The mass spectrum for a compound with molecular weight of...

Key Concepts

Mass Spectrometry

Mass spectrometry is an analytical technique that measures the mass-to-charge ratios of ions to provide information about a compound’s molecular weight and structure. It involves the ionization of molecules, followed by their fragmentation into smaller ions, which are then detected; this fragmentation pattern helps in identifying the parent molecule and deducing possible structures.

Infrared (IR) Spectroscopy

Infrared spectroscopy is a method used to identify functional groups within a compound based on the absorption of infrared light at characteristic frequencies. For example, a broad absorption around 3600 cm?¹ is typically indicative of an O–H stretch, which suggests the presence of hydroxyl groups frequently found in alcohols.

Fragmentation and Dehydration Reactions in Alcohols

In mass spectrometry, compounds such as alcohols often undergo fragmentation through dehydration reactions. This process involves the loss of a water molecule (18 atomic mass units) from the molecular ion, forming a new ion fragment. Understanding such dehydration mechanisms is crucial in explaining how specific peaks—like one that is 18 units less than the molecular ion—are generated.

Fragmentation Mechanisms

Fragmentation mechanisms refer to the pathway and processes by which a molecular ion breaks apart into smaller fragments during mass spectrometry. Common strategies, such as simple bond cleavage and rearrangement reactions, allow chemists to interpret the mass spectrum and correlate fragment ions with specific parts of the molecule, thereby aiding in the structural elucidation.

Transcript

00:01 So here, we are trying to elucidate the structure of a compound given its max spectrum and its infrared spectrum as well.

00:13 So to start off, we were given two peaks on the ir.

00:20 So if we just draw a makeshift spectrum over here with the y -axis, percent transmittance, and wave numbers, x -axis.

00:32 We were told at around 6 ,600, there is a broad, high intensity peak seen over there.

00:42 And then at 1 ,360, there is a medium intensity peak here.

00:51 Let's just try and draw this.

00:53 So something like that is how the spectrum might look like.

01:00 Okay, so yeah, now it's time to start assigning some functional.

01:05 Groups.

01:06 So yes, you probably already know what's over here in this region.

01:09 If we have a broad, high -intensity absorption in around 3 ,600 in this region, if you just check your correlation chart, you will see that, yeah, this is characteristic of a hydroxin group.

01:25 So very safe to say that there's an alcohol that we are dealing with in alcohol.

01:29 And even just to prove it further, this medium intensity peak here at 1 ,360 wave numbers is also characteristics of the carbon oxygen single bond vibrations.

01:45 So we have already identified one functional group.

01:48 Yes, we are dealing with an alcohol, for sure.

01:54 We're dealing with an alcohol.

01:57 Nice.

01:57 So now let's see what else we can elucidate.

02:00 So we were given the molecular weight from the mass spec as being 102 master charge units.

02:12 Okay, so now we're just going to use some math to count number of carbons.

02:19 So we know that a hydroxyl is 17 master charge units for the hydroxyl.

02:31 So if we just take that out of the way, get that out of the way, we get, let's just grab out our calculators, get 85 master charge left.

02:46 Okay.

02:47 So now to count carbons, what we're going to do is we're going to then divide that by 13.

02:55 And yeah, this is called the rule of 13.

03:00 It helps us count carbons.

03:03 So we know that the ch bond does have a mass charge of 13.

03:11 So then if we divide the remainder molecular rate from the 13, we should be able to count carbons.

03:20 And that came out to be around 6 .5.

03:23 So then we can say that we even have 6 or 7 carbons.

03:30 Okay, so let's try and draw out our structure to see if we can see if we have 6 or 7 carbons.

03:35 If we can get this molecular weight 102.

03:41 Okay, let's start off with six carbons.

03:46 Okay, so you draw it like that and start off with a primary alcohol, alcohol, just to start off, let's, yeah, let's count its molecular weight.

04:01 So its molecular weight will come out to being, so six carbons.

04:07 So we have to do six times 12, and oxygen, not even just an oxygen hydroxyl.

04:17 Those kind of hydrogens, there'd be three, five, seven, nine, 11, and 13, 13 hydrogens in the compound.

04:29 And yet, i believe we do get out to 102 grams from all for the molecular rate, which is nice.

04:37 We've sort of started to get our structure.

04:42 But we would also have this exact same molecular weight if it was a 1 hexanol versus 2 hexanol versus 3 hexanol.

04:57 So we still have to figure out which of the 3 it could possibly be.

05:02 So then now we really need to look at the mass spectra to figure out where we should place the hydroxyl.

05:10 Okay, so just to review some from the mass spec, we need to review base peak and what that is.

05:20 So, as you remember, the base peak is the highest intensity signal in a mass spectra.

05:35 And it comes from the stabulous fragment.

05:39 So that basically means that it'll be the signal that has the highest relative abundance.

05:48 That would be like 100.

05:51 And it is so abundant due to the fact that it's stable.

05:57 So after we irradiated our sample with whichever electrons ionized electrons to charge it, then whichever's, and then our sample will fragment, whichever of those would be the stabilist would be the base peak.

06:17 So here, let's see screen, our base peak was 45 mass to charge.

06:28 So then, okay, yes, so we just now need to assign some, you know, functionality to this basically.

06:39 So yeah, and with you doing that, we would be able to figure out the placement of this hydroxyl.

06:48 So you can't stop here and just call it hexanol.

06:57 Yeah, because we do know it's hexanol.

06:58 You just don't know whether it's one, two, or three.

07:00 And if you want to figure out whether it's one, two, or three, now we just need to look at how hexanol can fragment.

07:09 Okay, so, so, yeah, the most common way for alcohols to fragment is through something called alpha cleavage.

07:24 And that's just basically where the alpha position on alexenol, which is just the neighboring carbon, carbon adjacent to it.

07:35 Let's maybe use another color.

07:40 Yeah, it's just the neighboring carbon.

07:43 So this guy right here, this is alpha for this one, it's the alpha for that one.

07:51 So then if we just cleave the alpha bond or the bond adjacent to the alpha i mean, then, and for an oxonium ion, then we get alpha cleavage...

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