We now know, from our previous discussion, that the electrons orbit the nucleus at a very large distance (on a nuclear scale), but what is the nature of the “path” they follow? An electron orbiting a nucleus is described by a wavefunction. This is contrary to the common image of electrons being like little balls orbiting … Read more
Before we start to discuss more complicated areas of chemistry, it is necessary to ensure that certain concepts of atomic structure are understood. Firstly, the atom is composed of a nucleus (about 10-15m in diameter) at the centre which contains most of the mass of the atom, and orbiting electrons (negatively charged particles), which have negligible mass. The nucleus … Read more
The aromatic ring is a very stable and electron rich entity – the delocalised π electrons lie above and below the plane of the ring and make it more attractive to electrophiles than nucleophiles. Substitution is likely to be the mechanism because this will return the aromaticity to the ring which is energetically favourable. However, because benzene itself is so stabilised … Read more
Unsaturated hydrocarbons with a triple bond are known as alkynes or acetylenes. e.g. ethyne: The naming of alkynes follows the same rules we used for alkanes and alkenes earlier. i.e. ethyne (shown above), propyne etc. Ethyne is the most commonly encountered member of this group. It can be prepared by the following method: This process of addition of water to calcium carbide is used industrially. Other mechanisms … Read more
The main type of reaction that alkenes undergo is addition. This is not a trick, it is as simple as it sounds: an addition reaction is where two molecules react together completely to form one product (and only one product). e.g.: Above is shown the reaction of ethene and hydrogen bromide. The reaction above is an … Read more
We know from our discussion of hybridisation, that the overlap of two sp2 orbitals and the additional overlap of two p orbitals forms a double bond. A double bond can be considered as a functional group, just as a single bond can. Molecules containing a C-C double bond are known as alkenes. However, unlike alkanes, which were very unreactive, alkenes can react … Read more
The carbon carbon bond (C-C for short) is probably one of the simplest functional groups. A functional group is a group of atoms (or sometimes just one atom) that have a characteristic chemical property. We have already seen that a C-C single bond is formed with sp3 hybridisation. The name of the group of molecules that contain only C-C single bonds and C-H … Read more
Esters can be hydrolysed by either aqueous solutions of acid or base. The products in both cases are an alcohol and a carboxylic acid. Below is shown the mechanism for acidic ester hydrolysis: Basic hydrolysis proceeds via a tetrahedral intermediate: The addition of the –OH nucleophile causes the intermediate to become tetrahedral.
As with acids, there are two types of base catalysis; specific and general. Specific Base Catalysis With specific base catalysis, the rate is again found to be directly related to the pH. The only species capable of catalysing the reaction is OH–. An example of a base catalysed reaction: This is the reverse of an aldol … Read more
Catalysis provides a reaction a lower energy pathway, so that the reaction proceeds at a greater rate. Acids can act as catalysts in certain situations. There are two types of acid catalysis; Specific Acid Catalysis and General Acid Catalysis. Specific Acid Catalysis (SAC) This is acid catalysis involving only the H30+ (hydroxonium) ion. Characteristically, it can be detected because the rate is … Read more