Khan Academy Videos for Organic Chemistry, Part 4: Substitution and Elimination Reactions (Mostly)
This post covers videos 38-51 of Khan Academy’s videos for organic chemistry, covering mostly elimination and substitution reactions. For previous videos in this series, see Part 1, Part 2, and Part 3.
Video #38: E2 Reactions
Summary: The reaction of 2-chlorobutane with NaOCH3 is shown to form 2-butene, an example of an E2 reaction.
Key concepts/skills: Elimination reactions, concerted reactions
Nitpicky criticisms: No mention of the stereochemistry of the E2 here. Since this is such an important testable concept, it would be greatly valuable to include mention of it here.
Red flags: None
Video #39: E1 Reactions
Summary: The E1 mechanism is introduced by showing the reaction of 3-bromo-3-ethyl pentane in ethanol to give 3-methyl-pent-2-ene.
Key concepts/skills: Carbocation stability, elimination reactions, Zaitsev’s rule, E1.
Nitpicky criticisms: Since the rate determining step is carbocation formation, it would be helpful to talk about the factors that impact carbocation stability more. Wouldn’t say “the entire molecule becomes acidic” – more specific (and better) would be, “the C-H bond becomes more acidic”. Would help to say that bromide ion is not only a good leaving group but also a weak base.
Red flags: None
Video #40: Zaitsev’s Rule
Summary: Elimination of 2-bromobutane is shown to give 2-butene instead of 1-butene, illustrating that the more substituted alkene is formed preferentially.
Key concepts/skills: Elimination, Zaitsev’s rule.
Nitpicky criticisms: Discussion of “why” Zaitsev’s rule operates includes mention of “hyperconjugation”, but no mention is made of what hyperconjugation is. Why mention something if you’re not going to explain it, or say 3 times that “the jury is not out on this?”. It would be helpful to mention – clearly – merely that the stability of the double bond increases with increasing substitution by carbon. Finally, the alkene drawn at 11:59 is a cis (Z)- alkene, when in fact it’s much more likely that the more stable trans (E) alkene will be formed.
Red flags: The hyperconjugation conversation would be better placed in a discussion on carbocation stability.
Attaboy: Points for saying that elimination is the opposite of addition.
Video #41: Comparing E2 E1 SN2 SN1 Reactions
Summary: The reaction of bromocyclopentane plus CH3O(-) in DMF is shown to give methoxycyclopentane and cyclopentene.
Key concepts/skills: Substitution reactions, polar protic and aprotic solvents, elimination reactions.
Nitpicky criticisms: It’s not very precise to say that in an aprotic solvent “we can’t have protons floating around” (1:21). Also it would be nice to have an example where the stereochemistry of both reactions (SN2 and E2) is highlighted.
Red flags: I think this video gives the impression that the most important factor governing whether a reaction goes SN1/SN2/E1/E2 is the solvent (see 1:14). There’s no mention of analyzing whether the alkyl halide is primary, secondary, or tertiary – which is truly the most important factor. What’s missing here is a method for logically determining whether E2 or SN2 would predominate. As such, I don’t think the video is very useful for helping students with this decision.
Video #42: E2 E1 SN2 SN1 Example 2
Summary: The reaction of the t-butoxide ion with bromocyclopentane is shown to only give cyclopentene.
Key concepts/skills: Bulky bases, substitution, elimination.
Nitpicky criticisms: Maybe this is just a difference in focus but I wouldn’t say that seeing that the solvent is DMF would immediately point us in the “SN2 or E2 direction”. I’d look at the type of substrate first (primary, secondary, tertiary) then look at the type of nucleophile/base, and only then would analyze the solvent. Also it isn’t made clear why t-butoxide would have a hard time getting in to react with the substrate for a substitution – mention needs to be made of backside attack here.
Video #43: E2 E1 SN2 SN1 Reactions Example 3
Summary: The reaction of 1-(2-iodobutan-2-yl)-1-methylcyclopentane in methanol is shown to give a mixture of E1 and SN1 products.
Key concepts/skills: Comparing the SN1 and SN2 reactions.
Nitpicky criticisms: It would be best to draw the carbocation as flat, to show that it’s planar. Also, there’s no mention of the fact that a stereocenter is involved in the SN1, and therefore you’d get a mixture of retention and inversion of stereochemistry. Finally, it’s best not to draw a wedge on a (flat) alkene.
Red flags: Not to harp on things here, but I wouldn’t start with solvent as the first thing to look at in these reactions (0:26). It’s only third in importance behind the type of substrate and the type of nucleophile. I would start with the fact that it’s tertiary first.
Video #44: Free Radical Reactions
Summary: The reaction of methane with Cl2 in the presence of light is shown to give chloromethane.
Key concepts/skills: Free radicals, homolytic cleavage, initiation, propagation, termination.
Nitpicky criticisms: I am grouchy about constantly referring to molecules as “guys” and “he”. Other than the arrow pushing however, the video is OK.
Red flags: I’ve said a lot already about how the arrow pushing notation in these videos is not the standard arrow pushing notation one sees in courses and textbooks; here, single-barbed arrows are meant to show the movement of a single electron (like his double barbed arrows, I might add) – but he interprets the single barb as meaning that the atom “doesn’t give its electron away”. I’d need a whole post to get into why this is problematic. I’ll just say this for now: it’s both frustrating and confusing that the decision was made to not use the universal convention for arrow pushing.
