Synthesis and Column Chromatography: Crash Course Organic Chemistry #25 - Free Educational videos for Students in K-12

Synthesis and Column Chromatography: Crash Course Organic Chemistry #25 - 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 Debbie Chakravarty and welcome to
00:08	Crash course Organic Chemistry when you get down to it
00:11	all life is kind of similar from plants to fungi
00:14	to humans . We share many of the same biological
00:17	organic reactions that breakdown molecules for energy and build the
00:21	molecules that make you you . But we are different
00:24	from a mushroom oratory . And those differences allow some
00:27	organisms to produce molecules that have wildly different effects in
00:31	other organisms . Sometimes harmful but sometimes beneficial , for
00:35	example will always contain salicylic acid that acts as a
00:38	plant hormone . And as far back as 4000 B
00:41	. C . E . The assyrians used this extract
00:43	to treat joint pain in humans . Natural products like
00:46	salicylic acid can be medicines themselves or they can be
00:49	inspiration for more effective treatments . Like in 18 53
00:52	french chemist Charles Frederic Gerhardt synthesized a similar compound acetyl
00:56	salicylic acid , which we know today as aspirin .
00:59	Gerhard aspirin wasn't pure acetyl salicylic acid though . Which
01:03	brings us to the topic of this episode . When
01:05	we're running reactions in a lab we need to purify
01:08	them to isolate the chemical we're trying to make from
01:10	any other reactant or side products hanging around in the
01:13	mixture . And one method to separate chemicals is chromatography
01:19	. Yeah . Mhm Many reactions produced mixtures sometimes due
01:30	to competing side reactions and sometimes because the starting material
01:34	didn't completely react . For example , let's look at
01:36	a synthesis of the antidepressant medication paroxetine , also called
01:40	Paxil . We're starting with an alcohol and our goal
01:42	is to turn it into an ether . Remember that
01:45	hydroxide is a poor leaving group . So in the
01:47	synthesis we react the alcohol to form um easily .
01:50	Now we have a great leaving group and can use
01:52	the Williamson and verification reaction to make the ether .
01:55	Unfortunately , those reactions aren't super efficient and the ether
01:58	is only 56% of our products . So before we
02:01	move on to the last step to make Paxil will
02:03	need to separate the either we want from the side
02:05	products that we don't . This is where chromatography comes
02:08	in . Chromatography is what we call a group of
02:11	different techniques that let us separate the components of a
02:13	mixture . Usually to quantify or identify them . Chromatography
02:17	always involves two key pieces to help separate different molecules
02:20	in a sample . There's a stationary phase or absorbent
02:23	and a mobile phase or l . You win the
02:25	inventor of chromatography . Russian , italian botanist Mikhail seven
02:29	first use solid calcium carbonate as a stationary phase .
02:32	Two separate a sample of colored plant pigments , that's
02:35	how chromatography got its name . We can do a
02:37	simple kind of paper chromatography using a piece of paper
02:40	as our stationary phase to separate the component pigments .
02:43	In washable marker ring paper is made up of cellulose
02:46	, which is a bunch of sugar molecules strung together
02:49	all of those hydroxyl groups hanging off the sugars have
02:51	disciples , which means this paper is polar and interact
02:54	a lot with molecules that are also polar . We're
02:56	not just tearing off a piece of our notebook here
02:58	, notebook paper is often coated to prevent ink from
03:01	spreading and we need the ink to spread out .
