hi folks we are starting a new chapter here chapter 9 notice that we're going backwards a little bit from the previous chapter and this is video 9 - 1 so in chapter 9 we're going to start a very new and very different topic spectroscopic identification of organic compounds so last semester we had a very brief introduction to this where we looked at how to use IR spectroscopy and also hydrogen deficiency to at least have a brief introduction of how we could use different methods and techniques for identifying the structure of an unknown organic compound in this chapter we're going to pull together a lot more things involving IR spectroscopy as well as hydrogen efficiency and adding to that nuclear magnetic resonance spectrometry or NMR and you'll be using both IR and NMR in lab this semester and if time allows and that means we don't have too many snow days we will also be looking at mass spectrometry so my advice would be that since all of you should have been introduced to IR spectroscopy and hydrogen efficiency in organic chemistry one that you should go back and you should review that material ok let's start out with a very brief overview of I are starting a little bit about what IR does and also about the theory this is an instrumental method and we use it to identify whether certain types of functional groups are present in an organic compound IR cannot be used to identify all types of functional groups and also in organic chemistry for this class we're going to keep it pretty simple it's going to be only four functional groups that are obviously present in an IR spectrum the basic theory is actually pretty simple you take your unknown compound you put it on a plate on the IR and that compound gets irradiated with electromagnetic radiation in other words I in our case EMR at the frequency consistent with infrared remember infrared is not just one specific wavelength or one specific frequency just like blue light is not just a specific wavelength or frequency there is a small range in there we take advantage of that small range in order to determine if certain functional groups are present so as you're scanning your molecule with different types of infrared energy if a particular organic molecule has a vibrational frequency that matches the IR frequency energy is absorbed and we see that absorbance in the IR spectrum so on the right here we see different types of vibrational modes of carbon hydrogen bonds the same thing would apply if you have carbon-carbon bonds and since all organic compounds have carbon carbon and carbon hydrogen bonds you're always going to see the absorbance of infrared radiation at specific wavelengths or frequencies the question is what happens if you have functional groups will they have unique absorbance bands or will they absorb IR energy at unique wavelengths and unique frequencies the answer to that is yes well of course as we'll see on the very next slide so let's look at an example of an IR spectrum of a simple organic compound you see the organic molecule shown here obviously we're gonna have carbon-carbon bonds carbon hydrogen bonds and in this case a carbon oxygen double bond a carbon oxygen single bond and an oxygen hydrogen single bond so will those will those bonds of those functional groups have vibrational frequencies that we can detect using infrared and the answer is yes here is an infrared spectrum notice that on the x-axis and that's the only axis we need to be worried about is we have frequency units these are in the units of reciprocal centimeters of course frequency and wavelength are related to each other we could have had this axis as way as wavelength but the frequency numbers are easier to work with the x-axis goes from about 500 reciprocal centimeters to about 4,000 reciprocal centimeters and those ferry ends have been clipped off so that this spectrum fits on the page you see so we see a number of absorbance bands notice that on the top here the line seems to be kind of flat that means that infrared energy is not being absorbed wherever we see dip that represents energy that has been absorbed by some vibration of some type of a bond in this molecule so what we want to do is focus on bands that tell us specific information about functional groups for example the carbon oxygen double bond has this characteristic sharp strong band at approximately 1700 reciprocal centimeters furthermore the oxygen hydrogen bond has this broad characteristic band somewhere between approximately 3200 and maybe 3500 reciprocal centimeters those two functional groups have absorbance bands that are diagnostic in other words no other functional groups will have an absorbance in these particular regions the other absorbance bands that you see on this particular spectrum are always going to be there they are related to the carbon carbon and the carbon hydrogen bonds that all organic molecules have so we always see this and we're always going to see stuff down here absorbance bands down here what we want to be careful of is that we do not over interpret our IR spectra I also don't want you to memorize this information so this table is a correlation table of functional groups the frequent and the frequencies in which they're going to have absorbance bands I will always give you this this chart during an exam and I'll also put a PDF of this on the course web page I also don't want you to have to memorize every single one of these different functional groups there are about five here that are important to us for example the carbon-carbon triple bond if your if your molecule has a carbon-carbon triple bond there is a diagnostic band very very short one very small one at about 2100 2200 reciprocal centimeters if the triple bond is at the end of a carbon chain in other words if it is all also a carbon hydrogen bond here then there is an additional band at 3300 reciprocal centimeters the next chapter we're going to cover will be about the benzene ring the benzene ring has a very character characteristic absorbance bands we'll talk about that when we get to that chapter the carbon oxygen double bond as we saw in the example on the previous previous slide about 1700 reciprocal centimeters the OAH bond of an alcohol we saw that broad very characteristic absorbance band and I'll also point out the O H bond of a carboxylic acid which is near that of the alcohol but notice that the the frequencies are a little bit too there to the right a little bit smaller these tend to overlap with the band for the carbon hydrogen carbon-carbon bond so we have to be a little more careful in interpreting that particular absorbance the key here is keep it simple do not try to analyze every single absorbance band in an in an IR spectrum instead ask yourself do I have a triple bond do I have a benzene ring do I have a carbonyl and etc if finally I want to briefly review hydrogen efficiency again this is a topic that we covered last semester hydrogen deficiency measures unsaturation what that means is that you are comparing the number of hydrogen's that you see in the unknown to the number of hydrogen's you would expect versus the number of carbons a deficiency means that you have certain types of functional groups present if you want to know how this works how this actually works draw out the structure of pentane with all the carbons in all the hydrogen's count up the number of carbons count up the number of hydrogen's that is a saturated molecule if you apply the formula 2n plus 2 where n is the number of carbons you will find the number of hydrogen's for that saturated molecule so 5 carbons times 2 is 10 plus 2 that's 12 5 carbons you have 12 hydrogen's now put a double bond anywhere in that carbon chain and make sure you make Corrections so you don't have 5 bonds to a carbon now add up the number of hydrogen's and you find you have 2 fewer hydrogen's that is a hydrogen deficiency do the same thing by taking pentane again and making it into a ring again make corrections so you don't have 5 bonds to carbon once again you will see you have a hydrogen deficiency because of those functional groups so if you are given an a formula for an unknown what you want to do is calculate the maximum number of hydrogen's first that is related to the number of carbons 2n plus 2 where n is the number of carbons okay this little formula gives you the max hydrogen's you look at the formula of the unknown that is the actual number and that difference is your hydrogen deficiency HD of 2 is a double bond and HD of 4 is a triple mine HD of 2 is also could be low alkane and the benzene ring has an HD of eight will be doing examples of these in class and remember that combinations are possible so for example if you have at HD of 4 it could be a triple bond you might also have two double bonds in the molecule or two rings or one double bond and one ring so as part of our analysis we will be considering all possible combinations okay so that was a review for IR and for hydrogen deficiency and that's the end of this recording