Ask A Scientist Chemistry Archive

(Created prior to 1993)

Question: I am a curious junior in high school.  I just finished my first 
semester of chemistry but have always loved it.  Here is a LARGE block of 
questions I would love to have answered by anyone who can.  Here goes:
1) Why is mercury a liquid?  It is surrounded by solids!
2) How are cyclics (cyclopropane, cyclohexane, etc...) made from ordinary 
hydrocarbons?  Why can cyclopropane be used as an anesthetic?
3) Why would KNO2 reduce the number of H+ ions in a solution of HNO2?
4) How can arsenic be considered to have 5 valence electrons?  Are not valence 
electrons those in the s and p orbitals?
5) Does exothermic truly mean the reaction releases heat?  Could it also be 
light or is it always heat, then converted into light?
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These are GREAT questions!  Okay, let us take these questions one 
at a time.  
1) Why is mercury a liquid?  It is surrounded by solids!  Mercury is a sort of 
special case. Its valence shell is 5d10 6s2, which means that the two 
outermost electrons are in a spherically symmetric, energetically stable 
quantum state.  So are the elements in the same column of the periodic table 
(Zn, Cd).  However, for reasons that are not completely understood (at least 
by me), 6s electrons are particularly inert, i.e., unavailable to form 
chemical bonds.  This is also true for the 6s electrons in thallium, lead, and 
bismuth, but they have additional 6p electrons which can form chemical bonds 
(note that these are very soft metals though!).  Since Hg does not "like" to 
share its outer electrons, each atom only interacts weakly with the other 
atoms, and therefore it tends to be liquid at room temperature.  This is also 
the reason that mercury is so volatile (vaporizes easily) and has a low 
electrical conductivity (can you guess why?).  Zn, Cd and Hg are very 
different from their "neighbors" in the periodic table (copper, silver, gold), 
not just in the states they exist in at room temperature, but also in just 
about every thermodynamic property you can think of.  This is because they 
have the electrons in their d orbitals are apparently excited into s states by 
only a tiny bit of additional energy (like heat energy), and these form 
metallic bonds.
2) How are cyclics (cyclopropane, cyclohexane, etc.) made from ordinary 
hydrocarbons?  Why can cyclopropane be used as an anesthetic?  I think that 
the answer to the first part of this depends on which cycloalkane you are 
interested in.  The most straightforward way to make cyclohexane is to 
hydrogenate benzene, i.e., to react benzene with hydrogen gas in a closed 
chamber using solid platinum as a catalyst (catalysts are "reaction-rate 
accelerators"):  C6H6 + 3 H2 ---(Pt)---) C6H12 (g)(g)(g)   Benzene is commonly 
available and can be obtained, for example, from petroleum distillates.  
However, if you want to make the RING from scratch, well, the chemistry is a 
lot more complex. Basically, making organic compounds which have the desired 
number of carbons in a ring is an art form, and the smaller the ring is, the 
harder it is to make (due to "ring strain"). However, here is a synthesis for 
cyclopropane.  Let "Et" stand for CH4CH3....
 /\  CO2Et
 (1) CH2(CO2Et)2  ---(2NaOEt,ClCH2CH2CH2Br)---) /  \/(54%
yield) 
  \  /\ 
   \/  CO2Et
   (A)
/\  CO2Et /\  H
 (2) /  \/  ---(1.H2SO4,H2O;2.heat)---) /  \/  (80%
yield)
\  /\\  /\ 
\/  CO2Et \/  CO2H
   (B)
/\  H /\  H
 (3) /  \/  ---(heat)---) /  \/+ CO2  (complete)   
\  /\ \  /\ 
\/  CO2H   \/  H

Here is a question for you which will give you an idea of what synthetic 
chemists have to deal with as they make compounds.  Since 54% of the 
CH2(CO2Et)2 is converted to A and 80% of A is converted to B and 100% of B is 
converted to cyclobutane, what percentage of CH2(CO2Et)2 is converted to 
cyclobutane?  A second question: If a chemist has to go through 10 synthetic 
steps like the ones above to make a product and each step has a 50% yield, how 
much of the original molecule would be converted into the product?  Just some 
food for thought.  Oh, I almost forgot about your anesthesia question! I have 
never heard of cyclopropane being used as an anesthetic!  A brief survey of my 
textbooks makes no reference to this either.  The most commonly known 
anesthetic is diethyl ether, also known as ethyl ether (C2H5 - O - C2H5, or Et 
- O - Et), which is, I believe, still used somewhat as a general anesthetic.  
Then there are local anesthetics, such as procaine (Novocaine) and lidocaine 
(Xylocaine).  Note that the names are similar to cocaine; this is because 
cocaine was probably the first local anesthetic ever used by physicians 
(Leopold Koenigstein, the great Austrian physician and contemporary of Sigmund 
Freud, pioneered its use as an anesthetic in eye surgery).  However,cocaine is 
also THE MOST ADDICTIVE DRUG EVER PURIFIED BY MAN, especially when used in the 
form of crack.
Topper
=========================================================
Robert's example of benzene to cyclohexane requires at 
temperature of 225 degrees under 35 atmospheres of pressure!  Doable in the 
lab but costly.  This is due to the incredible stability ofbenzene.  I was 
able to make a cyclohexene compound in the lab at atmospheric pressure and 
only 200 degrees.  This was done starting with a straight chain compound.  
Cyclohexene is easily hydrogenated with the Pt catalyst then with hardly as 
much energy.

