(1) Structure of organic molecules
(a) connectivity and structural isomerism
(b) electron delocalization and drawing/evaluating resonance contributors
(c) conformational analysis (draw/evaluate chair forms and Newman projections)
(d) stereochemical analysis (E/Z alkenes; 1, 2 similar, or 2 dissimilar sterecenters)
(2) Prediction and explanation of Bronsted Acid-Base reactions (use of pKa table)
(3) Reactions of polar C-X sigma bonds (SNand E reactions & mechanisms)(4) Electrophilic addition reactions of C-C pi bonds (regioselectivity & mechanisms)(a) addition of H/eN (strong & weak Bronsted acids)
(b) addition of X/eN (halogenation; halogens & other nucleophiles)
(c) oxidation (mCPBA; OsO4; ozonolysis)
(d) reduction (hydrogenation with regular & poison catalyst; dissolving metal redn)
(5) EAS: Electrophilic Aromatic Substitution (regioselectivity & mechanisms)
Welcome back to part 2 of our organic chemistry I introduction crash course. Remember to follow the blog and comment if it was helpful to you! Let’s dive right in to the content.
Bronsted Acid Base Chemistry
Acid-Base chemistry is a continuous part of organic chemistry and it does not go away. There are many different ways that it is applied in different mechanisms moving forward. It is important to get a good grasp of the definitions and ways that acid base chemistry works. First, we will define some important terms.
Bronsted Acid: H+ donor (also read as proton donor)
Bronsted Base: H+ acceptor (also read as proton acceptor)
Conjugate Acid: The stronger the base the weaker the conjugate acid. The base had negative character as a proton acceptor and the conjugate acid will have a proton accepted.
Conjugate Base: The stronger the acid the weaker the conjugate base. The acid had positive character as a proton donor so the conjugate base will have a proton donated or lost.
Nucelophile: This is electron rich and has negative character. It will attack a + source.
Electrophile: This is electron deficient and has positive character. It wants electrons from a negative source.
Why is it important to use a PKA table and what is PKA?
Acids that are strong will have a lower pKa number. The smaller the pKa value the stronger the acid.
Bases that are strong will have a higher pKa number. The bigger the pKa value the stronger the base.
It is crucial to know how to use a pka table so as to determine which H+ should be deprotonated or what site should be protonated if there are multiple options available.
What do you do if you’re given the pH of a solution and you have pka to worry about?
The rule of 3 is an important rule to remember.
As an example let’s say you have an environment of a pH of 4.4 for the reaction and two pka for each proton as 1.25 and 4.34 respectively.
1.25+3= 4.25. The pH here is 4.4 and it is more than 3 above the pKa therefore we will deprotonate the H+ that has the 1.25 pka.
There is a partial deprotonation in this case for the 4.34 pka H+ but because of the 3 rule we cannot fully deprotonate it given these conditions.
We also wouldn’t want the second H+ to deprotonate any way because then there will be some charge repulsion within the molecule as both regions would gain negative character.
The 1.25 pKa proton would also be the one to come off if we didn’t know the conditions because it is the more acidic proton. The acidity of the proton is related to the stability of the conjugate base.
As you can see while pKa is the important consideration here, you’ll also have to grapple with other factors and concepts relating to acid-base reactions. It is important to know acid-base reactions very well.
Examples of very common Strong Acids and Strong Bases
Mechanism of Strong Acid with Alkenes
For an acid to be an effective enough electrophile for an alkene, the acid must be a strong one. The strong acids are the haloacids (HCl, HBr, HI) and peroxyacids (H2SO4, HNO3, HClO4). Strong acids have low pKas and have a proton (H+) and thus positive character. This is a 2-step mechanism
Mechanism of Weak Acids with Alkenes
Another common mechanism includes using a weak acid and thus a weak electrophile. This is usually not as efficient a mechanism as substitution by a strong electrophile.