If they’re on opposite sites, it’s denoted as the ‘trans’ or ‘E’ isomer. If the highest priority groups for each carbon are on the same side of the molecule, that molecule is denoted as the ‘cis’ or ‘Z’ isomer. These atoms or groups can be given ‘priorities’, with atoms with higher atomic numbers given higher priorities. This means that, if there are two different atoms, or groups of atoms, attached to each carbon of the carbon carbon double bond, they can be arranged in different ways to give different molecules. Rotation of these bonds is restricted, compared to single bonds, which can rotate freely. This type of isomerism most frequently involves carbon carbon double bonds (shown by two lines joining each carbon instead of one). However, ‘geometric isomerism’ is still consistently used in many A Level courses to refer to both, so for that reason I’ve used that name here. Geometric isomerism is actually a term that is ‘strongly discouraged’ by IUPAC (the International Union of Pure & Applied Chemistry), who prefer ‘cis-trans’, or ‘E-Z’ in the specific case of alkenes. They’re a little more complicated to think about than the structural isomers, so let’s have a look at each of them in turn. These, as the difference in name suggests, aren’t to do with any large scale rearrangements of the structure of molecules instead, they involve different arrangements of parts of the molecule in space. There are two main types of stereoisomerism – geometric isomerism, and optical isomerism. Different functional group isomers are possible for different functional groups. As an example, a standard straight-chain alkane (containing only carbon and hydrogen atoms) can have a functional group isomer that is a cycloalkane, which is simply the carbons bonded together in such a way that they form a ring. This is possible by rearranging the atoms within the molecule so that they’re bonded together in different ways. Nothing else about the molecule changes, simply where the functional group in it is, and the name simply alters slightly to indicate whereabouts in the molecule it is located.Īlso referred to as functional group isomers, these are isomers where the molecular formula remains the same, but the type of functional group in the atom is changed. There are a range of different functional groups, the more common of which were summarised in a previous post here. A functional group in organic chemistry is the part of a molecule that gives it its reactivity. Position isomers are based on the movement of a ‘functional group’ in the molecule. Obviously, there’s often more than one way of branching off groups of carbons from the main chain, which leads to the large numbers of possible isomers as the number of carbons in the molecule increases. The name of the molecule can be changed to reflect this, but we’ll save the naming of molecules for another post. Organic molecules are based on chains of carbon atoms, and for many molecules this chain can be arranged differently: either as one, continuous chain, or as a chain with multiple side groups of carbons branching off. We’ll consider structural isomers first, which can be split again into three main subgroups: chain isomers, position isomers, and functional group isomers. Structural isomerism can quickly get quite out of hand in terms of the number of possible isomers butane (four carbons) has two possible isomers, decane (ten carbons) has seventy-five, and a simple hydrocarbon containing 40 carbon atoms has an estimated 62,000,000,000 structural isomers.Ĭhain isomers are molecules with the same molecular formula, but different arrangements of the carbon ‘skeleton’. Isomers can be split into two broad groups – structural (or constitutional) isomers, and stereoisomers. This graphic looks at the 5 main types of isomerism in organic molecules, with a more detailed explanation of each given below, as well as the reason why isomerism is important in our day-to-day lives. the same number of atoms of each element), but different structural or spatial arrangements of the atoms within the molecule. The reason there are such a colossal number of organic compounds – more than 10 million – is in part down to isomerism. In organic chemistry, isomers are molecules with the same molecular formula (i.e.
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