03:03	Now I'll draw a little line on the bottom of
03:05	our chromatography paper in pencil to mark the origin ,
03:08	which is where we put our washable marker ink sample
03:11	. This ink is a mixture made up of several
03:13	different colored compounds that have different polarity . Then we
03:16	can stand the paper up in a solvent chamber here
03:19	. I just have a jar with our solvent or
03:20	mobile phase , a mixture of isopropyl alcohol and water
03:24	. We have to make sure the marker ink spot
03:25	is above the solvent or our sample will just diffuse
03:28	into the solution at the bottom of the chamber ,
03:30	giving us a murky solution and no separation on the
03:33	paper . Oops . What we want to happen here
03:36	is for a process called capillary action to move the
03:38	solvent up our chromatography paper . We actually saw capillary
03:42	action way back in episode six when we looked at
03:44	candle wax to polar compounds in the marketing sample ,
03:47	spend a lot of time interacting with the polar stationary
03:50	phase and stick to the cellulose molecules at the bottom
03:53	of the paper . Less polar compounds don't interact as
03:56	much with the paper molecules and move up with the
03:58	mobile phase . More easily . Different colored marker inks
04:00	have different compounds . So let's check out the beautiful
04:03	color separations from some paper . Chromatography like I mentioned
04:06	are solvent chamber was filled with isopropyl alcohol and water
04:09	because that's safe and easy to do in a crash
04:11	course studio . But in organic chemistry labs , Diet
04:14	the ether and hexane is a common chromatography solvent system
04:18	. Also because paper is porous , the colors in
04:20	the paper Chromatography experiment were pretty spread out , which
04:23	isn't that useful for working with organic chemicals . So
04:26	are stationary phase in the lab is usually silica gel
04:29	or alumina , which has the consistency of very fine
04:32	sand and gives a better separation . A thin layer
04:35	of the stationary phase is adhered to a glass ,
04:37	aluminum or plastic back plate . And we call the
04:40	technique thin layer chromatography or TLC for short . Now
04:43	that we've got a new experimental setup , let's look
04:46	at what happens to a sample of an organic compound
04:48	. When we alter the ratios of solvents , we'll
04:50	start with an equal mixture of diet , all ether
04:52	and hexane as our mobile phase , a 1 to
04:54	1 solvent system . We can quantify the movement of
04:57	a spot of organic compounds on the plate by calculating
05:00	its retention factor or R . F . A way
05:02	to describe the eluding power of a solvent system or
05:05	how much the solvent can move a compound up a
05:07	stationary phase during chromatography . To calculate our f ,
05:10	we measure the distance traveled by the spot from the
05:13	origin and divided by the distance from the solvent front
05:15	to the origin , which is how far we let
05:17	the solvent move up on the plate . We have
05:19	to remove the plate from the chamber before the solvent
05:22	reaches the top . Otherwise , a spot tends to
05:24	spread out too much doing the math . In this
05:26	1 to 1 solvent system are spot has a retention
05:28	factor of 0.41 Now , let's try a different ratio
05:31	by increasing the heck saying the non polar solvent in
05:34	our mobile phase for 1 to 5 ratio of either
05:37	to hexane . After doing the chromatography experiment , measuring
05:40	and calculating are f we only get 0.16 So we
05:43	can say that the 1 to 5 solvent system has
05:45	less eluding power than the 1 to 1 solvent system
05:48	. The non polar solvent , hexane is less effective
05:51	at pulling polar organic compounds away from the polar stationary
05:54	phase and moving them up the TLC plate . However
05:57	, if we switch the ratio and increase the ether
05:59	, the polar solvent in our mobile phase , we'll
06:01	get different results with a 5 to 1 ratio of
06:04	either . To heck saying . Well , expect polar
06:06	organic compounds to interact to a greater extent with the
06:08	more polar mobile phase , increasing the eluding power .
06:12	Let's do the math . And what do you know
06:14	in the solvent system ? R R F is 0.70
06:17	in the lab . We're not just calculating R .
06:18	F values of spots of pure organic compounds in different
06:21	solvents . We use chromatography to separate reaction products like
06:25	the either from the side products when making Paxil .
06:27	And that requires us to examine and actually physically separate
06:31	mixtures from a sample . We use TLC to monitor
06:33	our reaction . And using a thin capillary tube ,
06:36	we put a spot of our starting material in lane
06:38	A . On the left we spot our reaction mixture
06:41	which usually contains multiple compounds that we want to separate
06:44	from each other in lane . See on the right
06:46	, in the middle lane B . We spot both
06:48	on top of each other . A coast spot .
06:50	If all the lanes look like Lane A . After
06:53	our chromatography is done , we could have made a
06:55	completely pure product from the reaction with an identical R
06:58	. F . To the starting material . However ,
07:00	it's more likely that something has gone wrong with the
07:02	reaction . So basically TLC lets us preview if our
07:05	reaction actually worked or if we're stuck with the same
07:08	molecules that we started with Lane be the co spot
07:11	helps us make sure there really is a difference between
07:13	our starting material and our reaction mixture and that we
07:16	know are starting material was used up in the reaction
07:18	. For example , if Lane B looks like it's
07:20	missing spot from Lane A . But lanes B and
07:23	C look the same . A starting material could have
07:26	hitched a ride with reaction solvent to move further up
07:28	the plate and fool us into thinking it's a new
07:30	compound when it's not . But if Lane B includes
07:33	the same separations as both lane A and Lindsay were
07:36	good . It's really important to triple check all of
07:38	this with these three lanes . Now we're not always
07:40	working with colored compounds and plants like Mikhail Civet or
07:43	separating bright mark . Arinc with paper chromatography often with
07:46	TLC , our reaction mixtures are light yellow or clear
07:49	, which can make them hard to see . Luckily
07:52	many organic compounds have aromatic rings , double bonds or
07:55	triple bonds which absorb UV light . So we keep
07:57	a UV light in the lab to shine on TLC
08:00	plates and reveal are separated components . So we can
08:02	circle are spots with the pencil . If we're working
08:04	with compounds that don't show up under UV light ,
08:07	we use chromatography stains that can undergo chemical reactions with
08:10	the compounds on our plate . Conveniently the effects of
08:13	these stains can be seen with the naked eye .
