Disclosed herein are methods for synthesizing N-(4-fluorobenzyl -N-(I-methylpiperidin)4-yl)-N'-(442-methylpropyloxy)phenylmethyl)carbamide,Formula (I). Also disclosed herein is the hemitartrate salt of N-(4-fluorobenzyl)-N-(I-methylpiperidin-4-yl)-N'-(4-(2-methylpropyloxy)phenylmethyl)carbamideand methods for obtaining the salt. Further disclosed are various crystallineforms of N-(4-fluorobenzyl)-N-(I-methylpiperidin-4-yl)-N'-(4-(2-metylpropyloxy)-phenymethyl)carbamideand its hemi-tartrate salt including various polymorphs and solvates.