Radical Reactions & Hammond's Postulate: Crash Course Organic Chemistry #19 - Free Educational videos for Students in K-12

Radical Reactions & Hammond's Postulate: Crash Course Organic Chemistry #19 - By Math and Science

Transcript


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
00:08	Crash course organic chemistry antioxidants have gotten a lot of
00:12	hype as a superfood , possibly helping us fight illnesses
00:14	like heart disease and cancer . On a chemical level
00:18	, antioxidants are fairly straightforward . They react with radicals
00:21	which are molecules with a single unpaid electron . We
00:24	need radicals to stay alive . They combine with oxygen
00:27	as part of our normal metabolism . But sometimes these
00:30	radical reactions produce too many reactive oxygen species or roos
00:34	like the super oxide radical , it's not great to
00:37	have too many chaotic roos around . So our bodies
00:40	have ways to control them , like an enzyme that
00:43	turns super oxide back into oxygen . But sometimes along
00:46	the way , a more dangerous roos is produced .
00:49	The hydroxyl radical can damage cell membranes , proteins and
00:52	DNA . When our enzymes are overwhelmed , antioxidants can
00:56	come to the rescue , giving up single electrons to
00:58	neutralize cell damaging radicals . For example , vitamin C
01:02	can donate a hydrogen with a single electron to neutralize
01:05	roos . But nutrition isn't as simple as chugging gallons
01:09	of green tea or blueberry smoothies to live a long
01:12	life . Roos have a purpose signaling to ourselves that
01:15	something is wrong . So the cell can correct it
01:18	or die . Very high doses of antioxidants can overwhelm
01:21	those natural signals that too much roos are being produced
01:24	in the first place . So basically we need balance
01:28	now that we know a little bit about neutralizing radicals
01:30	. Let's learn how to make them and explore some
01:33	of the reactions they perform . Mm Yeah . Yeah
01:44	. Yeah . Yeah . So far in this series
01:47	, we've talked a lot about pairs of electrons ,
01:49	there are lone pairs , shared pairs of electrons and
01:52	covalin bonds and pushing pairs of electrons around . In
01:55	reaction mechanisms , we've seen many examples of hetero politic
01:58	cleavage where a bond breaks in a pair of electrons
02:01	migrate to one of the two atoms , like when
02:03	a bases reacted with the hydro nia . My on
02:05	the two electrons from the O . H . Bond
02:07	, both end up on the water molecule . But
02:10	electrons don't always have a buddy . A radical is
02:13	an atom or a group of atoms with a single
02:15	compared electron . Some radicals are stable like the chemical
02:19	nitric oxide , which ends up with a radical on
02:21	nitrogen radicals can also form during chemical reactions and make
02:25	some interesting stuff happen . Like the reactive oxygen species
02:28	we were just talking about this process is called hemolytic
02:31	cleavage where electrons and a broken bond go and equal
02:34	but different directions forming two radicals . For example ,
02:38	with heater light , the bond and die atomic chlorine
02:40	is weak enough to be split equally between the two
02:42	chlorine atoms , with each one getting a single electron
02:46	. Instead of using a full arrowhead to push the
02:48	pair of electrons , we use half barbed arrows to
02:51	split the bond , pushing single electrons onto each chlorine
02:54	atom . We're finally using those fish Lecaros . We
02:57	talked about an episode 13 radical reactions take place in
03:00	three stages initiation propagation and termination . The initiation stages
03:05	where a reactive radical forms . We need this reaction
03:08	to get the party started . Like the first person
03:10	who breaks the ice and starts dancing their heart out
03:13	propagation is where a few radicals bounce around reacting with
03:16	other molecules . It's the step that keeps people on
03:20	the dance floor but it's sort of important to limit
03:22	the number of high energy guests so things don't get
03:25	too rowdy . Importantly , propagation also regenerates the reactive
03:28	radical we made in the initiation step . Finally ,
03:31	the termination stage is when the radical reaction stops .
03:35	We've had enough high energy dancing . It's time to
03:37	end the party so everyone can go to sleep talking
03:40	metaphorically about radical reactions and parties might be fun .
03:43	But actually looking at examples will help us get more
03:46	comfortable with them to start . Let's look at the
03:48	radical halogen nation of al canes to keep things simple
03:51	, will use methane as our al Kane . The
03:53	thing that's going to be radically how originated Like the
03:56	name of the reaction our initiation step makes chlorine radicals
03:59	are high energy party people . The propagation stage kicks
04:02	off when a chlorine radical bumps into a methane molecule
04:05	. We can think of the chlorine radical as being
04:07	so charismatic that a hydrogen electron wants to party to
