Intro to Reaction Mechanisms: Crash Course Organic Chemistry #13 - Free Educational videos for Students in K-12

Intro to Reaction Mechanisms: Crash Course Organic Chemistry #13 - By CrashCourse

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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 , whether it's gps a
00:11	foldout map or directions like head to the big maple
00:14	tree and turn left . We need navigational tools to
00:17	help us nowhere to go , especially as we ventured
00:20	in new places . But there are highways , freeways
00:23	, roundabout , stoplights , construction signs , bicycle lanes
00:27	in so many other kinds of navigational signs and symbols
00:30	. All the details might be overwhelming at first ,
00:33	but with time we can build a familiarity so we
00:36	can travel quickly and maybe even effortlessly . Chemical reactions
00:39	have their own navigational language to in reaction mechanisms .
00:43	As we get more practice interpreting these roadmaps , we'll
00:46	be able to use reaction mechanisms to see patterns ,
00:49	predict products and successfully track electrons as they make and
00:52	break bonds . For instance , in the last episode
00:55	we learn that we only show nuclear filic attack happening
00:58	with an arrow drawn from the nuclear file to the
01:00	electro file . That's an important freeway . But there
01:03	are a lot of organic chemistry navigational skills left to
01:07	learn . So let's get to it . Mhm .
01:17	Yeah . Every chemical reaction has a beginning and an
01:22	end . We start with one or more reactions and
01:24	our destination is one or more products . Reaction mechanisms
01:28	are detailed maps that show us the routes we can
01:30	take and many notable pit stops along the way .
01:34	In non metaphorical words , a reaction mechanism is a
01:37	step by step sequence that helps us keep track of
01:39	electron movements , bonds that break and form and any
01:43	molecules that show up over the course of a chemical
01:45	reaction . Before we get into specific examples , we
01:48	need to get comfy with some basic navigational symbols .
01:51	Lots of arrows . Some you might recognize from earlier
01:55	in the series , but I want to make sure
01:56	that we're all on the same page . We can
01:58	use six types of straight arrows to describe the relationship
02:01	between different molecules , like chemical reactions or resident structures
02:05	. A straight arrow pointing in one direction means a
02:08	forward reaction , showing reactant is becoming products to straight
02:12	arrows , one on top of the other mean the
02:14	reaction can go in both directions forward and reverse if
02:18	they melt into each other , This means a step
02:20	is at equilibrium . Sometimes the reaction in an equilibrium
02:24	will be unbalanced and we'll see more of some chemicals
02:27	and less of others in our mixture . The longer
02:29	arrow tells us the direction the equilibrium lies . If
02:32	the long arrow points right , the forward reaction happens
02:35	more and the products are favored if the long arrow
02:38	points left , the backward reaction happens more and the
02:41	reactions are favored . Next we have an arrow to
02:43	represent resident structures where we move a charge around one
02:47	molecule . This arrow has a single line with two
02:50	heads going in opposite directions , so it's distinct from
02:53	an equilibrium era . We used curved arrows to show
02:56	electrons moving around within or between molecules . Whether we're
03:00	dealing with resonant structures within molecules or chemical reactions .
03:04	A curved arrow with a regular arrowhead means we're pushing
03:06	two electrons and a curved arrow with the harpoon head
03:09	or fishhook arrowhead means we're only pushing a single electron
03:13	, also known as radical . In general , electron
03:16	rich atoms or molecules called nuclear files are attracted to
03:19	electron poor atoms or molecules called electro files . And
03:23	these arrows will connect them as electrons get shuffled around
03:26	and bonds get made and broken . We're going to
03:28	put all the arrows we're going to see throughout organic
03:30	chemistry in a handy chart . So take notes if
03:33	you want , it might be useful as we're still
03:35	getting familiar with all these symbols now that we know
03:37	a few basic symbols , we can start looking at
03:39	whole maps of chemical reactions . Some reaction mechanisms can
03:43	be multi step and really complex , while others are
03:47	rather simple . In the last episode we talked about
03:49	nuclear feel like attack , which involves two types of
03:52	molecules , electro files and nuclear files and electro file
03:56	is electron loving . So it's something like a carbo
03:58	cat eye and that might have an empty orbital and
04:01	a positive formal charge . A super detailed way to
04:04	represent a carbo cat ion in a reaction mechanism is
04:07	by drawing out everything , including the MTP orbital and
04:11	its molecular geometry a nuclear file is a molecule with
04:14	electrons that are ready to react . So it usually
04:16	has a negative charge or lone pairs . Nuclear files
04:19	are ready to attack any molecule with a complete or
04:22	partial positive charge . So our nuclear file is the
04:25	starting point for our electrons and the electro file is
04:28	their destination . We can fill in the rest of
04:31	our reaction mechanism map with directional curved arrows . When
04:34	our bromide ion sees the carbo Catalans empty orbital ,
04:37	the nuclear file attacks the electro file and donates a
04:40	pair of electrons . Using the symbols we've learned ,
04:43	we show that electron movement with a regular curved arrow
04:46	head that makes a bond , which creates a neutral
04:48	molecule as the product . To show that it's our
04:50	next and final stop . In this reaction mechanism we
04:53	draw a straight arrow pointing in one direction and the
04:56	product that's made done this kind of reaction mechanism map
04:59	has a lot of detailed information but drawing out orbital's
05:03	and the geometry of molecules takes a while and makes
05:06	it hard to focus on just the reaction . So
05:08	in reaction mechanisms moving forward will mostly focus on how
05:11	the electrons are moving . We just draw arrows without
05:14	the extra flair , like orbital's . Sometimes we need
05:17	maps to check that we know where we're going .
