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Name: Nicole
Status: student
Age: 9-12
Location: TX
Country: N/A
Date: 1/3/2006


Question:
I am researching CH3COOH (acetic acid), and my question is why do acid's have the extra OH at the end? Why is that extra OH not add in to the formula? Basically I wish to know why it is not C2H4O2 instead of CH3COOH?


Replies:
Both notations are correct but the notation that has the group 'COOH' is used to make it obvious that you are dealing with an acid. All organic acids have this grouping - an Oxygen atom double-bonded to Carbon and a Hydroxy group bonded to the same carbon. The Hydrogen atom in this group is readily available for acid chemistry.

Greg Bradburn


Nicole,

Writing the chemical formula (called a compressed chemical structure) makes the formula easier to read and allows the writer to be very specific about which compound the writer is talking about. Think of the empirical formula for acetic acid: C2H4O2 - using that formula we can write many different -and correct- chemical structures: (1) O=CH-CH2-OH, (2) HO-CH=CH-OH, (3) CH3-O-CH=O, (4) CH3-COOH - all of which are acceptable Lewis Structures. So, if we want to be specific, we need to state that we are talking about acetic acid or the compound we the arrangement: CH3COOH.

Greg (Roberto Gregorius)


The formula C2H4O2 only gives the molar amounts of the elements C, H, and O. The notation CH3COOH attempts to describe how the atoms are connected in more detail, specifically: CH3--C//O--OH which does not translate very well in the fonts available. Talking through the structure, a CH3-- (methyl group) is connected to a carbon atom: CH3--C. The second carbon atom, on the right is doubly bonded to an oxygen atom and to a hydroxyl group
             O
            //
CH3--C
             \
              OH.
              

The "H" connected to the "O" ionizes in water to form the ionized form of acetic acid. This provides more information regarding the molecular structure of acetic acid. It is difficult to draw the structure given the font limitations. The "H" 's bonded to the carbon atom in the fragment "CH3--" are covalently bonded and do not ionize.

Vince Calder



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