IN this post (the 5th of 6) we go through videos 52-64 of Khan Academy for Organic Chemistry, covering mostly aromatic compounds and aldehydes/ketones. [For previous posts, see Part 1, Part 2, Part 3, Part 4]
Video #52: Aromatic Compounds and Huckel’s Rule
Summary: Benzene has unusual stability, a property called “aromaticity”; the p orbitals of benzene are drawn (very nicely I might add); cyclodecapenta-ene is also drawn, which is similarly aromatic; cyclobutadiene and cyclooctatetraene are drawn and are not aromatic.
Key concepts/skills: Aromaticity, Huckel’s rule.
Nitpicky criticisms: It’s not necessarily true that 99% of the time, aromatic compounds seen will be benzene (1:00), as “heteroaromatics” such as pyridine and pyrrole and furan are commonly encountered. It would also be good to mention that aromaticity also requires there to be a continuous ring of p orbitals, and that the molecule must be flat [a non-conjugated molecule such as cyclopentadiene might be instructive]. At 9:41 it’s said that there will be a correction video saying that cyclooctatetraene is “non aromatic”, not “antiaromatic”; no sign of the video yet. A subsequent video might also be good to show the energy levels of the different orbitals in benzene.
Red flags: None
Video #53: Naming Benzene Derivatives Introduction
Summary: Benzene, 1-bromo-2-chlorobenzene, phenol, 2-bromophenol, toluene, 3-fluorotoluene, aniline, benzoic acid, and benzaldehyde are drawn and named.
Key concepts/skills: Naming of simple benzene derivatives.
Nitpicky criticisms: 1-bromo-2-chlorobenzene is shown and named; this would be a good opportunity to include something on the ortho-meta-para naming convention. Also re: 0:15 (same as noted above) it’s not true that 99% of the aromatic molecules you’ll see in an organic chemistry class are benzene or are derived from benzene.
Red flags: None
Video #54: Electrophilic aromatic substitution
Summary: The reaction of benzene with a generic electrophile (E+) is drawn, the resonance forms of the intermediate carbocation drawn, and the final deprotonation shown to give electrophilic aromatic substitution.
Key concepts/skills: Generic mechanism for electrophilic aromatic substitution
Nitpicky criticisms: It’s said that an electrophile is “good at getting electrons”, and wants electrons “really, really, really, really, really badly”, but it’s still not really clear exactly what this means. Would be better to say it is “electron poor” or “bears a positive charge”. One suggestion – at 10:30, instead of saying, “now we have resonance stabilization”, say “now we have restored aromaticity”.
Red flags: Just my usual complaint about arrow pushing.
Video #55: Bromination of benzene
Summary: The reaction of benzene with Br2 and FeBr3 is shown to give bromobenzene and HBr.
Key concepts/skills: Lewis acids, electrophilic aromatic substitution, aromaticity
Nitpicky criticisms: It’s not really clear from the discussion *why* the end bromine gets attacked and not the Br with the positive charge. This would be a good opportunity to talk about either leaving group ability, or dipoles.
Red flags: Arrow pushing as always
Video #56: Amine Naming Introduction
Summary: Methylamine, dimethylamine, trimethylamine, 2-aminoheptane, and 5-ethoxy-1,2-dimethylcyclohexylamine are drawn and named.
Key concepts/skills: Naming of amines, priority of functional groups for nomenclature.
Nitpicky criticisms: Priority of functional groups is mentioned (1:51) but no further resources are given. It would be helpful to either have a video on this or to direct students to somewhere where they could find more resources.
Red flags: None to report
Video #57: Amine Naming 2
Summary: The molecules 8-amino-5-phenyl-3-octanol and 3-bromo-4-nitroaniline are drawn and named.
Key concepts/skills: Extensions of amine naming. Functional group priorities for nomenclature
Nitpicky criticisms: If a second video on amine nomenclature is done, it would be better to include one of the nomenclature terms unique to nitrogen, such as the “N-” terminology (such as in N,N-dimethylformamide). Also [3:12] while many explosive compounds have nitro groups, it’s not quite correct to say that “the nitro group is highly explosive”. Finally (same as above) it’s not clear *how* we know that the amine has a higher priority than Br or the NO2. A pointer to an outside resource would be helpful.
Red flags: Nothing significant
Video #58: Amine As A Nucleophile In SN2 Reaction
Summary: The reaction of ethylamine with ethyl bromide is shown to give diethylamine.
