Why does nucleophilicity increase down a column

In either case, it is important that the nucleophile be a good Lewis base, meaning it has electrons it wants to share. The following diagram is just a reminder of some of the nucleophiles that were presented in the section covering nucleophilic substitution. In looking at these two types of nucleophiles, you should notice that a reactive atom, such as oxygen, in a neutral species can also be a reactive atom in a negatively charged species.

It has been experimentally shown that a nucleophile containing a negatively charged reactive atom is better than a nucleophile containing a reactive atom that is neutral. The next diagram illustrates this concept.

Notice that when oxygen is part of the hydroxide ion, it bears a negative charge, and when it is part of a water molecule, it is neutral.

Similarly, when nitrogen is part of NH 2 , it bears a negative charge, and when it is part of NH 3 , it is neutral. To say that nucleophilicity follows basicity across a row means that, as basicity increases from right to left on the periodic table, nucleophilicity also increases. As basicity decreases from left to right on the periodic table, nucleophilicity also decreases. When it comes to nucleophilicity, do not assign this same rule when making comparisons between the halogens located in a column.

In this case of moving up and down a column, nucleophilicity does not always follow basicity. It depends on the type of solvent you are using. In the section Nucleophilic Substitution, we assigned a relationship to leaving groups containing C, N, O, and F, showing that the strength of the leaving group follows electronegativity. This is based on the fact that the best leaving groups are those that are weak bases that do not want to share their electrons. The best nucleophiles however, are good bases that want to share their electrons with the electrophilic carbon.

The relationship shown below, therefore, is the exact opposite of that shown for the strength of a leaving group. In general chemistry, we classified solvents as being either polar or nonpolar.

Polar solvents can be further subdivided into protic and and aprotic solvents. First, it is important to recognize that the two charged species, and are the two strongest nucleophiles. This is because the destabilizing negative charge present in these species may be neutralized by donating a lone pair to the formation of a chemical bond.

As we know, opposite charges attract, so species bearing a full negative charge are drawn to electron-poor regions. Uncharged species such as water and ammonia carry a lone pair capable of bonding, but are less energetically drawn towards positive charges. Ammonia is a stronger nucleophile than water because nitrogen is less electronegative than oxygen. What this means is that the nitrogen-bound lone pair of ammonia is more loosely contained than the oxygen-bound lone pairs of water.

As a result, they are more easily donated to form a bond at an electron-poor carbon. From this trend, one might expect that fluoride ions would be less nucleophilic than chloride ions since fluorine is more electronegative. However, moving down a group of the periodic table, atomic radius increases. Anions are stabilized by spreading electron density across an electron cloud of greater volume, such as that of compared to the smaller. In aprotic solvents, nucleophilicity increases with electronegativity when dealing with atoms in the same group column on the periodic table.

When discussing nucleophilic strength, we can begin to see trends. However, it is important to note that the question asked for the strongest nucleophile in aprotic solvent. The correct answer is. In polar protic solvents, electronegative nucleophiles tend to hydrogen bond with the solvent, inhibiting the nucleophile's nucleophilicity. However, in aprotic solvents, this does not occur and so basicity correlates to nucleophilicity.

Nucleophile are electron-rich species that form bonds with electron-poor species. When thinking in terms of acids and bases, bases tend to form bonds with protons making them strong nucleophiles while, acids usually donate protons making them weak nucleophiles. Which of the following sets of nucleophiles are correctly listed from strongest to weakest in a protic solvent? In a protic solvent, the larger the atom the better the nucleophile.

Atomic radius increases as you go down a group on the periodic table. Smaller molecules are better nucleophiles than larger ones they are not as sterically hindered. Thus, the lone pair of electrons on nitrogen are not as stable as those on oxygen, and will readily donate its pair of electrons to another species.

The best nucleophiles are negatively charged II. Smaller molecules are better nucleophiles than larger ones III. The smaller the atom, the better the nucleophile protic solvent. The larger the atom, not molecule, the better the better the nucleophile. Nucleophilicity can also be determined according to strength of the anion as a conjugate base. If you've found an issue with this question, please let us know. After all, basicity and nucleophilicity essentially describe the same phenomenon, except basicity concerns donation of lone pairs to hydrogen, and nucleophilicity concerns donations of lone pairs to all other atoms.

So as electron density increases, so does nucleophilicity. A handy rule to remember for this purpose is the following: the conjugate base is always a better nucleophile. Bottom line: as electronegativity increases, nucleophilicity decreases. Going down the periodic table, another factor comes into play next.

