E/Z Alkenes, Electrophilic Addition, & Carbocations: Crash Course Organic Chemistry #14 - Free Educational videos for Students in K-12

E/Z Alkenes, Electrophilic Addition, & Carbocations: Crash Course Organic Chemistry #14 - 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 dr Chakravarty and welcome to
00:08	Crash course Organic Chemistry , the Australian Blue Mountains and
00:12	the american Blue Ridge Mountains both get their colorful names
00:15	from a blue haze that blankets them on hot summer
00:17	days . This haze comes from small molecules that scatter
00:21	sunlight , specifically small wavelength blue light . One of
00:24	these small molecules is I Supreme , a volatile biogenic
00:27	organic compound or in simpler terms and organic chemical made
00:31	by living things . In this case , trees that
00:34	readily evaporates into the air . Like many Elkins ,
00:37	I Supremes pi electrons in double bonds , make it
00:40	a reactive molecule . It reacts with ozone , nitrogen
00:43	dioxide and other atmospheric pollutants . Not always in good
00:48	ways and I Supreme Problem arises to make a major
00:51	component of natural rubber . The reactions will learn over
00:54	the next few episodes will let us add different things
00:56	to Elkins . But first we have to revisit some
00:59	Alki nomenclature and another familiar friend , Carbo cat adds
01:03	those molecules would positively charged carbon atoms and the AL
01:17	came as a molecule containing carbon carbon double bonds .
01:20	Unlike single bonds , double bonds are rigid , They
01:23	can't easily rotate because the pi bond would need to
01:26	break first and that costs energy . In episode two
01:29	, we talked briefly about CIS and trans . I
01:32	summers of Alcan's as a refresher . This naming system
01:35	helps us describe two different geometric customers around a double
01:38	bond . For example , let's look at penn tooling
01:41	. If the methyl and ethyl groups are on opposite
01:43	sides around the double bond , we call it transplant
01:46	. Oohing and if the methyl and ethyl groups are
01:48	on the same side around the double bond , we
01:50	call it Cis , spent doing . But in the
01:53	context of alcohol in CIS and trans is an old
01:55	school naming system that only works from the double bond
01:58	carbons are attached to two hydrogen and to our groups
02:01	. If we consider something like to clear up into
02:03	in the CIS and trans system fails to help us
02:06	accurately communicate where the groups are . So we need
02:09	a better way , thankfully , organic chemists have us
02:11	covered similar to assigning R and S and anti immersive
02:15	molecules . We can prioritize the groups on each carbon
02:18	of the double bond using the rule that higher atomic
02:21	number means higher priority . In this first summer of
02:24	two Clara pent doing , the carbon on the left
02:26	side of the double bond is attached to an ethyl
02:28	group and a hydrogen atom . The ethyl winds priority
02:32	because it's carbon has a higher atomic number than hydrogen
02:35	carbon on the right side of the double bond is
02:37	attached to a chlorine and a methyl group here ,
02:40	the chlorine winds priority . If we market priority winners
02:43	and losers , we can see that high priority groups
02:45	on each double bonded carbon are on the same side
02:48	of the double bond . So this is the Z
02:50	. I . Simmer which comes from the german word
02:53	zaman for together . Or as I remember it on
02:57	the same side . So we have Z to Clara
03:00	pen to win . The other is a mother of
03:02	two . Clara Penta . Queen has high priority groups
03:05	on opposite sides with respect to the double bonded carbon
03:08	atoms . So this is the E . I .
03:10	Simmer derived from the german word and vegan or opposite
03:14	. I don't have quite a clever way to remember
03:15	this , but I remember it as E cross for
03:18	across . So it's E to clara Pent two in
03:21	assigning priorities isn't always so easy . So if we
03:23	have a tie , we need to keep hopping along
03:25	until one group . Wind . For example , in
03:28	this molecule , the left hand carbon of the double
03:30	bond is straightforward bro . Ming winds priority over the
03:33	metal group . But on the right hand carbon ,
03:36	the first position gives us the same thing . A
03:38	carbon with two hydrogen is attached a tie . So
03:42	we have to keep going and compare the next two
03:44	atoms there . We can see that the triple bonded
03:47	carbon winds priority because it's like three carbons at once
03:50	compared to one carbon on the other side . Now
03:52	we can see our priority winners are on the same
03:55	side . So it's a Z . I simmer and
03:57	we call this guy Z . Five promo for ethel
04:00	hex for . And one returning to our first example
04:03	trance pen to win can be more precisely called E
04:06	pen to win . And the system is smart is
04:08	the Z . Al Qaim . And if one side
04:10	of the alcan has two of the same group ,
04:12	like to metal pen to in here , we don't
04:14	need to use E N . Z . So now
04:16	we know how to name all kinds a little bit
04:18	better and we can start playing around with some reaction
04:20	chemistry . Many of the chemical reactions associated with falcons
04:23	are addition reactions , which means the pi electrons are
04:27	attracted to electro files and groups get added to the
04:29	carbons on each side of the double bond . In
04:32	episode 12 , we saw what happens when we mix
04:34	hydrogen bromide and sis between . It's a pretty straightforward
04:37	nuclear filic attack where the falcon double bond attacks a
04:40	proton and one of the previously double bonded carbons gets
04:44	a bro me , no matter which carbon bonds to
04:46	the bro . Ming the product is the same but
04:49	things aren't usually so simple . For instance , if
04:52	we add hydrogen bromide to to metal beauty queen ,
04:54	which isn't symmetrical , we can potentially make two different
04:57	products . Here's the catch though . When we do
05:00	this reaction in the lab , we only make one
05:03	of those two products . This seemingly mysterious observation actually
05:06	has a pretty good explanation . It's because of the
05:08	stability of different carbo cat ions , which are positively
05:11	charged carbon atoms . To see where carbo Canadians come
05:14	in , we have to break this reaction into steps
05:17	. Remember that hydrogen bromide is a strong acid ,
05:19	so it's fully dissociated into H plus and br minus
05:22	ions . Most textbooks show an attack on an unassociated
05:26	molecule of hydrogen bromide . So that's how we'll show
05:29	it in this series . So first the elkin initiates
05:31	a nuclear filic attack on the proton formed by the
05:34	dissociation of hydrogen bromide , donating a pair of electrons
05:38	, snagging the proton and leaving the remaining carbon of
05:41	the double bond short two electrons . This creates a
05:44	positive charge . A carbo cat eye on . There
05:47	are two possible carbo Catalans that can form here .
