Electrophilic Aromatic Substitution

Electrophilic Aromatic Substitution (EAS) definition

Reaction where an electrophile substitutes for H on an aromatic ring.

General EAS reaction format

Ar-H + E⁺ → Ar-E + H⁺

Overall rate law

Rate = k₂[Ar-H][E⁺]

Rate-determining step (RDS)

Formation of the σ-complex (arenium ion).

Step 1 of EAS

Formation of electrophile (E⁺).

Step 2 of EAS

Electrophile attacks aromatic ring → σ-complex.

Step 3 of EAS

Deprotonation to restore aromaticity → Ar-E.

Bromination overall reaction

Ar-H + Br₂ → Ar-Br + HBr

Color change in bromination test

Bromine (red-brown) disappears as aryl bromide (colorless) forms.

Purpose of Part A experiment

Compare how electron-donating substituents affect bromination rate.

Purpose of Part B experiment

Compare oxygen vs nitrogen substituents as activators.

Examples of oxygen EDGs

Phenol, anisole, phenyl acetate, diphenyl ether.

Examples of nitrogen EDGs

Aniline, N-methylaniline, acetanilide, N-phenylaniline.

Strong activating group example

Phenol (OH strongly donates by resonance).

Nitrobenzene reactivity

Brominates ~10⁷ times slower than benzene (strong deactivator).

Toluene vs benzene reactivity

Toluene reacts ~600x faster (methyl is activating).

Effect of substituents on rate

EDGs speed up EAS; EWGs slow it down.

Effect of substituents on orientation

EDGs → ortho/para; EWGs → meta.

EDGs that donate via resonance

Alkoxy (OR), amido (NHC=O), amino (NH₂).

Resonance stabilization in ortho/para addition

Positive charge placed adjacent to EDG → stabilized.

σ-complex definition

Cationic intermediate formed after electrophile addition.

Why meta is favored for EWGs

Positive charge avoids being adjacent to electron-withdrawing group.

Why ortho/para favored for EDGs

EDG donates electron density into ring → stabilizes σ-complex.

Pseudo-first-order condition

Use large excess of arene; Br₂ concentration is limiting.

Pseudo-first-order rate law

Rate = k₁[Br₂]

Integrated pseudo-first-order rate law

[Br₂]ₜ = [Br₂]₀ e^(-k₁t)

Linearized form using natural log

ln([Br₂]₀/[Br₂]ₜ) = k₁t

Linearized form using absorbance

ln(A₀/Aₜ) = k₁t

Why absorbance tracks Br₂ concentration

Only Br₂ absorbs light at 400 nm (Beer's law).

Beer's Law equation

A = εcl

Electrophile in bromination

Br⁺ (from Br₂)

Role of Lewis acid in bromination

Ferric bromide (FeBr₃) increases formation of Br⁺.

σ-complex resonance forms

Indicate positive charge delocalized to 3 ring positions.

Ortho product statistic effect

2 ortho positions vs 1 para → ortho:para = 2:1 if equal reactivity.

Diphenyl ether activation

Strongly activating due to resonance donation.

Phenyl acetate activation

Weaker EDG due to electron-withdrawing carbonyl.

Acetanilide activation

Moderate activator; resonance donation from amide nitrogen.

N-Methylaniline activation

Very strong activator (alkyl-substituted amine).

Steric effects on O/P ratio

Tert-butyl groups strongly block ortho positions.

Nitration directing example

Chlorobenzene → ortho and para products (sterics matter).

Nitration of methyl benzoate

Meta-directing due to CO₂Me (EWG).

σ-complex structure

main intermediate with positive charge on ring carbon.

Increasing strength of activation (oxygen series)

Diphenyl ether > phenol > anisole > phenyl acetate

Increasing strength of activation (nitrogen series)

N-methylaniline > aniline > N-phenylaniline > acetanilide

Br disappears in reaction

Indicates the rate of EAS bromination.

Semi-quantitative measurement

Description of color disappearance to estimate rate.

Quantitative measurement

Use UV-Vis absorbance of Br₂ vs time to calculate k₁.