Question about acid structure and strength

[Zellmer, Robert]

A fellow student sent a question about acid strength based on structure and the strength of the conj. bases.

"I thought that when looking at the differences between acid strength in a group (especially in halides), bond
strength was the most important factor, not electronegativity. Thus, HBrO would be a stronger acid (weaker
bonds) than HClO. I also thought this because my TA taught me that acid strength increases as you go down
a group (with HI being the strongest acid), so wouldn't HBrO be a stronger acid than HClO, and ClO- would be
the strongest base?"

This is conflating two different things about acid structure and strength.  Also, this is taking something which
applies to binary acids and applying it to ternary acids (oxyacids).

A binary acid has a H atom and one other atom, and the acidic H atom is attached to the other atom.  The
acidity of a binary acid depends on two things, the polarity of the H-X bond and it's bond strength.  Within
a group (like HF, HCl, HBr and HI) the bond strength is the primary factor.  Going down the group the H-X
bond gets longer and weaker and is more easily broken (takes less energy to break the bond) so the H atom
comes off more easily as an H+.  The acid strength inc. going down the group.  You can also think about this
in terms of the conj. bases.  As the X- gets larger it allows the negative charge to spread out.  When it spreads
out it's not as available to react with an H+.  That makes the X- a weaker base and thus its conj. acid is
stronger.  So, in a column the bond strength is the more important factor.

Within a row the size of the X atoms doesn't change much.  X gets smaller going from left to right in a row
but the size doesn't change much, certainly not like it does going down a column.  However, electroneg.
of X changes a lot.  You can see the EN of the atoms in Table in Fig 8.8.  The EN changes a lot as you got
from left to right in a row, especially in row 2, ending with F with an EN of 4.0.  This really changes the bond
polarity, which increases a lot as you go from left to right in a row.  The bond strength doesn't inc. much
going left to right as it gets shorter because the size of the X doesn't change much.   So, in a row the polarity
of the bond is the more important factor.  The polarity of the H-X bond inc. going left to right so acid strength
inc. from left to right in a row.  So, in a row the EN is the more important factor.

Thus, for BINARY acids ONLY, there is a periodic trend:

            Acid strength inc. left to right and top to bottom.  I have this in the notes.

*************************

For oxyacids (ternary acids) you're considering the acidity of an OH bond with the O connected to another
atom (such as in H2SO4, HNO3, HClO3, etc.).  There are two things you need to consider; both depend
on the polarity of the OH bond and what affects it.

1)  Same structure, different central atom.

            Examples:  HIO2 < HBrO2 < HClO2
                               H2TeO4 < H2SeO4 < H2SO4

Notice for these examples there's the same number of H and O atoms but the central atom is different.
Note also, the central atoms are from the same group.  This is why they have the same structure.

For these the only difference is the central atom.  The central atom draws electron density away from the
OH bonds.  The more it can draw density away the more polar the OH bond and the easier it loses the H
atom as an H+, stronger acid.  This depends on the electronegativity of the central atom.  The more
electroneg. the central atom the more polar it makes the OH bond and the stronger the acid.  Thus, the
acid strength inc. going UP the group, opposite the behavior of binary acids.

2) Same central atom but a different # of O atoms.

            Examples:  HClO < HClO2 < HClO3 < HClO4

The only difference here is the # of O atoms on the central atom.  The more O atoms you have
(technically O atoms w/o a H atom) the more electron density is pulled away from the OH groups on
the central atom.  So, more O atoms the stronger the acid.

You can NOT compare binary acids to oxyacids based on structure.

*************************

Then there's carboxylic acids.  You can't compare them to the above based on structure.

These have an -COOH (-CO2H) group,

           OH
            |
     X - C = O

The X can be a H atom (formic or methanoic acid) or a carbon group.  We often use acetic acid
(ethanoic acid), CH3-CO2H, as a weak acid example.  Vinegar is about a 5% acetic acid solution.
This formula is often written as HC2H3O2, with the acidic H written to the left even though it's on
an O atom (like in H2SO4, HNO3, etc.).

The H atom of the OH bond is acidic in carboxylic acids (Ka values on the order of 10^-3 to about
10^-6 for many of them).  The OH bond in an alcohol is not acidic, like CH3CH2OH (Ka of about
10^-16), although phenols are acidic (Ka of about 10^10), as explained in lecture and a previous
e-mail.  The reason the OH is acidic in carboxylic acids can be explained in two ways:

            1) The C=O functional group (carbonyl group) is a very polar group and draws electron
                density away from the OH bond making it more polar and making the H atom more
                acidic (easier for the OH bond to break and the H to leave as an H+).

            2) The conj. base, X-CO2^- is resonance stabilized.  Two resonance structures can be
                 drawn for the -CO2^- group.  This spreads the neg charge out over the three atoms
                 in the -CO2^- group.  That means the charge is not as readily available to react with
                 an H+.  That means it's a weaker base and thus its conj. acid is stronger.  The
                 conj base of an alcohol, like CH3CH2O- for ethanol, can not spread out the neg. chg.
                 The conj. bases of alcohols are strong bases (alcohols do not act as acids in water,
                  the soln remains neutral, so their conj. bases are strong bases).

            3) Also, any electron withdrawing groups on an alpha carbon can inc. the acidity of the
                H atom of the OH bond in the CO2H group.  The alpha carbon is the C atom attached
                to the C=O carbon atom (carbonyl carbon atom).  In acetic acid,CH3CO2H, the alpha
                C is the CH3 carbon atom.  Any electron withdrawing groups on the alpha C atom
                pull electron density away from the OH group making it a stronger acid.  The more
                EN the electron withdrawing group the stronger the acid. The more electron
                withdrawing groups on the alpha carbon the stronger the acid.

I would suggest if you're still struggling with this you watch the pre-recorded lecture.

I hope this helps with this.

Dr. Zellmer
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