Video #45: Alcohols
Summary: Four alcohols are named: 2-pentanol (here pentan-2-ol), cyclohexanol, 7,7-dibromo-oct-5-yn-4-ol, and 1,3-propanediol.
Key concepts/skills: Alcohol nomenclature; priority of functional groups for nomenclature.
Nitpicky criticisms: Seeing alkynes drawn in zigzag form makes me grouchy.
Red flags: None
Video #46: Alcohol properties
Summary: The hydrogen bonding properties of alcohols are drawn to explain their properties such as water miscibility (butanol specifically) and boilng point.
Key concepts/skills: Hydrogen bonding, water solubility, boiling point, Van der Waals forces, dispersion forces.
Nitpicky criticisms: Nothing significant.
Red flags: None
Video #47: Resonance
Summary: The resonance forms of benzene and the carbonate ion are drawn out.
Key concepts/skills: Resonance as a hybrid of two or more states.
Nitpicky criticisms: None really, other than the molecules are pretty simple. It could be stated more explicitly that the molecule doesn’t flip back and forth between two states – that it’s a hybrid. Explaining the resonance arrow more explicitly would help. Saying that 2/3 of the time the oxygens on carbonate ion have an extra electron isn’t correct (11:00). Not really much discussion on why resonance might be stabilizing.
Red flags: Nothing significant.
Video #48: Ether naming and introduction
Summary: Diethyl ether, isopropyl methyl ether, and cyclohexyl propyl ether are drawn out and named.
Key concepts/skills: What ethers are, and how to name them. Why ethers have lower boiling points than alcohols (no hydrogen bonding).
Nitpicky criticisms: None really.
Red flags: None.
Video #49: Cyclic ethers and epoxide naming
Summary: The molecules 1,4-epoxybutane (tetrahydrofuran), 1,2-epoxypentane, and cyclohexene oxide are drawn and named.
Key concepts/skills: Epoxides, ring strain, naming epoxides.
Nitpicky criticisms: None really.
Red flags: None.
Video #50: Ring opening SN2 reaction of epoxides
Summary: The reaction of cyclohexene oxide with aqueous acid is shown to give trans-1,2-cyclohexanediol.
Key concepts/skills: Epoxide ring-opening, ring strain, substitution, backside attack, how acid catalysis makes oxygen into a better leaving group.
Nitpicky criticisms: None major to report, other than now the earlier assertion that polar protic solvents indicate that an SN1/E1 reaction will occur has been contradicted.
Red flags: Just my old arrow pushing complaint.
Video #51: Sn1 and SN2 epoxide opening discussion
Summary: In contrast to the previous discussion, it is asserted that the reaction between isobutylene oxide and aqueous acid is SN1 and proceeds through a carbocation.
Key concepts/skills: Epoxide opening.
Nitpicky criticisms: It would have been good to use an epoxide with a stereocenter.
Red flags: This video seems a little bit trivial and the discussion is somewhat muddled. If it was meant to convey that an SN1 reaction can occur with epoxides, would it not be better to show an actual reaction with actual evidence that *demonstrates* that an SN1 reaction occurs, by showing a mixture of products that proceed with either inversion or retention of configuration? On a positive note, it’s good to see that solvent is abandoned as the primary predictive tool for determining SN1/SN2 (relative to substitution pattern); it would be good to re-do the previous substitution/elimination discussion videos so that they all consistently reflect this point of view.
These videos mostly concern themselves with substitution and elimination reactions, although there are also some videos on nomenclature. The videos on nomenclature are fine. As in the previous post, my biggest problem with the Khan Academy videos on organic chemistry is that they are not particularly useful for understanding chemical reactivity.
For example, the videos on predicting substitution/elimination reactions are not effective in imparting the skill of being able to predict whether a reaction will proceed SN1/SN2/E1/E2 because discussion of the importance of substrate structure is minimized. As much is admitted in video #51 where an entire video is devoted to attempt to clear up a misconception that arises from previous discussion of the subject in videos #41-43.
Another major weakness in this series of videos is that there is very little discussion of stereochemistry in the substitution and elimination reactions (no mention of the requirement for anti stereochemistry in the E2!!!) . For a student in a typical college course, stereochemistry is the key theme of Org 1; therefore, if relying solely on these videos, one might get the mistaken impression that stereochemistry is not crucial for the E2 (and SN1, for that matter). The examples provided are far too simple.
One last nitpicky criticism is as follows. A recurring habit in these videos is to start off with the statement, “Let’s think about what would happen if we added these molecules together”. I don’t mean to create a strawman here, but one might get the impression from these videos that predicting a reaction in organic chemistry is as simple as putting two molecules together and thinking about it. As I’ve said before, almost everything we know about organic chemistry we learned by looking backwards, not forwards. Reactions weren’t discovered by “thinking” about what would happen (as much as we might like to say so in retrospect). Nobody was smart enough to do that. These reactions were discovered. Observations were made and we then attempt to understand and explain what is going on. My suggestion for Khan is to start with the observations first, and then move on to understanding.
I don’t enjoy slagging these videos, by the way. I wish there was less to criticize when it came to the reactions, but this series of posts won’t be useful to anyone if I’m not completely honest.