How to Identify Molecules - Proton NMR: Crash Course Organic Chemistry #26 - Free Educational videos for Students in K-12

How to Identify Molecules - Proton NMR: Crash Course Organic Chemistry #26 - By Math and Science

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00:0-1	You can review content from Crash course Organic Chemistry with
00:02	the crash course app available now for android and IOS
00:05	devices . Hi , I'm dave okay Chakravarty and welcome
00:08	to Crash course Organic chemistry chemists used to have a
00:11	real problem when it came to identifying the crystals liquid
00:15	or general group they found themselves with at the end
00:18	of a chemical reaction . Of course they knew what
00:21	they were trying to make . But have they actually
00:24	made it ? They could check boiling points , melting
00:26	points , smell color or even taste in the bad
00:30	old days , which is definitely not recommended now .
00:33	But all of that took weeks of diligent effort and
00:36	was still somewhat inconclusive , fortunately we've moved on to
00:39	a much more high tech analytical technique , nuclear magnetic
00:43	resonance or NMR Yeah . Mhm Yeah . First off
00:56	let's straighten one thing out . The nuclear part of
00:58	any mars name doesn't refer to nuclear power . It
01:01	refers to the nuclei of atoms . All atomic nuclei
01:04	are made up of protons and neutrons except for hydrogen
01:07	, which doesn't have any neutrons adding up the protons
01:10	and neutrons gives us the atoms mass number . Nuclei
01:13	with odd mass numbers have a property called spin and
01:16	having spin makes them observable using NMR . Obviously there
01:19	are quite a lot of elements with odd mass numbers
01:22	, but we're only interested in a handful of them
01:24	for NMR nitrogen 15 flooring 19 , phosphorus 31 .
01:28	I'll show up occasionally , but the real stars are
01:31	carbon 13 and hydrogen . We'll focus on hydrogen today
01:34	and do proton NMR spin is not the nuclei literally
01:38	spinning . It's one of those weird quantum mechanical things
01:41	to avoid getting too into the physics , we'll just
01:43	say that when charged particles like protons in a nucleus
01:46	move , a magnetic field with the magnetic moment is
01:49	created . If you've played with iron filings before ,
01:51	you know , they move if you put a magnet
01:53	near them , each iron filing has a magnetic moment
01:57	and under normal circumstances there directions are random . However
02:01	, in the presence of a magnet which has a
02:02	magnetic field , they all line up similarly , if
02:06	you stick a nucleus with a magnetic moment in an
02:08	external magnetic field , it either lines up with that
02:11	field or exactly against it . It takes less energy
02:14	to align with the magnetic field and more energy to
02:17	go against it . So there's a clear energy difference
02:20	between the two different spin states . Now we've got
02:22	the nuclear and magnetic parts of NMR covered . But
02:26	what about the resonance ? It turns out that particular
02:28	frequencies of radio waves will cause the nuclei to flip
02:31	from one spin state to the other when we hit
02:34	that perfect frequency . That's what we call resonance .
02:37	And the nuclei absorb the energy of the radio waves
02:40	. This is we are a detector , measures the
02:42	frequency and intensity of the radio wave that got absorbed
02:45	and plots it on a spectrum . In this episode
02:48	, we're going to concentrate on proton NMR , which
02:50	tells us about the hydrogen nuclei and therefore hydrogen atoms
02:54	in molecules in general will get a huge magnet and
02:57	dissolve our chemical sample in a special decorated solvent where
03:01	the hydrogen atoms are replaced by deuterium . If we
03:04	used a regular old solvent with protons , the signals
03:06	from our organic chemical would be overwhelmed by the hydrogen
03:09	is on all the solvent molecules . Then we put
03:11	our tiny solution of chemical sample in the middle of
03:14	the magnet blast a pulse of radio waves and measure
03:17	the energy released by the nuclei as the different hydrogen
03:19	atoms in the chemical go from residents back to their
03:22	ground state . Like IR spectroscopy that we talked about
03:25	in episode five , we get a spectrum from the
03:27	sample . Once we look at some proton NMR spectra
03:30	will get the hang of these patterns . Let's start
03:33	with a simple example . This is the spectrum for
03:35	chloroform , ethyl methyl ether or mom chloride to its
03:38	friends . The X axis is measured in PPM ,
03:41	which stands for parts per million . It has to
03:43	do with the ratio between the radio frequency source and
03:46	the energy required to cause the nuclei to flip spin
03:49	. The process of adding deuterium to solvents isn't perfect
03:52	. So a few hydrogen is from the NMR solvent
03:55	show up as a peak , which we can use
03:57	to set the scale of our NMR spectrum . Or
04:00	we can include a standard chemical called tetra metal silent
04:03	or TMS in the sample , which produces a very
04:06	strong signal . TMS shows up on the far right
04:09	of the spectrum and usually doesn't overlap with the peaks
04:11	from the hydrogen is on our organic chemical basically ,
04:14	the standard provides a comparison point for the other peaks
04:17	. It's a bit like measuring times against GMT .