04:10	or in reaction terms mega bond . The hydrogen only
04:14	needs to donate one of its two shared electrons to
04:16	make a bond and form hydrochloric acid . So as
04:20	a result , a metal radical forms with a single
04:22	electron on the carbon sitting happily in a p orbital
04:25	. Then this metal radical can react with di atomic
04:28	chlorine to make Clara methane plus another chlorine radical ,
04:32	which can continue the propagation stage by reacting with another
04:35	molecule of methane , generating a new chlorine radical .
04:38	The steps of the chain reaction repeat and the dance
04:41	party rages on Eventually . Most of our regions are
04:44	used up and it's time to get all the high
04:45	energy radicals out of here . So we enter the
04:48	termination stage basically to radicals make a bond using their
04:52	single electrons . All radicals are used up and the
04:55	party is officially over here . There are three possible
04:58	termination steps . We can end with two chlorine radicals
05:01	or instead one metal radical and one chlorine radical .
05:04	Or two methyl radicals can combine radical reactions happen quickly
05:09	and can create side products like how to metal radicals
05:12	can make ethane when we're trying to make chloral methane
05:14	. And there are some patterns in radical reactivity that
05:17	we might recognize from other organic reactions . First of
05:20	all , more substitution makes a radical more stable .
05:23	We've learned that tertiary carbo Canadians are the most stable
05:26	carbo cat ions and tertiary radicals are the most stable
05:30	radicals to . So for example , if we do
05:32	radical brahman ation of propane , we can create a
05:34	primary or secondary radical because the secondary radical is more
05:38	stable . We get way more of the product that
05:41	comes from it to Bmo propane . But here's the
05:43	weird thing if we do radical chlorination of propane ,
05:47	it's much closer to a 50 50 split of the
05:49	primary and secondary Clara propane products . Even though we
05:52	still see more to clara propane to explain why bromine
05:56	radicals and chlorine radicals act so differently . Even though
05:59	they follow the same general pattern will need to revisit
06:02	thermodynamics . The big difference between these two reactions is
06:05	that the first propagation step in the radical chlorination of
06:07	propane is eggs . A thermic , which means it
06:10	loses heat to the surroundings like a chemical hot pack
06:13	. On the other hand , the first propagation step
06:15	in the radical domination of propane is end A thermic
06:18	, which means the reaction takes in energy from its
06:20	surroundings like a chemical cold pack . We need to
06:23	look closely at the transition state of these reactions .
06:26	The peak of the hill on an energy diagram or
06:28	bonds are partially formed . Hammond's postulate is the idea
06:31	that the transition state of reaction resembles the species .
06:34	It's closest to an energy . In other words ,
06:37	the transition state for an exo thermic reaction will look
06:40	more like the reactant while the transition state for an
06:43	end to thermic reaction will look more like the products
06:46	. So in the first propagation step of radical chlorination
06:48	, which is X . A . Thermic , the
06:50	transition state looks a lot like the reactant and happens
06:53	earlier along the reaction path . It doesn't matter too
06:56	much that the secondary radical is more stable than the
06:58	primary one . Because the transition state doesn't resemble the
07:01	propel radical that much now are chlorine radical doesn't just
07:04	pull off the first hydrogen it collides with on propane
07:07	. The formation of the secondary propel radical is slightly
07:10	favored because the activation energy is just a little lower
07:14	, but plenty of primary and secondary radicals go on
07:17	to make the final products . But in the first
07:19	propagation step of radical domination , which is end a
07:21	thermic , the transition state happens later in the reaction
07:25	and resembles appropriate radical much more . in this late
07:28	transition state , the secondary carbon radical has a significantly
07:31	lower peak to overcome its activation energy . So much
07:35	more of the secondary propel radical forms and goes on
07:37	to make the major product . We've talked about the
07:40	differences . But one key thing these reactions have in
07:42	common is taking our couch , potato al canes and
07:45	adding reactive groups onto them . This is so important
07:49	to involve them in more exciting organic chemistry . So
07:52	with alcan's covered , let's move on . Like we
07:54	saw what super oxide and antioxidants radical reactions can be
07:58	in compounds with double bonds to for example the olympic
08:01	domination of al Hakim's . This is a more carefully
08:05	planned party . Under the right conditions , all kinds
08:07	can be brominated at the carbon next to the double
08:10	bond , which is called the olympic position . As
