Substitution Reactions - SN1 and SN2 Mechanisms: Crash Course Organic Chemistry #21 - Free Educational videos for Students in K-12

Substitution Reactions - SN1 and SN2 Mechanisms: Crash Course Organic Chemistry #21 - 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 Crash
00:09	Course Organic Chemistry and to my home where we are
00:13	filming now because of the pandemic . When your fancy
00:16	clothes say dry clean only . Maybe you just never
00:20	wash them when I was little I thought dry cleaners
00:23	dusted laundry detergent on clothes and then just ran them
00:26	through a machine to get rid of the powder .
00:28	Dry cleaning isn't actually dry though , it's just using
00:32	a liquid other than water to wash it away .
00:35	The most common liquid is tetrachloride ethylene also called perchloroethylene
00:39	or Perk for short . But dry cleaning like everything
00:43	comes with accidents . And unlike a water spillage a
00:45	perks village can be harmful to the environment . We
00:48	don't have a great way to clean up for chemical
00:50	spillage is right now . So we may need to
00:52	get creative industrial plants use some species of soil bacteria
00:56	to clean up waste products like one to die plural
00:59	ethane . These bacteria have an enzyme that removes the
01:02	halogen through S . And two reactions , replacing them
01:05	with more palatable alcohols . So the waste chemicals can
01:08	be eaten by bacteria , processed and even reused in
01:11	things like plastic manufacturing perks double bond makes it unable
01:15	to do in SN two reaction . So it's a
01:17	challenging compound for bioremediation . But maybe someday chemists and
01:21	hungry bacteria could help with this pollution . For now
01:24	let's dive deeper into S . And one S .
01:26	And two reactions to find out how substrate structure and
01:29	other reaction conditions can help us pick a likely mechanism
01:33	. Mhm . Mhm . In episode 20 we learned
01:45	about S . And one in SN . Two pathways
01:47	for substitution reactions . Being able to figure out which
01:50	one takes place will let us predict the correct reaction
01:53	products . Remember SN one has a carbo cat eye
01:55	on intermediate and result in a mixture of stereo is
01:58	a merciful Cairo center is reacting . The S .
02:01	N . Two mechanism is a concerted process and gives
02:03	inverted stereo chemistry at a reacting Cairo center . There
02:07	are three key players in our substitution reaction dance or
02:10	married around or whatever visual you like . An sp
02:13	three hybridized carbon substrate , a leaving group that can
02:16	accept electrons as it departs from the molecule and a
02:19	nuclear file . Something with the lone pair or pi
02:22	bond , recognizing . Good leaving groups can help us
02:24	determine if a substitution reaction can take place . For
02:27	example , weak bases with strong conjugate acids are good
02:31	leaving groups , weak bases are happy to float around
02:33	by themselves in solution as an ions . Most salads
02:37	fall into this category because the P . K .
02:39	Is for hcl H . I and hbR are big
02:42	negative numbers . Other good leaving groups are Sultanate's compounds
02:46	that come from acids with negative P . K .
02:48	A values . They're big molecules , their electrons are
02:51	spread out and stabilized by multiple resonance structures and they
02:54	have special non aipac names like to oscillate missile it
02:57	and Tripoli . If the group we want to leave
02:59	, doesn't want to let go of the carbon chain
03:01	substitution does not happen . We call these poor leaving
03:05	groups . Poor leaving groups tend to be strong bases
03:08	which are less stable than weak bases out on their
03:10	own in solution , examples of poor leaving groups include
03:13	hydroxide and H two minus ch three minus anhydride ,
03:17	all with weak conjugate acids that have PKS above 12
03:21	. However , sometimes we can turn a poor leaving
03:23	group into a good one and entice it to leave
03:26	by protein eating . It sort of like offering your
03:28	friend candy to give you their spot on the S
03:30	. N . One married around . So like I
03:32	just said , hydroxide is a poor leaving group .
