Base or Nucleophile

[marymatthews]

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Chad says the first thing to look for is the Nucleophile to determine if the reaction is E2, E1, SN2, or SN1,....
How can you tell the difference between a Nucleophile or Base?
In order to determine if a reaction will go via SN1 or E1 (both prefer tertiary and aprotic), you need to know if it is a weak Nucleophile or Base, so how do you determine this exactly?/ Or In order to determine if a reaction will go via SN2 or E2, you need to know if it is a strong Nucleophile or Base, so how do you determine this exactly?
For example, some strong nucleophiles are also strong bases? so how do you know?

Thanks

 

[marymatthews]

How am I supposed to know that NaCN a stong nucleophile but a weak base? Why is NaOCH3 a strong nuc and strong base?

 

[DQLEUCD]

How am I supposed to know that NaCN a stong nucleophile but a weak base? Why is NaOCH3 a strong nuc and strong base?

These are strong nucleophiles because it's attached to a metallic group Na+ so it's automatically a negative anion.

For the reason why it's a stronger base, would probably have to look at pKb. It doesn't follow the CARDIO rule though so I believe it's because it's one of those exceptions he was talking about. Probably because cyanide is sp while the methoxide anion is sp3? or the fact that nitrogen is a more electron donating group then carbon is so it stabilizes the anion better making it a weaker base? Or it could be that nitrogen is more electro negative then carbon and the inductive effect is in effect making the cyanide anion a weaker base?

 

[MrWilson]

Not only would you get a quicker answer if you posted on the *********** forum, but you may actually get the answer from the horse's mouth. Not trying to be rude here, but Chad and Alan may appreciate if you post his material on his website rather than here.

 

[DQLEUCD]

Not only would you get a quicker answer if you posted on the *********** forum, but you may actually get the answer from the horse's mouth. Not trying to be rude here, but Chad and Alan may appreciate if you post his material on his website rather than here.

What does it matter where we post the rules? Did he invent the rules of CARDIO? People post stuff that are not from SDN all the time i.e. kaplan, achiever, topscore and etc... I don't see you going to them telling them to go to their respective websites and posting on their forums. Not to be rude or anything, there are a lot more people here to help out with answer with the question then the forum over there. If you looked at their forum, there are maybe, 5 people at most browsing through it. Here there are 50-100 viewing this section alone. If your not going to help with finding the answer, then those comments are not needed.

 

[marymatthews]

Ok, so if given a Nucleophile or Base, how should I go about determining whether it is strong or weak? What steps do I take exactly?
thanks

 

[DQLEUCD]

Ok, so if given a Nucleophile or Base, how should I go about determining whether it is strong or weak? What steps do I take exactly?
thanks

pka will ultimately determine it if the cardio rule fails.

http://www.brynmawr.edu/Acads/Chem/mnerzsto/hcn.htm

Here is a site that gives you the pka for HCN which is 9 and ROH which is about 18. as you can see HCN is the stronger acid, so it's the weaker base and ROH is the stronger base. I haven't gotten to the SN/E reactions on CHAD's video's, but if you are asking why they are more nucleophilic or basic, then there goes the answer.

 

[shiftypoptart]

yep I felt the same as you do when I was studying for my DAT. I even asked my friend who was a chemistry major and he said there aren't any concrete rules, you kinda just have to memorize the basic ones that are commonly used like NaCN, NaOH. Dont stress too much, as long as you know the basic ones, those are the ones they ask on the DAT. They wont pull something ridiculous out at you and not give you a pKa or pKb to work with.

 

[marymatthews]

I am just confused.... how can you tell the difference between a Nucleophile or Base exactly?
how would you know if something is weak or strong, or base or nucleophile?
so if given Something, how should I go about determining whether it is strong or weak, or nucleophile or base? What steps do I take exactly please?

 

[Thundercatz]

Mary,

If you watch Chad's video, he gives you a list of strong nucleophiles that are weak bases.

Strong nucleophiles that are weak bases (SN2-only):
NaCl, NaBr, NaI, NaCN, NaN3

Strong base but weak nucleophile (E2 only):
t-buytl

For something like NaOH, which is a strong base and nucleophile, it isnt as cut and dry, and you will have to look at the molecule it will react with. If its a primary halide, the major product is the SN2 and the minor is the E2.

For a secondary halide, the E2 will be the major product because an SN2 reaction is slower with a secondary halide.

For tertiary, its obviously an E2.

 

[marymatthews]

I have seen other strong Bases that are weak nucleophiles... I dont think t-butyl is the only one....
so how do you know whether it is a weak nucleophile that is a weak base?
Both SN1 and E1 require a weak base, so how do you know if it will go via SN1 or E1?

 

[AfroChapin]

^That can be determined by the solvent used. Protic solvents stabilize the carbocation and lead to Elimination and aprotic solvents favor substitution

 

[Ibraiz]

In order to know if it will go SN1 or E1 you have to know the temperature of the reaction. High temperature favors E1 over SN1 and vice versa. That is the only different I know of to distinguish between SN1 and E1.

 

[polkadds]

Mary,

Watch Chads new video on SN1 2, E1 2, it explains this idea perfectly! I was having the same problem then watched his new HD video and it is much clearer now

 

[marymatthews]

My subscription expired yesterday 🙁

 

[luv8724]

Any negatively charged molecule other than on oxygen are strong nuc but weak bases. negative Oxygen is happen to act as both strong....

 

[marymatthews]

I am confused because Nucleophile strength and Base strength follow the same trend on the periodic table... up and to the left.....

 

[luv8724]

They follow the trend depending on the sovlents... their strength differs by in which solvents they are used in

 

[Ibraiz]

Nucleophile means nucleus loving (nucleo = nucleus phile = loving) which makes nucleophile a positive charge loving. Nucleus has proton embedded in them which makes them positive. Nucleophile have a negative charge attack the nucleus. Anywhere they see even a positive partial they will attack.

Base on the other hand are proton loving, not necessarily a positive charge loving. Base usually attacks the acidic proton.

Reiterate: Nucleophile attacks positive (partial) charges
Base attacks the acidic proton

Trend for base increases from right to left and up the group
Trend for nucleophile increases from right to left AND increase up the group if solvent is APROTIC and increase down the group if the solvent is PROTIC

know your strong nucleophile that are weak bases: NaCl, NaI, NaBr, NaCN, NaN3
know your strong base that is a weak nucleophile: t-butyl ethoxide