today we are going to learn about one saccharides and stereoisomerism stereoisomers are those molecules having the same molecular formula they have the same atomic connections they have a different three-dimensional shape and are interchanged only by breaking bonds there is more to understanding the structure of a monosaccharide than knowing the number of carbon atoms than it has and whether it is an aldehyde or keto we must also identify which particular stereoisomer is present now for instance we have this two stereoisomers so we have cyst2butene and trans2butine stereoisomers like this to exist as a result of hindered rotation about covalent bonds they have the same molecular formula they have the same atomic connections they have a three dimensional shape which are different from each other and it can only be interchanged by breaking this ones an important property of many molecules including most carbohydrates is the property of handedness everything has a mirror image so if you hold your left hand up to a mirror you see its mirror image which matches your right hand if you turn your palms down and toward each other one hand is the mirror image of the other now if you try looking at the palms of your hands your thumbs are now on opposite sides if you then place your right hand over your left hand you cannot match up all the parts of the hands the palms the back the thumbs and the little fingers the thumbs and the little fingers can be matched but then the palms or blocks of your hands are facing each other your hands are mirror images that cannot be superimposed on each other when the mirror images cannot be completely matched they are called non-super impossible mirror images can either be superimposable or non-super non-superimposable we define superimposable mirror images as images that coincide at all points when the images are laid upon each other just like this example of a flask which has no markings so the mirror image can be superimposed to the original image non-superimposable mirror images are those images were not all points coincide when the images are laid upon each other just like our left and right hand the term chiral is used to describe objects that cannot be superimposed on their mirror image so our left and right hand are chiral objects left and right shoes are chiral left and right handed golf clamps are also chiral what is a chiral center an atomic molecule that has four different groups bonded to it in a tetrahedral orientation is a chiral center let us look into the simplest carbohydrate for an instance glyceraldehyde if you will look at this molecule there are actually four groups bonded to the central carbon atom so we have cho we have hydrogen we have the oh or the hydroxyl group and we have ch2oh this are four different groups which make glyceraldehyde a chiral molecule any molecule containing a chiral center or carbon is a chiral molecule and will exist as a pair of enantiomers now what are the guidelines for identifying chiral centers so these guidelines can help us to identify if there is a chiral center present in the molecule making the molecule chiral first carbon atom with multiple bonds do not have a chiral center so in this example we have here carbon as a central atom or as one central atom and we notice that there are three groups bonded to carbon and we have here a multiple bond so this means that there would be no four groups that can attach to carbon therefore carbon here this carbon is not chiral center carbon atom with two similar groups bonded to it is not chiral center here we have this carbon and this carbon is bonded to four groups of atom it is bonded to hydrogen and it is also bonded to this group over here however we have two methyl groups or two ch3 groups which are similar groups so this means that this carbon is not a chiral center carbon atoms in a ring system so here this carbon over here is not a chiral center therefore this molecule is a chiral this one we have a chiral carbon because the two halves of the ring are different from each other and therefore there are two different groups of atoms bonded to our carbon which makes carbon chiral and which makes the molecule chiral this time let us take a look into this molecule for example let us identify if there is any chiral center present let us start with carbon 1. carbon 1 is bonded to 2 hydrogen and therefore cannot be chiral even though carbon 1 is bonded to 4 groups of atoms but the presence of two similar group of atoms will not make it chiral carbon two is bonded to four different groups so we have the o h here we have the ch2oh we have hydrogen and we have this group over here which are four different groups and therefore carbon number two is a chiral center carbon number three is also bonded to four different groups so we have this group over here bromine is another group chlorine is another group and ch3 is another different group so that means carbon 3 is also a chiral center carbon 4 is bonded to three hydrogen atoms and therefore cannot be chiral since the molecule has chiral centers it is chiral molecule or it exhibits handedness the presence of even one chiral center will make the whole molecule chiral what are enantiomers stereoisomers that are mirror images are called enantiomers this means that they have the same molecular formulas but different spatial arrangements of their atoms so no amount of rotation can convert one of these structures into the other glyceraldehyde is again an example and here we have a pair of it's a natural so we have l or left handed glyceraldehyde and the glyceraldehyde or right-handed glyceraldehyde the dnl designations for the handedness of the two members of an enantiomeric pair come from the latin words dextro which means right and level which means left now what happens when there is more than one chiral center present as the number of chiral carbon atoms in a molecule increases so does the number of stereoisomers that can exist so this is a general formula used to predict the maximum number of stereoisomers possible for a molecule where n is a number of chiral carbon atoms so that means if there are two chiral carbon atoms present in one molecule then the maximum number of stereoisomers is four now what about diastereomers diastereomers are those stereoisomers that are not related as mirror images and are therefore not a naturals they are not mirror images and non-superimposed so just like this example this two stereoisomers have the same molecular formula they also have the same arrangement of atoms but are not mirror images and are therefore diastereomers so if we will look at the example given the molecule on the left has all its bromine atom on the right side while on the other molecule the bromine molecule are separated one is on the left side and the other is on the right side another type of stereoisomers are epimers epimers are actually diastereomers that differ only in the configuration at one chiral center so let us take a look at one example so here we have this two molecules which are diastereomers since they are not mirror images and they are non-superimposable but specifically cold epimers if we will look at these two images or at least two molecules the positioning of the o h groups of these two o h groups are on the same side but then on this chiral center the position of the o-h group differs so therefore this is an example of a pair of epimers