Aromatic Substitution, NMR Spectroscopy, and Aldehyde/Ketone Chemistry

Electrophilic Aromatic Substitution (EAS) and Substituent Effects

Substituent Effects on Aromatic Rings

Substituents on aromatic rings significantly influence both the rate and orientation of electrophilic aromatic substitution (EAS) reactions. These effects are classified as activating or deactivating, and as ortho/para or meta directing.

• Activating Groups (EDGs): Increase electron density in the ring, making it more reactive than benzene. Typically direct new substituents to the ortho and para positions.

• Deactivating Groups (EWGs): Withdraw electron density, making the ring less reactive. Most direct new substituents to the meta position.

• Halogens: Are deactivating due to induction but are ortho/para directors due to resonance donation of lone pairs.

Examples:

• EDG: -OCH3 (methoxy), -NH2 (amino), -OH (hydroxy)

• EWG: -NO2 (nitro), -CN (cyano), -COOH (carboxy)

Table: Substituent Classification and Directing Effects

Group Type	Examples	Directing Effect	Activation
Strong Activating	-NH2, -OH, -OR	Ortho/Para	Strong
Moderate Activating	-NHCOR, -OCOR	Ortho/Para	Moderate
Weak Activating	-R (alkyl)	Ortho/Para	Weak
Weak Deactivating	-F, -Cl, -Br, -I	Ortho/Para	Deactivating
Moderate Deactivating	-COOR, -COR, -SO3H	Meta	Deactivating
Strong Deactivating	-NO2, -CF3, -CCl3	Meta	Strong

Additional info: The table is inferred from standard organic chemistry knowledge and the notes' lists.

Mechanistic Basis for Directing Effects

• Resonance Effects: Groups with lone pairs (e.g., -OH, -NH2) can donate electron density via resonance, stabilizing the carbocation intermediate at ortho/para positions.

• Inductive Effects: Electronegative atoms withdraw electron density through sigma bonds, destabilizing the intermediate, especially at ortho/para positions (e.g., -NO2).

• Meta Directing: EWGs destabilize the ortho/para intermediates, making meta substitution more favorable.

Example: Nitration of toluene (methylbenzene) gives mainly ortho and para products due to the methyl group being an EDG.

Multiple Substituents

• When more than one substituent is present, the strongest activator dominates the directing effect.

• Ortho/para directors usually "win" over meta directors.

• Steric hindrance can prevent substitution between two meta groups.

Nuclear Magnetic Resonance (NMR) Spectroscopy

Principles of NMR

NMR spectroscopy is a powerful tool for determining the structure of organic molecules by studying the magnetic properties of certain nuclei (e.g., 1H, 13C).

• Nuclear Spin: Nuclei with an odd number of protons and/or neutrons possess spin and can be studied by NMR.

• Magnetic Field: In an external magnetic field, nuclear spins align either with (lower energy, α) or against (higher energy, β) the field.

• Resonance: Absorption of radiofrequency energy causes spins to flip from α to β state; this absorption is detected as an NMR signal.

Equation:

$E = \gamma \cdot B_0$

where $E$ is the energy difference, $\gamma$ is the gyromagnetic ratio, and $B_0$ is the magnetic field strength.

Key Features of 1H NMR Spectra

• Number of Signals: Indicates the number of distinct proton environments (types of H).

• Position (Chemical Shift, δ): Measured in ppm; reflects the electronic environment (shielded = upfield, deshielded = downfield).

• Intensity (Integration): Area under each peak is proportional to the number of protons of that type.

• Splitting (Multiplicity): Number of neighboring protons causes splitting (N+1 rule).

Example: Ethyl bromide (CH3CH2Br) shows a triplet (CH3) and a quartet (CH2).

Table: Typical 1H Chemical Shifts

Type of Proton	δ (ppm)
Alkane (R-CH3)	0.9
Alkane (R2CH2)	1.2-1.4
Alkene (C=CH)	4.5-6.5
Aromatic (Ar-H)	6.5-8.5
Aldehyde (CHO)	9-10
Carboxylic Acid (COOH)	10-12

Additional info: Table values are standard and inferred from the notes' ranges.

Splitting Patterns and the N+1 Rule

• A proton with N neighboring (nonequivalent) protons is split into N+1 peaks.

• Common patterns: singlet (no neighbors), doublet (1 neighbor), triplet (2 neighbors), quartet (3 neighbors), etc.

• Complex splitting occurs when a proton is adjacent to two or more sets of nonequivalent protons (e.g., doublet of triplets).

Example: CH3CH2Br: CH3 (triplet), CH2 (quartet).

Hydrogen Deficiency Index (HDI)

The HDI (degree of unsaturation) indicates the number of rings and/or π bonds in a molecule.

Equation:

$HDI = \frac{2C + 2 + N - H - X}{2}$

where C = number of carbons, N = nitrogens, H = hydrogens, X = halogens.

Example: For C7H8O: $HDI = \frac{2 \times 7 + 2 - 8}{2} = 4$ (suggests aromatic ring).

