Substitution Reactions - SN1 and SN2 Mechanisms: Crash Course Organic Chemistry #21 - By Math and Science
Transcript
00:0-1 | You can review content from Crash course Organic Chemistry with | |
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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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