[ffar5]

Phenacetin and 4-hydroxyacetanilide look very similar, how would you be able to tell which has a higher melting point from the chemical structure?

[wildfyr]

There is a quite critical functional group that gives a good hint here that one has and the other doesn't.

Granted, I think its a crappy homework question to ask an undergrad because such things seldom follow any true rules

[ffar5]

There is a quite critical functional group that gives a good hint here that one has and the other doesn't.

Granted, I think its a crappy homework question to ask an undergrad because such things seldom follow any true rules

What about the -OH functional group would give the 4-hydroxyacetanilide a higher melting point?

[wildfyr]

I think it is the hydroxy yes. What are hydroxy groups very good at, in terms of colligative properties? Better than amides or ethers.

[Enthalpy]

Yes. Confirmed by figures: +169°C for the alcohol and +134°C for the ether.

BUT

1) They decompose, so the melting point isn't very useful
2) Melting points do NOT follow relatively simple rules like boiling points do. Don't get fooled by this crappy question.

Just as one example: cubane C8H8 stays solid up to +133°C, wow. 1-octanol melts already at -16°C.

[Babcock_Hall]

It would have been more straightforward to predict R_f values on a TLC plate for these two compounds.