08:15	For example , a potassium permanganate stained reacts with compounds
08:18	by oxidation and makes the oxidized compound spots appear yellow
08:22	on a pretty purplish background . Kind of like our
08:24	set TLC gives us a great preview of how are
08:27	compounds will separate , but a tiny plate isn't practical
08:30	to purify grams of our reaction mixture . For that
08:32	. We'll need a bigger batter technique called flash chromatography
08:36	. To set up our flash chromatography . We actually
08:37	need to play around with TLC to find a solvent
08:40	system that gives the component we want to separate from
08:42	the reaction mixture in our f of about 0.2 .
08:46	We also need to know the mass of the sun
08:47	purified sample we want to separate . And from there
08:50	we can pick an appropriately sized flash column with a
08:52	simple table , thanks to the work of chemists before
08:55	us . So let's say that . We figured that
08:57	all out . And now we'll head into the thought
08:58	bubble lab to start . We'll push a plug of
09:01	cotton firmly into the bottom of the chromatography column and
09:04	cover the cotton with a little sand for reinforcement .
09:06	We're going to use a method called slurry packing ,
09:09	where we mix the mobile phase solvent with silica gel
09:12	as our stationary phase and pour that into the column
09:15	through a funnel . Next will apply pressure to the
09:17	top of the column using an air hose until the
09:19	solvent is level with the silica gel . Let's add
09:22	a little more sand to keep the silica gel even
09:24	. And then we're ready to load our sample into
09:26	the column with a long pasture pipette . We add
09:28	our reaction mixture to the column . Then we can
09:31	fill the reservoir with our mobile phase and run the
09:33	column will apply pressure with an air hose again ,
09:36	forcing the liquid through all the silica gel , sand
09:39	and the cotton plug . And we'll collect fractions of
09:42	the mobile phase in test tubes . We have to
09:44	be careful not to let the solvent move below the
09:46	level of the silica gel in the column . That
09:49	would mess up our carefully balanced mixture of mobile and
09:52	stationary phases so we can add solvent to the top
09:55	to prevent the column from going dry . The less
09:57	polar compounds move quickly through the silica gel in the
10:00	flash column into the first few test tubes . After
10:03	we collected all the fractions , we can check if
10:05	our compound is alluded by doing TLC . The less
10:08	polar compounds will also move fastest via capillary action upward
10:12	on the TLC . Play lastly , we'll combine the
10:14	fractions that have the compound we want in a round
10:17	bottom flask and concentrate using a rotary evaporator . A
10:20	vacuum reduces the pressure inside the road of app ,
10:23	which lowers the boiling point of the solvents to remove
10:25	them easily from the sample . We spin the flash
10:28	to create more surface area and encourage the solvent molecules
10:31	to move into the vapor phase . Hopefully we've isolated
10:34	a good size pure sample of a compound we want
10:37	. Thanks thought bubble with a purified sample . We
10:39	can calculate a percent yield to see how efficient our
10:42	reaction was , analyze our compound using spectroscopy and then
10:46	move on to the next reaction in our synthesis .
10:49	Maybe now we have our purified either and can move
10:51	on to the final step to make Paxil or we're
10:53	doing any number of other reactions . Flash chromatography is
10:57	pretty common in organic chemistry in this episode . We
11:00	learned that chromatography separates organic compounds based on polarity .
11:04	Then layer chromatography tells us how well our reaction went
11:07	. And flash chromatography lets us separate larger quantities of
11:11	compounds . In the next episode , we'll talk about
11:13	how we can use proton NMR , one of the
11:15	most important spectroscopy techniques to an organic chemist to see
11:19	if we really made our desired product . Until then
11:22	, thanks for watching this episode of Crash Course organic
11:24	chemistry . If you want to help keep all crash
11:26	course free for everybody forever , you can join our
11:29	community on Patreon .

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