Key concepts/skills: SN2, nucleophiles, electrophiles.
Nitpicky criticisms: [3:35] While it’s mentioned that nitrogen is partially negative, it’s not mentioned why it might be giving away electrons here, or *why* they are one of the most basic of the neutral functional groups. [Answer: N is less electronegative than O]. Should clarify this by comparing nitrogen to oxygen and mentioning the lower electronegativity. It would also be more precise to talk about nucleophilicity as how *fast* something is at reacting, now “how good” – [5:40] – “good” is too general a term.
Red flags: Just the arrows
Video #59: Amine in SN2 Part 2
Summary: The reaction between diethylamine and ethyl bromide continues to give tetraethylammonium bromide.
Key concepts/skills: Substitution, SN2, nucleophilicity, basicity
Nitpicky criticisms: It would be good to talk about *why* this particular reaction doesn’t stop after one substitution reaction. I don’t think it’s explicitly stated that the first product (diethylamine) is itself a nucleophile, and in fact is a better nucleophile than ethylamine itself [and triethylamine is a better nucleophile than diethylamine] hence the reaction can’t be stopped after just one reaction.
Red flags: Arrows
Video #60: SN1 Amine Reaction
Summary: The reaction of ethylamine with 2-bromo-2-methylpropane is drawn as giving the product N-methyl-tert-butylamine.
Key concepts/skills: SN1; the “N-” terminology for naming amines.
Nitpicky criticisms: Better late than never, but it would have been good to introduce the “N-” naming protocol a little earlier, maybe in the 2nd amine nomenclature video.
Red flags: The biggest problem here is the choice of example. This reaction will not work as stated. In practice, an elimination [E2] between ethylamine and the substrate will occur to give an alkene before there is any dissociation to give the carbocation; should the carbocation form, there is still the potential for elimination through the E1 pathway. I’m not sure this video really adds anything new that hasn’t been discussed before.
Video #61: Aldehyde introduction
Summary: The aldehydes formaldehyde, benzaldehyde, cinnemaldehyde, acetaldehyde are drawn and named, as is 2-methylpentanal.
Key concepts/skills: Aldehyde nomenclature, including common names.
Nitpicky criticisms: Nothing major
Red flags: None
Video #62: Ketone Naming
Summary: The molecules acetone [2-propanone], 2,2-dichlorocyclohexanone, 2-pentanone [methyl propyl ketone], acetophenone, and benzophenone are drawn and named.
Key concepts/skills: Ketone nomenclature
Nitpicky criticisms: None
Red flags: None
Video #63: Friedel Crafts Acylation
Summary: The reaction between benzene, acyl chloride, and AlCl3 is shown to give acetophenone.
Key skills/concepts: Electrophilic aromatic substitution, aromaticity, resonance
Nitpicky criticisms: I wouldn’t say that “aluminum can pretend that it as 8 electrons.” [4:05] It *does have* eight electrons. Also I would grumpily object to saying that “it wants to steal other people’s electrons” [5:00] Molecules aren’t people! The resonance form at [6:14] is mentioned to be more stable, but it’s not mentioned *why* : it’s more stable because every atom has a full octet. Also, “acylated”, not “acylized”. H-Cl isn’t drawn as a product but the correction video shows this.
Red flags: Just arrow pushing
Video #64: Addendum
Summary: H-Cl was not drawn in the previous video and this video corrects that.
Overall impression for these videos: These videos cover some concepts with aromaticity and aromatic reactions, as well as nomenclature. The videos covering nomenclature are fine; again, it’s the videos which discuss chemical reactivity which are problematic. While the videos on electrophilic aromatic substitution and SN2 of amines do show the proper sequence of bonds being formed and broken, they are lacking deep insight into how chemical reactivity operates, or discussions that might shed light on how to apply these concepts to other situations. As mentioned in the notes to each video above, there are numerous opportunities to explain the differences in reactivity between, say, nitrogen and oxygen, or why a Lewis acid makes Br2 more electrophilic, but the discussion doesn’t go there in a specific enough way to be useful. Finally, there’s a lot of things missing – understandable for a series of only 72 videos – but it’s striking to me that there is no discussion of directing groups in electrophilic aromatic substitution [i.e. ortho-para vs. meta directors] . This is a pretty fundamental topic to be absent, as it is discussed in even the most rudimentary organic chemistry courses. I think fewer videos on nomenclature and more videos on reactions would likely be a more useful path to take for the future.
In the next post we’ll wrap up the final series of videos.