Planetary nebulae are not interstellar clouds, but rather the expanding shells of gas and dust ejected by dying stars.

Gas-phase reactions are the primary mechanism for the formation of complex molecules in space. These reactions occur between gas-phase atoms and molecules, and are driven by the high temperatures and densities found in interstellar clouds.

Dissociative recombination is not a type of gas-phase reaction that can lead to the formation of complex molecules. Instead, it is a process in which a molecular ion recombines with an electron, resulting in the dissociation of the molecule.

Grain-surface reactions play a role in the formation of complex molecules in space by providing a surface for gas-phase reactions to occur, allowing for the formation of molecules that are not possible in the gas phase, and helping to protect molecules from destruction by ultraviolet radiation.

Formaldehyde is not a prebiotic molecule, as it is not found in significant quantities in the interstellar medium. The other three molecules, however, are all found in interstellar clouds and are thought to be essential for the formation of life.