To explore the feasibility of our approach, we began by investigating the photocatalytic ethylation of commercially available lepidine (1) using ethane as a C2 synthon under flow conditions. After optimizing key reaction parameters, we found that a 0.1 M acetonitrile solution of lepidine (1), containing 20 mol% FeCl₃, 1.2 equiv. N-fluoro-succinimide (NFSI), and 3.5 equiv. trifluoroacetic acid (TFA), when combined with a stream of gaseous ethane (40:1 (V/V) = 23 equiv.) and irradiated with 365 nm light, yielded the desired alkylated heteroarene (2) in 65% ¹H NMR yield (Table , Entry 1). Specifically, the reaction mixture was introduced into a continuous-flow microreactor (ID = 0.76 mm, 2.8 mL volume) and irradiated using UV-A light (Chip-on-board LED, λ = 365 nm, 144 W optical power) (see Supporting Information) for a total residence time of 60 minutes. A back pressure regulator at the reactor outlet ensured constant reaction pressure (52 bar), allowing for the complete liquefaction of ethane. Prolonging the residence time to 90 minutes, however, resulted in a lower yield, likely due to product decomposition via HAT (Table , Entry 2, 60% yield). Increasing the amounts of either FeCl₃ or TFA also led to reduced yields (Table , Entries 3 and 4, 60% and 42%, respectively). Adjusting the gas-to-liquid ratio (G:L) from 40:1 (V/V) to 60:1 (V/V) caused a further drop in yield (Table , Entry 5, 50% yield). Interestingly, the use of PIFA significantly reduces the yield of the reaction ( With the optimized conditions in hand, we began investigating the scope of the photochemical ethylation of heteroarenes using ethane as the alkylating agent (Figure ). Notably, lepidine and the more electronrich 4-methoxyquinoline afforded the desired products in moderate to good yields (2-3). Similarly, 2phenylquinoline was efficiently alkylated under the optimized conditions (4). As for electron-deficient heteroarenes, we observed that decreasing light intensity (14-29 W) and lowering the residence time (30-60 min) were necessary to maximize yield and selectivity during this study. Specifically, under these modified reaction conditions, a variety of halogenated quinolines were effectively converted into their corresponding ethylated analogues in moderate to good yields (5-7). Moreover, derivatives of quinoline-4carboxylic acid were successfully alkylated using this photocatalytic protocol (8-10). Of particular note, allylic and menthol ester analogues, which present C(sp 3 )-H bonds with lower bond dissociation energy (BDE) compared to ethane, afforded the desired products in good yields (9 and 10). The method was successfully extended to other classes of pharmaceutically relevant heterocycles such as phenanthridine, benzothiazole, 4-hydroxyquinazoline, phthalazine, and 2-chloro-3-ethylquinoxaline. In all cases, the corresponding ethylated analogues were isolated in synthetically useful yields (11-15). To further demonstrate the synthetic utility of this protocol, we targeted the late-stage incorporation of ethyl fragments into natural products and drugs. As shown in Figure , the xanthine core of caffeine and pentoxifylline could be effectively alkylated (16-17). Interestingly, selective mono-ethylation at the C1 position of fasudil was achieved despite the presence of multiple weak aliphatic C(sp 3 -H) bonds prone to undergo HAT (18, 35%). C4-selective ethylation of the anticancer natural product Camptothesin, bearing sensitive functional groups such as alcohol, lactone, and pyridone, was successfully achieved in 53% yield (19). Similarly, selective C2 functionalization of an EGFR tyrosinase inhibitor was accomplished, yielding the corresponding alkylated analogue (20, 25%). Fenzaquine, a quinazoline-based insecticide, could be readily ethylated, offering straightforward access to a previously undescribed analogue (21, 22%). Next, we explored whether the protocol could be extended to the incorporation of different small alkyl fragments using other abundant light alkanes. By simply switching gas bottles, both propane and butane served as effective alkylating agents under the optimized photocatalytic conditions, affording the corresponding functionalized heterocycles in moderate to good yields (22-29). For both gases, preferential functionalization occurred at the methylene site, yielding regioisomeric mixtures (i.e., secondary vs. primary C-H bond) that could be readily separated via column chromatography, thus providing access to both regioisomers in a single experiment.