3) Why would KNO2 reduce the number of H+ ions in a solution of HNO2?  HNO2 is 
a weak acid.  Therefore NO2- is a strong conjugate base.  Weak acids do not 
dissociate completely in solution (the reason for their "weakness").  So if 
you have free NO2- ions floating around, they are oing to grab the H+ ions and 
form the more stable HNO2 acid.  NO2-ions come from the very soluble KNO2.  
The K+ ions left will just hang around and do nothing in the solution (called 
a "spectator ion").
Joe Schultz
=========================================================
About cyclopropane as an anesthetic:  it use to be used, but it 
has now been replaced by others.  Cyclopropane is rather flammable, and so it 
was not too safe to use.  How does cyclopropane and other anesthetic work? The 
basic ideal is that anesthetics inhibit neural function.  Specifically, they 
increase the neurons' membrane threshold for excitation, thus decreasing 
synaptic transmission. Thus, anesthetics can "shut down" many points in the 
nervous system.  Of course, you do not want to shut down too many neural 
systems, otherwise, the patient will die!  Overdosing on anesthetics or 
narcotics may result in irreversible depression of respiratory function, 
leading to respiratory arrest.  That is what happens when someone OD's.  So, 
an ideal anesthetic depresses the respiratory center the least, while 
depressing the cerebral cortex (pain, consciousness, etc.) the most.  Using 
this criteria, ether, halothane, cyclopropane, ethylene, nitrous oxide are 
good.  On the other hand, sodium pentobarbital and morphine are poor, because 
they depress the respiratory centers to a great degree.
=========================================================
Thanks to Joe Schultz on describing an easier synthesis of 
cyclopropane.  This is what happens when a theoretical chemist tries to answer 
organic questions by whipping out the old textbooks!
Topper
=========================================================
Just to update the synthesis of cyclic compounds.  You may want 
to get an organic chemistry text to get more information.  The field of 
organic chem is so vast that there are literally hundreds of different ways to 
cyclize an "ordinary hydrocarbon."  Carey and Morrison & Boyd are two fairly 
"easy" to read texts.
Joe Schultz
=========================================================
4) How can arsenic be considered to have 5 valence electrons? Are 
not valence electrons those in the s and p orbitals?  No, actually valence 
electrons are the outermost electrons, i.e., those which are farthest from the 
nucleus and available (sometimes) to form chemical bonds.  For example, the 
valence electrons of the transition metals can involve d orbitals.  However, I 
believe that Arsenic has a valence configuration of 2 4s electrons and 3 4p 
electrons, or 4s^2 4p^3 (total of 5 valence electrons).  Maybe you are 
confused because nitrogen (in the same column) has only 3 valence lectrons 
(2p^3).  That is because atomic orbitals become more closely spaced in energy 
as a function of n (n=2 for a 2p orbital, etc).  Thus, although 2s and 2p are 
widely separated in energy, 4s and 4p are very close in energy.  So, if 4P 
electrons are available for bonding, it can be possible for 4s electrons to be 
available also.  This is a complex phenomenon, and you have asked an excellent 
question about it.  As an exercise, find out what the valence electrons are of 
some of the transition metals in the lanthanide series and see if you can 
rationalize the patterns.  Ask your teacher for help!
5) Does exothermic truly mean the reaction releases heat?  Could it also be 
light or is it always heat, then converted into light?  There are several ways 
to think of this, but I think of "exothermic" or "endothermic" as a 
classification of whether a reaction loses energy to its surroundings (exo) or 
absorbs it from its surroundings endo).  Then, one can imagine different 
mechanisms by which a reaction could absorb or emit energy, on a macroscopic 
scale.  One might be by atomic/molecular collisions, which we associate with 
heat transfer.  Another might be by emitting/absorbing photons, which could 
happen if, instead of doing the reaction in a beaker, we put it into a 
supersonic jet and fire a laser beam into the jet stream (believe it or not, 
this particular experiment is very common in physical chemistry).  In other 
words, whether an exothermic reaction gives off light or heat depends on 
several factors, including (1) the reaction conditions and (2) the types of 
molecules involved.
Topper
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