A Quick Introduction to Supramolecular Chemistry Quiz

Answer: Chemistry beyond the molecule

While it is true that supramolecules are generally large, it is not necessarily true that all supramolecules are. Supramolecular chemistry is primarily concerned with the intermolecular forces between specific molecules. It is said that molecules are to supramolecules as atoms are to molecules.

Answer: George Wittig

In 1987 Lehn, Pedersen and Cram were honoured with a Nobel Prize for chemistry due to their work in the field of supramolecular chemistry. If the name Wittig rings a bell then it is probably due to the Wittig Reaction, a popular reaction in undergraduate teaching.

Indeed, Wittig was also a Nobel Prize winner in chemistry with Herbert Brown for their work on boron and phosphorus chemistry.

Answer: Non-covalent bonding

"Traditional organic chemistry" is mainly making and breaking covalent bonds to form new molecules and this is where supramolecular chemistry departs from this discipline. Supramolecular chemistry uses non-covalent forces to achieve the formation of new supramolecules from molecules.

However, that is not to say that there is an absence of covalent bonding in supramolecular structures, merely that the non-covalent bonds are what binds the molecules present together and hence in essence, transforms a collection of molecules into a supramolecule.

Answer: DNA

The DNA double helix is a very good example of a supramolecule. The hydrogen bonding present is the primary method by which the two strands are held together and the structure of each base provides an ideal match for only one of the four bases. This is the basis for the recording of genetic information that DNA is known for.

Answer: True

There is an entropic price to pay if molecules require rearrangement to the correct geometry for binding. An example of this rearrangement is when a bond needs to be rotated to allow proper access to a lone pair for hydrogen bonding. If the lone pair can be fixed in the correct geometry on synthesis, by making the molecule aromatic and hence with no free rotation about bonds for example, then this decreases or eliminates the entropic penalty for reorganising the molecule to the correct geometry for binding from the geometry adopted when free.

This is just one of the ways that binding affinity can be increased.

Answer: False

While it is true that Van der Waal's forces are very weak, there is still often a large component from them due to the sheer number of Van der Waal's interactions present, particularly with large molecules. However, the non-directional nature of Van der Waal's forces makes them very difficult to utilise when attempting to design an artificial supramolecule.

Answer: All of these

Cations can generally be bound fairly effectively by ion-dipole interactions as well as hydrogen bonding. The same principles can be applied to anions, although this is an area which has been less extensively studied. It is even possible to bind a zwitterionic (both positively and negatively charged) guest by designing two separate domains in the host to bind both types of charge, this ditopic receptor is much more difficult to design however, as effectively you must design two binding sites and find an appropriate way of linking them.

The binding of neutral species usually relies on non-electrostatic interactions, such as pi-pi stacking and hydrogen bonding.

Answer: Cations

Crown ethers (developed by Pedersen) are good binders of metal cations, e.g. K+. The oxygen atoms form electrostatic interactions with the cation, holding it in place in the middle of the ring. There is a degree of size selectivity here, i.e. 18-crown-6 (a ring with 18 atoms, 6 of which are oxygens) binds K+ much more strongly than any of the other alkali metals due to the fact that K+ is the best 'fit' in the ring.

Answer: It would allow very efficient and stereoselective catalysis of often very difficult reactions.

A supramolecular catalyst would be capable of bringing the two (or more) reactants into such close proximity and in the correct geometry (thereby reducing the activation energy of the process) so as to facilitate the reaction. In reactions where there is more than one outcome, in terms of stereochemistry, the binding of the reactants may block attack to particular sides of the molecule or make a particular geometry of either a product or intermediate much more favourable. An example of this would be blocking attack on one side of a double bond and so limiting the product to the Z isomer, where under non-catalysed circumstances the outcome would be a racemic mixture.

While supramolecular catalysts may be capable of replacing metal catalysts these supramolecular catalysts, due to their complexity, would be unlikely to be cheaper than all but the most expensive metals. Metal catalysed reactions are also very efficient, palladium catalysed cross couplings, for example, are known to produce almost 100% isomerically pure products. To improve upon these would be nigh on impossible! However it may be possible to improve upon other, less efficient reactions.

Answer: False

The lock and key principle actually states that in order for effective and selective binding to take place the size, shape and position of a binding site/s in a host is ideal for a specific guest. This principle is often seen in nature, where the active site of an enzyme is exactly the right size, shape and position to easily bind the correct substrate.

In fact the active site of an enzyme is often so well designed that very few other molecules, if any, will fit in the active site suitably for binding.

The same is true for receptors and this gives rise to the challenge in designing novel drugs.