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When cyanide ions act as nucleophiles in substitution reactions, why is KCN the reagent of choice? Would some other source of cyanide such as NaCN or HCN work equally well in the reaction, since they should dissociate to give cyanide ions as well?

orthocresol
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Anilo
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NaCN is quite alright. The main problem with HCN, on the other hand, is that it's a relatively weak acid which is barely dissociated (pKa 9.71). So, trying to use HCN as a source of CN− is like trying to use H2O as a source of OH− ions, which is not likely to work. On top of that, it's gaseous, and getting it into your reaction without killing yourself can be quite tricky.

orthocresol
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    Well, then why actually NaCN, or other salt isn't used? Why is this one of these cases that potassium salt is common and not sodium? BTW if OP did his research he wouldn't ask about HCN. – Mithoron Nov 21 '19 at 16:32
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    @Mithoron not sure what you're aiming for. NaCN is used quite regularly http://commonorganicchemistry.com/Common_Reagents/Sodium_Cyanide/Sodium_Cyanide.htm – orthocresol Nov 21 '19 at 16:34
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    @Mithoron Maybe the OP tried to research and couldn't understand as the literature he/she may have found wouldn't have been very easy to understand. Also I believe SE aims to answer questions which the users face trouble with (If they aren't very basic but that again is subjective from person to person) and all question which are asked aren't necessarily open questions or award winning questions. At different stages of ones studies one can have different understanding and maybe a beginner might find the same page much difficult to read than someone who does so on a regular basis. –  Nov 21 '19 at 16:42
  • Well, it sure is used, but I think KCN is the stereotypical cyanide. – Mithoron Nov 21 '19 at 17:12