Isomers are molecules that have the same molecular formula but different arrangements of their constituent atoms. Constitutional isomers (structural isomers) have different bonding arrangements of their atoms (connectivity) and usually show very marked differences in physical and chemical properties. Connectivity differences can involve the carbon skeleton or the nature and position of functional groups. Stereoisomers are molecules with identical connectivity but different spatial arrangements of their constituent atoms that cannot be interconverted by bond rotation. geometrical isomer One of two or more compounds that have the same chemical composition but differ in the spatial arrangements of the atoms. Geometrical isomers are isomers that have the same molecular formula but the atoms are in different non-equivalent positions to one another. Geometrical isomers occur as a result of restricted rotation about a carbon-carbon bond. Single bonds between two carbons in a non-cyclic structure may rotate around each other unhindered at room temperature because as the carbons rotate around each other the degree of overlap between the atomic orbitals is not changed and remain at maximum overlap as rotation occurs. However if there is a double bond between two carbons, these sp2 carbons have a sigma overlap or bond between two sp2 hybrid orbitals and a Pi overlap as a result of the pure p overlap. The sigma overlap would not change its maximum overlap as a result of a rotation about the two sp2 carbons. However if the two sp2 carbons attempt to rotate about each other this will reduce the p-p overlap of the Pi bond. This has a destabilizing influence on the molecular system and so is resisted. The Pi bond prevents the carbons from rotating about each other. As a result, the atoms attached to these sp2 carbons will always remain fixed in three dimensional space. If the sp2 carbons have different atoms attached to themselves then we would be able to differentiate between two like groups (one on each sp2 carbon) being on the SAME side of the double bond and two similar groups being on the OPPOSITE sides of the double bond. If the similar groups are fixed on the same side as a result of this restricted rotation, then the isomer would be classified as "cis". If, on the other hand, the similar groups are fixed due to restricted rotation by the double bond on opposite sides of the double bond, then the isomer would be the "trans" isomer. Cis and trans isomers will differ in their physical and chemical properties. In order to identify them we prefix the cis or trans onto the front of the name of the isomer. For example, cis-1,2-dichloroethene would have two Chlorines on the same side of the double bond. Trans-1,2-dichloroethene would have the two Chlorines on opposite sides of the double bond. If the two atoms or groups of atoms attached to the same sp2 carbon are the same, then there cannot be geometrical isomerism since the two like groups on the same sp2 carbon could not differentiate between the two like groups. there is no geometrical isomerism in Alkynes. The triple bond has two Pi overlaps and a sigma overlap between two sp hybrid orbitals. If one tries to rotate the two sp hybrid carbons there is resistance. Then why is there no geometrical isomer. That is because of the linear geometry around the sp hybrid carbons. The atoms attached to these carbons are in the same line as the carbons, so there is no difference in the geometrical orientation of these groups. Geometrical Isomerism for Cyclic Hydrocarbons The carbons found within the ring of a cyclic hydrocarbon also has restricted rotation. The carbons within the ring are sp3 hybridized. If you attempt to rotate the carbons around each other again there will be a lessening of the overlap between two sp3 orbitals. This, in turn, will cause instability if the overlap is decreased so there is a resistance to the rotation. Consequently, like the atoms attached to the sp2 carbons in an Alkene, the atoms attached to the sp3 carbons within the ring become fixed as long as the ring remains intact. If two similar atoms or groups of atoms find them selves on opposite sides of the ring, then the groups are trans and the isomer is the trans form. If the two similar groups are on the same side of the ring structure then we have the cis isomer. For example, Cis-1,2-dimethylcyclohexane would have the two methyl groups on the same side of the six-member ring system. Chirality (Stereogenicity) Chirality (cheir, Greek for "hand") refers to objects which are related as non–superimposable mirror images and the term derives from the fact that left and right hands are examples of chiral objects. sp3–Hybridised carbon atoms possessing four different substituents display this property due to their tetrahedral geometry. Such an asymmetrically substituted carbon atom is a stereogenic centre and is the commonest source of chirality in organic molecules. Unambiguous definition of the spatial arrangement of substituents