Molecular weight has NOTHING to do with the boiling point of compounds. Any apparent connection is merely a coincidence. Boiling point differences are due to differences in the strength of the intermolecular attractions.
Both molecules are liquids at room temperature, and both exhibit hydrogen bonding along with all three of the van der Waals forces: London dispersion forces, Keesom forces (dipole-dipole attraction) and Debye forces (induced attraction).
More on intermolecular forces: van der Waals forces (van der Waals forces consist of Keesom forces, Debye forces and London dispersion forces)
1. London dispersion forces .... forces between nonpolar molecules caused by the formation of temporary dipoles. All molecules exhibit London dispersion forces, LDF's. The strength of LDF's is proportional to the polarizability of the molecule, which in turn, depends on the number of electrons and the surface area of the molecule. Contrary to what some teachers and even some authors say, in many cases London dispersion forces can be stronger than Keesom or Debye forces and are second only to hydrogen bonding.
2. Keesom forces (dipole-dipole attraction) ... the attraction of one polar molecule for another. Oppositely charged ends of the molecules undergo electrostatic attraction.
3. Debye forces .... the attraction between a polar molecule and one which is nonpolar, where the polar molecule induces a charge separation in the nonpolar molecule. Debye forces can also exist between two polar molecules, even the same kind of molecule.
4. Hydrogen bonding .... The weakly covalent bonds found between the hydrogen atom of one molecule where it is bonded to N, O or F, and the N, O, or F of an adjacent molecule. The hydrogen atom functions as a bridge, forming a bond between two molecules. Hydrogen bonding is more than an electrostatic attraction between molecules, instead there is evidence of orbital overlap and covalent bond formation.
Acetic acid has a higher boiling point due to more capability of hydrogen bonding/stronger intermolecular interactions, despite having a shorter hydrocarbon chain than 1-propanol. 1-propanol has hydrogen bonding capability and more opportunity for van der waals hydrophobic-hydrophobic interactions, but...
When you put two 1-propanol molecules together, they can only form monomers by hydrogen bonding.
CH3CH2CH2OH-----(OH)-CH2CH2CH3
When you put two acetic acid molecules together, they can form dimers by hydrogen bonding.
Answers & Comments
Molecular weight has NOTHING to do with the boiling point of compounds. Any apparent connection is merely a coincidence. Boiling point differences are due to differences in the strength of the intermolecular attractions.
Ethanoic acid .... CH3COOH ............. BP = 119C ........... (aka acetic acid)
1-Propanol ........ CH3CH2CH2OH..... BP = 98C
Both molecules are liquids at room temperature, and both exhibit hydrogen bonding along with all three of the van der Waals forces: London dispersion forces, Keesom forces (dipole-dipole attraction) and Debye forces (induced attraction).
But with acetic acid there is twice as much hydrogen bonding due to -COOH. Two acetic acid molecules can form a dimer. https://upload.wikimedia.org/wikipedia/commons/b/b... (See the diagram below.)
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More on intermolecular forces: van der Waals forces (van der Waals forces consist of Keesom forces, Debye forces and London dispersion forces)
1. London dispersion forces .... forces between nonpolar molecules caused by the formation of temporary dipoles. All molecules exhibit London dispersion forces, LDF's. The strength of LDF's is proportional to the polarizability of the molecule, which in turn, depends on the number of electrons and the surface area of the molecule. Contrary to what some teachers and even some authors say, in many cases London dispersion forces can be stronger than Keesom or Debye forces and are second only to hydrogen bonding.
2. Keesom forces (dipole-dipole attraction) ... the attraction of one polar molecule for another. Oppositely charged ends of the molecules undergo electrostatic attraction.
3. Debye forces .... the attraction between a polar molecule and one which is nonpolar, where the polar molecule induces a charge separation in the nonpolar molecule. Debye forces can also exist between two polar molecules, even the same kind of molecule.
4. Hydrogen bonding .... The weakly covalent bonds found between the hydrogen atom of one molecule where it is bonded to N, O or F, and the N, O, or F of an adjacent molecule. The hydrogen atom functions as a bridge, forming a bond between two molecules. Hydrogen bonding is more than an electrostatic attraction between molecules, instead there is evidence of orbital overlap and covalent bond formation.
Acetic acid has a higher boiling point due to more capability of hydrogen bonding/stronger intermolecular interactions, despite having a shorter hydrocarbon chain than 1-propanol. 1-propanol has hydrogen bonding capability and more opportunity for van der waals hydrophobic-hydrophobic interactions, but...
When you put two 1-propanol molecules together, they can only form monomers by hydrogen bonding.
CH3CH2CH2OH-----(OH)-CH2CH2CH3
When you put two acetic acid molecules together, they can form dimers by hydrogen bonding.
https://archive.cnx.org/resources/776807da02f51996...
This means that it is more difficult to break the bonds between two acetic acids than the bonds between two 1-propanols