Consider the following reaction sequence:
CH4 + Cl2 (UV light) → P → NaOH (aq) → Q
Identify P and Q:

The halogenation of alkanes with chlorine or bromine in the presence of UV light or heat proceeds by a free radical chain mechanism, which consists of three distinct phases: initiation, propagation, and termination. In the initiation step, absorbed UV photons provide the energy to homolytically cleave the halogen-halogen bond (X2 + hv → 2X•), generating highly reactive halogen radicals. During propagation, the halogen radical abstracts a hydrogen atom from the alkane to generate an alkyl radical and HX (X• + R-H → HX + R•), and then the alkyl radical reacts with another molecule of halogen to give the alkylhalide product and regenerate a halogen radical (R• + X2 → R-X + X•). The chain terminates when two radicals combine (X• + X•, R• + X•, or R• + R•). The relative reactivities of hydrogen types toward abstraction (which determines selectivity) are: tertiary H (about 5× for Cl, 1600× for Br) > secondary H (about 4× for Cl, 80× for Br) > primary H (1× for both). Fluorination is explosive and uncontrollable; iodination is too slow (thermodynamically unfavourable because C-I bond formation does not release enough energy to compensate for the endothermic I• + H → HI + H• step); only chlorination and bromination are practically useful, with bromination being much more selective than chlorination.

Nucleophilic substitution reactions at saturated carbon are among the most fundamental and thoroughly studied reaction types in organic chemistry, providing the basis for synthesising a vast array of organic compounds from alkyl halides. In SN2 (Substitution Nucleophilic Bimolecular) reactions, the nucleophile attacks the electrophilic carbon bearing the leaving group in a single concerted step from the side diametrically opposite to the leaving group (backside attack), generating a trigonal bipyramidal transition state at the reacting carbon in which the nucleophile, the central carbon, and the leaving group are collinear. As the nucleophile forms its bond to the carbon, the leaving group simultaneously departs, so no true intermediate is formed. This mechanism results in INVERSION of configuration at the stereogenic centre (Walden inversion) — an R configuration is converted to S and vice versa. In SN1 (Substitution Nucleophilic Unimolecular) reactions, the reaction proceeds through a two-step mechanism: first, the leaving group departs to generate a planar, sp2-hybridised carbocation intermediate (the rate-determining step, which is why only the substrate concentration appears in the rate law), and then in a fast second step, the nucleophile attacks the planar carbocation from either face with equal probability. This leads to racemisation (approximately equal amounts of R and S products, though in practice slight inversion predominates due to ion pairing effects). The selectivity between SN1 and SN2 depends critically on the structure of the substrate, the nature of the nucleophile, and the solvent.

Halogen derivatives (alkyl and aryl halides) are formed by replacement of one or more hydrogen atoms in hydrocarbons with halogen atoms (F, Cl, Br, I). They are classified by the number of halogen atoms (mono-, di-, tri-, polyhalides), the type of carbon bearing the halogen (primary, secondary, or tertiary for alkyl halides; allylic or benzylic for special cases; vinyl or aryl halides), and the type of hydrocarbon (alkyl halides from aliphatic hydrocarbons, aryl halides from aromatic rings). Physical properties: alkyl halides are generally polar molecules with higher boiling points than the corresponding alkanes of similar molecular weight due to dipole-dipole interactions and, for iodo compounds, London dispersion forces. They are generally immiscible with water but soluble in organic solvents. The C-X bond is polar with carbon being slightly positive (delta+) and halogen being slightly negative (delta-), making carbon electrophilic and susceptible to nucleophilic attack. Reactivity of the C-X bond toward nucleophilic substitution follows the order: R-I > R-Br > R-Cl > R-F, because bond strength increases in the order R-I < R-Br < R-Cl < R-F (fluorine forms the strongest C-X bond, making C-F most difficult to break despite being most polar).

One of the most powerful synthetic applications of alkyl halides is their conversion to Grignard reagents (organomagnesium halides), discovered by Victor Grignard in 1901 (Nobel Prize 1912). Preparation: R-X + Mg (in dry ether or THF) → R-MgX. The Grignard reagent R-MgX contains a highly polarised C-Mg bond in which carbon carries a significant negative charge, making it an extremely powerful nucleophile and a strong base. Reactions of Grignard reagents: With formaldehyde (HCHO): R-MgX + H2C=O → R-CH2-OH (primary alcohol after H3O+ workup). With other aldehydes (R'CHO): → R-CH(OH)-R' (secondary alcohol). With ketones (R'COR''): → R-C(OH)(R')-R'' (tertiary alcohol). With CO2: → R-COOH (carboxylic acid, chain elongation by 1C). With esters: → R-C(OH)(R')2 (tertiary alcohol). Critical requirement: all reagents and solvents must be completely anhydrous because water protonates and destroys the Grignard reagent (R-MgX + H2O → R-H + Mg(OH)X), and the Grignard reagent must be prepared and used in strictly anhydrous conditions. Grignard reactions are enormously important in synthetic organic chemistry for forming carbon-carbon bonds.

Competing with substitution reactions in alkyl halides is elimination — the removal of HX to form an alkene (or alkyne if dihalocompounds are used). The two main mechanisms are E1 (Elimination Unimolecular) and E2 (Elimination Bimolecular). E2 elimination: a strong base abstracts a beta-hydrogen simultaneously as the leaving group departs, in a single concerted step, typically giving the more substituted (more stable) alkene as the major product (Zaitsev's rule). Favoured by: bulky strong bases (like KOtBu), high temperatures, secondary and tertiary substrates, non-polar solvents. E1 elimination: first step is formation of carbocation (same as SN1 first step), then elimination of beta-H to form alkene; favoured by weak bases, polar protic solvents, tertiary substrates. The competition between substitution and elimination depends primarily on the substrate structure (primary favours substitution, tertiary favours elimination), the nucleophile/base strength and size (strong, bulky bases favour E2; strong small nucleophiles favour SN2; weak nucleophiles in polar solvents favour SN1/E1), and temperature (higher temperature favours elimination over substitution because elimination has greater entropy of activation). Practical rule: NaOH(aq) with RX gives alcohol (SN2); NaOH(alc) with RX gives alkene (E2).

The haloform reaction is a specific reaction of methyl ketones (and acetaldehyde) with excess chlorine, bromine, or iodine in alkaline conditions: CH3-CO-R + 3X2 + 3NaOH → CHX3 + R-COO-Na+ + 3NaX. For chlorine: yields chloroform (CHCl3) and the carboxylate salt. For iodine: yields iodoform (CHI3, yellow precipitate with distinctive smell) — the iodoform test is a qualitative test for methyl ketones and methyl carbinols (CH3-CH(OH)-R, which are oxidised to methyl ketones by the hypiodate in the alkaline iodine solution). Chloroform (CHCl3) was historically the first widely used general anaesthetic (1847). Carbon tetrachloride (CCl4) was used as a fire extinguisher and dry cleaning solvent but is now restricted due to toxicity (liver damage) and ozone depletion. Chlorofluorocarbons (CFCs, like CCl2F2 Freon-12): formerly used as refrigerants and aerosol propellants, now banned under the Montreal Protocol because they catalyse ozone layer destruction. Polyvinyl chloride (PVC, from vinyl chloride CH2=CHCl) is one of the most widely used plastics. Chlorobenzene is an important industrial solvent and intermediate in synthesis of phenol, DDT, and herbicides.