05:50	One were the positive charge ends up on a carbon
05:53	surrounded by three other carbons called a tertiary carbo cat
05:56	eye on and one where the positive charges on the
05:59	carbon surrounded by two carbons and hydrogen called a secondary
06:03	carbo cat eye . On the next step is to
06:04	form the product , a bromide nuclear filic attack on
06:07	the carbo cat ion to form a bond where the
06:10	carbo Catalan forms , ultimately determines what product we get
06:13	. And for some reason the tertiary carbo Catalan is
06:16	what forms because that's what leads to are observed product
06:20	. So , I don't know about you , but
06:21	I'm wondering why does this reaction happen this way ?
06:25	Well , we describe carbons with more carbon carbon bonds
06:29	as more substituted and more stable . If we were
06:32	to draw four similar carbo cat ions as we replace
06:35	carbon carbon bonds with carbon hydrogen bonds , the carbo
06:38	cat and becomes less and less stable . In other
06:41	words , a tertiary carbo Catalan is more stable than
06:44	a secondary one , which is more stable than a
06:47	primary one , which is more stable than a plain
06:49	old metal carbo Catalan , the more substituted carb acadian
06:53	is stabler than a less substituted one . This pattern
06:56	instability is caused by two things , an inductive effect
07:00	and hyper conjugation . The inductive effect is where the
07:03	electron density is sort of spread out through the sigma
07:06	bonds to stabilize the positive charge . It's like if
07:10	you're trying to balance on a uni cycle , the
07:12	carbon carbon bonds are your friends with their feet firmly
07:15	on the ground , who can stick out a hand
07:17	or some electrons and bonds for support . However ,
07:21	the carbon hydrogen bonds are friends that aren't strong enough
07:25	to help basically more alcohol group surrounding the carbo cata
07:28	and lead to a bigger stabilizing effect . Type of
07:31	conjugation is an even more spread out kind of stabilization
07:35	, borrowing electron density from sigma bonds , kind of
07:38	sideways from the positive charge . To sort of continue
07:41	the uni cycle metaphor . It's like instead of just
07:43	a hand , your friends actually lean their body up
07:46	against you to help you balance sort of like a
07:49	big bear hug . So here two additional carbon atoms
07:53	have more sigma bonds that can stabilize the carbo cat
07:56	eye . So when you add a hydrogen Halide to
07:58	an alkaline , the product will be formed from the
08:01	most stable carbo cada dia . This pattern is called
08:04	Markov because rule in these reactions , the proton will
08:07	add to the side of the double bond that has
08:10	the most hydrogen is that means the positively charged carbon
08:13	will be connected to more sp three carbons , which
08:15	means it's more substituted and therefore the more stable of
08:18	the two possible carbo cat ions . With practice ,
08:21	we can use more cosmic auvs rule to help us
08:23	predict lots of products . For example , let's try
08:26	adding hydrogen bromide to one methyl cyclo hex one in
08:29	we know the nuclear filic attacks starts at the double
08:32	bond and attacks the proton . Then to follow more
08:35	cosmic cause rule we have to put the proton on
08:37	the side of the double bond with the most hydrogen
08:40	. That's the carbon with one carbon hydrogen bond instead
08:43	of none . The positive charge will end up on
08:45	the other carbon , which gives it more stabilization ,
08:49	will make a tertiary carbon acadian instead of a secondary
08:52	one . The bromide does a nuclear filic attack and
08:55	our product is one brahma , one methyl cyclo hexane
08:59	. Seems pretty simple , right ? Almost too simple
09:03	. So let's try another example and use more Kalashnikovs
09:06	. Rule again to predict the product . Here we
09:08	have hydrogen bromide and 33 dimethyl butte one in .
