ChemicalData returns the correct result for "Sulfuric" but an empty list for "Sulfuric Acid". Why?

Consider using a dedicated search pattern making function. Here is an example.

molekule = "Sulfuric Acid";

Clear[MakeSearchPattern]
MakeSearchPattern[s_String] := 
  StringExpression @@ 
   Join[{BlankNullSequence[]}, 
    Riffle[StringSplit[ToLowerCase[s]], 
     BlankNullSequence[]], {BlankNullSequence[]}];

MakeSearchPattern[molekule]

(* ___ ~~ "sulfuric" ~~ ___ ~~ "acid" ~~ ___ *)

Select[ChemicalData[#, "StandardName"] & /@ ChemicalData[], 
 StringMatchQ[#, MakeSearchPattern[molekule], IgnoreCase -> True] &]

(* {"SulfuricAcid", "SulfuricAcidSolution", \
"DeuteratedSulfuricAcidDeuteriumOxide", "PeroxysulfuricAcid", \
"NitrosylsulfuricAcid", "SodiumSaltSulfuricAcid,MonohexylEster", \
"TitaniumIVOxysulfateSulfuricAcidHydrate", \
"CeriumIVSulfateHydrate,ComplexWithSulfuricAcid"} *)

Update : more complicated search

is there any way to fix the exmpty list that is returned when exchanging Acid with Sulfuric ("Acid Sulfuric" instead of "Sulfuric Acid")

The following function

1. does the search with the straight string and the reverse string first,

2. if the result is empty, searches for each search word are made and the most frequent results are returned first.

The second, fall-back step can be slow since multiple searches are made. Note that the result of the first step is extended to be pairs of {_String,_Integer} in order to make the return values of MoleculeSearch consistent.

Clear[MoleculeSearch]
MoleculeSearch[s_String] :=
  Block[{swords, pats, res},
   swords = StringSplit[ToLowerCase[s]];
   pats = Map[MakeSearchPattern, swords];
   res = Select[ChemicalData[#, "StandardName"] & /@ ChemicalData[], 
     StringMatchQ[#, 
       MakeSearchPattern[swords] | MakeSearchPattern[Reverse[swords]],
        IgnoreCase -> True] &];
   If[Length[res] > 0, Transpose[{res, ConstantArray[1, Length[res]]}],
    res = 
     Flatten@Map[
       Function[{sp}, 
        Select[ChemicalData[#, "StandardName"] & /@ ChemicalData[], 
         StringMatchQ[#, sp, IgnoreCase -> True] &]], pats];
    SortBy[Tally[res], -#[[2]] &]
    ]
   ];

MoleculeSearch["acid sulfuric"]

(* {{"SulfuricAcid", 1}, {"SulfuricAcidSolution", 
  1}, {"DeuteratedSulfuricAcidDeuteriumOxide", 
  1}, {"PeroxysulfuricAcid", 1}, {"NitrosylsulfuricAcid", 
  1}, {"SodiumSaltSulfuricAcid,MonohexylEster", 
  1}, {"TitaniumIVOxysulfateSulfuricAcidHydrate", 
  1}, {"CeriumIVSulfateHydrate,ComplexWithSulfuricAcid", 1}} *)

Take[MoleculeSearch["acid sulfuric ester"], UpTo[20]]

(* {{"SodiumSaltSulfuricAcid,MonohexylEster", 3}, {"1,1,4,4Tetramethyl1,4ButanediylEsterHexaneperoxoicAcid,\
2Ethyl", 2}, \
{"1,1Benzyl3Hydroxy2OxoPropylcarbamoyl2PhenylEthylCarbamicAcidBenzylEs\
ter", 2}, \
{"1,1DimethylethylEster1H,5H,11H1Benzopyrano6,7,\
8IjQuinolizine10CarboxylicAcid,2,3,6,7Tetrahydro11Oxo", 
  2}, {"1,1DimethylethylEster2ButeneperoxoicAcid", 
  2}, {"1,1DimethylethylEsterHexaneperoxoicAcid,2Ethyl", 
  2}, {"1,1FerrocenediboronicAcidBisPinacolEster", 
  2}, {"1,1Methyl4,\
5DioxoPent2Enylcarbamoyl2PhenylEthylCarbamicAcidBenzylEster", 
  2}, {"124CarbamimidoylPhenylaminoMethyl1Methyl1hBenzoimidazol5YlCycl\
opropylPyridin2YlMethyleneaminooxyAceticAcidEthylEster", 
  2}, {"1,2Chlorophenyl3,\
5Dimethyl1hPyrazole4CarboxylicAcidEthylEster", 
  2}, {"12HydroxyoctadecanoicAcidMethylEster", 
  2}, {"1,3Hydroxy2Oxo1PhenethylPropylcarbamoyl2PhenylEthylCarbamicAci\
dPyridin4YlmethylEster", 
  2}, {"1,4Aminophenyl3,5Dimethyl1hPyrazole4CarboxylicAcidEthylEster",
   2}, {"1,4BenzenediboronicAcidBisPinacolEster", 
  2}, {"1,4Methoxyphenyl3,\
5Dimethyl1hPyrazole4CarboxylicAcidEthylEster", 
  2}, {"1,4PhenylenebisMethyleneEsterSelenocyanicAcid", 
  2}, {"16DoxylStearicAcidMethylEster", 
  2}, {"1Benzylpyrazole4BoronicAcidPinacolEster", 
  2}, {"1BocIndole5BoronicAcidPinacolEster", 
  2}, {"1Cyclohexen1YlBoronicAcidPinacolEster", 2}} *)

You could use Interpreter:

molekule=InputString[];
Interpreter["Chemical"][molekule]

This approach won't work for strings like "Sulfuric" (too many matches) or "Acid sulfuric" (wrong order), but it will work with different cases, misspellings and formulas:

Interpreter["Chemical"]["sulfuric acid"]
Interpreter["Chemical"]["sufuric acid"]
Interpreter["Chemical"]["H2SO4"]

Entity["Chemical", "SulfuricAcid"]

Entity["Chemical", "SulfuricAcid"]

Entity["Chemical", "SulfuricAcid"]