strong base/strong nuc?

[sparkledoc44]

what's the easiest way to distinguish between a strong nuc and a strong base? Besides looking at the solvent to decide for E1/SN1/SN2/E2? Still confused about this topic. Any advice would be appreciated!

---

Members don't see this ad.

 

[music314]

This video series should help you:


Generally, remember that bases abstract protons and nucleophiles attack in substitution reactions.

Two important categories to have straight are strong base/strong nucleophiles compared to strong nucleophiles only. For example, RO- like HO- (hydroxide) CH3O-, CH3CH2O-, ect. are both strong bases and strong nucleophiles. On the other hand, CN-, H2S (or any sulfur that is neutral or with a negative one formal charge), N3-, halides, among others, are strong nucleophiles only.

An important thing to remember is that substitution and elimination reactions both require good leaving groups to occur.

For SN2 reactions, they most often occur on primary substrates, usually with a strong base/strong nucleophile OR with a secondary substrate with a strong nucleophile only. Preferentially, the solvent would be aprotic in most SN2 reactions.

For halogens acting as nucleophiles in PROTIC solvents, the nucleophile strength goes:
F- < Cl- < Br- < I-
For APROTIC solvents:
I- < Br- < Cl- < F-

For E2 reactions, they most commonly occur on secondary or tertiary substrates with strong base/strong nucleophiles or strong bases only (only one you really need to remember is tert butoxide). Remember that bulky bases like tert butoxide favor the Hoffmann product while strong bases that aren't bulky favor the Zaistev product. Remember E2 has the huge limitation: the beta proton must be antiperiplanar to the leaving group.

Big thing to notice: a secondary substrate reacted with a strong base/strong nucleophile will go E2 over SN2.

E1 and SN1 undergo carbocation intermediates and thus prefer tertiary substrates. A big indicator that a reaction will go E1 or SN1 is a weak base/weak nucleophile. The only example I can think of off the top of my head is some form of ROH, like HOH (water), CH3OH, CH3CH2OH, ect. So basically if you get ROH and a tertiary substrate, you are likely looking at an E1 or SN1 reaction. Heat can be used to favor E1 over SN1.

Remember SN2 reactions kinetically depend on concentration of substrate and concentration of the nucleophile, whereas for SN1, it only depends on concentration of substrate.

Please let me know if you have questions. I tutor/SI for organic chem and would love to help you if you are still struggling.

 

[DAT Destroyer]

what's the easiest way to distinguish between a strong nuc and a strong base? Besides looking at the solvent to decide for E1/SN1/SN2/E2? Still confused about this topic. Any advice would be appreciated!

For the DAT exam,,,,,keep the following in mind,,,,,,CN- SH-, CH3S- are all very very powerful nucleophiles and are the superstars of the SN2 reaction. I- is fairly good as well especially when an aprotic solvent is employed. Nucleophilicity often parallels basicity.....BUT NOT ALWAYS. For example, I- is a good nucleophile but a poor base. Strong bases you need to know for the DAT exam include,,,,HO-....CH3O-, C2H5O-, NH2-, t-butoxide, and LDA. These bases are also nucleophiles. When doing reactions on a secondary or tertiary halide they will do the E2 mechanism. Heating always favors the elimination process as well. Zaitsev elimination for most of them, Hofmann elimination with t-butoxide and LDA. Be on the lookout for these bases,,,,,E2 mechanism will be the likely mechanism path. For a primary halide you will do SN2 with the exception of t-butoxide or LDA....which gives the E2 product. Many examples of this methodology are shown in Destroyer . For further clarification, I think the David Klein text as well as the Francis Carey text do a fine job to present examples if you need it.

Hope this helps...keep hammering away.

Dr. Romano

 

[music314]

For the DAT exam,,,,,keep the following in mind,,,,,,CN- SH-, CH3S- are all very very powerful nucleophiles and are the superstars of the SN2 reaction. I- is fairly good as well especially when an aprotic solvent is employed. Nucleophilicity often parallels basicity.....BUT NOT ALWAYS. For example, I- is a good nucleophile but a poor base. Strong bases you need to know for the DAT exam include,,,,HO-....CH3O-, C2H5O-, NH2-, t-butoxide, and LDA. These bases are also nucleophiles. When doing reactions on a secondary or tertiary halide they will do the E2 mechanism. Heating always favors the elimination process as well. Zaitsev elimination for most of them, Hofmann elimination with t-butoxide and LDA. Be on the lookout for these bases,,,,,E2 mechanism will be the likely mechanism path. For a primary halide you will do SN2 with the exception of t-butoxide or LDA....which gives the E2 product. Many examples of this methodology are shown in Destroyer . For further clarification, I think the David Klein text as well as the Francis Carey text do a fine job to present examples if you need it.

Hope this helps...keep hammering away.

Dr. Romano

Dr. Romano, I already took my DAT, but since I tutor organic chemistry, I have a question I was hoping you could help me clear up.

The question is regarding epoxides under acidic conditions (#111 in Destroyer 2016). The question presents the epoxide, 2-methyloxirane, which has both a primary and secondary carbon. This epoxide is in acidic conditions with ethanol, and we are asked to predict the product. The answer key says that the correct answer would result from a nucleophilic attack on the secondary carbon, as opposed to the primary carbon. I understand that acidic conditions, unlike basic conditions, are not only based on steric hindrance, but rather on both sterics and electronics. I agree that under basic conditions primary > secondary > tertiary. However, as far as I know, under acidic conditions, the order is tertiary > primary > secondary, where tertiary beats primary due to electronics and primary beats secondary due to sterics. This idea is also backed by 2nd edition Organic Chemistry by Klein. I noticed in my studying that all the resources I used (Destroyer, QVault, and Bootcamp) all propose that electronics trumps sterics when comparing the primary and secondary carbons as well. So, is Klein wrong about that? Thanks for any input.