Free Radical Reactions - Free Educational videos for Students in K-12

Free Radical Reactions - By The Organic Chemistry Tutor

Transcript


00:01	in this video , we're going to talk about free
00:03	radical reactions . But first let's focus on radicals .
00:08	What are radicals And how are radicals formed ? A
00:12	radical is basically any species or any atom with an
00:17	unpaid number of electrons . So any atom that has
00:21	an odd number of electrons , we'll have at least
00:24	one unpeeled electron . And so that's going to be
00:26	a radical . Now let's talk about how they're formed
00:32	. You need to be familiar with two types of
00:34	bond cleavage is home olympic bond cleavage and the hetero
00:40	politic bond cleavage . Let's talk about a hetero politic
00:47	bond cleavage . The prefix hetero means different whereas the
00:54	Suffolk's lighting or license means to split apart in the
00:59	heterocyclic bond cleavage . Typically you have two different atoms
01:03	attached by means of a bond . And when that
01:06	bond breaks the electrons go towards the more electro negative
01:10	atom . Yeah . And so it's going to separate
01:15	into charges . One of the atoms will acquire a
01:19	positive charge , the other one will acquire a negative
01:23	charge . So let's consider two examples . So let's
01:28	consider the carbon brahman bond and carbon likes the form
01:36	for bonds . Soon let's drive out around it .
01:43	So when the carbon grooming bond breaks , what's going
01:46	to happen ? Which element will acquire the two electrons
01:51	in its bond ? In order to answer that question
01:54	, we need to consider election negativity . The the
01:59	election negativity of roaming is about 2.8 For carbon ,
02:03	it's 2.5 . So brahman is more electro negative than
02:07	carbon . Therefore , when that bond breaks , blooming
02:11	is going to pull the electrons toward itself , brahman
02:16	has a partial negative charge . Carbon has a partial
02:19	positive charge when they brought it together . When that
02:23	bond breaks , we're going to get a carbon canteen
02:30	, we're also going to get a bromide ion .
02:33	So this is an example of a hetero logic bond
02:37	cleavage . Now , let's consider another example . Mhm
02:46	. So this time we're going to attach carbon two
02:49	hydrogen . Now , when a carbon hydrogen bond breaks
02:55	, which element will receive the two electrons , hydrogen
03:02	Has an election negativity value of 2.1 for carbon ,
03:06	it's 2.5 . So relative to hydrogen , carbon is
03:10	partially negative , hydrogen is partially positive . So because
03:15	carbon is more electro negative than hydrogen . When this
03:18	fond breaks , the electrons will go to carbon .
03:24	And so we're going to get a carbon nine instead
03:27	of a car mechanic and hydrogen will have a positive
03:32	charge . So ahead of leg bond cleavage when that
03:37	occurs , you get two different ions , you get
03:40	an ion with a positive charge and an ion with
03:43	a negative charge because the electrons , they will be
03:47	distributed unequally when that bond breaks well in a homoerotic
03:51	bond cleavage . When the bond breaks , the electrons
03:54	will be distributed equally and a hemolytic bond cleavage .
04:01	You have two atoms of the same kind bonded to
04:04	each other . So a good example of this would
04:07	be two bryan Adams . Mhm . If you were
04:15	to add heat to bremen or if you are to
04:19	irradiated with ultraviolet light , The bond between the two
04:24	bromine atoms will break now because each burning adam are
04:28	identical , they will pull on the electrons in that
04:32	bond equally . So half of the electrons will go
04:37	to the booming on the left and the other half
04:40	will go to the booming on the right now .
