RCH2I

RCH2I denotes a family of organic compounds in which an iodine atom is bonded to a methylene carbon that bears the substituent R. The R group can be alkyl or aryl, giving derivatives such as methyl iodide (R = H) and benzyl iodide (R = phenyl). These are primary alkyl or benzylic iodides. The carbon–iodine bond is relatively weak, and the iodide is a good leaving group, making RCH2I substrates reactive in nucleophilic substitution and in radical processes. They are typically liquids or low-melting solids and can be lachrymatory and toxic.

Preparation commonly proceeds from the corresponding alcohols. The Appel reaction (PPh3 with I2) converts RCH2OH to RCH2I, and phosphorus triiodide (PI3) formed in situ is another route. Other methods include halide exchange on pre-formed benzyl or primary alkyl halides.

Reactivity: RCH2I substrates readily undergo SN2 reactions with nucleophiles such as alkoxides, thiolates, amines, or organolithium or organomagnesium reagents to give RCH2–Nu derivatives. Benzylic iodides are particularly reactive. They can also participate in radical processes under photochemical or thermal conditions, enabling coupling and cyclization reactions. In metal-catalyzed cross-couplings, alkyl iodides serve as versatile electrophiles.

Applications: Used as alkylating agents to form ethers, amines, and esters; as building blocks in the synthesis of pharmaceuticals, natural products, and polymers; and as precursors in radical or cross-coupling chemistry.

Safety: RCH2I compounds are typically toxic and irritant; methyl iodide is a known carcinogen. Handle in a fume hood with appropriate PPE, and store away from light and moisture where applicable.