03:34	But if we had a proton to it , the
03:36	leaving group changes to water , which is happy to
03:38	swim away in solution . Leave behind a carbo cat
03:41	eye on and get substituted by a nuclear filic bromide
03:44	. We need a good leaving group for both SN
03:46	one and sN two to happen . But in organic
03:49	chemistry will often be asked to choose which mechanism is
03:52	more likely for a given substitution reaction . So let's
03:55	look at three key factors that can help us decide
03:58	whether to do S . N . One or S
04:00	. N . To the first . And most important
04:02	factor is the structure of the substrate substrates with leaving
04:05	groups on primary carbons . Use SN two substrates with
04:09	leaving groups on secondary carbons can use SN one or
04:12	S . N two , and substrates with leaving groups
04:14	on tertiary carbons use exclusively SN one . These guidelines
04:18	will work for most molecules you'll come across in organic
04:20	chemistry but there are important exceptions for one . This
04:24	neo pencil primary substrate won't do an sN two mechanism
04:27	very smoothly are leaving group is one carbon away from
04:30	hysterically hindered tertiary carbon . So backside attack is really
04:34	tough and the reaction is too slow to be practical
04:36	. We also have substrates like perk claro eh thin
04:39	or chloral benzine where the leaving group is directly attached
04:42	to a double bond substitution by S and one or
04:45	S . N two doesn't happen at all . The
04:47	sp two geometry is all wrong for S and two
04:49	backside attack in sN one is pretty unfavorable to because
04:53	the vinyl or fennel carbo kati means that would have
04:55	to form our high energy . And then we have
04:57	substrates with ALEc and Ben's like leaving groups , a
05:00	leaving group that's one sp three carbon atom away from
05:02	a double bond . They use both SN one and
05:05	S and two reaction mechanisms . Looking at one of
05:08	these compounds , we might say , oh there's a
05:10	leaving group on a primary carbon , so it's got
05:12	to be an sn two mechanism . But depending on
05:14	reaction conditions , like if there's an excellent leaving group
05:17	, a carbo Catalan can form because its resonance stabilized
05:21	, That's the first step of an sN one mechanism
05:23	. This resonance stabilization of an intermediate cast iron is
05:26	what makes a lilic and Ben's Ilic substrates special .
05:29	It also makes things even more complicated for a lilic
05:31	substrates . If a reaction proceeds by an S .
05:34	N . One mechanism , the two resonance forms of
05:36	the carbon kata and intermediate can lead to a mixture
05:39	of constitutionalism . MERS as products . On the other
05:41	hand , Ben's Ilic substrates only give one substitution product
05:45	because they don't react on the benzene ring after the
05:47	structure of the substrate . The second key to picking
05:50	a path is the role of the nuclear file .
05:52	The nuclear file in an S . And two reaction
05:54	is an active participant in the rate determining step in
05:58	our marriage . Around analogy , they were the playground
06:00	bully who push someone off rather than waiting for them
06:02	to leave . Nuclear Felicity is a term to describe
06:05	just how pushy the nuclear files behavior is . Adam
06:08	size plays a key role in nuclear file strength .
06:10	The more polarized , able to adam , the easier
06:13	it is to get those electrons to attack and make
06:15	a new bond . So nuclear Felicity increases moving down
06:18	a group on the periodic table halogen , which we
06:21	talked about as good leaving groups are also good nuclear
06:24	files and really strong nuclear files are often charged ,
06:27	so violates hydroxide and Alcock sides are all examples of
06:31	strong bullies or nuclear files that use SN . Two
06:34	mechanisms , however , compounds like methanol , acetic acid
06:38	, water and other alcohols are relatively weak nuclear files
06:42	. Even with lone pairs on an oxygen atom ,
06:44	these molecules are uncharged and are less nuclear filic than
06:47	their deep resonated cousins . They patiently wait to hop
06:50	on the merry go round and are more likely to
06:52	promote an sN one reaction and chunky sulfates , which
06:56	are good leaving groups are very poor nuclear files .
06:59	Their electrons are tied up in resonance not available to
07:01	attack and make new bonds to a substrate . Our
07:04	third and final key to help us decide between an
07:06	S . N . One and S and two mechanism
07:09	is the solvent solvents are usually liquids that are able
07:11	to dissolve . Other things like how perk dissolves clothing
07:14	, grime in dry cleaning and water dissolves table salt
07:17	. There are two classes of solvents we need to
07:19	consider in substitution reactions . Polar product and polar a
07:23	product . Polar protic solvents like water and ethanol both
07:27	have a proton on an electro negative atom . This
07:29	type of polar bond with a proton lets them hydrogen
07:32	bond to both cat ions and and ions . This
07:35	is good for SN One mechanisms because the rate determining
07:38	step is the substrate breaking up into ions . So
07:42	polar products solvents favor the SN . One mechanism on
07:45	the other hand , in an SN two reaction ,
07:47	we need the nuclear file to be really available to
07:50	push out the leaving group in the rate determining step
07:53	. If our nuclear file is tied up by the
07:54	solvent , it loses its pushing power and becomes weaker
07:58	. So a polar A product solvent , which has
08:00	no available hydrogen can't hydrogen bond to our nuclear file
08:04	and an S . And two reaction is favored .