04:42	Keep in mind a full arrow represents the flow of
04:45	two electrons . A half arrow represents the flow of
04:49	one electron . And so we're going to get is
04:53	to radicals or to burning Adams . So as we
04:59	can see , each bromine atom has an unpaid electron
05:03	. So therefore they're radicals . Radicals are very reactive
05:09	bro . Ming wants to have eight electrons , so
05:11	it's going to react with something to strip off an
05:14	electron . So you can have a So radicals tend
05:17	to be very , very reactive species . Now when
05:21	dealing with free radical reactions , you need to be
05:24	able to identify three important steps , initiation , propagation
05:30	and termination . So I'm going to show you how
05:33	you can do that . The first step is initiation
05:39	and here's how you can tell whenever you have a
05:43	neutral molecule , Turn it into two radicals . That
05:47	step is called initiation . So any time you have
05:50	to radicals on the right side of the equation ,
05:53	it's initiation during propagation . You have one radical on
05:58	the left and one radical on the right . And
06:02	the last step , termination , this occurs anytime you
06:05	have two radicals on the left . So remember if
06:08	you have two radicals on the right , it's initiation
06:10	. If you have two radicals on the left ,
06:13	it's a termination step . And if you have a
06:15	radical on the left and a radical on the right
06:18	, it's a propagation step for the sake of practice
06:23	, go ahead and pause the video and then identify
06:27	each reaction as either an initiation step , propagation or
06:32	termination step . So let's look at the first one
06:37	. Is this initiation propagation determination ? Well , we
06:41	have a radical on the left and the radical on
06:44	the right . Since we have a radical on both
06:46	sides of the equation , this is going to be
06:49	a propagation step . Let me use a different color
06:53	to identify it . Now , what about # 2
06:56	? Is it initiation propagation or termination ? Notice that
07:01	we have to radicals on the left , and we
07:03	don't have any on the right . So we're terminating
07:06	the radicals . This is going to be a termination
07:08	step for number three . We don't have any radicals
07:14	in the left , but we're creating two radicals on
07:17	the right . So any time you have to radicals
07:20	on the right , it's initiation for number four .
07:25	We have two radicals on the left , The same
07:28	as # two . So that is a termination step
07:33	for number five . We also have to radicals in
07:35	the left , so that's termination . Number six ,
07:39	we have a radical on the left and one on
07:41	the right . That's a propagation step . And number
07:44	seven , the same is true . We have a
07:46	radical on the left side and on the right side
07:48	of the chemical equation . So that's another propagation stuff
07:53	. So now that we've considered how to identify the
07:55	steps of a radical reaction is being either initiation propagation
07:59	of termination . Let's talk about the chlorination of methane
08:05	. So methane is an out Kane and we're gonna
08:08	reacted with chlorine gas . Now we can either add
08:13	heat to it or we can irradiate the mixture with
08:19	ultraviolet light . Doing any one of these things will
08:24	create radicals . And so this is gonna be a
08:26	free radical reaction . A free radical reaction is a
08:30	substitution reaction . We're going to substitute one of the
08:35	hydrogen atoms with chlorine . So we're going to get
08:38	CH three cl as one of the products . So
08:41	this is method chloride and the other product will be
08:45	hydrochloric acid . Mhm . Yeah . So that's a
08:52	free radical substitution reaction . But now let's talk about
08:55	the mechanism for this process . Mhm . So the
09:00	first thing that happens is that we generate two radicals
09:07	. So this is going to be initiation . So
09:11	we can either we can add heat or we can
09:14	add ultraviolet light . And what's gonna happen is this
09:17	pond is gonna break . So we're gonna get a
09:20	home a little cleavage . We're going to get to
09:25	chlorine radicals . Now , if you want to you
09:32	can show all of the lone pairs on the chlorine
09:35	atom . So chlorine actually has seven valence electrons .
09:43	But just to keep it just to keep things simple
09:45	Going forward . Sometimes I'm just going to write one
09:49	like this to indicate it's a chlorine radical . So
09:54	that's the first step . It's initiation . We need
09:57	to generate the radicals . Now , once we have
10:02	our chlorine radical chlorine can then react with methane ,
10:11	which I'm going to write it like this . So
10:20	one electron from chlorine will be used to form a
10:24	body between agency out and one electron between carbon and
10:28	hydrogen will also be used to create that bond .
10:33	So we're gonna get hydrochloric acid and the other electron
10:38	we'll go back to carbon . So we're going to
10:41	get a method radical . Mhm . Yeah . Now
10:47	what type of step is this would you say ?
10:49	It's initiation , propagation or termination ? So notice that
10:54	we have a radical on the left and one on
10:56	the right . So this step is a propagation step
11:03	. And so far we have one of the two
11:06	products in the reaction . We need to get the
11:08	other product . So here's what we can do to
11:16	get it . We're going to start with the method
11:18	radical actually let's write chlorine first . So we're gonna
11:24	start with cl two and then we're going to react
11:27	it with this method radical that we just formed .
