Lecture Notes from CHM 1341
9 July 1996
Lewis Structures Demystified
Phosphorus
When the tetrahedral P4 element reacts with oxygen, it makes
the tetrahedral P4O6 where the O atoms insert between each P in P4
and further oxidation caps each P with one more O to make P4O10.
It's easy to remember the structures since every tetrahedron has 4 corners (P atoms) and 6 edges (O atoms):
O
||
P P P
/|\ /|\ /|\
/ | \ O | O O | O
/ | \ / | \ / | \
P---|---P + 3 O -----> P--O|---P + 2 O -----> O = P--O|---P = O
\ | / 2 \ O / 2 \ O /
\ | / O | O O | O
\|/ \|/ \|/
P P P
||
O
Now we've drawn those edge oxygens as if their bond angles are 180 degrees, but you know they're bent; so the edges distort out (symmetrically).
Both of the oxides hydrolyze (react with water) to produce acids:
P4O6(s) + 6 H2O(l) -----> 4 H3PO3(aq) phosphorous acid
P4O10(s) + 6 H2O(l) -----> 4 H3PO4(aq) phosphoric acid
both very weak, but (predictably) phosphorous acid is weaker than phosphoric. A pedantic point: chemists routinely emphasize the syllable -phor- in pronouncing the acids rather than the syllable phos- which is emphasized in the pronuciation of the element.
The oxidations states of phos'-phor-ous aren't as numerous as those of chlorine or even sulfur. It is found in only:
Ox. #: [-III] [0] [III] [V]
e.g., P3- P4 PO33- PO43-
phosphide phosphorous phosphite phosphate
In keeping with the naming of the ammonium ion, the corresponding
PH4+ is the phosphonium ion.
However in contrast to the naming of ammonia itself, the corresponding PH3 is phosphine (not to be confused with "phosgene," Cl2CO, although probably as toxic!).
Where phosphate compounds shine for us obligate aerobes is in their ability to condense, that is to collapse many phosphoric acids (or phosphate anions) together to form a chain. They polymerize in this way by yanking a water molecule from the acidic moieties (reactive pieces) between them and satisfying the now unsatisfied valencies by making a phosphate-phosphate bond. Let's look at an example of condensing a pair of monohydrogenphosphates:
:O: H :O: :O: :O:
_.. || .. \ || .._ _.. || .. || .._
:O - P - O: + :O - P - O: -----> :O - P - O - P - O: + H2O
¨ | \ ¨ | ¨ ¨ | ¨ | ¨
:O: H :O:_ :O:_ :O:_
¨ [water to ¨ ¨ ¨
leave]
Now while it's true that the "condensation" of phosphoric acid (stick protons everywhere you see a negative charge above) it actually does get thicker, more viscous, that is not the meaning of a "condensation" reaction. It merely means molecules which have bonded together by elimination of some "good leaving group" like water! Indeed, there are lots of examples of such reactions; one we've seen before is the condensation of (monosaccarides, or single unit sugars) glucose and fructose to yield (the disaccaride) sucrose (common sugar) also by elimination of water:
CH2OH CH2OH CH2OH CH2OH
| | O | | O
C-----O | _/ \_ C-----O | _/ \_
H /| \ H | _/ \_ H H / \ H | _/ \_ H
|/ H \| |/ \| |/ \| |/ \|
C C + C C -----> C C C C + H O
|\ OH H / \ / \ H HO /| |\ OH H / \ / \ H HO /| 2
HO \| |/ O-H H-O \| |/ CH2OH HO \| |/ O \| |/ CH2OH
C-----C [water C-----C C-----C C-----C
| | to go] | | | | | |
H OH OH H H OH OH H
But in the case of the phosphate condensation, called "phosphorylation" in Biology, there's an energy price to be paid. Of course, you get the energy back if you reverse the process (dephosphorylate). And therein lies the real blessing our mitochondria bestowed upon us: they know how to use the energy of the oxidation of carbohydrates (glucose, for example) to add a phosphate to adenosine diphosphate (ADP) to make the higher energy adenosine triphosphate (ATP), harnassing the destructive potential of oxygen to generate the molecule (ATP) which powers the metabolism of all Life.
NH2
|
C adenosine triphosphate (ATP)
// \
N C---N
| || \\ O O O
| || C-H || || ||
| || / O O---P---O---P---O---P---O
H-C C---N _/ \_ / |_ |_ |_
\\ / \ _/ \_ CH O O O
N \ / \| 2
C C
|\ H H /|
H \| |/ H
C-----C
| |
OH OH
We've a small supply of ATP which is constantly being replenished by our mitochondria. It's a good thing that they never rest; that supply lasts only 3 seconds at the rate we use up energy!
Lewis Structures Revisited
The compounds we've been building of late violate the octet rule left and right. If we want to know what shape they are, we'd best revise the Lewis structure building rules to accommodate the 10 and 12 electrons we sometimes find around Period 3 and higher central bonding atoms as a result of the availability of their d shells for hybridization. Fortunately, we do not need to know how they hybridize exactly to determine these more complicated structures. VSEPR theory is still applicable as a guideline. What we do need to know is what to do with too many electrons for an octet, and the answer is "don't worry...make multiple bonds to the central atom."
For example, take chloric acid, HClO3. Cl is the central atom in a structure:
O - H
|
O - Cl - O but this is only the "sigma" bond skeleton of the molecule!
And it neglects most of the valence electrons which number 7(Cl)+18(O)+1(H)=26.
Those 4 sigma bonds use 8 of the 26 leaving 18 unassigned.
..
:O - H This structure uses all 18 electrons in such a way
.. | .. that every atom has at least an octet ('cept H's duet).
:O - Cl - O: But it has a weakness in that unnecessary ionization
¨ ¨ ¨ has been implied. If we subtract lone pair and HALF the
-1 +2 -1 sigma electrons from each atom's core charge we get the
formal charges listed below those atoms.
..
:O - H If instead we use the extra lone pair on each of the
.. | .. two formally charged oxygens to form a double bond to
O = Cl = O the central chlorine, all of the atomic formal charges
¨ ¨ ¨ are now zero. The closer we can get to zero, the better.
In either case, the central chlorine ends up AX3E for trigonal pyramidal.
(And, of course, the OH oxygen's AX2E2 implies its bonding is bent.)
It is interesting to note that Gillespie refuses to call either structure "wrong" when they are both simplistic approximations to the actual 3d electronic distributions. We will learn later in chemistry that the more such not-unreasonable Lewis structures one can imagine, the greater freedom is implied for the molecule's electrons. They can "explore" such distributions as a component of their actual state, and such "exploration" (constructive interference) permits their matter waves to relax into different domains. The consequent broadening of their wavelengths implies greater stability than if only one Lewis structure can be drawn.
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Chris Parr
University of Texas at Dallas
Programs in Chemistry, Room BE3.506
P.O. Box 830688 M/S BE2.6 (for snailmail)
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Last modified 11 July 1996.