hey everyone Victor is here your guide to All Things organic chemistry and today I want to talk about the IR spectroscopy and before you yawn at me no this is not going to be one of those videos where I bore you with 30 minutes of theory no this video is going to be a practical guide that will teach you everything you need to know about the IR with the emphasis on how to solve the IR Spectra and answer the exam questions which is exactly what you're going to be tested on so grab your cup of coffee and notebook to jot down the most important parts hit the like button for good luck on the test and let's get started IR stands for infrared spectroscopy this is a vibrational spectroscopy which means that if something vibrates it is going to be visible in the IR spectrum and guess what all bonds vibrate there are different types of vibrations and some of those vibrations are visible in so-called analytical region of the IR so the idea is quite simple you Shine the IR light at your molecule and different functional groups will absorb that light and since different bonds and functional groups absorb light at different frequencies we can tell exactly what we have in our molecule so for instance on this spectrum I have something that absorbing light here here and here but the question is what exactly does all of that mean when it comes to the IR spectrum we have two regions we have a fingerprint region region which is below 1500 reciprocal cm and we have the analytical region above 1500 reciprocal cimet anything we care about is going to be in the analytical region right over here the fingerprint region will only be useful for you if you are trying to identify your molecule by comparing it to a known sample the fingerprint region works well like a fingerprint it's pretty much unique for each molecule however it is nightmarishly difficult to interpret unless you really know what you're looking for so we'll leave these to the pros and focus on the analytical region of the spectrum so the fingerprint region is something that we don't really care about so what type of information are we going to get from the analytical region first of all the wave number which is these numbers over here it's typically expressed in reciprocal centim and it's related to the vibrational frequency which is directly proportional to the energy with which our bonds are vibrating so the higher the wave number the higher the energy with which the bond vibrates so for instance on this spectrum I have a couple of signals one at about 3,400 reciprocal cm and another one at around 2850 2950 in addition to the wave number we'll also look at the signals shape so like for instance this signal is Broad and smooth while this one is very spiky and finally the least bit of information for us is going to be the signal strength which can be strong if it deeps down below 30% transmittance medium if it is somewhere in the middle of the transmittance scale and weak if the signal is higher than 70 or so now the signal strength is the least important piece of information as the strength will depend on how the Spectrum was obtained and U what exactly the molecule is all about this same molecule can have differently looking signals in terms of their strength depending if the Spectrum was let's say obtained from the thin film or potassium bromide mold or Neal mix Etc what will be the same though is the position of the signal and its shape so once you get your wave numbers and the shape you can mat those to the reference table so for instance here is the reference table I give to my students this one visually represents the position of the signals in the Spectrum so it's kind of easy to match what you are seeing in the Spectrum to what we have on the table if you like this one you can always download it from organic chemistry tutor.com but I'm pretty sure your textbook has something similar to that well maybe not that pretty or the reference table can be given to you in the form of an actual table I suggest you choose whichever table works for you and learn how to use it efficiently some people like the visual one some people like the table one whatever works will work for you doesn't matter which table you choose But whichever table you choose you got to learn how to use it properly I'm not going to tell you to memorize most common numbers but I'm going to tell you to memorize most common numbers here is a deal though even if you are efficient at finding your reference numbers and know your IR table like the back of your hand it is still going to take a few seconds here and a few seconds there to compare those numbers and those seconds will add up very quickly on the test the good news though is that you'll memorize those numbers anyways if you do enough problems which brings me to the most important point of this video you must work through a ton of problems that part is not optional if you want to be able to solve these problems you need to practice and when I say practice I mean attempt to solve as many problems as you can not watch somebody solving them recognizing how someone solved a puzzle and solving a puzzle yourself are two huge differences but enough of me renting here let's get back to our IR so when I solve my IR Spectra I always look at the Spectrum from left to right the area around 3500 reciprocal CM typically houses the O and an H groups so for instance in this case we have a broad and smooth signal which is the O group but how do I know it's not the NH you might ask well uh that's where the shape of the signals come into play the O is typically Broad and smooth while the NH is typically weaker and sharper so here is the NH signal and how it looks like for the comparison see it's much weaker and much sharper than the O group then we have this 3,000 line this is a very important line in the IR because this is where you