Another biochemical question - (Dec/23/2008 )
This question is also excerpted from Deborah Goldberg's AP Biology:
"4. Which of the following is NOT a base?
© Mg(OH)2 [2 is the subscript to OH, but I can't type subscripts here]
Many lines, again, are skipped so that the reader can formulate his or her answer...
This again, is not a homework problem and is merely a question that I am using to study for a big test coming up soon.
Solution: "(d) This is ethyl alcohol and is not a base. The other compounds contain hydroxide (OH-) [Note: in the OH-, the minus is a superscript] and are all bases."
The progress that I made: Literally none. Even after reading the solution many times, I still have no clue. I mean, if you somehow memorized that C2H5OH was ethyl alcohol then you would probably get this question right. But I don't think that the author of this book wanted the reader to memorize a whole bunch of molecular formulas (or are they structural formulas?). So I thought that maybe there was some sort of way of just looking at the molecular formula and somehow deducing that it was not a base. I looked through the chapter, but I could not find it. So I was wondering if someone could tell me how to, just by looking at the molecular formula, somehow deduce that it is, or not, a base. Or in other words, if one did not memorize that C2H5OH was not a base, then how would one answer the question (correctly)????????? My other question is this: The solution, at least to me, makes it sound so that if it's an alcohol, then it's not a base. Is that proposition correct? I did do some research, but the definitions of alcohol (at least the ones that I have looked at) don't seem to mention anything about it being (or not) a base.
Meanwhile the oxygen atom has lone pairs of nonbonded electrons that render it weakly basic in the presence of strong acids such as sulfuric acid.
Thanks a lot!!!!!!!
However, I still don't really get it
So someone explain it to me?
If it fits the form CnH(2n+1)OH, it's an alcohol. Thus, in ethyl alcohol n is 2 (because it's C2), so we're looking for H = 2 * 2 + 1 = 5 and OH, which is what you've got...
So does that fact that C2H5OH is an alcohol imply that it must not be a base???????????
Anyway, if you didn't know the CnH(2n+1)OH form, then how would you answer this question???????
Thanks in advance.
i think, you should read more advance chemistry books. You have to read the definition of an acid or base.
And I guess you have to define what kind of acid-base definition you are using
Arrhenius definition (very very old and disused)
acid - react with water to form hydronium (H3O+) ions
base - reacts with water to form hydroxide (OH- ) ions
acid - proton donor
base - proton acceptor
or the more modern Lewis definition
acid is a electron pair acceptor
base is a electron pair donor
For the answer given, it appears to use the old Arrhenius definition.
A base is a compound that react with water to form a hydroxide ions.
All the compound aside from C2H5OH are ionic compounds. Due to the large dissimilarity in electronegativity between the atoms involve in the compound, the electron pair is localised primarily to the most electro negative atom (oxygen). The atoms in an ionic compound are thus held together almost solely by electrostratic force. Upon reaction with water, the water forms a water cage due to the electrostatic force, which tears apart the molecule. This leads to the formation of OH- ions. And by Arrhenius definition are bases.
In C2H5OH, the electro negativity between carbon and oxygen are not so dissimilar. Although the electron pair spends a bit of time around the oxygen (given the oxygen atom a slight negative change), the electron pair spends sufficient time between the nucleii of the carbon and oxygen. This form a covalent bond. The covalent bond is quite strong and isn't broken up by electrostatic attraction by water. Since no OH- ions are form upon reaction with water, by Arrhenius definition, C2H5OH is not a base.
However, chemistry has move on in the 130 years since Arrhenius definition of acid and base was first used. We now realize that water can also act as a base or an acid. Water doesn't just sit there. And chemist start to wonder which acid is the strongest? And so we have better and newer definitions.
As the wiki article has noted we can react EtOH with strong base. Ethanol can donate the H+ ion and produce C2H5O- (ethoxide). Similarly ethanol can react with sulphuric acid and become protonated (oxygen as a two lone pair electrons that can accept H+). So by the Brønsted–Lowry definition and Lewis definition, ethanol can be either a base or an acid depending on circumstance. By the Brønsted–Lowry definition ethanol can either donate or accept proton depending on circumstance. (Similarly Lewis definition, for electron pair.)
You will find that, a substance is an acid or a base depends first by its chemical structure and secondly in comparison to what. And I guess in school... depending on the definition that the teacher is using.
PS: To be cheeky... BaOH is not a base. BaOH is not a real compound. It can not be synthesised. As BaOH is not real, it can't be a base. However Ba(OH)2 is real and is a base.
Very interesting insight, perneseblue. Thank-you
So, clarifying, just by looking at C2H5OH, would I be able to deduce all of that (very interesting) information? If so, can you give me a brief outline of how I would do that??????
Thanks once again.
yes you can.
1 - you need to know the definition of an acid and base.
2 - you need to have a feeling of how electronegative the first 20 atoms are relative to each other.
3 - you need to know how the structure of the molecule.
C2H5OH - gives you the structure. And from prior knowledge you know that atoms which composed this compound have similar electronegativity and thus ethanol is not an ionic compound but composed of covalent bonds.
And then you need to know what is the definition of acid-base in use. Then see if you can move electron pair (which follow electronegativity and valence shells) and hydrogen ions around to cause the formation of an acid or base.