What is the difference btwn a nucleophile and a base...don't they both have an extra pair of electrons to donate? To elaborate...my book says that SN2 rxns require a strong Nu-, while E2 rxns require a strong base.

A strong base makes a strong nucleophile. They are usually drawn "Nu:-" with the pair of electrons showing.

I am a little mixed about this as well, i keep them separated by memorizing that nucleophiles attack carbons and base attack Hydrogens. Can anyone else help?

good point

From what I understand, E1 and E2 have been eliminated from the MCAT along with the Alkenes section. I check the Organic Chem Topics thoroughly and found NO MENTION of ANY elimination reactions.
Am I wrong?????????????????

Jon Davis
I just looked through the Ochem topics from AAMC and E1, E2 isn't mentioned anywhere.

they aren't even covered in EK orgo, just slight references. know the sn1 and sn2 stuff and you will be fine.

E1 and E2 isnt on the mcat anymore because alkenes are not on there anymore. I would still like to know the answer to this question though. I know that nucleophicity and basicity are directly correlated as u move down periods, but they do not correlate as u move down groups. I still do not understand what that means though, the two words mean the same thing as far as I can see.

From Audio Osmosis, a nucleophile's electrons attack a carbon... a base's electrons attack a proton. They can be the same species, but the behaviors are different.

From Audio Osmosis, a nucleophile's electrons attack a carbon... a base's electrons attack a proton. They can be the same species, but the behaviors are different.

This is true. Bases and nucleophiles aren't necessarily the same or different. Strong bases usually make strong nucleophiles. One species can act as both.

think subset. A base is a subset of a nucleophile. -OH is both a base and a nucleophile, so is NH3. Now, an alkene is not a base, but it can act as a nucleophile.

How is this important for the mcat? Well, simply said, it isn't.

exactly. basicity and nucleophilicity depend on what the species does in a reaction. If it abstracts a proton,then it's acting as a base. If it attacks an electrophile, for instance, a carbonyl carbon or a carbon in the alkyl halide, then it's acting as a nucleophile.

thanks everyone for clearing that up...it makes sense now!

1,3-dioxane has two oxygen atoms and it is well known as aprotic solvent.
How it enhances nucleophilicity of chloride ion in the reaction of alcohol with thionyl dichloride?
Thanks

I think we have to look at conjugate bases of the acids.
The weakest acid has the strongest conjugate base and therfore the strongest nucleophile.
Example
compare
which nucleophile is stronger methoxide OCH3 or acetate?
Answer
methanol is weaker than acetic acid
thus the conjugate base methoxide is stronger than acetate.
Methoxide is stronger nucleophile than acetate

are you fing serious?

if you have taken ochem and don't know the difference between a nucleophile and a base, then JESUS CHRIST. also, just because it isn't on the mcat doesn't mean you shouldn't know it. the end of ochem1 and half of ochem2 is spent on this topic (nucleophiles, baes, and E1 and E2)! i sure hope most premeds aren't like this....only learning what's on the mcat and only studying for the sake of good grades as opposed to wanting good grades AND for the sake of learning. jesus christ!

I think that a nucleophile is a base ( Lewis base) if it attacks hydrogen or any species.

oh but to prevent from being a complete troll...
bases and nucleophiles generally use the same mechanism (think Lewis acid/base mechanism). the easiest way to distinguish the two in your head is bases deal with transfer of H+ while nucleophiles generally are more associated with carbon bonding. nucleophiles can act as bases tho and vice versa. however, not in every case, such as if an anion is acting as a base it will result in E1 but if acting as a nucleophile (through carbocation intermediate) it'll result in S1.

how can the mcat not include elimination rxns? that is one the fundamentals of ochem! i just started studying for the mcat a few days for the first time and so far i'm not impressed by it haha. they'll only test you on the mere basic of each topic. pretty lame

Why you are so upset, I am trying to say that the species itself can play two roles the role of a bronsted base when it attacks hydrogen and the role of a nucleophile in substitution reactions.

nucleophillicity is a KINETIC term that describes how efficient something is at seeking out an electrophile. ie, how high is the energy barrier for the rate limiting step in an sn2 reaction?

basicity is a THERMODYNAMIC term that describes the relative stabilities of the conjugate acid/conjugate base, think acid/base equillibria--> what is the difference in stability between CH3COO- and CH3COOH?

something can be a strong base and a poor nucleophile if it is so sterically hindered that it cannot subsitute. think tert-butoxide: strong base because tert-OH is more stable, but poor nucleophile because of steric hindrance .

You are absolutly right, I will add something now have started to glow in my mind. If the base and substarte are bulky, does a reaction occur?

If the base is bulky E2 will be favored. Bulky substrates by definition favor E2 over subsitution also because
(1) They are not open for nucleophillic attack and
(2) they form more stable alkenes.

If the base is bulky E2 will be favored. Bulky substrates by definition favor E2 over subsitution also because
(1) They are not open for nucleophillic attack and
(2) they form more stable alkenes.

Don't think in terms of bulky though, remember it's in reference to how substituted the carbon is. So, Sn2 rxns favor primary carbons (ie, methyl carbons), and E2, Sn1 and E1 favor tertiary carbons. While the ones that need to be considered when trying to decide between Sn2 or E2, E1, Sn1, are in relation to the secondary carbons.

Also, Reaction temperature can be a good indicator as well: If you have a temp <50 degrees it is likely either going to be Sn2 or Sn1.