08:06	Polar . A product solvents have atoms like oxygen or
08:09	nitrogen with a partial negative charge and a polar bond
08:12	just not to a hydrogen atom . The polar bonds
08:15	help these solvents dissolve organic compounds . And the ionic
08:18	nuclear files often used in SN two reactions . Some
08:21	examples of polar a product solvent or acetone , dimethyl
08:24	, former meid and dimethyl sulfide oxide or D .
08:27	M . S . So I'm going to be honest
08:28	evaluating these key reaction conditions can feel complicated , but
08:32	here's a trick . We can use weak nuclear files
08:35	are often polar products solvents and favor SN one since
08:38	their product , the reaction is carried out under acidic
08:41	conditions or generates a molecule of acid as a byproduct
08:44	. So when we see acid as a reactant or
08:47	product , think SN one . And on the flip
08:49	side , reactions that take place under neutral or basic
08:52	conditions tend to favor SN two . We can summarize
08:55	all we've learned by adding different types of nuclear files
08:58	to the table . We started an episode 20 like
09:02	I've mentioned over and over again . The best way
09:04	to learn reaction mechanisms and organic chemistry is to practice
09:08	. So let's do some rapid fire problems . We're
09:11	going to put four substitution problems on screen and predict
09:14	the likely mechanism and the products . Then we'll work
09:17	through the answers . So pause right after the question
09:20	. If you want to solve them yourself . Ready
09:22	, here's the first problem . A reaction between a
09:24	. Tosel it and sodium benzene dial it carried out
09:28	in the solvent D . M . F . We
09:29	have a secondary substrate with an excellent leaving group .
09:32	So there's no definitive answer . There are nuclear file
09:36	is the real deciding factor . We have sulfur with
09:39	a negative charge on a large atom . So it's
09:41	a strong nuclear file . A playground bully . This
09:44	reaction is S . N . Two , which gives
09:46	us the inversion of stereo chemistry in our product here
09:49	is the second problem . A reaction between an alcohol
09:52	and hydrochloric acid . First of all , hydroxide is
09:55	a poor leaving group . But remember hcl is a
09:58	strong acid that dissociates into H plus and cl minus
10:02	. So there are protons floating around that can protein
10:04	eight the alcohol and make it an excellent leaving group
10:07	water . We have two keys here , a good
10:09	leaving group and a tertiary substrate that blocks backside attacks
10:13	. We get an SN one reaction . This is
10:16	reinforced by our little trick . There's acid in the
10:18	reaction . So think sN one the products are a
10:21	mixture of the same and inverted stereo chemistry at the
10:24	reacting Cairo carbon . For a third problem , How
10:27	about this ? Iodide reacting with acetic acid ? Let's
10:30	assume acetic acid is also our solvent here . We
10:33	have a secondary substrate , a benzene ring with the
10:36	leaving groups separated from it by one sp three carbon
10:39	. So actually this is a Benz Ilic substrate that
10:41	can make a resonance stabilized secondary carbo cat eye on
10:44	that means it could do SN one or S and
10:47	two . And we have to look at another factor
10:49	to make the final call . Acetic acid is our
10:51	weak nuclear file and our polar products solvent . And
10:54	we clearly have acidic conditions . All science point to
10:57	S . N . One . Finally , our last
11:00	problem , a reaction between a bromide and in a
11:02	civilized and I on our substrate here is a primary
11:05	alcohol bromide , which rules out SN one right away
11:08	because SN one reactions can only happen with secondary or
11:12	tertiary substrates . To double check our nuclear file is
11:15	the strongly basic A satellite and ion and are solvent
11:18	is polar . A product that ticks all the boxes
11:21	for an SN two reaction . That's all we have
11:24	for now . But keep practicing nuclear Felix substitution reactions
11:27	are some of the most common reactions in organic chemistry
11:30	. So we'll see them again and again . In
11:33	this episode we learn that the main determining factor of
11:36	SN one or S . N two mechanism is the
11:38	structure of the substrate , weaker nuclear files . Favorite
11:41	Sn one while stronger nuclear files . Favorite SN two
11:44	polar products , solvents , favorite Sn one while polar
11:47	a product . Sullivan's favorite SM two and acidic conditions
11:51	characterized SN one mechanisms while neutral or basic conditions are
11:55	typical of SN two in the next episode will add
11:58	to our table even more as we learn about elimination
12:00	reactions where groups are lost from the substrate . Until
12:03	then , thanks for watching this episode of Crash Course
12:05	Organic Chemistry . If you want to help keep all
12:08	Crash course free for everybody forever , you can join
12:11	our community on Patreon .

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