2C-T-x Substitution Size and Potency
The 2C phenethylamines typically refer to the 2,5-dimethoxy 4-substituted phenethylamines. Generally, small lipophilic substitutions at the 4-position tend to produce compounds which act as agonists, while larger substitutions are partial agonists or antagonists.
This statement appears to be borne out by investigation of the 2C-T-x series of compounds developed and studied by Shulgin. If we sort compounds with entries in PiHKAL by molecular mass to obtain a rough measure of substitution size, we can graph the reported dosage ranges to help illustrate this relationship.
2C-T-15 is shown as a dashed line due to the fact that it was only tested at 30mg and found to be underwhelming with no upper range established, in contrast to the more rigorously established dosage ranges for other compounds. This lack of potency could be a consequence of the inclusion of the atypical cyclopropyl group, which is also found in the disappointing 2C-T-8.
2C-T, 2C-T-2, 2C-T-4, 2C-T-7 and 2C-T-21 have smaller substitutions at the 4-position and are considered rather potent psychedelics. Similar to the relationship of 2C-D (the smallest alkylated substitution) to the rest of the active alkylated 2Cs, 2C-T is less potent per milligram but has a very similar mental state to the previously cited compounds if dosage is adjusted upwards appropriately. As the substitution size grows, compounds like 2C-T-9, 2C-T-13 and 2C-T-17 are encountered which are less potent and not as classically psychedelic, most notably possessing decreased open-eye visual activity and an increased focus on stimulant effects relative to smaller substitutions even when dosage levels are adjusted for.