If the temp is at Reflux or very high, it is likely going to be E1 or E2.

Also, I believe your confusion about the base comes about because it doesn't seem like you're considering the solvent.

So, for Sn2:

Nucleophile: Strong
Base: Weak

Solvent: Aprotic (ie, DMSO or DMF; Think ROR)


For E2:

Nucleophile: Weak
Base: Strong

Solvent: Protic (Think ROH)


Also, remember that basicity is dependent upon the type of solvent. So, what is a weak base in one solvent can be a strong base in another.

The difference between a nucleophile and a base is simply what the substrate is. If the thing being attacked is a H, then the attacker is a base. If the thing being attacked is anything else, the attacking species is a nucleophile.

A good nucleophile is USUALLY a good base, and a weak nucleophile is usually a weak base.

One exception are halogens. For ex, Br- is a good nucleophile, a poor base, which is why it can act as a nucleophile even in an acidic solution (whereas most other nucleophiles would get protonated, and thus inactivated as good nucleophiles).

This is why an alcohol can be converted into an alkyl halide simply through adding an acid such as HBr. The hydroxyl group acts a base to abstract H from HBr. Then, water leaves, and Br- acts as a nucleophile to attack the resultant carbocation (Sn1).

What is the difference btwn a nucleophile and a base...don't they both have an extra pair of electrons to donate? To elaborate...my book says that SN2 rxns require a strong Nu-, while E2 rxns require a strong base.

Nucleophiles and bases are almost synonymous. The don't have to be negatively charged, but both have at least one lone pair of electrons.

It's mostly a definition game.

Let's take water. When it acts like a nucleophile, it participate in a substitution reaction. Because, substitution, e.g. SN2, reactions involved at least two reactants swapping substituents.

Water, in an elimination reaction, e.g. E2, acts as a base. Bases are proton acceptors. Most common reaction, is when water accepts a proton and becomes the hydronium ion.

There's more involved, but I can't remember at this time. good luck

are you fing serious?

if you have taken ochem and don't know the difference between a nucleophile and a base, then JESUS CHRIST. also, just because it isn't on the mcat doesn't mean you shouldn't know it. the end of ochem1 and half of ochem2 is spent on this topic (nucleophiles, baes, and E1 and E2)! i sure hope most premeds aren't like this....only learning what's on the mcat and only studying for the sake of good grades as opposed to wanting good grades AND for the sake of learning. jesus christ!

yeah ... most people were getting their grades bumped unlike you... sucks to be you :laugh:

E1 and E2 isnt on the mcat anymore because alkenes are not on there anymore. I would still like to know the answer to this question though. I know that nucleophicity and basicity are directly correlated as u move down periods, but they do not correlate as u move down groups. I still do not understand what that means though, the two words mean the same thing as far as I can see.


what about alkynes? Because eliminations reactions, such as E2, do make alkynes

such as if an anion is acting as a base it will result in E1 but if acting as a nucleophile (through carbocation intermediate) it'll result in S1.

If I recall correctly, an anion acting as a base will surely go E2. It is so strong of a base that no carbocation will have time to form.

I have a quick question and this well help me a bunch on a lot of my problems. What do you look for when determining what's a good nucleophile? Same thing for leaving groups. Thanks.

I have a quick question and this well help me a bunch on a lot of my problems. What do you look for when determining what's a good nucleophile? Same thing for leaving groups. Thanks.

A good nucleophile is essentially a strong base that is not too bulky. Consider an SN2 reaction where a nucleophile attacks a tertiary carbon. In order for the nucleophile to effectively act perform this task it needs to

A) Have a strong desire to form a bond because doing so makes it more stable (A strong base for example)
B) Be small enough to interact with the carbon without being repelled by the substituents on the carbon.



Now a good leaving group is something that is stable once it leaves. a lot of organic chemistry can be solved by considering stability. Consider Ethanol's OH group. Lets say OH would leave and become OH-, what would happen? OH- is a strong base because it is much more stable when it binds to something. Thus the OH on ethanol is more stable than free OH-. With this in mind, the OH on ethanol is a bad leaving group because once it leaves it becomes less stable. Water is a much better leaving group than hydroxide because it is more stable once it leaves.

Hope this clarifies things a bit.

I also learned the rule that Nucleophiles trended with bases across a period, but not down the columns of the periodic table.
I took this to mean that if the group was more polarizable, then it was a better nucleophile than a base.

What is the difference btwn a nucleophile and a base...don't they both have an extra pair of electrons to donate? To elaborate...my book says that SN2 rxns require a strong Nu-, while E2 rxns require a strong base.

A nucleophile wants the delta plus on the backside of carbon.

Bases like hydrogens/protons.

I have a quick question and this well help me a bunch on a lot of my problems. What do you look for when determining what's a good nucleophile? Same thing for leaving groups. Thanks.

The minimum requirement for anyone to be a nucleophile is to have lone pairs. Think about it-a nucleophile loves positive charge, so it must have some negative charge associated with it-hence the delta minus from lone pairs.

A good nucleophile will exceed the minimum and will usually have a full minus charge associated with it versus delta minus from lone pairs.

Weak bases are good leaving groups. The whole point is to not have that leaving group come back and attack your intermediate (i.e., your carbocation) because then you'd have no yield on your product! So, the general rule is incoming nuclephile/base has to be stronger than the leaving group. Some great leaving groups are H20, Cl-, Br-. OH- is a terrible leaving group-it's a strong base.