Acidity: Crash Course Organic Chemistry #11 - Free Educational videos for Students in K-12

Acidity: Crash Course Organic Chemistry #11 - 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 . A fun little detail in
00:11	that sci fi movie alien . You know the one
00:13	with Sigourney weaver is how the zona moore has acidic
00:16	blood . When the humans top one open , its
00:19	blood eats right through the metal floor and leaves a
00:22	hole . It's creepy . Xena more flood is way
00:25	stronger than most assets produced by earth creatures . For
00:28	example , tawny ants produce formic acid to protect themselves
00:31	from the basic venom of fire ants , but formic
00:35	acid is relatively weak , kind of like vinegar or
00:38	lemon juice , which definitely won't eat through a spaceship
00:42	. As organic chemists , we need to know how
00:44	to predict the strength of weak acids and bases .
00:47	Not just a dream of cool and scientifically accurate aliens
00:51	, but because acids and bases are molecular hotspots where
00:54	reactions can take place . So let's add acid base
00:57	chemistry to our toolkit for predicting chemical reactions . Mhm
01:08	Yeah . Part of what makes acid base chemistry tricky
01:13	is that there are different but overlapping definitions of acids
01:16	and basis are in use . Bronze stood Lowry and
01:19	Louis all had their opinions on how an acid and
01:22	base should be defined for this video . We'll stick
01:24	with the bronze stood Lowry definition proposed in 1923 .
01:28	And to help understand this definition will need to push
01:30	some electrons around . Last episode we learn how to
01:33	use arrows to push electrons within a molecule to understand
01:36	resident structures , but we can also push electrons between
01:39	molecules to show how bonds break and form . Using
01:42	the same rules , we start on electrons and point
01:45	to where new bond is made in the Bronx did
01:47	larry definition and asset is anything that loses a proton
01:51	. Also known as a plus one charged hydrogen ion
01:54	in a base is anything that accepts a proton car
01:57	back . Cilic acids like acetic acid and protein .
01:59	OIC acid are Bronston . Lowry acids . When we
02:02	dissolve these acids and water , the water acts as
02:04	a base and removes a proton to form a hydro
02:07	nia , my on and the corresponding carb oxalate .
02:09	I am we say that hydro knee um is the
02:11	conjugate acid of water and the car box lit is
02:13	the conjugate base of the carb oxalic acid . Inorganic
02:16	chemistry , we want to know how readily a molecule
02:19	will gain or lose a proton . This is a
02:21	physical property like boiling point or melting point that we
02:24	describe with K . A . The acid dissociation constant
02:28	thing talks more about K . A . And episode
02:30	30 of crash course chemistry if you want to start
02:32	with the basics but essentially it describes the relationships between
02:35	products and reactant when the rate of the forward reaction
02:38	is equal to the reverse reaction . In other words
02:41	, when the reaction has reached equilibrium , the size
02:44	of the K . A . Tells us if we
02:45	have more product or reactant . If the K .
02:48	Is large , the equilibrium favors the product side and
02:51	the molecule is a strong acid one that's very willing
02:54	to get rid of a proton if the K .
02:56	Is small than the equilibrium favors the reactant . So
02:59	the molecule is a weak acid one that's a little
03:01	less willing to get rid of a proton . So
03:03	that logic is all fine and good . But it
03:05	can be a little hard to tell whether the K
03:07	. A small or large from looking at a number
03:09	like 1.8 times 10 to the negative five . We
03:12	can get rid of the exponents and make these numbers
03:14	more manageable by taking the negative log of K .
03:17	A . To get PK . A . Just like
03:19	the concentration of protons becomes ph with a little math
03:22	and acids equilibrium constant becomes PK . We can use
03:25	P . K . A . To compare to acids
03:27	. A lower P . K . A means the
03:29	equilibrium prefers the product side and a molecule is a
03:32	stronger acid and a higher P . K . A
03:34	means the equilibrium prefers the react inside and a molecule
03:37	is a weaker acid . Let's look at a couple
03:39	examples . We can see that hydrochloric acid has a
03:42	negative P . K . A . Which is really
03:44	low . We consider it completely dissociated into products .
