alkyl group tert-butyl group 2-methylpropan-2-ido 2-methylpropan-2-yl 0 tBu -C(CH3)3 C4H9 (CH3)3C- t-butyl 57.07043 CC(C)(C)* 1,1-dimethylethyl -CMe3 chebi_ontology CHEBI:30355 tert-butyl 57.11426 isobutane