normally see your carbon hydrogen stretches signals leaning onto the 3,000 from the right or in other words signals that are hair under 3,000 so about 2950 or so are the regular sp3 hybridized carbon hydren stretches you should see those signals for the majority of your compounds signals leaning onto your 3,000 from the left side or in other words signals that are just a little bit over 3,000 like 3050 those are SP2 hybrid died carbon hydrogen stretches so you'll expect to see those guys if you have double Bonds in your molecule and those double bonds have hydrogens on them this molecule doesn't have any so we are not seeing anything there the next important region we are going to look at is going to be around 2500 to 2,000 reciprocal CM this is where the triple bonds leave those signals are typically pretty weak and can sometimes completely disappear but if it is a terminal triple bond like what we have here we are going to have a sharp Spike at almost exactly 3300 CM which is the SP hybridized carbon hydrant stretch it's a somewhat rare signal since we are only going to see it for the terminal triple bond but nonetheless you should be familiar with this signal the next important region is the 1,800 to 1500 CM which is where we have various double bonds for instance in this case I have a CO double bond at roughly 7 00 and I also have a cc double bond at about 1650 these are of course the most prominent functional groups you'll be looking at in your molecule there are many other ones that can potentially pop up in the analytical region and with practice you will learn to recognize those as well with time and practice you'll learn to recognize all of these functional groups and will memorize most of these numbers as well you'll also be able to recognize some signals in the fingerprint region but unless you are comfortable with the analytical region first which is above 1500 reciprocal CM don't even poke your nose into the fingerprint you'll get lost there and you will identify things that are not in your molecule uh just because you don't know what you're looking for Beware of the fingerprint and only go there if you really know what to look for all right let's look at a few examples here here I am not seeing anything in the 3500 region so I do not expect any o or age groups next I'm seeing these spikes uh leaning onto 3,000 from the right side this is the indication of the regular sp3 hybridized carbon hydren stretches we expect to see those in the majority of our molecules then I'm also seeing the carbonal co double bond at about 176 here which means that we are most likely looking at some sort of a ketone and while I can't tell you the exact structure of your molecule I know what types of functional groups to expect from this molecule so if I had more data like let's say data from the hydrogen anmr or carbon NMR I'd be able to tell you what exactly I'm looking at here in this molecule for instance I have a broad smooth signal at 35 3300 which is a straight up indication of an alcohol I'm also seeing some sp3 hybridized carbon hydren stretches at the right about uh 3,000 reciprocal centimet leaning on it from the right side and that's about it which means that this molecule is most likely some sort of simple alcohol and here is another example here I'm seeing some signals from the SP2 hybridized carbon hydren stretches because those signals are a little bit above 3,000 I'm also seeing the typical sp3 hybridized carbon hydrogen stretches which I would be more surprised if I didn't see honestly I'm also seeing a carbonal at 1730 and there is also a tiny signal over here for the carbon carbon aromatic Bond So based on this data we can conclude that this is likely an aromatic Ketone or other type of a carbonal looking at the fingerprint region I can say that this is actually an aster but I wouldn't expect a typical sophomore organic chemistry student to be able to fish out this information from the IR alone so normally the IR will be the first step in your spectroscopic deter ation of a compound and you'll be able to come up with the complete structure using the combination of the IR NMR and maybe some other techniques as I've mentioned before signals can look differently from Spectrum to Spectrum and from molecule to molecule so solving spectroscopy questions is a bit of an art and you can only become good at it if you do a ton of practice all right folks in these past few minutes we've journeyed through the landscape of the IR spectroscopy together decoding the hieroglyphics of the Peaks and valys on those Spectra you now know how to interpret the wave numbers the shape of the signals and even their strength to a certain extent you've got your reference table either from me or your textbook or some other place and you've equipped with tips on how to efficiently read the IR spectrum based on the approach that I outlined in this video but remember it's one thing to be a spectator and another to be a player in the field you're learning Curve will only Skyrocket when you roll up your sleeves and get down to solving real world problems starting today take up my challenge solve at least two to three spectroscopy problems every single day until your final exam you'll stumble you'll scratch your head you'll get frustrated but eventually you'll conquer this skill by the time the big test rolls around you'll handle those spectroscopy questions like a pro and if you found this video helpful don't forget to hit that like button and share it with your friends and classmates every thumbs up is like a high five to me for more daily organic chemistry tutorials and solutions subscribe and hit that notification Bell and if you have questions or need further clarification drop a comment below I always love to hear from you guys thank you for tuning in watch this video next and I'll see you tomorrow