Hi shadowrider-ga,
I will give a basic explanation of organic reactions, nucleophiles and
electrophiles, and provide you with answers to your question.
1) The key to remember in all organic reactions is that:
a) all compounds are conserved. You never "lose" an atom, ion, or molecule.
b) all electrons are conserved. You can transfer electrons and
charges, but you cannot create charge from nothing.
Definitions:
A nucleophile is a nucleus-loving agent. Think of audiophile -
audio-loving, or pedophile (I'm sure you know what that means). What
likes positive nuclei? Negative electrons! (Opposite charges attract.)
Nucleophiles are generally negatively charged or neutral, and have
spare electrons. Lewis bases are nucleophiles.
An electrophile is a electron-loving agent. What likes electrons?
Positive nuclei! Electrophiles are generally positively charged or
neutral, and have empty orbitals. Lewis acids are electrophiles.
(If you don't know what Lewis acid/bases are, don't worry about it. If
you are curious:)
http://www.meta-synthesis.com/webbook/12_lab/lab.html
When dealing with organic reactions, a nucleophile (almost) always
attacks a electrophile. It invariably ends up with a transfer of
(usually) two electrons from the nucleophile to the electrophile. All
transfers are indicated by an arrow, indicating the transfer of two
electrons from the tail of the arrow to the head of the arrow.
Arrows leading AWAY from a bond indicate bond-breaking; arrows leading
from a negative charge to somewhere else indicate bond formation. All
bonds (i.e. H-Cl) denote two shared electrons, or a covalent bond.
I have drawn the reaction mechanisms for the reactions you mentioned.
You can download the image file of my work from my website:
http://individual.utoronto.ca/dtsang/ga/acidbase.gif
Let me know if you have problems opening the file. It was scanned at a
high resolution so may take a little while to load.
In some cases I left off the Na+ icons. This is correct, as Na+ is a
spectator ion and doesn't react per se.
Here are captions for the mechanisms I posted. The numbers are in the image.
(1) Bicarbonate (nucleophile) attacks the hydrogen (electrophile).
Because the O-H bond is polar (O is electronegative), the hydrogen has
a positive dipole - which is why the negative bicarbonate attacks it.
(2) Benzoate (nucleophile) attacks the proton of HCl. This is because
H-Cl is a polar molecule; chloride is electronegative, leaving
hydrogen with a partial positive charge (3). Thus, the two are
attracted, and results in a reaction. Furthermore, Cl- (chloride) is
stable; chlorine stabilizes a negative charge readily. Since the
product is stable, the reaction mechanism is plausible.
(4) Here, there are many places from which OH- (hydroxide) can
abstract a proton to form water. It can steal a proton from the O-H
group, from the benzene ring, or from the tert-butyl group. You need
to ask yourself: what can best stabilize a negative charge after
proton abstraction? The answer is the oxygen; it is electronegative
and doesn't mind the negative charge. The removal of the proton also
allows resonance (6), which stables the phenoxide ion further. The
benzene ring is notoroiously stable as it is, so it doesn't give up a
hydrogen ion. C- (carboanions) are relatively unstable, so the
tert-butyl group won't give up a proton. Thus, we deal with the
hydroxyl group.
(5) Hydroxide, a strong nucleophile, attacks the phenol, abstracting a proton.
(6) Resonance stabilizes phenoxide, indicating it is a plausible product.
For more information on resonance in benzene:
http://www.chemhelper.com/benzenetutorial.html
(7) Here, phenoxide, the nucleophile, attacks the nucleophilic (and
partially positive) H on HCl. It recreates the phenol, and produces
Cl-. Again, Cl- is stable because chlorine is electronegative.
I hope I have provided a comprehensive explanation to your questions.
If you have questions about my answer, please request a clarification
and I will respond promptly.
If you want more information about these kinds of reactions, I
recommend you purchase the organic text I referenced below. It is an
exemplary and authoritative source on organic chemistry, and all
aspects of nucleophilic and electrophilic reactions. It may be
overkill for your studies, but if you continue on in organic
chemistry, it will be very useful.
- supermacman-ga
Search strategy
benzene resonance
lewis acid base
Reference
J McMurry, Organic Chemistry, 6th edition, United States of America,
Thomson-Brooks/Cole, 2004. |