In so doing, the nucleophile is considerably less reactive; everywhere it goes, its lone pairs of electrons are interacting with the electron-poor hydrogen atoms of the solvent. The ability of nucleophiles to participate in hydrogen bonding decreases as we go down the periodic table.

Hence fluoride is the strongest hydrogen bond acceptor, and iodide is the weakest. A polar aprotic solvent does not hydrogen bond to nucleophiles to a significant extent, meaning that the nucleophiles have greater freedom in solution. Under these conditions, nucleophilicity correlates well with basicity — and fluoride ion, being the most unstable of the halide ions, reacts fastest with electrophiles. If we want a reaction to take place, we need to use solvents that will actually dissolve our nucleophile.

The bottom line here is that the bulkier a given nucleophile is, the slower the rate of its reactions [and therefore the lower its nucleophilicity]. Note: Are there other factors? This list of four covers the basics, but several other factors are worth noting. Iodide, being larger, will have a lower charge density and interactions with hydrogen will be weaker. Does that make sense to you? Its actually the opposite. Since flourine is smaller, its charge is confined to a smaller space and it therefore has a higher electron density.

They are correlated most of the time but not always. How is polarizability related to nucleophilic strength? Would it fit into any of these categories? Polarizability plays a role when you take the solvent into account. In polar protic solvents, hydrogen bonding occurs between the partial positive hydrogen H attached to N or O usually and the nucleophile. Smaller nucleophiles become more solvated than larger nucleophiles, which means that smaller nucleophiles in polar protic solvents will not be able to react as well and thus are poorer nucleophiles.

For example, Florine anions become so heavily solvated in polar protic solvents that they wont even react, but Iodine, being much larger, is much less solvated and can still react. In aprotic solvents, hydrogen bonding does not occur to any significant extent and the stronger base is usually the stronger nucleophile.

If this list does not take into account all the factors that make a good nucleophile, where is a more detailed treatment of the ones that are remaining? Also, in the case of polar aprotic solvents, one may mention the idea of the cation being solvated, while the anion nucleophile less so, and so it is more reactive. One example is the differing selectivity of enolates for C vs. O alkylation; depending on the nature of the solvent, counter-ion, and electrophile, either dominant O vs.

C alkylation can be achieved. I understand that the presence of electron donating groups eg. Great question. These types of tradeoffs are what can make organic chemistry difficult. How do we decide that from anisole, nitrobenzene and benzene, what will be the correct order of rate of electrophillic substitution? And can you possibly link me to an article related to it? You are soon gonna get a lot of Indian visitors.

In OCH3 oxygen has got a lone pair due to which can be shared with the benzene ring through resonance hence increasing its charge density whereas nitrogen in NO2 has not got lone pair due to formation of dative bond with O and has higher electronegativity than C so it withdraws charge from benzene ring. Hope it helps! Hello, Is it accurate to say that primary amines are more strongly nucleophilic than carboxylic acids? Many thanks for your help and time. Which one is it?

It seems to me that you are contradicting yourself and making me more confused than I was previous to visiting this page…. Maybe I should have made this clearer. Also, that rule only applies for polar protic solvents. F - tends to H-bond with the solvent more, making it less reactive as a nucleophile, as compared to a nucleophile containing carbon. The reverse of the rule is what actually applies in polar aprotic solvents.

Since the solvent does not H-bond to the halide nucleophiles, fluorine basically becomes the most reactive among the halides. It took me way too long before I finally understood this whole nucleophile thing, but I hope my answer helped. Pyridine or Morpholine? Anyway, I think the answer is morpholine but I do not know how to explain it. Could anyone please help me on this?

I expect that, in pyridine nitrogen atom surrounded by three bons all of them with carbon atoms while in morpholine there are three bonds two of them with crbon and the thired one with hydrogen which is lower electronegative than carbon so the availability of unshered electron pair in morpholine more than that in pyridine.

I mean which polar aprotic solvent may retard reaction? That drives the equilibrium forward. Not quite. But steric hindrance due to the fact that a sigma star orbital is being attacked on carbon, versus an S orbital on hydrogen is the key difference.

My question is why flouride ion behaves as a strong nucleophile in aprotic polar solvent when nucleophilisity is related to polarizability. Then why not iodide is a strong nucleophile in aprotic polar solvent and also iodide is less electronegative than fluoride so it should easily donate its lone pair of elctron.

It does not prefer to accommodate any other atom with it. Second, we see the same trends going across a row of the period table examples a and b. The more electronegative an atom the less nucleophilic or basic it is because it wants to keep its electrons and not share them. Third, resonance stabilized anions are less basic and less nucleophilic than similar anions without resonance example c.

This is particularly problematic for very strong nucleophiles such as NH 2 - which will usually act as a base and not a nucleophile.