Sources and References

Supplementary texts — Palleros, D.R. Experimental Organic Chemistry. Wiley, 2000. | Mohrig, J.R., Hammond, C.N. & Schatz, P.F. Techniques in Organic Chemistry, 5th ed. W.H. Freeman, 2019. | Clayden, J., Greeves, N. & Warren, S. Organic Chemistry, 2nd ed. Oxford University Press, 2012. Online resources — SDBS Spectral Database (sdbs.db.aist.go.jp); PubChem (pubchem.ncbi.nlm.nih.gov); Reaxys (elsevier.com/solutions/reaxys); SciFinder (cas.org/products/scifinder)

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Chapter 1: Laboratory Safety, Documentation, and Professional Practice

Section 1.1: Laboratory Safety as a Chemical Discipline

The organic synthesis laboratory is an environment in which hazardous chemicals, glassware under pressure or vacuum, open flames, and high-temperature heating devices are used in close proximity. A thorough understanding of laboratory safety is therefore not merely a regulatory requirement but a genuine chemical discipline — one that requires knowledge of the physical and chemical properties of the substances being used, an appreciation of the conditions under which those properties become hazardous, and the disciplined habits of mind that allow one to work efficiently without incident.

Before beginning any experiment, the competent chemist consults the Safety Data Sheet (SDS) for each reagent and solvent, paying particular attention to the sections on physical and chemical hazards (flash point, boiling point, reactivity), health hazards (acute and chronic toxicity, routes of exposure), and protective measures (appropriate personal protective equipment, ventilation requirements, spill response). The SDS system (formerly the MSDS system) has been harmonized internationally under the Globally Harmonized System (GHS) of classification and labelling, which assigns standardized hazard pictograms, signal words (Danger or Warning), and hazard statements.

Personal protective equipment (PPE) in the organic laboratory includes: a laboratory coat (cotton or flame-resistant synthetic, worn buttoned to the waist) to protect clothing and skin from splashes; safety glasses or goggles that protect the front and sides of the eyes from chemical splash and flying particles; and nitrile or neoprene gloves appropriate to the reagents being handled. Gloves must be inspected for integrity before each use and changed when contaminated — they are not a universal barrier and should be removed before touching shared surfaces (door handles, keyboards) to prevent secondary contamination.

Work with volatile organic solvents and toxic reagents must be conducted in a chemical fume hood — not merely a hood-shaped cabinet, but a device that maintains inward airflow sufficient (typically 0.3–0.5 m s\(^{-1}\) face velocity) to prevent vapors from entering the laboratory. The sash should be positioned at the designated working height marked on the hood, and equipment should be placed at least 15 cm behind the sash to allow the airflow to contain vapors effectively.

Section 1.2: The Laboratory Notebook as a Legal Document

The laboratory notebook is the primary record of all experimental work and is treated, in professional and legal contexts, as a contemporaneous record that can establish priority in patent disputes and provide evidence of due diligence in research. Entries in the notebook must be made in permanent ink, in real time (not reconstructed after the fact), and must never be erased — corrections are made by drawing a single line through the error (leaving the original entry legible) and writing the correction alongside it, initialed and dated.

A complete notebook entry for an experimental session includes: the date, a descriptive title for the experiment, a statement of the objective or hypothesis being tested, the relevant safety information (hazard class and appropriate PPE for each reagent), the exact quantities (mass, volume, moles) of all reagents used and their sources (manufacturer, lot number if critical), the step-by-step procedure as executed (not the procedure as written — recording what was actually done, with observations noted in real time), all observations (color changes, temperature changes, precipitate formation, gas evolution, odors), yield data (mass of crude and purified product, theoretical yield, percent yield), and characterization data (melting point, spectroscopic data, comparison with literature values).

The “plan of action” — the student’s pre-written, predicted procedure for the experiment, derived from the laboratory instructions — is entered before coming to the lab. This advance preparation is pedagogically critical: a student who has already written the procedure understands the experiment at a mechanistic level before touching a reagent, and can therefore respond appropriately to unexpected outcomes and deviations.

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Chapter 2: Core Laboratory Techniques

Section 2.1: Recrystallization

Recrystallization is the premier technique for purifying solid organic compounds. It exploits the differential solubility of the desired compound and its impurities in a solvent or mixed-solvent system as a function of temperature. The ideal recrystallization solvent dissolves the compound freely at the boiling point of the solvent but sparingly at room temperature or below, and dissolves the impurities poorly at all temperatures (so they remain in solution at the filtration stage) — or vice versa (so impurities that are more soluble are carried off in the hot mother liquor).

