Lecture Two - Chirality and nomenclature for enantimers Flashcards
Define and differentiate between isomers, constitutional isomers and stereoisomers.
Isomer - Different compounds with the same molecular formula.
Constitutional isomers - Isomers with the same molecular formula but a different connectivity, that is, a different order of attachment of their atoms.
Stereoisomers - Isomers with the same molecular formula and the same connectivity but a different orientation of their atoms in space that cannot be interconverted by rotation about a single point (mirror image).
Define chiral, achiral and a stereogenic centre.
A molecule (or object) that is not superposable on its mirror image is described as being chiral.
A molecule (or object) whih is superposable on its mirror image is achiral.
An object that is achiral has a plane of symmetry.
Many molecule contain an asymmetrical carbon centre, a so called stereogenic centre.
This is easily identified as a carbon atom attached to four different groups.
Define enantiomers.
Stereoisomers (differ only in the spatial orientation of their component atoms are called stereoisomers) that are mirror images of each other are enantiomers.
Enantiomers have identical physical and chemical properties in an achiral environment (same MP, BP, solubility, chemical reactivity etc.).
In a chiral environment, enantiomers exhibit different physical and chemical behaviours.
Define optically active and inactive substances.
An optically active substance rotates the plant of polerised light and this rotation, or optical activity, is detected and measure in a polarimeter.
After polerised light is generated it passes through the sample cell. After emerging from the cell, the light passes through a second polarising filter and finally the detector/eye.
If the sample is optically inactive, the plane is unchanged and no rotation is detected.
If the sample is optically active, the plane of polarisation is rotated. The polarimeter is used to determine both the direction and magnitude of the rotation.
How can enantomers be distinguished.
Enantiomers can be distinguished by looking at how the interact with a plane of polarised light.
They are optical isomers.
Ordinary light consists of waves vibrating in all planes perpendicular to its direction of propagation.
Plane polarised light consists of waves vibrating only in one plane.
Polarimeter - a device for measuring the extent of rotation of plane polarised light. Polarising filters are made from calcute or polaroid sheeting.
What does specific roation depend on?
Specific roatation (α) depends on the temperature (T) and wavelength (λ) at which the measurement is taken, together with the concentration of the sample (c) and length of the polarimeter cell (l). It is as much a property of a compound as its melting point, boiling point or density.
Observed rotation - the number of degrees, α, through which a chiral compound rotates the plant of polaraised light.
Define dexterity and levorotatory
Dexterity (d) or (+): rotation of the plane of polarized light to the right (clockwise).
Levorotatory (l) or (-): rotation of the plane of polarized light to the left (anticlockwise).
Define a racemic mixture.
Also known as a racemate, it is a mixture of equal amounts of the enantiomers. Because the mixture contains equal amounts of dextro and levotatory molecules the specific rotation is zero (optically inactive).
This is often prefixed with the symbol
How can enantiomers be named?
Using the Cahn, Ingold, Prelog system of nomenclature.
Used to designate configuration at a stereocentre.
The lables R and S are used to differentiate between enantiomers.
Do not confused the (+) and (-) with the R- and S- symbols.
The (+) and (-) signs indicate the direction that the enantiomer rotates polarised light.
The R- and S- indicate the arrangement of the groups at the stereogenic centre.
Nomenclature rule:
1) Locate the stereogenic centre.
2) Assign a priority (1 -> 4). 4 being the lowest atomic number and 1 being the highest atomic number.
3) Orient the lowest priority substituent away from you - usually hydrogen.
4) Read the groups 1 -> 3, if the reading is clockwise, it is designated as R. If the reading is anticlockwise, it is designated as S.
What is the value for the specific rotation of a pair of enantiomers?
The value for the specific rotation for a pair of enantiomers is the same, just with opposite signs.
In order to calculate the number of possible stereoisomers of a molecule, which rule can be used?
Are enzymes chiral?
Enzymes are chiral substances that only produce or react with substances that match their stereochemical requirements.
Enzymes can distinguish between different enantiomers of glyceraldehyde.