05:19	Like when you look up your favorite restaurant just to
05:21	be positive , you took the right freeway exit and
05:24	navigational tools , like maps and road signs are so
05:27	helpful because they can help us go somewhere . We've
05:29	never been before in organic chemistry , what that means
05:32	is we can use reaction mechanisms and electron pushing to
05:35	help us predict products and chemical reactions . We don't
05:38	have to memorize every single reaction that has ever existed
05:41	. We just have to navigate carefully through each step
05:44	of our journey . For example , let's look at
05:45	this reaction . Were given the start of a road
05:48	map with three compounds and an arrow that tells us
05:51	the reaction can go in both directions were not given
05:54	the product , but we can figure out what it
05:56	is to start . We have to look for the
05:58	functional groups on the starting molecule and then what's written
06:00	above the reaction arrow and think about what they might
06:03	do here . We've got an alkaline and some water
06:06	in sulfuric acid , sulfuric acid is a super strong
06:09	acid , almost as strong as hydrochloric acid or hydroponic
06:12	acid , which means it's most acidic proton completely dissociates
06:16	in water . We talk about acidity in more detail
06:19	and episode 11 . So rewatched that video if you
06:21	need a refresher but simply sulfuric acid doesn't argue when
06:24	the water molecule grabs a proton to form a hydro
06:26	nia My on H 30 Plus . After puzzling through
06:29	that road sign , we can properly start our reaction
06:32	mechanism journey with two reactions . We have sis beauty
06:35	queen which has an electron rich double bond , a
06:38	nuclear file and we have a hydro nia My on
06:40	which has a positive formal charge and electro file .
06:43	We can mark these regions in red and blue respectively
06:45	. To see our reaction hotspots because double bonds have
06:48	a lot of electrons between just two atoms . They're
06:51	attracted to positive regions . So we know where the
06:54	first reaction will happen . Let's draw a regular curved
06:57	arrow that starts on the electrons in the double bond
06:59	and does a nuclear filic attack on the electro filic
07:02	hydro nia . My on remember even though it might
07:05	be tempting to show the electrons going straight to the
07:07	positive charge on oxygen . That move would give oxygen
07:10	10 electrons . Which is not allowed . It's also
07:13	not okay for hydrogen to have two bonds in organic
07:15	chemistry mechanisms . So we have to draw one arrow
07:18	to attack hydrogen and a second arrow to move a
07:21	pair of electrons to the positively charged oxygen atom ,
07:24	which neutralizes the charge since the double bond attacks the
07:27	proton and electron file , this step is called electra
07:30	filic edition of a proton to an alky now we
07:33	have a plain old water molecule and a carb acadian
07:35	, which is quite reactive . So we're not done
07:38	yet . This is just a pit stop and we
07:40	saw farther to go on our roadmap . Specifically ,
07:43	this is a prime set up for another nuclear filic
07:45	attack , this time with water as the nuclear file
07:48	and the carbo cat iron as the electro file .
07:51	So next we can draw a curved arrow from a
07:53	lone pair of electrons on the oxygen and water to
07:56	the positively charged carbon in the carbo cat eye on
07:58	this electron push forms a bond and gives us a
08:01	pro donated alcohol called an ox ammonium ion . That's
08:04	the oxygen atom with a carbon oxygen bonds to hydrogen
08:07	oxygen bonds , a positive charge . And a lone
08:10	pair . The PK of this protein ated alcohol is
08:13	very low as another refresher from the acidity episode .
08:16	A low P . K . A means a stronger
08:19	acid that's more willing to give up its hydrogen .
