Analytical Techniques: Mass Spectrometry, IR, and NMR

Mass Spectrometry (MS)

Mass spectrometry is a powerful analytical technique used to determine the molecular weight and structure of organic compounds by analyzing the mass-to-charge ratio of ionized fragments.

• Fragmentation Patterns: Alcohols, ethers, and carbonyl compounds (ketones and aldehydes) exhibit characteristic fragmentation patterns.

• Functional Group Identification: Certain functional groups show specific features in their mass spectra:

  • Nitrogen Rule: Compounds with an odd number of nitrogen atoms have an odd molecular ion mass.

  • Halides: Presence of halogens (Cl, Br) leads to M+2 peaks due to isotopic abundance.

  • Alcohols: Often show absence of M peak due to loss of water (M-18).

• No Rule of 13 or M+1: These are not required for this review.

Infrared Spectroscopy (IR)

IR spectroscopy identifies functional groups based on characteristic absorption of infrared light, which causes molecular vibrations.

• Functional Group Recognition: Each functional group absorbs at a specific wavenumber (cm-1).

• Examples:

  • O-H stretch (alcohols): Broad peak around 3200–3600 cm-1

  • C=O stretch (carbonyls): Sharp peak near 1700 cm-1

  • C-H stretch (alkanes): 2850–2960 cm-1

Nuclear Magnetic Resonance (NMR) Spectroscopy

NMR spectroscopy provides detailed information about the structure of organic molecules by analyzing the environment of hydrogen (1H) and carbon (13C) nuclei in a magnetic field.

• Number of Peaks: Indicates the number of distinct proton or carbon environments.

• Multiplicity (Splitting): Determined by the number of neighboring protons (n+1 rule), but only for protons separated by less than four covalent bonds. Identical neighboring protons do not split each other.

• Integration: Area under each peak corresponds to the number of protons represented.

• Chemical Shift: Indicates the electronic environment (shielded vs. deshielded protons).

Example: In ethanol (CH3CH2OH), the methyl, methylene, and hydroxyl protons each give distinct signals with characteristic splitting and integration.

Radical Reactions: Free Radical Halogenation

Mechanism and Stereochemistry

Free radical halogenation involves the substitution of hydrogen atoms in alkanes with halogens via a radical chain mechanism.

• Mechanism Steps: Initiation, propagation, and termination.

• Stereochemistry: The reaction can generate chiral centers; review the stereochemical outcomes.

• Stability Order: Free radical stability follows the order: tertiary > secondary > primary > methyl (same as carbocations).

• No Rearrangement: Free radicals do not undergo rearrangement.

Organometallic Compounds

Formation and Reactivity

Organometallic reagents are compounds containing a metal-carbon bond, commonly used as nucleophiles and bases in organic synthesis.

• Types: Organolithium, Grignard (RMgX), and Gilman (R2CuLi) reagents.

• Formation: Prepared by reacting alkyl halides with lithium, magnesium, or copper salts in appropriate solvents (e.g., ether for Grignard).

• Reactivity: Decompose in acidic conditions; strong nucleophiles and bases.

• Gilman Reagents: Used for coupling reactions, replacing halides with R groups.

Alcohols, Ethers, and Epoxides

Characteristic Reactions

Alcohols, ethers, and epoxides undergo a variety of substitution, elimination, and oxidation reactions.

• Alcohols: Undergo substitution (SN1/SN2), elimination (E1/E2), and oxidation. Carbocation rearrangements can occur (1,2-hydride shift preferred over 1,2-methyl shift).

• Ethers: Reactivity depends on the stability of the carbocation formed from the R group attached to oxygen. If no stable carbocation, reaction proceeds via SN2 (attack at least hindered carbon).

• Epoxides: React in both acidic and basic conditions:

  • Basic/neutral: Nucleophile attacks least substituted carbon.

  • Acidic: Nucleophile attacks most substituted carbon (requires acid such as H3O+, H2SO4).

• Nucleophiles: Organometallics, hydrides, alkoxides, hydroxide, etc., can attack epoxides.

Aromatic Compounds: Benzene and Substituted Benzenes

Electrophilic and Nucleophilic Substitution

Benzene undergoes characteristic substitution reactions, with the type of reaction and mechanism depending on the substituents and conditions.

• Electrophilic Aromatic Substitution (EAS): Five main reactions:

  • Halogenation

  • Nitration

  • Sulfonation

  • Friedel-Crafts Alkylation

  • Friedel-Crafts Acylation

• Electrophile Generation: Alkylation requires a carbocation; rearrangements may occur to form the most stable carbocation. Alternative methods (e.g., addition of HF to an alkene) can generate carbocations for alkylation.

• Limitations: Friedel-Crafts reactions do not proceed on strongly deactivated rings or for a second substitution if the ring is deactivated.

• Nucleophilic Aromatic Substitution (NAS): Two mechanisms:

  • Direct displacement (requires strong ring deactivation at ortho/para positions).

  • Addition-elimination (benzyne mechanism; e.g., reaction with NaNH2).

• Sandmeyer Reaction: Involves formation of a diazonium ion, followed by substitution.

• Substituent Effects: Substituents affect reactivity and orientation (activation/deactivation, ortho/para/meta directors).

• pKa Effects: Substituents at ortho and para positions influence the acidity of groups on the ring.

Table: Substituent Effects on Benzene Reactivity

Additional info: Table entries inferred from standard organic chemistry knowledge.

Carbonyl Chemistry

Reactivity and Mechanisms

Carbonyl compounds (aldehydes, ketones, carboxylic acids, esters, amides, etc.) undergo a variety of nucleophilic addition and substitution reactions.

• Class I vs. Class II Carbonyls:

  • Class I: Acyl derivatives (acyl halides, anhydrides, esters, amides) undergo nucleophilic acyl substitution.

  • Class II: Aldehydes and ketones undergo nucleophilic addition.

• Order of Reactivity: Acyl halides > anhydrides > aldehydes > ketones > esters > amides > carboxylates.

• Reactions with Organometallics: Class I compounds undergo two additions (to ketone, then alcohol); stopping at ketone possible with Gilman reagent.

• Hydride Reductions:

  • NaBH4: Reduces aldehydes and ketones only.

  • LiAlH4: Reduces all carbonyls.

  • DIBAH: Can stop at aldehyde stage.

• Reactions with Nitriles and Acetylide Ions: Nitriles can be converted to amines or carboxylic acids; acetylide ions add to carbonyls.

• Protecting Groups: Used to mask reactive carbonyls (especially class II); typically removed by hydrolysis of cyclic acetals using diols.

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

• Alpha Hydrogen Acidity: Alpha hydrogens are acidic due to resonance stabilization of the enolate ion.

• Alpha Carbon Reactions: Halogenation and alkylation occur at the alpha position.

• Alpha-Beta Unsaturated Carbonyls: Strong nucleophiles add 1,2 (to carbonyl); weak nucleophiles add 1,4 (to beta carbon).

Condensation Reactions

• Aldol Reaction: Formation of β-hydroxy carbonyl compounds from aldehydes/ketones.

• Claisen Condensation: Formation of β-keto esters from esters.

• Michael Addition: 1,4-addition of nucleophiles to α,β-unsaturated carbonyls.

Key Equations

• General Nucleophilic Addition to Carbonyl:

• Wittig Reaction:

• Aldol Reaction:

Additional info: Mechanistic details and order of reactivity inferred from standard organic chemistry curriculum.