Chap. 5 NotesChap. 5 NotesOverview of Isomers Isomers: compounds that are constructed from the same atoms, but still differ from each otherCis stereoisomer – exhibits groups on the same side of the double bondTrans stereoisomer – exhibits groups on opposite sides of the double bond.ChiralitySuperimposable: when the mirror image of a compound is identical to the original image of the structure.- Objects that are not superimposable on their mirror images are called chiralSame molecular formula and constitution but different spatial arrangement of atomsSame molecular formula but different constitution (order of connectivity of atoms)IsomersStereoisomersConstitutional isomers- The two compounds above are non-superimposable mirror images and they represent 2 different compounds. These two compounds differ from their spatial arrangements of their atoms and therefore are called sterioisomers.Chiral Carbon: when a carbon is bonded to 4 different types of groupsExamples:- These are some of the examples of a chiral carbon, as shown the green areas are the chiralcarbons, bounded to 4 different groupsEnantiomersEnantiomers: when a compound is chiral and has one non – superimposable mirror image- Of course for all obvious reasons a chiral compound would never have more than one enantiomer. - A way to look at it is if you place mirrors in all 3 dimensions and draw the molecule the way it would look like in front of mirrors surrounding itDesignating Configuration using the Cahn – Ingold – Prelog (CIP) System- It is a nomenclature for identifying each enantiomer differentlySteps:1. Assign priorities to each of the four groups attached to the chirality center, based on atomic number – the atom with the highest number is assigned first priority (1) and the atom with the lowest number is assigned the last priority (4).2. After assigning the priorities, we then rotate the molecule so the fourth priority goes to the back, in other words, behind the page – If the fourth priority had to be switched with another priority than was originally at the back, then the other two priorities have to be switched with each other like so:3. Then we look to see if the sequence 1-2-3-4 is clockwise or counterclockwise – if it clockwise then it is labeled “R”, meaning recuts, and if it is counter clockwise it is labeled “S”, meaning sinister. - “R” and “S” are used to describe the configuration of a chirality center. - Usually, groups are prioritized over the electronegativity however, sometimes it is hard toprioritize molecules such as the one below.- In this case we look at the first carbon on the left and on the right and the atoms surrounding it, in this case both the carbons have hydrogens surround it which makes them indifferent. So then we go to the next one, which is highlighted in green, and as listed the carbon on the left is bonded to two carbons and one hydrogen, making that the first priority over the other carbon.- Wen naming a chiral carbon the configuration is indicated at the beginning of the name, italicized and in parentheses.Example:Optical Activity- Even though enantiomers have the same physical properties, they behave differently when exposed to plane – polarized light. - the image below is a representation of electromagnetic radiation (light) and as shown the planes of oscillating electric and magnetic fields are perpendicular to each other.- The red and blue are the different planes of polarization of light waves.- When light passes through a polarized filter, only photons of a particular polarization are allowed to pass through, giving rise to plane polarized light.- The figure above shows two parallel polarizing filters (a) and two perpendicular polarizing filters (b)PolarimetryOptically active: when organic compounds (chiral compounds) rotate the plane of plane polarized light and compounds (achiral compounds) that lack this ability were said to be optically inactive. Polarimeter: A device that measures the rotation of plane – polarized light caused by optically active compounds.- When a solution of a chiral compound is placed in a polarimeter, the observed rotation will be dependent on the number of molecules that the light encounters as it travels through the rotation. - Path length and concentration are directly proportional to observed rotation.- In order to compare the rotations for various compounds scientists had to choose a set of standard conditions, this is where specific rotation comes in. Specific Rotation: the observed rotation under these standard conditions.- Standard conditions include: concentration (1g/ml) and path length (1dm)specific rotation=[α]=αc∗l- α stands for observed rotation, c is the concentration and l is the path length- The specific rotation of a compound is sensitive to temperature and wavelength, however these factors cannot be incorporated into the equation, because the relationship between the factors and specific rotation is not linear. - A compound exhibiting a positive rotation (+) is called dextrorotatory, D- A compound exhibiting a negative rotation (-) is called Levorotatory, L- The first enantiomer is Levorotatory and is therefore called (-) – 2 – bromobutane and thesecond enantiomer is dextrorotatory and is therefore called (+) – 2 – bromobutane.Enantiomer excess- A solution containing a single enantiomer is said to be optically pure, or enantiomericallypure.Racemic mixture: a solution containing equal amounts of both enantiomers.- When plane – polarized light passes through such a mixture, the light encounters one molecule at a time, rotating slightly with each interaction. Since there are equal amounts of both enantiomers, the net rotation will be zero degrees, hence making the mixture as a whole optically inactive. - A solution containing two enantiomers of a different ratio will be optically active, such asmaybe a solution containing 70% (R) – 2 – butanol and 30% (S) – 2 – butanol. Another thing we must look at is a presence of enantiomeric excess and this case there is an excess of 40% of (R) - 2 – butanol. %ee=|observed[α]||[α]of pure enantiomer|∗100 %Stereoisomeric relationships: enantiomers and diastereomersDiastereomers: stereoisomers that are not mirror images of each other, like cis and trans isomers. - The main difference between enantiomers and diastereomers is that enantiomers have the same physical properties, however diastereomers have different physical properties.