11:36	So one of the electrons in the chlorine chlorine bond
11:40	will be used with the electron on the method radical
11:44	to create a bond between the method group and the
11:50	chlorine atom . Which I'm gonna write it like this
11:53	so that's the same as methyl chloride and then the
11:57	other electron will go to this chlorine atom regenerating the
12:02	coin radical so the process can be repeated . So
12:07	this step is also a propagation step because we have
12:12	a radical on the left and radical on the right
12:16	. So during the propagation step the presence of radicals
12:20	continue during an initiation step . You're creating radicals during
12:27	the termination step , you're eliminating radicals . But during
12:31	the propagation step the reaction is occurring without any increase
12:36	or decrease in the total number of radicals . So
12:40	we started with radical and we ended with radical .
12:45	So just by using these three steps we have our
12:49	two products we have hydrochloric acid and we also have
12:53	Method chloride . Now there are some other things that
12:57	can occur while this reaction is happening now let's talk
13:00	about it . Yeah . So while the reaction is
13:07	progressing to chlorine radicals , Mhm . Can react with
13:16	each other . Yeah . Mhm . And turned back
13:24	into chlorine gas . So this is a termination step
13:29	. This is completely possible . Something else that could
13:35	happen . Is that a chlorine radical can terminate with
13:43	a method radical ? Mhm . Yeah . Given us
13:57	the product method chloride . So that's another termination step
14:03	. Or we can get to method radicals terminating each
14:10	other and this will give us ethane . So ethane
14:18	is a byproduct of the chlorination of methane . Yeah
14:24	. So these are some termination steps that can occur
14:28	during the chlorination of methane . Now consider the reaction
14:34	between propane and chlorine gas . Yeah . Mhm .
14:42	What will be the products of this reaction ? And
14:45	let's compare it with propane and grooming , identify the
14:50	major products in each of these reactions . Now ,
14:56	what you need to know is that chlorine is highly
15:01	reactive . Chlorine is more reactive than grooming and when
15:05	it reacts with an arcane it's going to be non
15:08	selective bro . Ming is less reactive than chlorine and
15:19	as a result it's going to be selective . Mhm
15:27	. Now chlorine can replace the secondary hydrogen that is
15:31	the hydrogen on the secondary carbon . Or it can
15:35	replace the primary hydrogen . It's non selective . They
15:40	can replace either one of them . So in this
15:43	free radical substitution reaction we can replace the secondary hydrogen
15:48	with chlorine given us this product or we can replace
15:57	the primary hydrogen with chlorine . So we get a
15:59	mixture of products . We can get one clara propane
16:04	or two chloral . We actually have both . One
16:07	clear appropriate and to clear appropriate . Now blaming on
16:11	the other hand is highly selective . It's going to
16:17	preferentially replace the secondary hydrogen as opposed to the primary
16:21	hydrogen . Yeah , so the major product will be
16:25	two bromo propane . Now , as the reactivity of
16:35	the halogen increases , the selectivity decreases . So if
16:44	you were to focus on the periodic table , we
16:46	have the religions Florina , chlorine , brahmi and hiding
16:52	florian is highly reactive . Therefore it's going to be
16:57	the least selective . I had died is not reactive
17:02	enough to replace the hydrogen from an out Kane .
17:06	So the two that we tend to use are these
17:09	two . Flooring is too reactive to use . Its
17:13	reacts violently with al cane . So and it's basically
17:16	too dangerous . He is chlorine , on the other
17:19	hand , is less reactive than flooring is not as
17:23	dangerous as flooring but it's reactive enough where it'll abstracts
17:29	any of these hydrogen is that it encounters bro ,
17:33	ming reacts much more slowly than chlorine . It's less
17:37	reactive but it can react with either one of these
17:41	two hydrogen . But because it reacts slowly , it's
17:44	going to prefer two replace the secondary hydrogen over the
17:49	primary hydrogen . And the reason for this has to
17:52	do with radical stability . Yeah . A tertiary radical
17:59	is more stable than a secondary radical . Yeah .
18:04	And that's more stable than a primary radical . Which
18:09	is more stable than a method radical . Yeah .
