Water is a weak acid, so the hydroxide ion is a strong base. It "wants" to use its lone pair electrons to form a covalent bond. Water is the conjugate base of hydronium ion, the strongest acid that can exist in water.
Thus water is a weak base. The general idea is that "poor" leaving groups have a strong nucleophilicity , or a strong "desire" to not bring its electrons with it and allow the bond to break. You can determine how "poor" of a leaving group it is by knowing the pKa of the acid related to the leaving group. That alone tells you that it's almost always going to want to stay attached to the rest of the compound. Here's a video on leaving groups. Key Questions What are considered "good" leaving groups?
Answer: Good leaving groups are weak bases. Explanation: Consider a general nucleophilic substitution reaction. The second arrow always shows a pair of electrons going toward the leaving group.
The best leaving groups "want" those electrons. They don't want to share them with other atoms. In general, the weaker the base, the better the leaving group.
Exception : Fluorine is a poor leaving group. And stabilising a negative charge directly correlates with being a weak base. LG Wade has put those concepts in another direction, but his explanation is valid and have put it across in the following discussion:. When the C-X bond breaks in a nucleophilic substitution, the pair of electrons in the bond goes with the leaving group. In order to act as a proton acceptor, a base must have a reactive pair of electrons.
When evaluating the stability of the conjugate base that resulted from the proton transfer, basic concepts used include:. In other words, the trends in basicity are parallel to the trends in leaving group potential - the weaker the base, the better the leaving group.
Just as with conjugate bases, the most important question regarding leaving groups is this: when a leaving group leaves and takes a pair of electrons with it, how well is the extra electron density stabilized? In laboratory synthesis reactions, halides often act as leaving groups. Iodide, which is the least basic of the four main halides, is also the best leaving group — it is the most stable asa negative ion. Fluoride is the least effective leaving group among the halides, because fluoride anion is the most basic.
The more electronegative an atom, the better it is able to bear a negative charge. Because fluorine is the most electronegative halogen element, we might expect fluoride to also be the least basic halogen ion. But in fact, it is the least stable, and the most basic! It turns out that when moving vertically in the periodic table, the size of the atom outdoes its electronegativity with regard to basicity.
The atomic radius of iodine is approximately twice that of fluorine, so in an iodine ion, the negative charge is spread out over a significantly larger volume. The following diagram illustrates this concept, showing - CH 3 to be the worst leaving group and F - to be the best leaving group. This particular example should only be used to facilitate your understanding of this concept.
In real reaction mechanisms, these groups are not good leaving groups at all. For example, fluoride is such a poor leaving group that S N 2 reactions of fluoroalkanes are rarely observed. If we move down the periodic table, size increases.
With an increase in size, basicity decreases, and the ability of the leaving group to leave increases. The relationship among the following halogens, unlike the previous example, is true to what we will see in upcoming reaction mechanisms. Furthermore, note how we almost never see alkanes or hydrogens as leaving groups. You might note that I have carefully avoided discussing fluorine. In Org 2, you may see some examples where F can act as a leaving group when it is attached to a carbonyl carbon or an aromatic ring.
These reactions addition-elimination reactions are a little bit different in that the rate determining step is not so related to loss of the leaving group.
Which is a stronger acid? Now: the stronger the acid, the weaker the conjugate base. So which of these would have the weaker conjugate base? The weaker the base, the better the leaving group. Do I have this thought process correct? I have my MCAT exam in 2 days, well 1.
Ah… well the leaving group would be S CH3 2 which is a very weak base and an excellent leaving group! This is just what I need,Organic chemistry has been a part of chemistry that seemed to be impossible to understand.
But with the help of your explanations I see my self getting a straight A,beyond the shadow of doubt. Thanks to you James. Thanks for this amazing resource to help me through orgo. Just wanted to say thank you.
Wish I had found this website months ago. All the information is so accessible and very well organized. Keep up the tremendous work. There is a mistake in the second paragraph, I think. An atom can never be neutral and charged at the same time.
I recommend you to say an atom becomes an ion negatively charged to reach the full octect. You made very simple. Are there other factors beside PKa? So, if the base is weak, it will not be in a hurry to accept protons and react. Hence, as the base is not readily reacting, it is stable and a good leaving group. Weak bases are very stable as anions in solution on their own. A weak base has a strong conjugate acid e.
HCl is the strong conjugate acid of the weak base Cl-. Since the acid is strong, we know that the base must very readily leave.
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