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Name: Caren
Status: student
Age: 17
Location: N/A
Country: N/A
Date: 8/22/2004


Question:
What atomic/molecular structure (bonding) gives acetone its desired properties?


Replies:
I am not sure what you mean by desired properties. So what I will do is describe the molecular structure of acetone and relate that to the most common usage of acetone.


Replies:
For those who are not familiar with the molecular geometry and chemical structure of acetone, imagine a threebladed airplane propeller. At the center of this propeller there is a carbon atom. The three blades of the propeller are the bonds from the carbon atom. At the tip of one of the blades we find an oxygen atom. At the other two blades tips we find a CH3 group (for our purposes, we can just imagine these as carbon atoms as well).

Since two of the propeller blades are made up of carbontocarbon bonds (a carbon is at the tip of the blade and bonded to the central carbon), these two blades are essentially nonpolar. The third blade, however, is representative of a carbontooxygen bond (it is actually a doublebond, but that is not important right now). We know that oxygen is far more electronegative than carbon, that is, oxygen is better at attracting the pairs of electrons that form bonds between the central carbon and the oxygen. We can therefore imagine that the electrons spend a lot more time near the oxygen than near the carbon.

Since electrons are negatively charged, the oxygen becomes partially negative relative to the central carbon. We say that bond is polar, and, since there is no other polar bond within the acetone structure that could cancel out this polarity, we say that the molecule has a netdipole. Just like a magnet, it can be very good at attracting other molecules with a netdipole.

A lot of the chemical and physical properties of acetone can be traced back to the presence of this netdipole within its geometry. For example, acetone has a higher boiling point than other molecules with similar masses to acetone but which do not have the netdipole. It dissolves very well in water because water also acts like a polar molecule. It is a good solvent for a wide range of organic compounds, because most of these compounds have some polarity as well.

The chemical reactions of acetone are also dependent on this carbontooxygen bond. Most compounds that react with acetone will either have a positive functionality that tends to be attracted to the negative site found in the oxygen, or a negative functionality that tends to be attracted to the positive site found in the carbon.

I hope that helps.
R. Gregorious


I found searching Google for "flame temperatures" I found a nice little table:

http://www.engineeringtoolbox.com/9_422.html .

They are more like 1900C for fuel pre-mixed with air. Pre-mixed is what a Bunsen burner does, and your gas stove, too.

There is not really very much variation, they are all around1950C. Hydrogen about 2050C.

Acetylene with 2400C is exceptional. (Isn't an alkane, admittedly.) I have not yet guessed how you got your 1300C number.

More scatter, but lots of references, at:

http://hypertextbook.com/facts/1998/JamesDanyluk.shtml

It is likely a diffusion-flame can have a lower temperature than a pre-mixed flame. But it would not really be a constant of the gas, it would be an effect of a particular geometry of flame, loosing heat in all directions about as fast as it can make that heat. A larger diffusion flame, perhaps partly occupied by hot crumbs of rock to obstruct heat-radiation, would burn hotter inside. Eventually you have re-invented some ancient-style pottery furnace, and it might approach the temperature of a pre-mixed flame.

The alternatives are "pre-mixed" or "diffusion-flame" (in addition to choosing between air and pure oxygen, of course).

A diffusion flame is like a candle flame. Fuel gas on the inside, air on the outside, diffusing through each other as fast as they can so they can react. Not really very fast. A diffusion flame "drifts" upwards by gravity/convection.

A pre-mixed flame is a jet of dangerously mixed cool gas going one way, and a flame front racing the other way (upstream) as fast as it can. There is a narrow point in the nozzle where the gas flows faster than the flame can propagate, then the flow spreads out and slows down. That is why the flame front can stand still before your eyes. It is the round bottom of the little blue flame in your gas stove top burner. The gasses burn near-instantly as they cross the "flame front". Suddenly the gasses reach their ideal maximum temperature, then they start cooling down as they flow away. You therefore have a small place where the torch-flame has not yet cooled down by any significant amount.

If the distinction is not clear to you, think about the three components required for fire - fuel, oxidizer, and heat. If you mix fuel and oxidizer first, then add heat, that is pre-mixed. If you heat fuel and/or air first, then mix, it will be a diffusion-flame. When the fuel and oxidizer enter the hot zone by different paths, it must be a diffusion-flame.

hope that helps-

Jim Swenson



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