04:20	If you live in new york , your time zone
04:22	is gmt minus five . While someone in paris is
04:25	on GMt plus one and someone in Canberra Australia is
04:28	on GMT plus 11 . So ignoring the peak at
04:31	zero from our tMS standard , there are only two
04:34	peaks we need to analyze in the spectrum . Using
04:37	a computer , we can label each of these peaks
04:39	with the integral the area underneath them , which corresponds
04:43	to the ratio of the number of protons in that
04:45	part of the molecule . One of our peaks here
04:47	has an integral value of three , which means three
04:49	protons . So that means there's probably a medical group
04:52	, CH three in our sample . The other peak
04:54	has an integral of two , which suggests our chemical
04:57	also contains a CH two groups . So if we
04:59	were a chemist in the lab who didn't know but
05:01	just suspected that we had mom chloride , we could
05:04	check whether the structure of our suspected molecule fits with
05:07	our spectrum . And , yep , there sure is
05:10	a CH three and CH two groups in mom chloride
05:13	. One of the peaks is upfield or further to
05:15	the right and the other is more downfield or further
05:18	to the left . That's because the protons making up
05:20	these peaks are surrounded by different amounts of electrons because
05:24	of different neighboring atoms . Nearby electrons from other atoms
05:27	can shield the proton nuclei from the magnetic field so
05:30	they feel it less . And we need a lower
05:32	frequency radio wave to flip the spin . On the
05:35	other hand , electro negative atoms can draw electrons away
05:38	from proton nuclei . So there d shielded feel the
05:40	magnetic field more and need a higher frequency radio wave
05:44	to flip the spin . Taking a look at mom
05:46	chloride , we can see the CH three group is
05:47	attached to an electro negative element oxygen , but the
05:51	CH two group is sandwiched between two electro negative atoms
05:54	and oxygen and chlorine . So the electrons near the
05:57	CH two groups are more drawn away . The proton
06:00	nuclei are more D . Shielded and the peak is
06:03	down field of the CH three groups . To keep
06:05	it really simple , Just remember that the presence of
06:07	electro negative atoms or groups . She speaks to the
06:10	left in proton NMR . The peaks on the spectrum
06:13	actually a pretty consistent shifts based on the electro negativity
06:16	and hybridization of the atoms near the protons tables like
06:20	this . Can't cover every possibility though , which is
06:23	why we need to understand how to think about electro
06:25	negativity and chemical shift . Okay , now that we've
06:28	talked through the mom chloride spectrum , let's try another
06:32	example . This is the proton NMR spectrum of ethanol
06:35	. There are three peaks in this one , but
06:38	two of them are more like groups of lines .
06:40	There are lots of protons in organic molecules and remember
06:44	that each nucleus with an odd mass number has been
06:47	and a magnetic moment . So protons on one carbon
06:51	can influence protons on an adjacent carbon in a process
06:54	called coupling , this leads to the split peaks or
06:57	groups of lines that we're seeing here . In fact
06:59	, this has been observed so regularly that we can
07:02	predict how many times a peak will be split .
07:05	Thanks to the aptly named N plus one rule ,
07:07	we know that a peak will be split N plus
07:09	one times where N . Is the number of protons
07:12	on adjacent carbons . The best way to understand splitting
07:15	is by looking at examples . So let's start with
07:18	our most shielded peak . It's at about one PPm
07:21	with an integral of three . So we know it's
07:23	the CH three group , were still practicing . So
07:26	let's look at the structure of ethanol . The CHD
07:29	group is bonded to a CH two group . There
07:31	are two protons on the neighboring carbon that are coupled
07:34	. This means N . Is to using the N
07:36	plus one rule , we can do two plus one
07:38	equals three . So the CH three peak should be
07:41	split into three . Now going one peak downfield at
07:44	about three PPm . We see the CH two groups
07:47	in our spectrum . It's down field of the CH
07:49	three because the CH two group is bonded to an
07:51	electro negative oxygen atom in the structure of ethanol .