08:22	So we're still not done yet . After a second
08:25	pit stop , we have one final acid based reaction
08:28	. There are still water molecules floating around . So
08:30	our reactions are a water molecule as our base and
08:33	the ac sodium ion as our acid . We can
08:36	draw one last curved arrow from a lone pair of
08:38	electrons on the oxygen in water too deep resonate the
08:41	ox sodium ion and reform the hydro nia My on
08:44	catalyst we started with as final products . We've regenerated
08:47	our hydro nia , my on catalyst and made butin
08:49	to all , there's nothing left in solution that's interesting
08:52	to the beauty tool . So that means we've reached
08:55	our destination . We could draw out a comprehensive reaction
08:58	mechanism with all of these steps , including our starting
09:01	point , our destination and all the pit stops ,
09:04	but it's often simpler to sum up our work with
09:07	the overall reaction . It was a long and winding
09:10	road trip to get here , but we predicted a
09:12	product . Not even the most celebrated Nobel prize winning
09:16	organic chemist knows every single reaction from start to finish
09:20	. Knowing the navigational language of reaction mechanisms and how
09:24	to push electrons can help us solve any problem step
09:27	by step . Now , I don't want to amp
09:28	up the difficulty too much , but let's practice our
09:31	basic reaction mechanism skills once more with a slightly different
09:34	example in this reaction . Were given the start of
09:37	a road map with an arrow meaning a forward reaction
09:40	. But we're given lots of different chemicals here and
09:43	it might look a little bit overwhelming . But don't
09:46	worry . Let's start again by looking at the functional
09:49	groups and what's written near the reaction arrow and figure
09:51	out what it means in this puzzle , the one
09:54	and the to represent two steps that have to take
09:57	place in that order . So we need a little
10:00	more road signage to help us with our journey .
10:02	We'll start with the reactions labeled one and our cyclo
10:04	hexane known sodium is civilized , is dissociated in solution
10:08	as an ionic salt . The negative carbon of the
10:10	triple bond is very reactive . So it does the
10:13	driving while the sodium is just along for the ride
10:16	. That means the acidified is our nuclear file and
10:18	the partially positive carbon of the carbon oxygen double bond
10:21	in cyclo hexane . Known is its electro file target
10:24	. With our players identified . We can draw the
10:26	nuclear filic attack with a regular curved arrow from the
10:29	lone pair on the assad allied to the partially positive
10:32	carbon . We can't take a pit stop there though
10:34	because that would be five bonds to carbon and break
10:37	science well for this typical organic compound at least so
10:41	we have to push electrons to the electron negative oxygen
10:44	and then we're done . So this molecule is the
10:47	first pit stop on our roadmap and the positively charged
10:49	sodium ion is just hanging around stabilizing the negatively charged
10:53	oxygen . Now we could stop here for dinner .
10:55	The sodium salt is stable in our reaction flask ,
10:58	but we know that there's a better barbecue joint just
11:01	down the street so we'll keep on driving now that
11:03	we followed number one on the reaction era , we
11:05	can move on to the directions listed in number two
11:08	. We've got some water in hydrochloric acid , which
11:10	is super strong and completely disassociate . So just like
11:13	the last puzzle , this road sign basically means add
11:16	in hydro nia . My on . This negatively charged
11:19	ion doesn't have resonance stabilization . So just like the
11:22	conjugate base of ethanol , we met an episode 11
11:25	. This is a strong base . Our hydro nia
11:27	. My on is a strong acid . So the
11:28	proton is transferred from the acid to the base and
11:31	presto we've made a neutral product and with that final
11:34	bond were done , we've got some side products like
11:37	water and sodium chloride . But most importantly , we
11:39	have one final cyclo hexane one . All in summary
11:42	. The reaction mechanism looks like this . Don't panic
11:45	. All the items are there , I promise .
11:47	But by convention side products like water and sodium chloride
11:51	aren't shown in a summary reaction . We just show
11:54	the major organic product we made it through this journey
11:56	by using a few key navigational tools and without a
11:59	whole bunch of memorization , we'll keep building up our
12:02	toolkit even more in episodes to come in this episode
12:05	, we learned how to write reaction mechanisms or maps
12:08	of reactions that strong acids in water should be considered
12:12	hydro knee , um ions or sources of protons and
12:15	electrons , rich atoms or regions of molecules are attracted
12:18	to electron poor atoms or regions of molecules . Next
12:21	time we'll look more closely at that positively charged carbon
12:24	with the empty p orbital . As we begin reactions
12:27	of Calkins . Thanks for watching this episode of crash
12:29	course Organic Chemistry . If you want to help keep
12:32	all crash course free for everybody forever , you can
12:35	join our community on Patreon .

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