on a stereogenic centre that distinguishes mirror images gives the absolute configuration. A convention permitting structural distinction between two opposite absolute configurations is based upon the sequence rules. It is independent of the chemical or physical properties of the molecule and is equally applicable to tetrahedral stereocentres other than carbon. (i) Rank substituents on the stereogenic centre in order of decreasing priority using the sequence rules. (ii) View the stereogenic centre with the lowest priority substituent pointing away. (iii) If the order of priority of the three remaining substituents decreases in a clockwise manner the centre is defined as (R)– (rectus, Latin for "right"). (iii) If the order decreases in an anti-clockwise direction the centre is defined as (S)– (sinister, Latin for "left"). Anti-clockwise direction }(S)–Alanine Enantiomers An enantiomer is one of a pair of stereoisomers that are related as non–superimposable mirror images. Enantiomerism commonly results from the presence of one or more stereogenic centres in a molecule but may also occur in orthogonal structures (allenes, hindered biaryls), helical structures (E–cyclic alkenes, helicenes) and extended tetrahedra (differentially substituted adamantanes). Such molecules are chiral and display identical chemical and physical properties in an achiral environment. However, opposite enantiomers will react at different rates with a single enantiomer of a reagent. A solution of a single enantiomer will rotate the plane of plane–polarised light and is referred to as optically active; although this physical property cannot be directly related to absolute configuration of the molecule. An enantiomer is given the prefix (+)– if the rotation is clockwise (dextrorotatory) and (–)– if the rotation is anticlockwise (levorotatory). Examples An equal mixture of opposite enantiomers is a racemate and solutions of racemic mixtures do not rotate the plane of plane–polarized light. Clearly, unequal mixtures of two enantiomers will have a lower optical rotation than a pure enantiomer and the strength of this rotation will depend upon the enantiomeric excess (e.e.) of the mixture. Mixtures of unequal amounts of enantiomers are referred to as scalemic. Enantiomeric Excess = (%Enantiomer A –% Enantiomer B)% Specific Rotation The specific rotation enables comparison of optical activity between samples by standardising the analysis conditions and permits determination of the enantiomeric excess. For dilute solutions the degree to which a substance rotates plane–polarised light depends upon the number of molecules present in solution and their ability to interact with the light. This is in turn dependent upon the concentration of the solution, the path length of the cell and the wavelength of the light used for analysis. Commonly specific rotation is quoted for light at the wavelength of the D line of the emission spectrum of sodium (589.3nm). The temperature of the sample and the nature of the solvent may also affect the value and these must also be stated when quoting specific rotation. The sign of the rotation must also be quoted. If clockwise it is +ve and if anticlockwise –ve. Where: a = observed rotation of sample (symbol 176 \f "Symbol" \s 12°) c = concentration of sample (g 100mL–1) l = pathlength of cell (dm) Quoted as: [a]Dt = ± X (c = Y, solvent) • Note the non–standard units for deriving [a] and the fact that, by convention, the figure is always quoted dimensionless. The enantiomeric excess of a scalemic mixture can be deduced from the measured specific rotation: Diastereoisomers (Diastereomers) Diastereoisomers are stereoisomers with a different relative configuration and are not related as mirror images. They have different chemical and physical properties. Molecules possessing more than one stereogenic centre also exhibit diastereoisomerism because inverting one or more (but not all) of the centres leads to structures which do not have a mirror image relationship with the original. Inversion of a single stereogenic centre gives an epimer of the original structure. Inversion of all stereogenic centres gives the enantiomer. A molecule possessing n stereogenic centres has a maximum of: 2n stereoisomers, 2n–1 pairs of enantiomers and n epimers Molecular symmetry within the molecule may result in a reduction of the numbers of different isomers due to internal compensation. Example:2,3,4-Trihydroxybutanal Diastereoisomerism also occurs in alkenes, oximes and imines where interconversion of the double bond substituents is prevented by the energy barrier to rotation about the p–bond. The E–isomer (entgegen, German for "opposite") has the highest priority substituents on the double–bonded atoms pointing away from each other and the Z–isomer (zusammen, German for "together") has the highest priority substituents on the same side. In the case of oximes and imines the lone pair on the nitrogen is counted as the lowest priority substituent of that atom.