Leaving group (LG; nucleofuge): An atom or group of atoms which breaks away from the rest of the molecule, taking with it the electron pair which used to be the bond between the leaving group and the rest of the molecule. Bromide ion is the leaving group in this S_N2 reaction.

How do you determine a leaving group?

In order for a leaving group to leave, it must be able to accept electrons. A strong bases wants to donate electrons; therefore, the leaving group must be a weak base. We will now revisit electronegativity, size, and resonance, moving our focus to the leaving group, as well providing actual examples.

What is leaving group example?

In chemistry, a leaving group is a molecular fragment that departs with a pair of electrons in heterolytic bond cleavage. Common anionic leaving groups are halides such as Cl⁻, Br⁻, and I⁻, and sulfonate esters such as tosylate (TsO⁻). Fluoride (F⁻) functions as a leaving group in the nerve agent sarin gas.

Is the nucleophile the leaving group?

The nucleophile is a key part of every substitution reaction. In these reactions, it is the group that “substitutes” for the leaving group. A nucleophile has a lone pair of electrons that makes up the molecule’s business end.

How do you know if a reaction is E1 or E2?

The key differences between the E2 and E1 mechanism are: 1) E2 is a concerted mechanism where all the bonds are broken and formed in a single step. The E1, on the other hand, is a stepwise mechanism. 3) E2 is a second-order reaction and the rate depends on the concentration of both, the substrate and the base.

What are the best leaving groups?

Good leaving groups are weak bases. They’re happy and stable on their own. Some examples of weak bases: halide ions (I-, Br-, Cl-) water (OH2), and sulfonates such as p-toluenesulfonate (OTs) and methanesulfonate (OMs). The weaker the base, the better the leaving group.

Is NO2 a leaving group?

ABSTRACT: The “element effect” in nucleophilic aromatic substitution reactions (SNAr) is characterized by the leaving group order, F > NO2 > Cl ≈ Br > I, in activated aryl halides. An anionic nucleophile, thiolate, gives very early transition states and negative activation enthalpies with activated aryl halides.

Is water a good leaving group?

Water is the conjugate base of hydronium ion, the strongest acid that can exist in water. Thus water is a weak base. It has little tendency to share its electron pairs with another species. Water in the form of hydronium ion is an excellent leaving group.

Is NH a strong base?

The issue is similar with bases: a strong base is a base that is 100% ionized in solution. All strong bases are OH ^– compounds. So a base based on some other mechanism, such as NH ₃ (which does not contain OH ⁻ ions as part of its formula), will be a weak base.

Which is better leaving group NH3 or H2O?

Because NH3 is an excellent leaving group – even better than H20.

Why is NH2 a poor leaving group?

If you really mean NH2- as a leaving group, then you can rationalize this by noting that NH2- is somewhat basic (not super weak). Thus, they are somewhat reactive and make poor leaving groups. Ideally, you want your leaving group to be as unreactive as possible.

Is leaving group nucleophile or electrophile?

In nucleophilic substitution reactions, an electron rich nucleophile bonds with or attacks an electron poor electrophile, resulting in the displacement of a group or atom called the leaving group.

What is the difference between leaving group and nucleophile?

A nucleophile donates a pair of electrons. A leaving group accepts a pair of electrons.