I wish I had a smaller pKa

My name’s C2H6O and I’m a

weak acid you know …

(very!)

Weak Acids don’t dissociate very much in solution.

(a.k.a. ethanol)

I can’t donate H+ to a Brønsted

base

Don’t be so tight! That

vinegar didn’t mind

N

A Brønsted-Lowry base (like ammonia) acts as a proton

acceptor.

An ion’s stabilised, if

charge is delocalised

When a proton leaves acetic acid, the negative charge on its conjugate base is spread over 3 atoms. This can’t happen with

ethanol.

The ethanoate anion is stabilised by distributing its negative charge over more than one atom. It could be represented by the diagram on the right.

Negative charge distributed over 3 atoms

HA [H+] + [A-]

Protons leave with ease and so there is a decrease in

pKaaaaaaaaaaaaaaaa

If the conjugate base is stable, the equilibrium shifts right, increasing Ka and reducing pKa.

ppKKaa = -log = -log1010KKaa

-log10(x)

Ka = [H+] [A-]

[HA]

And I wish I had a smaller

pKa Cos I just can’t seem to

give my proton

awaaaayy

The anion formed from dissociation is very unstable because the charge is localised on one atom (the oxygen)

I wish I had a smaller pKa

I wish I was a stronger acid

Strong acids have low pKas. The pKa of ethanol is about 16, the pKa of acetic acid is about 5 so it’s 100,000,000,000 times stronger than

ethanol but even acetic acid is still a weak acid!

Through a -bonded

framework …

bond between carbon atoms

bond between C and H atoms

bond between carbon atoms

bond between carbon atoms

bond between C and H atoms

bond between carbon atoms

Mesomeric effects work when double

and single bonds

alternate there

This is sometimes known as a mesomeric effect

The ethanoate anion is stabilised by distributing its negative charge over more than one atom. It could be represented by the diagram on the right.

Negative charge distributed over 3 atoms

So canonical structures, form when resonance

occurs

These are two different ways of drawing the same structure we call them resonance structures or sometimes canonical structures

And where there’s

conjugation there’s

delocalisation of lone pairs

Conjugation is another way of saying “alternating double and single bonds” – the electrons are evenly spread across all the bonds in an

extended orbital

So I wish I had a smaller

pKaCos I just

can’t seem to give my proton

awaaaayy

The anion formed from dissociation is very unstable because the charge is localised on one atom (the oxygen)

I wish I had a smaller pKa

I wish I was a stronger acid

Strong acids have low pKas. The pKa of ethanol is about 16, the pKa of acetic acid is about 5 so it’s 100,000,000,000 times stronger than

ethanol but even acetic acid is still a weak acid!

My name’s C2H6O I’m not as

strong as H2O

Even water is better at dissociating – it has a pKa of

15.7

I can’t donate H+ to a Brønsted

baseNot YOU

again

A Brønsted-Lowry base (like ammonia) acts as a proton

acceptor.

N

But when systems can be found to

move electrons around

pKa of 11

Then their Ka is large because their negative

charge is spread

(hooray!)Large Ka means small pKa

And I wish I had a smaller

pKaCos I just

can’t seem to give my proton

awaaaayy

The anion formed from dissociation is very unstable because the charge is localised on one atom (the oxygen)

I wish I had a smaller pKa

I wish I was a stronger acid

Strong acids have low pKas. The pKa of ethanol is about 16, the pKa of acetic acid is about 5 so it’s 100,000,000,000 times stronger than

ethanol but even acetic acid is still a weak acid!

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Words: Aimee HartnellMusic: Wizzard