Melting/Boiling points (1 Viewer)

[mystify]

Im hopin dat sum1 can help me....im confused in a chemistry exam we had 2 explain why the melting points and boiling points of alkanols and alkanoic acids were higher than those of alkanes with similar masses. i said that it was bcoz both alkanols and alkanoic acids have strong intermolecular forces of attraction--> both have dipole-dipole attractions, dispersion forces and hydrogen bonding which gives rise to the high melting and boiling points. apparently, the teacher sed dat hydrogen bonding has nufin 2 do with it but he gave us notes saying that the hydrogen bonding was the main key. now im completely lost n d trial is soon any help would be greatly appreciated

 

[gershy]

Ok your chem teacher may have been drinking one of his/her concoctions because H-bonding explains everything here.
Alkanes have only dispersion forces as their intermolecular forces as they are more or less non-polar. Alkanols have an -OH group so they have not only dispersion forces but also stronger H-bonding, so their BPs are higher .
Alkanols have both an =O and an -OH so they can form 2 H-bonds with neighbouring molecules, hence an even higher BP.
Hope this clears up the confusion. Good luck in your trials, make sure you write in English, not in rappish.

 

[xiao1985]

lolz... agreed... ur chem teacher is drunk

alkanoaic acid is capable of forming most (3) hyrdrogen bonds... ---> highest bp
alkanol is capable of forming some hydrogen bonds (2) -------) high bp
alkanes only have dispersion forces, which, with short chain hydrocarbons is insignificant comparing to hydrogen bonds...

hence acid+alkanol has high bp than alkanes with similar masses...

 

[kow_dude]

What is the bond that connects the O and H in the hydroxy group ?

 

[tennille]

what do you mean by bonding? As in hydrogen bonding...?

 

[kow_dude]

nevermind.