07:54	We know that the CH two group is bonded to
07:56	the CH three group . So there are three protons
07:59	on the neighboring carbon that are coupled and N .
08:02	Is three . Using the N . Plus one .
08:04	Rule three plus one equals four . So the CH
08:07	two peak is split into four plus . It has
08:10	an integral of two , telling us there are two
08:12	protons . The split peak patterns , or multiplicity is
08:15	have special names too . The ch the peak split
08:18	into three is a triplet , and the CH two
08:20	peak split into four is a quartet . The third
08:22	peak at about four PPm is the proton on the
08:25	O . H . Group of our ethanol molecule .
08:27	O . H . Protons are tricky because they can
08:29	swap with other protons in the sample solution and can
08:31	turn up as a new split peaks pretty much anywhere
08:34	in the spectrum . In fact , sometimes they don't
08:37	show up at all . It's also important to mention
08:39	that the protons on oxygen and nitrogen don't split the
08:42	protons on adjacent carbons . So these protons don't count
08:46	when figuring out N plus one . After you look
08:48	at more peaks and more spectra solving these logic puzzles
08:51	can be fun . And because the whole point of
08:53	proton NMR is to figure out the structure of molecules
08:56	. This next spectrum doesn't come with a name ,
08:58	structure like mom chloride or ethanol , fortunately , we
09:02	have a high resolution mass spectrum that gives us our
09:04	chemical formula C five H 802 So let's start with
09:08	the most shielded part of the spectrum . We have
09:11	two single peaks that have integral of three protons each
09:15	. So it looks like we have two ch three
09:17	groups on our mystery molecule . Next we can see
09:19	that the most d shielded single peak has an integral
09:22	of one proton . Referring back to our handy chart
09:24	. we can see this is where the proton on
09:26	a carb oxalic acid shows up . And what do
09:29	you know ? We have two oxygen atoms in our
09:31	formula which fits perfectly with that thought , let's add
09:34	this fragment to the pieces of our molecule , puzzle
09:36	one more peek to go and this single peak corresponding
09:40	to one proton is a bit D . Shielded to
09:42	. In fact looking at our chart again , it's
09:45	showing up in the range of alkaline hydrogen atoms .
09:47	The trickiest part about putting these four pieces together is
09:50	figuring out where to place the two methyl groups and
09:52	the car back silic acid on . Are all keen
09:54	to start . Let's put the two ch three groups
09:57	on different sides of the alkaline . Wait a second
09:59	though , with this structure of the metal group is
10:01	next to a carbon atom with the hydrogen . This
10:04	would split a single peek into a doublet and we'd
10:06	expect the hydrogen atom to be split by the three
10:08	hydrogen on the metal to since we don't see splitting
10:12	in the spectrum , let's rearrange our structure . So
10:14	the hydrogen atom and both metal groups won't be split
10:17	by any adjacent protons , even though the two methyl
10:20	groups are attached to the same carbon double bonds are
10:23	rigid , so one of these groups is stuck on
10:25	the same side as the car back , silic acid
10:27	and one is on the same side as the hydrogen
10:29	. These metal hydrogen Zarin different chemical environments and they're
10:33	not chemically equivalent . So that's why they show up
10:35	as two different peaks . We'll talk more about chemical
10:38	equivalents in a later episode , when we look at
10:40	the proton and um are spectra of aromatic compounds .
10:43	But in this episode we've learned nuclear magnetic resonance helps
10:46	us visualize the atoms in molecules as peaks in the
10:49	spectrum . In proton NMR . The integral tells us
10:52	how many hydrogen each peak represents nuclei close to electro
10:56	negative atoms appear downfield on a spectrum and split peaks
11:00	or multiplicity give clues about how atoms are joined to
11:03	other atoms . In the next episode will work our
11:06	way through reactions of some other often fragrant compounds Aldo
11:09	hides and key tones . Until then , Thanks for
11:12	watching this episode of Crash Course Organic Chemistry . If
11:15	you want to help keep all Crash Course free for
11:17	everybody forever , you can join our community on Patreon
11:23	. Yeah .

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