The recrystallization process consists of five stages. First, the crude solid is dissolved in the minimum volume of hot solvent (just at or slightly below the boiling point); this requires patient, drop-wise addition to avoid adding too much solvent, which wastes yield. Second, the hot solution is filtered by gravity hot filtration through fluted filter paper (in a stemless funnel) to remove insoluble impurities — a step that must be performed quickly to prevent premature crystallization in the funnel. Third, the filtrate is cooled, ideally slowly (controlled cooling by placing the hot Erlenmeyer in a room-temperature water bath, then eventually in an ice bath), to maximize crystal size and purity. Fourth, the crystals are collected by vacuum filtration through a Buchner funnel (with suction), washed with a small volume of cold solvent, and dried on the filter. Fifth, the purified product is characterized by melting point and spectroscopy and compared with literature values.

The melting point of an organic solid is one of the most useful physical properties for assessing purity: a pure compound melts sharply over a narrow range (0.5–1°C), while an impure sample melts over a broad range at a depressed temperature. The melting point depression is a colligative property, proportional to the mole fraction of impurity according to the van’t Hoff equation. Melting points are measured in a capillary tube using a Mel-Temp or Fisher–Johns apparatus and compared with literature values from databases such as the Aldrich Handbook or Reaxys.

Section 2.2: Distillation

Distillation is the primary technique for purifying liquid organic compounds. It exploits differences in vapor pressure (and therefore boiling point) between the desired compound and its impurities. The fundamental principle is governed by Raoult’s law: for an ideal solution, the partial pressure of component \( i \) above the solution is \( p_i = x_i P_i^* \), where \( x_i \) is the mole fraction in solution and \( P_i^* \) is the vapor pressure of the pure liquid at that temperature. Components with higher vapor pressures (lower boiling points) are preferentially enriched in the vapor phase.

Simple distillation (using a distillation head, condenser, and receiver) is suitable for separating liquids with boiling point differences greater than about 25°C. Fractional distillation uses a fractionating column (packed with inert material or containing plates) placed between the flask and the distillation head. The column provides multiple theoretical stages (equilibrations between vapor and liquid), progressively enriching the vapor in the more volatile component. The efficiency of a fractionating column is measured in theoretical plates: a column providing \( n \) theoretical plates is equivalent to \( n \) separate simple distillations.

Vacuum distillation (reduced-pressure distillation) is employed for compounds that would decompose before reaching their atmospheric boiling point. The reduced pressure lowers the boiling point (by the Clausius–Clapeyron equation \( \ln P = -\Delta H_{\text{vap}}/RT + \text{const} \)), allowing thermally sensitive compounds to be distilled at much lower temperatures. This is common in the purification of high-boiling esters, natural products, and pharmaceutical intermediates.

Steam distillation exploits the fact that two immiscible liquids boil together at a temperature below the boiling point of either component alone, since the total vapor pressure is the sum of the individual vapor pressures (\( P_{\text{total}} = P_A^* + P_B^* \), where \( P_A^* \) and \( P_B^* \) are the vapor pressures of the pure components at that temperature). Steam distillation is used to isolate volatile natural products (such as essential oils) from plant material at temperatures below 100°C, avoiding decomposition.

Section 2.3: Extraction and Partitioning

Liquid–liquid extraction (solvent extraction) uses the differential partitioning of a solute between two immiscible liquid phases to separate it from the matrix. The distribution coefficient (or partition coefficient) \( D = c_{\text{org}} / c_{\text{aq}} \) governs how much of the solute resides in the organic vs. aqueous phase at equilibrium. For a compound with high \( D \), multiple extractions of the aqueous phase with fresh organic solvent are far more efficient than a single large-volume extraction.

In a separatory funnel, the two immiscible phases (typically aqueous and an organic solvent such as diethyl ether, dichloromethane, or ethyl acetate) are shaken together, allowed to separate into distinct layers, and the denser layer is drained from the bottom. The choice of organic solvent depends on the polarity of the target compound (like dissolves like), the desired density difference (for clean phase separation), and the ease of removal (low boiling point solvents like diethyl ether or dichloromethane are readily removed by rotary evaporation).

Selective extraction is a powerful tool for separating classes of compounds with different acid–base behavior. An organic mixture containing a carboxylic acid, a base, and a neutral compound can be separated by sequential extraction with dilute aqueous base (which extracts the acid as its water-soluble carboxylate salt), dilute acid (which extracts the amine as its water-soluble ammonium salt), and leaving the neutral compound in the organic phase. The separated fractions are then basified (to liberate the free acid from its salt) or neutralized (to liberate the free amine) and re-extracted with organic solvent.

