Do laws of physics prohibit direct conversion of particles to antiparticles? Can antiparticles only be created in pair production? How/which laws of physics prohibit direct conversion of say an electron to a positron? A neutron to an antineutron?
I have seen a comment that it is thermodynamically impossible. True? How exactly? Any other ways it is theoretically impossible?
 A: What prohibits a particular interaction from occurring are the applicable conservation laws which must be obeyed. In your two examples:

*

*The conversion of an electron to a positron would
violate the law of conservation of charge, and would also violate conservation of lepton number, meaning the reaction $e^+\rightarrow e^-$ is forbidden.


*The conversion of a neutron to an antineutron,
since baryon number is not conserved, $n\rightarrow \bar n$  is also forbidden.
You'll find that other interactions are allowed only if the relevant conservation laws are obeyed.
A: It depends on the particular kind of particle. Assuming the Standard Model holds, then:

*

*Electrons can't convert to positrons because that would violate conservation of charge.

*Neutrons can't convert to antineutrons because that would violate conservation of baryon number.

*When permitted by conservation laws, any particle can and generally does convert to its antiparticle. Examples include neutral kaons, $D^0$ mesons, and $B^0$ mesons. Kaon oscillations in particular have been measured with exquisite sensitivity, and provide some of the strongest known constraints on physics beyond the Standard Model.

*The story for neutrinos is more complicated, but to oversimplify it a bit, we still don't know whether they can or can't convert to their antineutrinos.

