Alkyl halides (R-X) bear a halogen on an \(sp^3\) carbon and are substrates for two competing substitution pathways: \(S_N2\) (bimolecular, backside attack, leading to inversion of configuration) and \(S_N1\) (unimolecular, via a planar carbocation intermediate that allows racemization, often imperfect because of ion pairing). Primary, unhindered substrates favor \(S_N2\), while tertiary substrates favor \(S_N1\) (often competing with E1 elimination). The two elimination mechanisms are E2 (concerted, anti-periplanar geometry, requires strong base) and E1 (stepwise via a carbocation, competing with \(S_N1\)). Both eliminations follow Zaitsev's rule, predicting the more substituted alkene as the major product. Carbocation stability follows the order tertiary > secondary > primary > methyl, reflecting hyperconjugation and inductive donation by alkyl groups. Bond cleavage can occur homolytically (one electron to each atom, generating radicals, as in alkane halogenation) or heterolytically (both electrons to one atom, generating ions). Good \(S_N2\) nucleophiles are strong, unhindered, and highly polarizable/electron-rich species such as I⁻, RS⁻, and CN⁻, and good leaving groups are weak bases such as halides (especially I⁻ and Br⁻) and tosylate/sulfonate esters. Polar protic solvents (water, alcohols) favor \(S_N1\)/E1 by stabilizing the carbocation, while polar aprotic solvents (DMSO, acetone) favor \(S_N2\) because they do not hydrogen-bond to and hinder the nucleophile. Vinyl and aryl halides are unreactive toward \(S_N1\)/\(S_N2\) because the halogen is on an \(sp^2\) carbon where backside attack is blocked and the C–X bond is stronger; geminal dihalides have both halogens on the same carbon, while vicinal dihalides have them on adjacent carbons, and a vicinal dihalide can be converted back to an alkyne by double E2 elimination with excess \(NaNH_2\).
Grignard reagents (R-MgX) are formed by reacting an alkyl or aryl halide with magnesium in dry ether; they are powerful carbon nucleophiles and bases and must be handled under rigorously anhydrous conditions because any acidic proton immediately protonates them to the corresponding alkane. In protecting-group strategies, an acetal can temporarily mask a reactive carbonyl group during a Grignard reaction elsewhere in the molecule, then be hydrolyzed back to the carbonyl afterward. In reactions with alpha,beta-unsaturated carbonyl compounds, hard nucleophiles such as Grignard and organolithium reagents favor 1,2-addition directly to the carbonyl carbon, while softer nucleophiles such as lithium dialkylcuprates (R₂CuLi) favor 1,4-(conjugate) addition at the beta carbon. The Baeyer–Villiger oxidation inserts an oxygen next to a ketone carbonyl using a peracid to give an ester (or a lactone from a cyclic ketone). The Wittig reaction converts an aldehyde or ketone into an alkene using a phosphorus ylide, with triphenylphosphine oxide as byproduct. The Claisen condensation joins two ester molecules with loss of an alkoxide to form a beta-keto ester, and when combined with an intramolecular aldol condensation forms the Robinson annulation. Acyl chloride-derived reactions install new C–C bonds: a primary amine reacting with an acid chloride forms an amide, a Grignard reagent attacking an ester proceeds through a ketone to a tertiary alcohol, and a Grignard reagent adding to a nitrile gives, after hydrolysis, a ketone.
Spectroscopy is essential for identifying functional groups. The IR fingerprint region (roughly 500–1500 cm\(^{-1}\)) contains complex, compound-specific absorption patterns useful for confirming identity by comparison with reference spectra. The C=O stretch frequency depends strongly on the functional group: acid chlorides and anhydrides absorb highest (≈1750–1820 cm\(^{-1}\)) due to strong electron withdrawal, followed by esters, then aldehydes and ketones, with amides lowest (≈1630–1700 cm\(^{-1}\)) because of resonance donation from nitrogen. In \(^1\)H NMR, the aldehyde C–H proton appears far downfield at 9–10 ppm due to carbonyl deshielding, carboxylic acid O–H protons appear at 10–13 ppm and are often broad, aromatic protons at 6.5–8.5 ppm from ring-current deshielding, vinyl protons at 4.5–6.5 ppm, and alkyl (\(sp^3\) C–H) protons at 0.5–2.5 ppm. In \(^{13}\)C NMR, carbonyl carbons appear in the range of roughly 160–220 ppm, with esters, amides, and acids near the lower end (160–180 ppm) and ketones and aldehydes near the upper end (190–220 ppm).
Qualitative tests further help identify functional groups. The Jones (chromic acid) test turns orange Cr(VI) green/blue-green Cr(III) and indicates oxidation of a primary or secondary alcohol or an aldehyde. Bromine water (Br₂) is decolorized by alkenes and alkynes via addition but not by alkanes or aromatics without a catalyst. Terminal alkynes form insoluble silver or copper acetylides with \(Ag(NH_3)_2^+\) or \(Cu(NH_3)_2^+\), while internal alkynes do not. Cyclic structures of biological importance include lactones (cyclic esters from intramolecular esterification of hydroxy acids), lactams (cyclic amides formed within amino-acid-like molecules), and beta-lactams (strained four-membered cyclic amides found in penicillin antibiotics). In IUPAC nomenclature, halogens are always named as prefixes (fluoro-, chloro-, bromo-, iodo-) and ether oxygens as alkoxy- (methoxy-, ethoxy-); the parent chain is numbered to give the principal functional group the lowest possible locant, with priority order roughly carboxylic acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine > alkene/alkyne.