After extraction, the organic layer typically contains traces of water, which must be removed with a drying agent (anhydrous Na\(_2\)SO\(_4\), MgSO\(_4\), or CaCl\(_2\)) before the solvent is evaporated. The drying agent is added in excess, swirled, allowed to settle, and then filtered off by gravity filtration before the filtrate is concentrated.

Section 2.4: Chromatography

Chromatography separates components of a mixture based on their differential distribution between a stationary phase (solid or liquid supported on a solid) and a mobile phase (liquid or gas) that flows over or through the stationary phase. Components that interact more strongly with the stationary phase travel more slowly through the column (are retained more) and are therefore separated from those that prefer the mobile phase.

Subsection 2.4.1: Thin-Layer Chromatography

Thin-layer chromatography (TLC) is the workhorse analytical technique of the organic chemistry laboratory, used to monitor reaction progress, assess sample purity, and optimize conditions for column chromatography. The stationary phase is typically a thin layer of silica gel (for polar compound separation, normal-phase mode) or reverse-phase C-18 silica coated on an aluminum or glass plate. The mobile phase (eluent) is a mixture of solvents chosen to give well-separated spots with \( R_f \) values in the range 0.2–0.7.

The retention factor \( R_f = d_{\text{spot}} / d_{\text{solvent front}} \) is a characteristic (though solvent- and stationary-phase-dependent) quantity for a compound under specified conditions. Comparing \( R_f \) values of the crude reaction mixture, the starting materials, and (ideally) an authentic standard of the expected product is the most rapid way to confirm that a reaction has occurred and to judge the purity of the product.

Visualization of spots on TLC is accomplished by UV lamp illumination (254 nm for compounds with UV-absorbing chromophores), followed by staining with chemical reagents: KMnO\(_4\) (potassium permanganate in base, stains alkenes, alcohols, and most organic compounds), cerium ammonium molybdate (CAM, stains carbohydrates and many polar compounds), iodine vapors (reversible, stains most organic compounds), or ninhydrin (selective for amines, particularly amino acids).

Subsection 2.4.2: Column Chromatography

Flash column chromatography (pressurized column chromatography) uses positive air pressure to push solvent through a column of silica gel, reducing the time for separation from hours (gravity flow) to 15–30 minutes. The column is packed with silica gel slurried in the eluting solvent to avoid trapping air pockets. The sample is loaded as a concentrated solution (or, for very polar or sparingly soluble compounds, pre-adsorbed on silica as a dry load). Fractions are collected in test tubes and analyzed by TLC to determine which fractions contain the pure product.

The choice of eluent polarity is crucial. On normal-phase silica, non-polar solvents (hexane, petroleum ether) elute non-polar compounds quickly (low retention); increasing polarity of the eluent (by adding ethyl acetate, dichloromethane, or methanol) causes more polar compounds to elute. The optimal eluent for column chromatography gives \( R_f \approx 0.3 \) on TLC, which translates to good separation and manageable elution volumes on column.

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Chapter 3: Reaction Mechanisms and Synthetic Transformations

Section 3.1: Nucleophilic Aromatic Substitution

Nucleophilic aromatic substitution (SNAr) proceeds by a two-step addition–elimination mechanism, unlike the one-step concerted mechanism of SN2. In the first step, the nucleophile adds to the ipso carbon bearing the leaving group, forming a negatively charged Meisenheimer complex (sigma complex) in which the ring carbon becomes sp\(^3\). The aromaticity is temporarily lost. In the second step, the leaving group departs, restoring aromaticity and giving the product.

This mechanism is favored when: (1) electron-withdrawing groups (particularly nitro groups) are present ortho or para to the leaving group — they stabilize the negative charge in the Meisenheimer complex by resonance; and (2) the leaving group is a reasonable anion (fluoride is an excellent leaving group in SNAr despite being poor in aliphatic SN2, because in SNAr the leaving group ability is governed by the C–LG bond strength in the product-forming step, not by the rate of SN2). The synthesis of dyes, pharmaceuticals, and agricultural chemicals frequently exploits SNAr reactions.

Section 3.2: Carbonyl Chemistry in Synthesis

A competent synthetic chemist uses carbonyl reactions as bidirectional tools — the forward aldol condensation builds carbon–carbon bonds and installs unsaturation, while hydrolysis of acetals or esters provides protecting group strategies or access to carboxylic acid building blocks.