18:14	Yeah . This is similar to the stability of carbo
18:19	kids radicals and caramel Karen's they both electron deficient .
18:25	So they want electrons . Therefore the transmissibility is the
18:30	same . A secondary carbon canteen is more stable .
18:34	I mean a tertiary carbon Karen is more stable than
18:36	the secondary one and that's more stable than a primary
18:39	carbon canteen and so forth . Mhm . Now carbon
18:47	ions are opposite to this trend . Carbon eines don't
18:51	want any more electrons . They already have plenty of
18:54	electrons . So um ethel carbon I is more stable
19:00	then a primary carbon I which is more stable then
19:06	the secondary carbon I . Mhm and tertiary carbon ions
19:14	are the least stable . So carbon IEDs are nuclear
19:22	filic , their electron rich and so tertiary carbon emissions
19:27	are the least able electro files . And carbon headlines
19:31	. I mean radicals and carbon credits are electro files
19:35	. They are electron poor and they want electrons .
19:38	So territory radicals and territory carbon cannons are the most
19:42	stable . The method groups that are attached to the
19:49	caramel canine , they can donate electron density by means
19:53	of the inductive effect and by means of hyper congregation
19:56	to stabilize the caramel canine and even stabilize the radical
19:59	as well . Yeah . Now you might be wondering
20:08	why the reaction between al cane and chlorine is less
20:13	selective than a reaction between al Qaeda and brahmi .
20:18	It has to do with the relative differences in the
20:21	activation energies of each reaction . Now let's consider the
20:28	reaction diagram between the al cane and chlorine . Would
20:34	you say this is an end a thermic reaction or
20:37	an extra thermic reaction ? Notice that the energy of
20:41	the reactant is greater than the energy of the products
20:46	. So therefore this is going to be an exotic
20:49	reaction . Now , according to they haven't postulate ,
20:58	does the transition state resemble more likely reactant or more
21:02	like the products ? So , notice that in an
21:07	extra thermic reaction , the transition state is closer in
21:11	energy to the reactant , so therefore it's going to
21:15	resemble more like the reactant . Notice that the relative
21:22	, like the energy differences between the products is large
21:26	, but the energy differences between the reactant are small
21:31	and because the transition state resemble more like the reactant
21:35	on the left side , Or for the reaction between
21:38	these two , the differences and activation energy will be
21:44	small because the differences in the energy of the reactant
21:49	is also small . Now , let's consider the situation
21:54	on the right , do we have an extra thermic
21:57	reaction or an endemic reaction ? Notice that the energy
22:01	of the products is greater than the energy of the
22:04	reactant . So we have an an atomic reaction for
22:12	the reaction between al cane and grooming . Mhm .
22:15	So according to the hammer partial it , the transition
22:17	state resembles more like the products because they're closer in
22:22	the energy now , because the products have a huge
22:26	difference in energy . Mhm . The energy of the
22:30	transition states will be significantly different . So therefore the
22:35	change in activation energy between each reaction , It's gonna
22:39	be larger . And here is the key . The
22:45	reason why chlorine is less selective is because the difference
22:51	is in the activation energy for abstract in a primary
22:57	hydrogen or a secondary hydrogen . The difference in the
23:01	activation energy is small . So that's why chlorine is
23:05	less selective in the case of bromine , the difference
23:09	in the activation energy is large and so brimming will
23:15	selectively abstract a tertiary hydrogen much more than it would
23:20	a secondary hydrogen . It's due to the large differences
23:24	in activation energy . Now , let's discuss the relative
23:28	reactivity rates of chlorine and bromine when they're abstract in
23:34	a hydrogen from an al caine . So we know
23:39	that chlorine is more likely to obstruct the tertiary hydrogen
23:45	compared to a secondary or primary hydrogen . Yeah ,
23:50	We're going to sign a primary hydrogen of value of
23:53	one at room temperature . A chlorine radical is 3.8
24:03	times more likely to abstract a secondary hydrogen than the
24:08	primary hydrogen . Or , more specifically , It's 3.8
24:12	times easier for chlorine radical to abstract a secondary hydrogen
24:16	from the primary hydrogen . And for tertiary hydrogen ,
24:22	it's five times easier for the corn radical to replace
24:26	the tertiary hydrogen as opposed to replacing the primary hydrogen
24:31	. So these are the relative rates of formation for
24:35	the different types of hydrogen on al caine when chlorine
24:38	is a reaction with that out , Kane . Now
24:43	let's talk about brahmi . So for the primary hydrogen
24:46	, let's assign it a value of one . Now
24:51	from bromine , It is 82 times easier for it
24:58	to abstract a secondary hydrogen than a primary hydrogen .
