Video #65 – Keto-Enol Tautomerism
Summary: 3-methyl 2-butanone is drawn, and the mechanism for its conversion into its enol form is shown via protonation of the carbonyl oxygen and deprotonation.
Key concepts/Skills: keto-enol tautomerism, constitutional isomers.
Nitpicky criticisms: A little bit of vaugeness about where H3O(+) comes from; there shouldn’t be too much in water at pH 7. Might help to say the reaction is assisted by adding acid. Could help to mention what factors favor the keto over the enol, or to give students a typical value for the position of equilibrium [about 104 favoring keto, for instance].
Red flags: None, this is fine [except for arrows]
Video #66 – Carboxylic Acid Introduction
Summary: A generic carboxylic acid is drawn, and the acid-base reaction between it and water is shown. It’s demonstrated that the stability of the conjugate base is due to resonance. Formic acid, acetic acid, and oxalic acid are also drawn.
Key concepts/skills: acidity, resonance stabilization, carboxylic acids
Nitpicky criticisms: It’s said that carboxylic acids are more acidic than alcohols. By how much? It would also be good to explicitly say that “the stronger the acid, the weaker the conjugate base”. When the Arhennius definition of an acid as a proton donor is mentioned [1:05] I think he means Bronsted.
Red flags: Nothing except for the arrow pushing
Video #67 – Carboxylic Acid Naming
Summary: Butanoic acid, hexanoic acid, 3-methyl hexanoic acid, and (E)-3-heptenoic acid are drawn and named.
Key concepts/Skills: Carboxylic acid nomenclature
Nitpicky criticisms: It’s fine. One weird thing though – around 5:15 it’s said that “this is only true if you’re assuming that I drew it in the actual 3 dimensional configuration in some way”. Not sure what is meant by this.
Red flags: None
Video #68 – Fischer Esterification
Summary: Heptanoic acid is drawn with ethanol and H2SO4. Reaction really begins at 5:00. Protonation of carbonl oxygen, addition of ethanol, ethanol losing proton, protonation of OH, and loss of water. Formation of ethyl heptanoate.
Key concepts/Skills: Equilibrium, carboxylic acids, addition, elimination.
Nitpicky criticisms: “Fischer”, ahem. The video begins with “let’s think about what might happen if we do this”. Little pet peeve of mine – we can appreciate things looking backward, but it’s difficult to look forward using first principles. The discussion up to 5:00 probably could have been skipped, since it’s just discussing the acidity of H2SO4. Would be good to mention that acid makes the carbonyl group a better electrophile; also, an opportunity is missed to explain why the nucleophile attacks the carbon instead of the (positively charged) oxygen. It’s said that we’ve seen this “many, many times” in SN2 reactions but this is actually his first time drawing the addition mechanism in carbonyls. It’s good to mention the reaction is in equilibrium, but it would be better to mention *why* equilibrium favors formation of the ester. Finally I know I’m a grouch, but I just don’t like statements like “protons are flying around everywhere” (11:25).
Red flags: The elimination step at 14:00 is drawn as a bimolecular mechanism, which is incorrect. Deprotonation should be shown as a separate step.
Video #69 – Acid Chloride Formation
Summary: The mechanism for formation of acetyl chloride from acetic acid using SOCl2 is shown.
Key concepts/skills: Conversion of carboxylic acids to acid chlorides
Nitpicky criticisms: Would be good to mention why Cl is a good leaving group. Why do we lose oxygen and not Cl in the elimination step? It would be good to mention that the gas SO2 is formed and this will drive the reaction to completion. The last step shouldn’t have an equilibrium arrow. Heh, nobody would perform this reaction in a beaker [9:15] it stinks too much!
Red flags: Just the arrow pushing (as always)
Video #70 – Amides, Anhydrides, Esters, and Acyl Chlorides
Summary: The structures of acetamide, N-methyl propanamide, methyl ethanoate, acetic anhydride, ethanoic anhydride, propanoic anhydride, acetyl chloride, and ethanoyl chloride are drawn.
Key concepts/Skills: Nomenclature of amides, esters, anhydrides, and acid chlorides
Nitpicky criticisms: None
Red flags: None
Video #71 – Relative Stability of Amides, Esters, Anhydrides, and Acyl Chlorides
Summary: Acetamide, methyl acetate, acetic anhydride, and acetyl chloride are drawn, along with their resonance forms. The higher reactivity of anhydrides is rationalized as being due to the lower stability of the resonance form.
Key concepts/Skills: drawing resonance structures, understanding stability of resonance structures.
Nitpicky criticisms: If nitrogen is electron rich – more electronegative than carbon, then why might it give up its electrons? [2:44]. Finally, for the ester, he mentions the real reason why oxygen is less nucleophilic than nitrogen – because it is more electronegative. This is the first time this is mentioned in the whole video series. How does one differentiate between “quite stable” and “just stable”? Would be better just to show an arrow beginning with “most stable” on the left going to “least stable” on the right. Finally, he says “we’ll explore some of the mechanisms in the next few videos”, but in fact it’s just mentioned in one video.
I think ranking leaving group ability is a more powerful way to explain the relative reactivity of carboxylic acid derivatives than the relative stability of resonance forms, but overall this is a useful video.
Red flags: No resonance structure is drawn for acetyl chloride because it is said that it “has no resonance structure” ; instead of saying it “has no resonance structure”, which is not true, it would be better to say that the resonance form is not “significant”; also, chlorine is less electronegative than oxygen, so the answer is not electronegativity (9:20) but poor orbital overlap.
Video #72 – Amide Formation From Acyl Chloride
Summary: The reaction between butanoyl chloride and dimethylamine is shown to give N,N-dimethylbutanamide.
Key concepts/Skills: Reactivity of acyl chlorides, nucleophilic acyl substitution.
Nitpicky criticisms: Another instance of “Let’s think about what might happen….”. The “stability” argument from the previous video is used to explain the reactivity of acyl chlorides relative to amides, but leaving group ability is more valuable. Compare Cl(-) as a leaving group versus Me2N(-)! This explains everything.
Red flags: Just arrow pushing.
Video #73 – Aldol Reaction
Summary: The aldol addition reaction between two aldehydes RCH2CHO in the presence of NaOH is shown.
Key concepts/Skills: Enolates, addition reactions, aldol reaction, acidity of alpha protons, enolates as nucleophiles.
Nitpicky criticisms: It’s said that this is a review of previous discussions of enolate ions, but in fact this is the first description of them. It would be better to consistently show the aldehyde hydrogen.
Red flags: The arrow pushing here makes it look like the carbonyl carbon is the nucleophile, which isn’t the case [6:32].
And that’s it: 73 videos for organic chemistry from Khan Academy. Again, lots of nomenclature videos here, plus a few on reactions. Kind of disappointing that there’s so few on the reactions of aldehydes, ketones, and carboxylic acids; nothing on reduction, oxidation, extensions of enolate chemistry… a lot of work still left to do, in other words. In the next post I’ll write an overall summary of these videos that answers the question, “How useful are Khan Academy videos for learning organic chemistry?”.