the topic of this lecture will be miso compounds we've seen miso compounds before we just haven't given them a name a miso compound is an a chiral compound that has chiral centres let's look at some examples we will start with one to die bromo cyclohexane another structure of this compound looks like this each of the carbons that have a bromine have the potential to be chiral centers because they are sp3 hybridized carbons that have four different groups on them for example we can put the top bromine coming out towards us and the bottom bromine going away from us this is trans one to die bromo cyclohexane this compound has a non super imposable mirror image that looks like this because these are non-superimposable mirror images they are enantiomers take a moment and pause the video and determine the absolute stereochemistry at each one of the chiral centers hopefully what you've determined is that the structure on the left has two are centers on the structure on the right has two s centers because they are mirror images of one another the absolute stereochemistry is on the Left will be mirror images of the absolute stereochemistry is on the right hence both of the ones on the left are are making both of the ones on the right have to be s now take a moment and pause the video and see if you can write the names for these two compounds the structure on the left will be called 1 R 2 R 1 to dye bromo cyclohexane the structure on the right will be 1 s 2 s 1 to dye bromo cycle axing now let's consider sis one to drive cyclohexane now let's draw the mirror image of this one to die bromo cyclohexane these two mirror images are superimposable making sis 1 to die bromo cyclohexane a chiral we can also determine determine that they are a chiral because they each contain an into internal mirror plane of symmetry however even because even though they're a chiral they both contain two chiral centers take a moment to pause the video and determine the absolute stereochemistry and each of the chiral centers insists one to die bromo cyclohexane hopefully you've determined that the top stereocenter is r and the bottom is s this is an example of a compound that contains Carroll centers but is not chiral itself in other words this is a meso compound we name the mesial compound by calling it me so one to dye bromo cyclohexane due to its internal mirror plane of symmetry a meso compound will always be our s or s R let's look at another bowl consider two three dye bromo butane this molecule stereochemistry is most easily considered when we draw it as a Fischer projection there are four different ways that we can orient the groups on the two chiral carbons we can put the bromine on the top carbon on the left and the bottom carbon on the right and now we can draw this the mirror image of this molecule or we can put both bro means on the left side and then we can draw this molecules mirror image with both o means on the right side take a moment to determine the absolute stereochemistry of each of the chiral centers we've drawn hopefully you've determined that both of the stereo centers in the first structure are are both of the stereo centers in the next molecule RS and that there is an R and an S the stereo Center in each of the last two molecules now take a moment and determine if each of these molecules that we've drawn as Fischer projections is chiral or achiral the first two structures are non-superimposable mirror images they are enantiomers and they are chiral the second two structures are mirror images of one another but they are superimposable because they contain chiral centers this is a meso compound these molecules contain an internal mirror plane of symmetry and are a chiral this example also nicely illustrates what is called the - and rule a molecule with n stereo centers can have up to two to the stereoisomers it's important to realize that they can have up to two and stereoisomers because if there is some internal symmetry to the molecule it will have less than two to the n stereoisomers as we see in this case this molecule has two stereo centers so it can have up to four stereo isomers but because of the symmetry in the molecule it only has three let's look at another example two bromo 3 chloro butane which looks like this because this molecule has two stereo centers it can have up to four stereo isomers again let's draw these as Fischer projections we can draw both the bromine and the chlorine on the right side of the molecule and we can draw its mirror image or you can put one bromine on one side and the chlorine on the other and we can draw this molecules mirror image pause the video for a moment and determine first the absolute stereochemistry is of each of the chiral centers we've drawn and then determine whether or not these molecules are chiral the first molecule is 2 s 3 R the second molecule its mirror image will be 2 R 3 s both of the chiral centers in the third molecule RS and both of the centers in the fourth molecule are are now four of these molecules are chiral what we have here are two pairs of enantiomers and the relationship of each of these molecules to the other molecules are diastereomers in this case there are two N equals 4 stereo centers and there are 4 stereo isomers because there is no internal symmetry