Because the earth isn't flat, conversion from a linear unit to decimal degrees (and vice versa) isn't constant. One decimal degree converted to kilometers will vary depending on where you are on the earth's surface. One way to do a realistic conversion is to use a projected coordinate system. Which PCS you use depends on where you are. The process would be to project your center point to a PCS that makes sense for where you are. Since you are wanting to convert to kilometers, a PCS that uses meters for its linear unit would make things easier (UTM for example). After your point is projected, you can create your envelope by adding/subtracting from the center point's x,y coordinates. Once the envelope is created, you can project it back to the original geographic coordinate system if you need to.