What are the intermolecular forces in methanal, HCHO?

A)London forces only

You will not get hydrogen bonds in methanal (or as it is more commonly known, formaldehyde) because whilst there are hydrogen atoms in the molecule, they are not bonded to the electronegative oxygen atom.

There will be some dipole-dipole interactions between the permanent dipole of the carbonyl group (partial +ve on the carbon, partial -ve on the oxygen). And there will be London dispersion forces as there are in all molecules.

So the answer is (D).

Permanent dipoles are imbalances of electron charge density over a covalent bond caused by a large difference in electronegativity of the two bonded atoms. Oxygen is much more electronegative than carbon, so it pulls a disproportionate share of charge towards itself giving itself a partial negative charge, whlist the carbon atom is electron charge deficient with a partial positive charge. The partially poistive C and partially negative O of adjacent molecules can align with one another to form additional intermolecular forces.

These are “additional” to London dispersion forces, exhibited by all molecules. These forces are very weak and are caused by correlated electron movements in adjacent molecules. This causes molecules to repel each other when in very close proximity, leading to instantaneous (but not permanent) dipoles.

Source(s): Chemist

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