The Wittig reaction converts an aldehyde or ketone to an alkene with complete stereochemical predictability. A phosphonium ylide (Ph\(_3\)P=CH–R, prepared from a phosphonium salt and strong base) reacts with the carbonyl compound to give a four-membered oxaphosphetane intermediate, which collapses by a retro [2+2]-like process to give the alkene and triphenylphosphine oxide. Stabilized ylides (where R = COR’, CO\(_2\)R’, CN) give predominantly the \( E \)-alkene; unstabilized ylides give predominantly the \( Z \)-alkene. The Wittig reaction is complementary to the aldol route because the alkene position is determined by the carbonyl, not by the position of a protic alpha proton.

The Michael addition (conjugate addition) adds a nucleophile to the \( \beta \)-carbon of an \( \alpha,\beta \)-unsaturated carbonyl compound. The acceptor (Michael acceptor) activates the \( \beta \)-carbon because the carbonyl withdraws electron density through the conjugated system. Common Michael donors are stabilized enolates (malonate, acetoacetate, etc.), and the process builds C–C bonds at the \( \beta \)-position without the regiochemical ambiguity that can affect direct aldol reactions.

Section 3.3: Reduction and Oxidation Reactions

The controlled adjustment of oxidation state is central to synthetic strategy. Organic chemists use a range of reducing agents with different selectivities.

Lithium aluminium hydride (LiAlH\(_4\)) is the most powerful common hydride reducing agent, reducing all carbonyl compounds (aldehydes, ketones, esters, amides, carboxylic acids) and also nitriles and nitro compounds. The reaction proceeds by delivery of hydride (H\(^-\)) as a nucleophile from the Al–H bond to the electrophilic carbonyl carbon, forming an alkoxide intermediate that is released on aqueous workup. LiAlH\(_4\) reacts violently with water and must be handled with strict exclusion of moisture (anhydrous THF or diethyl ether, addition of reagent to solvent with cooling and stirring).

Sodium borohydride (NaBH\(_4\)) is a milder hydride donor that reduces only the more electrophilic carbonyls (aldehydes, ketones) and not esters or amides. This selectivity arises from the lower hydride-transfer power of NaBH\(_4\) compared to LiAlH\(_4\). NaBH\(_4\) can be used in protic solvents (methanol, ethanol) where LiAlH\(_4\) would react destructively.

Diisobutylaluminum hydride (DIBAL-H) at −78°C reduces esters selectively to aldehydes (by quenching the tetrahedral intermediate before the second hydride delivery), while at room temperature it fully reduces esters to primary alcohols. This temperature-dependent selectivity makes DIBAL-H uniquely useful for the synthesis of aldehydes from esters.

For oxidations, the reagent choice determines which oxidation state is achieved and the tolerance of other functional groups.

Pyridinium chlorochromate (PCC) in CH\(_2\)Cl\(_2\) oxidizes primary alcohols to aldehydes (stopping at the aldehyde stage, unlike permanganate) and secondary alcohols to ketones. It does not affect alkenes, esters, or other Cr(VI)-resistant functional groups. Swern oxidation (oxalyl chloride/DMSO/triethylamine) is a milder alternative for oxidation-sensitive substrates, operating at −78°C with no risk of over-oxidation.

Ozonolysis cleaves C=C double bonds at the position of the double bond: the alkene reacts with O\(_3\) to give a cyclic molozonide (1,2,3-trioxolane), which rearranges to an ozonide (1,2,4-trioxolane). The ozonide is then cleaved reductively (with Me\(_2\)S or PPh\(_3\)) to give two carbonyl compounds (aldehydes from terminal carbons bearing H, ketones from internal carbons bearing alkyl groups) — or oxidatively (with H\(_2\)O\(_2\)) to give carboxylic acids from the aldehyde fragments. Ozonolysis is a critical tool for structural proof: by identifying the carbonyl fragments, one can deduce the original position of the double bond in the target molecule.

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Chapter 4: Spectroscopic Characterization of Products

Section 4.1: Integrating Spectroscopic Methods for Structure Confirmation

In the senior organic laboratory, determining that a synthesized compound has the expected structure requires the same systematic spectroscopic approach used in research: collecting and correctly interpreting IR, NMR, and (where available) MS data, and reconciling these data with the expected structure before the compound leaves the laboratory. The ability to recognize when data are inconsistent with the proposed structure — and to diagnose the discrepancy — is a hallmark of the trained organic chemist.

The logical hierarchy for interpreting spectroscopic data of an unknown or synthesized compound begins with the molecular formula (from elemental analysis or high-resolution MS), which yields the degree of unsaturation (also called the index of hydrogen deficiency, IHD):

\[ \text{IHD} = \frac{2C + 2 + N - H - X}{2}, \]

where \( C \), \( H \), \( N \), and \( X \) are the numbers of carbon, hydrogen, nitrogen, and halogen atoms. Each ring or double bond contributes 1 to the IHD; a triple bond contributes 2; an aromatic ring contributes 4. An IHD of 4 or more is consistent with an aromatic ring; zero IHD means the compound is fully saturated and acyclic.

