Hey guys when discussing why certain polysaccharides of glucose are less soluble is it better to say hydroxyl groups exposed or glycosidic links exposed? Also is the bonding between respective molecules hydrogen or dispersion? Like for amylopectin would I say due to its branched structure it undergoes long range dispersion forces or long range hydrogen bonding which causes more hydroxyl groups to be exposed?
Thanks guys!
I'm going to start with the second question because I think there might be some confusion around intermolecular forces. Both dispersion forces and hydrogen bonding are examples of intermolecular forces - and the third one you should be familiar with are dipole-dipole interactions. There are more, but in VCE, we're just going to think of these 3. They can also be examples of intramolecular forces, but we're just going to focus on them being intermolecular for now.
In order of strength, these forces go hydrogen bonding >> dipole-dipole interactions >> dispersion forces, and molecules are always going to preference the strongest interaction that they can. They might undergo multiple (for example, sodium dodecyl sulphate is a molecule that undergoes both hydrogen bonding, but also dispersion forces), but they will always preference the strongest available to them. Just because two molecules are the same, you cannot say that they will undergo hydrogen bonding or dispersion forces just because they're the same, because it depends on the molecule.
Some examples - hexane can only undergo dispersion forces. It cannot form hydrogen bonds or dipole-dipole interactions. So, hexane will form dispersion forces with other hexane molecules. Water can do all three, but it will favour hydrogen bonds with itself, because it is both a hydrogen-bond donor and a hydrogen bond acceptor. A weirder example is acetone. Acetone is a hydrogen bond acceptor, but not a donor - it is also soluble in both hexane and water. So in hexane, acetone will only form dispersion forces with hexane molecules. In water, acetone will form hydrogen bonds with other water molecules. However, since acetone is polar, it can form dipole-dipole interactions as well. So, acetone molecules will form dipole-dipole interactions with itself - and it will do that whether it's dissolved in hexane or with water, because that's the strongest force it can form with itself. So, if you imagine a giant pot filled with hexane, water, and acetone (note: this is a pretend pot, because in a real pot, the water and hexane will not mix), then you will see all forces happen at once. Whenever acetone gets close to water, it will start to form hydrogen bonds. However, when the acetone moves away from the water, it will start to lose those hydrogen bonds and form different interactions depending on what it finds while moving around - if it's another acetone molecule, that'll be dipole-dipole, and if it's hexane, it'll just be dispersion forces. I've attached a picture to hopefully get this idea across. Dispersion forces are not shown, but everywhere you see an acetone near a hexane molecule, you can assume there are dispersion forces there. Hydrogen bonding and dipole-dipole interactions are the red dotted lines
As for the first question - there's two contributing reasons that causes them to be less soluble. The first is the fact that it gets bigger - the bigger something is, the more bonds it can make with other molecules that are the same. It's the same reason that as you increase carbon chain length in methane, ethane, propane, etc., that the state of matter at room temperature changes from gas, to solid, to liquid. The other one is that you lose hydrogen bonding groups. Remember - water wants to form hydrogen bonds, because it can form them, so those are all that water will be looking for. As the chain gets bigger, and hydroxy groups turn into glycosidic links, you start to lose available hydrogen bonding sites, and so solubility in water will decrease. The key point, though, is that it's not one or the other - it's both, and both are equally correct.
Also, hydrogen bonding HAS to be short ranged. If the molecules move far away, then the hydrogen bond becomes weaker, until eventually it's only as strong as a dispersion force.