08:12	with all radical reactions . The first stage is initiation
08:16	. We need a bro mean radical to make one
08:19	will take a bro ming containing source called N bro
08:21	Mossack cinema or NBS for short and irradiated with light
08:26	. This what's the molecule into radicals . And some
08:28	of these radicals form a small amount of molecular bro
08:31	. Ming . An energetic grooming radical is irresistible to
08:34	Angelique hydrogen electron on cyclo heck scene . So the
08:37	stage is set for propagation . HBR is formed in
08:40	an aloe radical is left on the alkaline , basically
08:43	a lilic hydrogen are so ready to party because this
08:46	al radical is in a p orbital and is stabilized
08:49	by residents with a double bond resonance stabilization and radicals
08:52	is super important . In fact . The extensive resonance
08:55	stabilization in the radical anti and formed from vitamin C
08:59	. Is partially why it's such a good antioxidant .
09:01	Anyway to go back to a little abomination , the
09:03	propagation stage continues as the a little radical reacts with
09:07	the small amounts of molecular bro ming we formed even
09:10	though we have an alkaline and molecular bro ming which
09:12	are ingredients in the addition reactions we've learned in the
09:15	past few episodes . The radical propagation steps are really
09:19	fast so they take control . The radical reaction keeps
09:22	propagating until the re agents are all used up and
09:25	all the radicals pair up , terminating the party .
09:27	Now we've seen al canes and Calkins , but we
09:30	can't forget about all kinds specifically , the dissolving metal
09:33	reduction reaction produces e Elkins from all kinds . The
09:37	dissolving metal part of the name comes from to re
09:40	agents that you can see above the reaction arrow sodium
09:43	metal dissolved in liquid ammonia to produce solve ated electrons
09:46	that are floating around and stabilized by the ammonia solvent
09:50	. These solve ated electrons form a beautiful dark blue
09:53	solution . Will actually talk about colors in organic chemistry
09:56	much later in the series . So this is just
09:58	a sneak peek for now we'll focus on how salivated
10:01	electrons do radical reactions . It's a little tricky .
10:04	So we'll use a reaction mechanism diagram with orbital's to
10:07	really see what's happening . To kick off the reaction
10:10	. A salivated electron can jump into one of the
10:12	plain old p orbital's of a pi bond in the
10:14	AL kind . As the bonhomme politically cleaves the solve
10:18	ated electron pairs up with one electron from the triple
10:21	bond . So now we have a new molecule .
10:23	The new electron pair causes one carbon to have a
10:25	negative charge since it has five valence electrons . Meanwhile
10:29	the other electron from the triple bond forms a radical
10:32	on the other carbon one . This very basic and
10:35	I . And finds a source of protons like the
10:37	ammonia floating around the negative charge , will actually remove
10:40	a proton from ammonia in an acid base reaction .
10:43	To finish this reaction , another salivated electron comes along
10:47	adds to the orbital holding that radical and makes another
10:49	negative charge , putting five valence electrons on the formally
10:53	radical carbon . At this point , the our group's
10:55	reorganize themselves to different sides of the double bond to
10:58	make a more stable an ion . Finally , another
11:01	molecule of ammonia is deep resonated and we get an
11:03	E alkaline and the party can end . Even though
11:06	we explored antioxidants to solve ated electrons , we didn't
11:09	even get to how radicals help us make different kinds
11:12	of plastics . So don't worry , this won't be
11:14	the last time we hear of them in this episode
11:16	. We learn that radicals are highly reactive single electron
11:20	species , which are important in antioxidant chemistry . There
11:23	are three steps in radical reactions , initiation propagation and
11:27	termination . Hammond's postulate helps us predict transition states and
11:30	explain product distributions . And radical reactions can be used
11:35	to halogen al canes and al keens and reduce all
11:38	kinds in the next episode of crash course organic chemistry
11:41	will use these halogen It'd all canes in a new
11:43	type of reaction substitution . Thanks for watching this episode
11:47	of Crash course Organic chemistry . If you want to
11:49	help keep all crash course free for everybody forever .
11:52	You can join our community on Patreon .

Summarizer

DESCRIPTION:

Quality Math And Science Videos that feature step-by-step example problems!

OVERVIEW:

Radical Reactions & Hammond's Postulate: Crash Course Organic Chemistry #19 is a free educational video by Math and Science.

This page not only allows students and teachers view Radical Reactions & Hammond's Postulate: Crash Course Organic Chemistry #19 videos but also find engaging Sample Questions, Apps, Pins, Worksheets, Books related to the following topics.