25:02	And it's 1600 times easier for the Brahmin radical to
25:06	replace the treasure hydrogen relative to a primary hydrogen .
25:11	So we could see why based on these numbers ,
25:14	bruning is much more selective than chlorine . So both
25:24	radicals prefer to abstract a tertiary radical . I mean
25:28	a tertiary hydrogen and the reason for that is tertiary
25:31	radicals are more stable than primary radicals . But looking
25:35	at the difference in the case of chlorine , it's
25:38	five times easier for it to obstruct a tertiary hydrogen
25:42	than a primary hydrogen , But for broadening its 1600
25:47	times more easier so that we can see why based
25:51	on these numbers , chlorine is , even though it's
25:53	more reactive , it's less selective and abstracting a proton
25:58	than roominess . Now , let's work on some math
26:01	problems . So let's go back to propane and we're
26:06	going to react with chlorine in the presence of ultraviolet
26:10	light . Yeah . Now , as we mentioned before
26:15	, chlorine is not very selective . So we're gonna
26:20	get a mixture of products , we're gonna get to
26:23	claro Propane and one Clara Propane . But now ,
26:30	using the relative rates of formation that we mentioned earlier
26:33	, how can we calculate the percent yield Of each
26:37	of these two products ? Feel free to pause the
26:40	video if you want to try it . So remember
26:46	the relative rates for each hydrogen is five , 2
26:54	3.8 - one . So the first thing we need
26:59	to do is identify the different types of hydrogen atoms
27:05	. There's only two hydrogen atoms that will give us
27:10	this particular product , let's call it hydrogen A .
27:17	Now here we have two method groups . If we
27:20	replace any one of those hydrogen , we can get
27:23	one chloral propane . So there's six potential hydrogen atoms
27:30	That can give us one chlor appropriate . Yeah .
27:36	Yeah . So now let's focus on the secondary hydrogen
27:43	atoms that is hydrogen A . So this is secondary
27:49	because it's on the secondary carbon and this is primary
27:52	. The reason why this carbon is secondary is because
27:55	it's attached to two other carbons . And the reason
27:58	why this carbon is primary is because it's attached to
28:01	only one other carbon . Now , for the secondary
28:05	hydrogen , we only have two of them . And
28:11	the reactivity rate For secondary hydrogen when dealing with chlorine
28:16	, that room temperature is 3.8 . So if we
28:20	multiply two and 3.8 , we get 7.6 . Now
28:24	for the primary hydrogen , we know that there's six
28:27	of them . We're gonna multiply by The relative reactivity
28:33	rate for a premier hydrogen , which is one .
28:36	And so we get six And then we're gonna add
28:38	these two numbers . 7.6 plus six is 13.6 .
28:44	Now to get two yield , We're gonna take 7.6
28:47	and divided by this total number , Which is 13.6
28:52	. And then we're gonna multiply that by 100% .
28:57	So if you take 7.6 and divided by 13.6 times
29:03	100 , that will give you a percent yield for
29:07	to clear a propane Which is 55.9 if you around
29:13	it . So this came from the secondary hydrogen ,
29:18	this chlorine , uh the secondary hydrogen was replaced with
29:21	chlorine . Sure , That for the other one simply
29:27	take this number six , divided by 13.6 , and
29:31	then times 100% Then you get 44.1% . So that's
29:42	the relative percent yield for one clara propane . So
29:47	, for this particular example , to clear a propane
29:50	is the major product . one Clove of Propane is
29:54	the minor product .

Summarizer

DESCRIPTION:

This video provides a basic introduction into free radical reactions. It explains the reactivit-selectivity principle between Chlorine and Bromine and it explains how to calculate the relative percent yields of each monochlorinated product. It discusses heterolytic and homolytic cleavage as it relates to the formation of radicals as well as how to identify initiation, propagation and termination steps in a radical reaction.

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