acid. The -Cl has a -I effect, which pulls density away and stabilizes the chloroacetate anion.
• Electron-donating groups (+I, +R) *destabilize* the conjugate base, decreasing acidity.

Acidity of Phenols vs. Alcohols: Phenol (C₆H₅OH) is *millions* of times more acidic than ethanol (CH₃CH₂OH).
Reason: The conjugate base of phenol (the phenoxide ion, C₆H₅O⁻) is highly stabilized by resonance. The negative charge is delocalized over the entire benzene ring. The conjugate base of ethanol (ethoxide, CH₃CH₂O⁻) has its charge localized on the oxygen and is further destabilized by the +I effect of the ethyl group.

Basicity

Strength of a base (B:) depends on the availability of its lone pair of electrons to donate.

B: + H⁺ ⇌ BH⁺
More available lone pair = Stronger base

• Electron-donating groups (+I) *increase* the electron density on the central atom (e.g., Nitrogen), making the lone pair more available. This increases basicity.
  Example (Amines): (CH₃)₂NH (2°) > CH₃NH₂ (1°) > NH₃ (Ammonia)
  The +I effect of the methyl groups pushes electrons onto the N, making it a stronger base.
• Electron-withdrawing groups (-I, -R) *decrease* the electron density on the central atom, making the lone pair less available. This decreases basicity.
  Example (Aniline): Aniline (C₆H₅NH₂) is a *very weak* base compared to ammonia.
  Reason: The lone pair on the nitrogen of aniline is delocalized into the benzene ring (+R effect). It is "busy" in resonance and not available to accept a proton.