09:11	If we go through the same steps , do a
09:14	nuclear filic attack , put the proton on the carbon
09:17	with two carbon hydrogen bonds instead of one and get
09:20	a positive charge on the other carbon . Are expected
09:23	product would be two bromo 33 dimethyl butane . But
09:27	when we do this reaction in the lab , we
09:29	get a mixture of two products and are expected product
09:33	is just the minor one . Not even the majority
09:37	. We just learned Markov because rule and now this
09:40	reaction puts the bombing in a place where we didn't
09:42	even have a double bond to start . Okay ,
09:45	deep breath organic chemistry is tricky , but it does
09:49	make sense with lots of practice . Are we really
09:51	breaking science or is there something more subtle going on
09:55	here ? Let's see where and why ? This reaction
09:58	gets weird first . Like we've seen before , the
10:01	alcan attacks the proton from hydrogen bromide and the positive
10:05	charge ends up on the carbon . That creates the
10:07	more stable secondary carbo cat eye on instead of the
10:10	less stable primary carbo cat eye on that , we
10:12	didn't make . This is where I tried to do
10:14	the next step of the reaction and how we got
10:16	our minor product , but for the major product to
10:19	form the positive charge ends up on a different carbon
10:22	. Somehow , that somehow is a sneaky move called
10:25	a one to alcohol shift . As a reminder ,
10:28	alcohol groups are metals , ethel's and things like that
10:31	. And in this case it's a 12 metal shift
10:34	where a metal group ch three moves over to the
10:37	positively charged carbon next door , taking its electrons with
10:41	it . This move only happens with groups on the
10:44	adjacent carbon atoms that creates a new even more stable
10:47	tertiary carbo cat eye on . So when the bromide
10:50	does its nuclear feel like attack , we get the
10:52	major product that we see experimentally to bromo 23 dimethyl
10:57	butane . So technically my earlier prediction wasn't wrong ,
11:01	it was just incomplete depending on how we follow our
11:05	reaction mechanism , roadmap and whether the 12 metal shift
11:08	happens , this reaction can have two different products .
11:11	Not all carbo Catalans rearranged like this as we saw
11:14	with the first two reactions . This multi dimensional map
11:17	only happens when there's an even more stable carbo catalon
11:20	that can form . So we have to keep an
11:23	eye out for carbons branching out next to the spot
11:26	where the carbo catalon forms and not just for shifty
11:29	alcohol groups , turns out , hydrogen atoms can also
11:32	do their own sneaky move called a 12 hydride shift
11:35	to see this in action . Let's look at an
11:36	example of hydrogen bromide in three metal pent one en
11:40	we do the same nuclear filic attack initiated by our
11:42	alkaline and pick the secondary carbo Catalan as our product
11:45	because it's more stable than the primary option . Then
11:49	a reaction with the bromide would form the product to
11:52	bromo three metal painting . But this time we won't
11:55	be fooled , we're keeping an eye out for more
11:57	stable carbo Catalans . And what do you know ?
12:00	There's another possible product bumping the positive charge left one
12:03	carbon would give us a more stable tertiary carbo cat
12:06	eye on and we can make that shift happen with
12:08	a 12 hydride shift , it's a very similar dance
12:11	where a hydride or hydrogen with its pair of electrons
12:14	shifts over to the positively charged carbon next door by
12:17	taking the electrons from its bond with it . The
12:20	hydrogen leaves behind a positive charge on the carbon it
12:22	abandoned . Now there is a metal group here too
12:25	. So I guess we could try a 12 metal
12:27	shift again , but that actually wouldn't help us much
12:31	. See the hydride shift creates a tertiary carbo cat
12:33	ad when we had a secondary carbo cat eye on
12:36	, but if we were to do a metal shift
12:38	, it doesn't help with the stability . These two
12:41	secondary carbo cat ions are energetically the same . So
12:44	we need to remember that rearrangements don't just happen for
12:48	the heck of it . They form a more stable
12:50	intermediate . After we do the hydrate shift again ,
12:53	stability and the bromide does its nuclear filic attack .
12:55	We get the major product that we see in the
12:57	lab three bromo three metal painting . This is another
13:01	reaction that has two possible road maps and a major
13:04	and minor product . It just used a different kind
13:06	of shift to help increase carbo caddy instability throughout organic
13:09	chemistry . Energy and stability will continue to be guiding
13:13	forces as we puzzle our way through reactions . But
13:15	for this episode we learn that this is trans nomenclature
13:19	for al Keynes is limited so we can use the
13:22	easy system for more precision Mark Kalashnikovs rule can help
13:25	us predict products of addition reactions involving Calkins . Carbo
13:29	cat ions are stabilized by the inductive effect in hyper
13:31	conjugation and we have to inspect all key in addition
13:35	reactions for possible rearrangements through 12 shifts that make more
13:39	stable carbo cat ions . Next episode will get into
13:41	thermodynamics and how to use free energy and kinetics to
13:44	help us predict reaction products , things for watching this
13:47	episode of Crash Course Organic Chemistry . If you want
13:50	to help keep all Crash Course free for everybody forever
13:53	, you can join our community on Patreon Yeah .

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