03:47	So it's a strong acid , propane OIC acid and
03:50	acetic acid have similar PK A values which makes sense
03:54	because propane OIC acid only has one extra CH two
03:56	group . They're both weak acids . But the PK
03:59	of ethanol is dramatically different from acetic acid even though
04:03	they're both losing a proton from an O . H
04:05	. Group . That's the key though . They're similar
04:07	but not the same when it comes to charge distribution
04:10	. When acetic acid loses a proton , it forms
04:12	its conjugate base acetate . And remember last episode we
04:15	talked about the resident structures of an acetate ion and
04:19	how the negative charge is spread out over its two
04:22	oxygen atoms . This is kind of like going backpacking
04:24	with a group of friends and splitting up the gear
04:26	. So like instead of carrying everything yourself , you
04:29	take the 10 , another friend takes the cooking gear
04:31	and another takes the food , distributing the weight means
04:34	you all carry some of the burden . The same
04:36	is true here distributing the negative charge over the two
04:39	oxygen atoms . Makes it easier for the acetate ion
04:42	to carry the burden of the negative charge because of
04:44	the resonance stabilization in the conjugate base , it's not
04:48	too tough for acetic acid to lose a proton .
04:50	On the other hand , when ethanol loses a proton
04:53	, it forms the conjugate base at fox side with
04:55	oxide doesn't have any resonance structures , which means it
04:58	doesn't have friends to share camping gear with . So
05:01	it's oxygen atom is feeling the full burden of the
05:04	negative charge . This makes ethanol a weaker acid than
05:07	acetic acid . But since it's pretty tough to take
05:09	the proton off of the ethanol in the first place
05:12	, if we do form the conjugate bases with oxide
05:15	is a much stronger base than acetate . Another great
05:17	way to see how resonance stabilization can affect acidity is
05:20	to consider to kind of similar ring compounds . Final
05:24	in psychology , external finals PK is about 10 Encyclopedic
05:28	styles P . K . Is 16 in psychology .
05:30	External conjugate base that negative charges stuck on oxygen because
05:34	there are no double bonds or other oxygen's to share
05:37	the burden . So like athoc side , the conjugate
05:40	bases less stable and really wants a proton back .
05:43	But in females conjugate base monoxide , the negative charge
05:46	can be pushed around the benzene ring to make four
05:49	different resonance structures . This stabilizes the conjugate base so
05:53	final is more acidic resonance stabilization is one of four
05:57	major factors that help us understand the role of PK
06:00	in our reactions . Another key is the atom that
06:02	loses the proton within a row on the periodic table
06:05	. More electro negative elements stabilized negative charge better and
06:09	within a group , larger elements . For more stable
06:12	conjugate basis , bigger atoms have more electrons which end
06:15	up in orbital's that are pretty diffuse . So with
06:17	bigger atoms , the electron cloud is easily smeared out
06:20	and distorted a property we call polarize ability . Imagine
06:24	a cup of water as the electron density on a
06:26	small atom . If we pour that water into a
06:29	frisbee , which is like a large atom , it
06:31	has more space to move around and spread out .
06:33	Adam polarize ability affects acidity and PKK because this smeared
06:37	out nous stabilizes negative charges . It's always useful to
06:41	look back at an example . So let's go back
06:42	to final and compare it with its closely related cousin
06:45	thio final , which has a sulfur atom instead of
06:48	an oxygen . The only difference in structure is the
06:51	size of the sulfur atom compared to the size of
06:53	the oxygen atom . To be precise , sulfur has
06:55	eight more electrons than oxygen . So in thio finals
06:58	conjugate base electrons are more smeared out on the polarize
07:02	herbal sulfur atom stabilizing the conjugate base and the more
07:06	stable the conjugate base , the more acidic , the
07:08	acid making by a fetal more acidic than final .