Aldehydes and Ketones: Structure, Nomenclature, and Synthesis

Structure and Properties

• Carbonyl Group (C=O): Polar, with partial positive charge on carbon (electrophilic) and partial negative on oxygen.

• Physical Properties: Higher boiling points than alkanes due to dipole-dipole interactions, but lower than alcohols (no H-bonding).

Nomenclature

• Aldehydes: Suffix "-al" (e.g., butanal). Always at the end of the chain; no need to number the carbonyl.

• Ketones: Suffix "-one" (e.g., 2-pentanone). Number the chain to give the carbonyl the lowest possible number.

• Rings: Aldehyde: add "carbaldehyde" to ring name; ketone: use "-one" suffix.

Examples: 3-methylbutanal, 2-hexanone, cyclopentanecarbaldehyde.

Synthesis of Aldehydes

• Oxidation of 1° Alcohols: Mild oxidants (PCC) yield aldehydes.

• Ozonolysis of Alkenes: Cleavage of double bonds forms aldehydes/ketones.

• Hydroboration of Terminal Alkynes: $1. HB(Sia)_2, 2. H_2O_2$ yields aldehydes.

• Reduction of Acid Chlorides: $1. LiAlH(OtBu)_3, -78^\circ C, 2. H_2O$ yields aldehydes (selective, less reactive hydride).

• Reduction of Esters/Nitriles: DIBAL-H at low temperature, then water, yields aldehydes.

Synthesis of Ketones

• Ozonolysis of Alkenes: Internal alkenes yield ketones.

• Friedel-Crafts Acylation: Aromatic rings react with acid chlorides and AlCl3 to give aryl ketones.

• Oxidation of 2° Alcohols: PCC or Jones reagent yields ketones.

• Organometallic Addition to Nitriles: $1. R-MgBr, 2. H_3O^+$ yields ketones.

• Hydroboration/Oxymercuration of Alkynes: Internal alkynes yield ketones.

• Organocuprate Addition to Acid Chlorides: Gilman reagents (R2CuLi) react with acid chlorides to give ketones.

Reactions of Aldehydes and Ketones

Nucleophilic Addition Mechanism

• General Mechanism: Nucleophile attacks the electrophilic carbonyl carbon, forming a tetrahedral intermediate.

• Activation: If the nucleophile is weak, acid catalysis can activate the carbonyl by increasing its electrophilicity.

• Reactivity: Aldehydes are generally more reactive than ketones due to less steric hindrance and fewer electron-donating groups.

Common Nucleophilic Addition Reactions

• Organometallic Addition: Grignard reagents (R-MgBr) and organolithiums add to carbonyls to form alcohols after aqueous workup.

• Hydride Addition: NaBH4 and LiAlH4 reduce aldehydes/ketones to alcohols.

• Cyanohydrin Formation: $1. NaCN, 2. H^+$ adds CN- to carbonyl, forming a cyanohydrin.

• Wittig Reaction: Converts carbonyls to alkenes using phosphonium ylides.

• Horner-Wadsworth-Emmons Reaction: Modified Wittig reaction using phosphonate esters, typically gives E-alkenes.

Reactions with Oxygen and Alcohol Nucleophiles

• Hydration: Addition of water forms gem-diols (hydrates).

• Alcohol Addition: Forms hemiacetals (one equivalent) and acetals (two equivalents, acid-catalyzed). Acetals are useful as protecting groups for carbonyls.

Reactions with Sulfur and Nitrogen Nucleophiles

• Thiol Addition: Forms thioacetals, which can be used to remove carbonyls (Raney Ni reduction).

• Imine Formation: Reaction with primary amines yields imines (Schiff bases).

• Enamine Formation: Reaction with secondary amines yields enamines.

• Hydrazone and Oxime Formation: Reaction with hydrazine or hydroxylamine yields hydrazones or oximes, respectively.

• Wolff-Kishner and Clemmensen Reductions: Methods to reduce carbonyls to alkanes.

Oxidation of Aldehydes

• Strong Oxidants: KMnO4, CrO3 oxidize aldehydes to carboxylic acids.

• Tollens' Test: Ag2O/NaOH selectively oxidizes aldehydes (silver mirror test).

Summary Table: Key Reactions of Aldehydes and Ketones

Reaction Type	Reagents	Product
Reduction to Alcohol	NaBH4, LiAlH4	1° or 2° Alcohol
Grignard Addition	R-MgBr, H3O+	Alcohol (new C–C bond)
Cyanohydrin Formation	NaCN, H+	Cyanohydrin
Imine Formation	RNH2, H+	Imine
Enamine Formation	R2NH, H+	Enamine
Wittig Reaction	Ph3P=CHR	Alkene
Acetal Formation	2 ROH, H+	Acetal
Oxidation	KMnO4, CrO3, Tollens	Carboxylic Acid

Additional info: All tables and some mechanistic details are expanded for clarity and completeness.