Section 4.2: Interpreting NMR Spectra of Laboratory Products

The \(^1\)H NMR spectrum provides information about the chemical environment of each distinct proton (through chemical shift), the number of protons in each environment (through integration), and the number of adjacent protons (through multiplicity). When interpreting the NMR of a newly synthesized compound, one should begin by counting the number of distinct \(^1\)H environments predicted by the expected structure, and then systematically match each observed signal to its structural origin.

The \(^{13}\)C NMR spectrum (broadband decoupled) shows one signal for each chemically distinct carbon. The chemical shift of \(^{13}\)C signals follows the same trends as \(^1\)H (electron-withdrawing groups deshield, electron-donating groups shield), but over a much wider range (0–220 ppm vs. 0–12 ppm for \(^1\)H). Carbonyl carbons (aldehyde, ketone, ester, amide) are particularly distinctive, appearing at \( \delta \) 160–220 ppm. The DEPT (Distortionless Enhancement by Polarization Transfer) experiment distinguishes CH\(_3\), CH\(_2\), and CH carbons from quaternary carbons (which have no \(^1\)H directly attached): in DEPT-135, CH and CH\(_3\) carbons appear as positive peaks, CH\(_2\) carbons as negative peaks, and quaternary carbons are absent.

Two-dimensional NMR experiments — particularly COSY (Correlation Spectroscopy, which shows \(^1\)H–\(^1\)H coupling through 2–3 bonds) and HMBC/HSQC (heteronuclear correlation, which connects \(^1\)H signals to the directly bonded \(^{13}\)C or to \(^{13}\)C two to three bonds away) — allow the complete connectivity of the molecule to be established without ambiguity, even for complex natural products or pharmaceuticals. In the senior laboratory setting, these experiments provide definitive structural confirmation.

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Chapter 5: Laboratory Reporting and Scientific Communication

Section 5.1: Journal-Article Format for Laboratory Reports

A defining feature of CHEM 360 is the expectation that students communicate their experimental work in the format of a peer-reviewed journal article, specifically following the conventions of the Journal of Organic Chemistry (JOC). This format trains the same scientific writing skills that are required in graduate school and in research careers.

A full journal-format experimental report contains the following sections. The abstract (50–100 words) summarizes the objective, the key synthetic transformation performed, the major result (yield and characterization), and the significance of the work. The introduction provides context: why is the target molecule of interest? what is known about the synthetic method? what does this experiment contribute? The results and discussion section presents the data (yield, spectroscopic data, physical properties) and interprets them: are the NMR data consistent with the proposed structure? what side products or byproducts were observed and why? how does the yield compare with literature, and what factors affected the outcome? The experimental section (in JOC format) gives the procedure in a tightly standardized format: “To a stirred solution of compound X (amount, mmol) in solvent (volume, concentration) was added reagent Y (amount, mmol) at temperature. The mixture was stirred for time, then worked up by…” — past tense, passive voice, all quantities in SI units or mmol/mL, compound characterizations listed in a standard order (\(^1\)H NMR, \(^{13}\)C NMR, IR, HRMS, melting point, optical rotation if chiral). The references section cites all primary literature and databases used.

Section 5.2: Yield Calculation and Error Analysis

The theoretical yield is the mass of product that would be obtained if the limiting reagent were completely consumed and the reaction proceeded with perfect selectivity and no loss during workup. It is calculated from the moles of limiting reagent and the molar mass of the product.

\[ \text{Theoretical yield} = n_{\text{limiting}} \times M_{\text{product}}. \]

The percent yield relates the actual (experimental) yield to the theoretical:

\[ \% \text{ yield} = \frac{\text{actual yield (g)}}{\text{theoretical yield (g)}} \times 100\%. \]

A rigorous discussion of yield in a laboratory report addresses: the purity of the isolated product (confirmed by melting point, NMR, or TLC), the sources of yield loss (solubility of product in the aqueous wash, loss during transfer between vessels, incomplete reaction, competing side reactions), and a comparison with the literature yield for the same procedure or closely analogous conditions.

Error propagation ensures that the reported uncertainty in a derived quantity reflects the contributions from the uncertainties in each measured quantity. For a product of two quantities \( q = a \cdot b \), the relative uncertainty \( \delta q / q = \sqrt{(\delta a/a)^2 + (\delta b/b)^2} \). Understanding uncertainty propagation allows the chemist to identify the step in the procedure that contributes most to the overall uncertainty and to prioritize improvements in technique accordingly.