07:11	The third key to Pekka is hidden in covalin bonds
07:15	. The inductive effect has to do with electro negativity
07:17	throughout a molecule , with more electro negative atoms pulling
07:21	the negative charge toward them through bonds . As an
07:24	example . Let's compare to other very similar compounds ,
07:27	acetic acid and try flora acetic acid , both of
07:30	their conjugate bases have resonance stabilization across the two oxygen
07:33	atoms . But the resonance stabilization is all that acetate
07:36	has going for it , giving acetic acid . It's
07:39	PK of 4.76 . Now try floor acetate also contains
07:43	flooring an electro negative atom . Those three florins pull
07:47	on the negative charge and stabilize the conjugate base even
07:50	more . It's sort of like having even more backpacking
07:53	buddies who carry the poles and other pieces of the
07:55	tent for you . So the product side of the
07:58	equilibrium is favored and the PK of try flora acetic
08:01	acid is 0.23 The last in least powerful key to
08:06	understanding PK also has to do with orbital shapes ,
08:09	specifically the s character of hybrid orbital's . Remember an
08:12	episode for we talked about orbital's places where we're most
08:15	likely to find electrons around atoms . And we talked
08:18	about orbital's combining . When atoms bond to form hybrid
08:21	orbital's . The three most common hybrid orbital's for organic
08:25	compounds are sp three , Sp two and Sp and
08:29	this is approximately what they look like . Sp hybrid
08:32	orbital's can be thought of as being 50% s orbital
08:35	and 50% p orbital . So we can say that
08:37	they have more s character than SP two and Sp
08:40	three hybrid orbital's which combine additional P orbital's into the
08:44	mix . So sp orbital's are more similar to a
08:47	plain old s orbital . These shapes mean their electrons
08:50	are closer to the nucleus and the atom can stabilize
08:53	the negative charge better . By contrast , sP two
08:56	and sp three hybrid orbital's hold the electrons a little
08:59	further away from the nucleus , and the atom doesn't
09:01	stabilize the negative charge as well . To look at
09:04	this in action , let's compare the PK of F
09:06	in F in and F in The carbons and a
09:09	thin have sp hybridized orbital's , which means the conjugate
09:12	base has more s character and can stabilize the negative
09:15	charge better . So effin is the most aesthetic of
09:18	this trio . But to be totally realistic , topeka
09:21	of 25 is not very acidic at all in the
09:23	grand scheme of things and will definitely not be eating
09:27	through spaceships or anything like that . We can see
09:29	the importance of the s character of hybrid orbital's by
09:32	looking at the acidity of different protons within a molecule
09:35	. For example , this molecule has an alkaline at
09:38	one end with the protons highlighted in blue and an
09:40	alkaline at the other with a proton highlighted in red
09:44	. So which of these two protons is more acidic
09:47	. By pulling off a proton from each end of
09:49	the molecule , we can create two different conjugate bases
09:52	with a negative charge in different places . Using what
09:54	we know about hybrid orbital's . The alkaline end has
09:57	an sp two hybridized carbon and the al Qaeda and
10:00	has an sp hybridized carbon . That means the al
10:03	Qaeda and has more S character and can stabilize the
10:05	negative charge better so that conjugate base is more stable
10:08	and that proton is more aesthetic . So throughout this
10:10	episode we've learned four key things that we can use
10:13	to predict relative acidity . Each of these factors stabilizes
10:17	negative charge and a conjugate base , which makes the
10:20	corresponding acid more acidic . Number one adam identity ,
10:24	more electro negative and larger elements stabilize charge better .
10:27	Number two resonance stabilization . If we can draw multiple
10:31	LewiS structures for conjugate bases , they are more stable
10:35	. Number three , the inductive effect electro negative atoms
10:38	can pull negative charge toward themselves through Covalin bond's and
10:42	number four the s character of the orbital . More
10:45	S characters stabilizes negative charge . Better acidity is definitely
10:49	tricky , but it's an important part of organic compounds
10:52	and will help us predict the products of chemical reactions
10:55	in our next episode . Will start to use all
10:57	of the tools we've learned so far and start forming
10:59	Covalin bonds at our molecular hotspots . Thanks for watching
11:03	this episode of Crash Course Organic Chemistry . If you
11:06	want to help keep all crash course free for everybody
11:08	forever , you can join our community on Patreon .

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