Active low-pass butterworth filter design
Question: Is it possible that the required corner frequency (3 dB) is 1 kHz and the stopband frequency (with a certain required damping specification) is given with 5 kHz ? 
Please note that it is NOT possible to "filter out" a frequency of 2.5 kHz.
You only can provide a certain degree of damping - unless you design a filter with a passband zero at 2.5 kHz. However, this has no Butterworth response.

The filter order necesary to meet your requirements is determined by the stop band attenuation, which is not given up to now.

As an example: A Butterworth low pass of 4th order can be realized as a series connection of two active lowpass stages with the same 3dB cut-off frequency (1 kHz) but with two different Qp values (Qp: pole Q).