What are isomers?
Isomers are molecules with the same molecular formula, but different arrangements of atoms. There are different types of isomers, they are
Structural isomerism occurs when two or more organic compounds have the same molecular formulae, but different structures. These differences tend to give the molecules different chemical and physical properties. There are three types of structural isomerism that you need to be aware of at school: chain isomerism, positional isomerism and functional isomerism. There is a fourth type, known as tautomerism (where there are two isomers are known as the keto and enol isomers) but you won't come across this at school.
Stereo-isomerism occurs when the atoms in a molecule can have different arrangements in space. There are two types of stereo-isomerism: geometrical isomerism and optical isomerism. Geometrical isomers can have very different physical properties, such as different melting points, but they tend to have the same chemical properties.
Optical isomers have the same chemical and physical properties, except that one structure rotates the plane of polarised light to the right and the other rotates it to the left.
Geometric isomers have the same structural formulas but differ in the arrangement of groups at a single atom, at double bonds, or in rings. Cis- and trans-platin (see Figure 37) are examples of geometric isomers based on the different arrangement of groups at a single atom. Cis- and trans-2-butene differ in the arrangement of the methyl groups about the double bonds.
Although geometric isomers have completely different physical and chemical properties (for example, cis- and trans-2-butene have different boiling points and densities), optical isomers (also called enantiomers) differ in only one characteristic--their interaction with plane polarized light. When a beam of light is passed through a certain...
(A) Illustration of the application of the Cahn–Ingold–Prelog rules for describing the configuration of chiral centres. In the Cahn–Ingold–Prelog system, the four substituents around the asymmetric carbon atom are assigned a priority based on three rules: (i) consider the first atom in each substituent, highest priority is given to the atom with the highest atomic number; in case of isotopes, a higher atomic mass gives a higher priority. (ii) If two substituents have the same first atom, move away from the asymmetric carbon to the next bonded atom until a difference is reached. (iii) If the substituent contains a double or triple bond, then the atom farthest from the asymmetric carbon counts two or three times respectively. The molecule is then drawn with the lowest priority substituent at the back (into the page). If the priority of the other three substituents increases clockwise, the centre is assigned ‘R’ stereochemistry, otherwise it is ‘S”. (B) All natural proteinogenic amino acids are the L-form under the Fischer rules, when using the Cahn–Ingold–Prelog rules the natural form of cysteine is the R isomer, this is in contrast with the other amino acids, exemplified by serine, which is the S isomer.