Hydrogen Bonds – Alcohols

00:00 This is important in the physical and chemical characteristics of this class of compound.

00:01 Physically, there are hydrogen bonds that can be formed. These are a special type of dipole-dipole interaction. And what I’ve shown here is an example of four alcohol molecules all close together. Because you have a partial positive charge on the hydrogen and a partial negative charge on the oxygen, it’s possible to form a dipole-dipole bond between hydrogens and oxygens on neighbouring molecules. These are a particular type of dipole-dipole interaction known as hydrogen bonds. And whenever you see a hydrogen attached to an oxygen like that, you should be thinking about a possibility for hydrogen bonding not just by itself with another molecules of the same, but with other molecules as well.

00:49 Hydrogen bonds are a special type of attractive interaction that exist between an electronegative atom and a hydrogen atom bonded to each other.

01:00 It has only 5-10% strength of the comparative covalent bond, but it has a profound effect on the boiling point and solubility of the alcohols.

01:12 The cells of living things are made up of a vast variety of different molecules, but two important classes, which I’d like to bring to your attention now, are nucleic acids found in DNA and proteins.

01:26 Parts of these very, very large molecules are involved in hydrogen bondings with other parts of the same molecules. And this is what gives them their unique structural characteristics in many cases. Not just the strong covalent bonds, but indeed the shape of the molecules is informed largely by the intermolecular interactions which, in this case, are manifest in an intramolecular fashion.

01:50 This is very important in establishing their structure and their properties, particularly when you’re looking at things like proteins; the structure is essential for its biological activity.

02:00 Let’s have a quick look at hydrogen bonding as it occurs in DNA: deoxyribonucleic acid.

02:06 The double helical structure of DNA is due largely to hydrogen bonding between the base pairs. Here, we have an example of so-called duplex or Watson-Crick base pairing, after the people who have discovered this phenomenon.

02:23 Here you can see the hydrogen bonding is possible between a hydrogen and a nitrogen. Note that as before, when we were talking about electrophilic species, nitrogen and oxygen and also sulphur fall into that category. And so, hydrogen bonding, as you can see, between the oxygen and the delta positive of the hydrogen directly attached to the nitrogen at the top facilitates the interaction of guanine and cysteine, the pairing of the bases.

02:51 You’ll also see, in the case of adenine and thiamine, the other two bases found in DNA, that hydrogen bond interactions take place there as well. And it’s this binding together that holds the complementary bases aside from each other on either strand.

03:10 Let’s have a quick look at hydrogen bonds in proteins.

03:14 If we look, for example, at hydrogen bonds, they play an important role in the three-dimensional secondary structures of proteins. The primary structure of a protein is given by one amino-acid attached to the next which is attached to the next in a covalent polyamide fashion.

03:30 But, the reality is that the complex nature of the proteins is down to, in many cases, the three-dimensional structure which is informed not just by the pH, but also by the interactions of amino-acid with neighbouring amino-acids via hydrogen bonding.

03:47 Bearing in mind, if you look for example at an amide bond, which we’ll do a little later on, it’s possible for it to act both as a hydrogen bond donor and also a hydrogen bond acceptor.

03:59 Hydrogen bonds form between a backbone oxygen and amide hydrogens to form the more frequently either alpha-helix or the beta-sheet secondary structure of the proteins, as shown here.

04:11 It also, as I said, strongly influences the physical characteristics of this homologous series. Let us compare the corresponding alkane to the alcohol.

04:24 As you can see, in row one, we have one carbon. In the alkane form, this is methane. The boiling point of methane is -161 degrees. However, the corresponding alcohol, with only one carbon in it, actually has a boiling point of around 65 degrees.

04:44 If you actually compare, more energy is needed to break the hydrogen bonds between molecules of alcohols, so the boiling points are higher than the compare... the alkane in comparison, which has a similar molecular weight.

04:58 If we go down all the way down to number 5, where we are looking at the equivalent pentanol on the right hand... left hand side, you’ll see it has a boiling point of 136 degrees.

05:10 And this compares to 35 degrees in the case of the pentanol on the other side.

05:19 Solubility.

05:19 As we said before, the difference between alkanes and alcohols is related, to a large extent, to their ability to hydrogen bond, not just with each other, as we’ve shown, and therefore, to increase their boiling point, but also with water. So, for relatively low molecular-weight alcohols such as methanol and also the favourite ethanol, solubility in water can also be achieved.

05:43 Water-miscible alcohols include methanol, ethanol and propanol. Butanol is partially soluble at around 8% by weight and pentanol slightly less. The larger the alcohol chain, the more it will define the degree of solubility of a given alcohol in water.

06:03 Again, as I showed you, the water solubility is predicated on the ability of an OH dipole to interact with another OH dipole.

06:12 Now, let’s have a quick look at the reactivity.

06:16 You may recall this style of diagram from when we were talking about haloalkanes, where we’re trying to find which is the delta negative and which is the delta positive species in this case.

06:26 As we know that oxygen is more electronegative than carbon, we can be reasonably confident that that bond, that R-O bond, is polarised. Oxygen bears a partial negative charge and the alkyl carbon, shown there as R, bears a partial positive charge. And the reactivity depends on the structure of the alcohol group. The properties, in terms of the physical properties observed, are determined by the presence of the OH group. But, in either cases, as we saw in the case of the haloalkanes, substitution or elimination reactions may be the result.

07:00 So, let’s have a look at the formation of alkyl halides from alcohols.