Question

Pre-lab

1. Provide the Lewis structures for the organic molecules of benzoic acid, C6H5CO2H, and o-methoxybenzaldehyde, CH3OC6H4C(O)H.

2. Describe the intermolecular forces involved with each compound.

3. Formulate a hypothesis regarding the melting point relation between benzoic acid, C6H5CO2H, and o-methoxybenzaldehyde, CH3OC6H4C(O)H, using intermolecular forces terminology.

Answer 1

1) The lewis structure of benzoic acid is in fig-I

The lewis structure of o-methoxy benzaldehyde is in fig-II

2) The intermolecular forces in the benzoic acid are intermolecular hydrgen bonding and also weak intermolecular dipole-dipole interaction.The intermolecular hydrogen bonding is shown in fig-III

thus benzoic acids actually remain as a long chain array i.e (PhCOOH)_n in liquid phase and in solid phase it remains as a dimer.

In o-methoxy benzaldehyde there is an weak intermolecular dipole-dipole interaction shown in fig-IV

3)Now we know melting point depends on several intermolecular forces among which intermolecular hydrogen bonding and intermolecular dipole-dipole interaction play a key role.Now as per described above in benzoic acid there is a strong intermolecular hydrogen bonding along with weak dipole-dipole interaction.On the other hand in case of O-methoxy benzaldehyde there is only weak dipole-dipole interations.Thus for benzoic acid to reach the melting point we've to break the strong intermolecular hydrogen bonding hence it has a much higher melting point of 122.41C compared to O-methoxy benzaldehyde whose melting point is 30-40 C only.