Transcript for:
Infrared Radiation and Molecular Structures

hello this is dr hannah asseel and today we're talking about infrared radiation and uh how to use it to deduce molecular structures this is for ias at excel chemistry okay so we know that infrared is a region in the spectrum which is right next to the visible it's just above the red end of the visible light and some substances can absorb the ir radiation now when molecules absorb the ir radiation they change their polarity when they vibrate so a compound like carbon dioxide for example can absorb ir radiation uh change its polarity and this can be recorded on a spectrum that shows peaks at specific uh sections of the uh spectrum at specific wave numbers and the presence of these peaks can be used to identify the compound now only molecules which can change their polarity as they vibrate can absorb ir radiation so for example molecules of water we said co2 methane and all these can absorb ir radiation but molecules such as oxygen and nitrogen do not absorb ir radiation now each functional group in a molecule gives a characteristic peak at a specific wavelength so for example what we will be having is we will have a spectrum like this and our job is to decide if it has a certain structure by the presence of certain peaks so notice that for example this is ethanol can you see the peak for the oh so if i look at a spectrum and i find a peak a broad peak like that in that area uh 3 300 that means that this must be an alcohol now if we're looking for other things we will look for peaks at other uh wave numbers so you will be provided with this but it's always a good idea to know what you're looking for so that you don't spend time looking all through this table but you will have this table in front of you now let us take a look at what we should be looking for if i have an asset i must have a peak for oh at around 3300 or 2500 or in between so can you see the peak for the oh always look in that area around 3 000 if we have a peak in that area that means i have either an alcohol or an acid because the o h will give a peak in that now if this is an acid that means it also has in addition to the o h it has a c double bond o now c double bond o will always give you a strong peak at around 1700 or 1720 so that long peak at 17 21 indicates that i have a c double bond o so that means that this is a compound that has a c double bond o and then o h that indicates that it must be a carboxylic acid are we paying attention it's very easy if you just look in that area at around 17 20 you should have a big peak that means i have a suitable bundle if you don't have a peak long strong peak in that area then there is no suitable bundle oh is very clear in that area around 3000 you should have a broad peak at uh with around 3 000 that would indicate the presence of oh presence of both these peaks indicates carboxylic acid but can you see the peak for the alcohol there is no peak at around uh 1700 like the previous spectrum there is only a peak at around 3300 that is an alcohol are we paying attention okay ketone ketone is something that has a c double bond o no o h is nothing the c double bond is in the middle of the compound so all you're looking for is a peak at the c double bond o that is the peak at around 17 10 or 17 20. that strong peak indicates the presence of c double bond o so if i have that peak only and no peak for o h do you remember where the peak for the o h was something around 3 000 should be broad and very clear at that area around 3 000 there is no peak in that area for o h so this means that i have only c double bond o that means i have a ketone and aldehyde would have c double bond o but can you see what it has before 3000 it doesn't have a peak at 3300 like the other one so there is no peak for oh but there is a peak for the ch that is next to the c double bond o so this small peaks on top at around 2800 2900 these indicate the presence of aldehyde in addition to the peak for the c double bond so let's take a look at a typical question he says the infrared spectrum for one of these compounds is given below and his he gives you this table o h for alcohols or in this range c h alkanes and so on are in this range so he gives you specific uh wave numbers and he's asking okay this spectrum that we have is for which of these compounds well looking at it i can see there there is no o h there is no broad peak for the alcohol so it is not an alcohol it's not a or b now could it be butanol yes because there is a peak for the c double bond o the peak for the c double bond at around 17 40 or 1700 yes we have so it could be either c or d though so is it an aldehyde butanol or a ketone butanone we said the aldehyde has these three these peaks tiny peaks at around before 3000 so these are the weak peaks for the ch of the aldehyde so this is a butanol now another spectrum which of the following could be produced uh could have produced the above spectrum okay we look at this there's obviously a peak for the oh can you see the broad peak at around 3000 that's an orange so that could be an alcohol or an acid now i also have a peak for the c double bond o at around seventeen hundred so this has an o h and acetabult so it must be a carboxylic acid a sample of butane to all was oxidized by heating on the reflux with an oxidizing agent and then the product was separated for ir analysis apart from the peaks due to the c c and ch bonds which peaks would be present in the ir spectrum of the oxidation product remember you're oxidizing butane to all you think too all when it is oxidized it would give what it would give a keto this is a secondary alcohol so it will give a ketone so you you will not expect a peak for oh at all so you should actually have only a peak due to the c double bond or the heat okay which of these molecules does not absorb infrared radiation remember that we said molecules like oxygen or nitrogen that cannot change their polarity will not absorb ir radiation now the next question says what effect does ir radiation have on the covalent bonds in water molecules remember that we said if water molecules absorb the ir this is because the bonds vibrate more vigorous infrared spectroscopy can be used to distinguish different functional groups such as alcohols and aldehydes state how this analytical technique is used to do this and explain the effect of radiation on the molecule so we said what happens when a molecule absorbs ir the ir causes changes in the bond polarity different bonds absorb different ir wavelengths and the peaks obtained are then compared with the data booklet to determine the structure explain how ir spectroscopy could be used to detect whether butane to all has been oxidized well we said you should have if it has been oxidized it's the secondary alcohol that means it will change into a ketone so you should have the peak for the oh should disappear and a peak for the c double bond o of the ketone would appear carbon dioxide and water vapor both contain polar bonds what effect does infrared radiation have on the bonds of these molecules remember that we said the ir hydration causes the bonds to change polarity three compounds can each have the same molecular formula are known to be alcohols the ir spectra of compounds a b and c are shown so he has three spectra and he's saying identify one feature common to all three infrared spectra which shows that a b and c are all alcohols of course when you look in that region of about 3300 you will find that there is a broad peak there in all of them that indicates that there is an alcohol or an o h group state given a reason for your answer if it is possible to identify each of these three alcohols so can you tell if this alcohol is a primary or secondary tertiary no we cannot remember that the ir cannot be used to determine if it is a primary or secondary or tertiary since all alcohols have the same type of oh thank you for listening that was the last uh lesson in unit two and we will be doing um past papers questions from past papers from now thank you