Elimination (E1) with 1,2-alkyl shiftby James

Description: When secondary (or primary) carbocations are formed adjacent to a quaternary carbon, 1,2-alkyl shifts can occur.

Notes: The acids in these reactions is often sulfuric acid (H2SO4) or tosic acid (TsOH) because the conjugate base of these acids are very poor nucleophiles.

Examples:

Notes: In the first and second cases a methyl group shifts over by one carbon (in the second example, silver nitrate  AgNO3 (-) is a Lewis acid that makes halides such as Br better leaving groups).

In the third example the cyclobutane ring expands to give the five-membered ring, which has less ring strain.

In the fourth example the ring expands to form a seven-membered ring from a six-membered ring (recall that primary carbocations are unstable relative to secondary carbocations).

Mechanism: Formation of a carbocation is achieved here through protonation of the alcohol with strong acid (Step 1, arrows A and B) followed by loss of water to give the carbocation (Step 2, arrow C) . Next, migration of a methyl group from the adjacent quaternary carbon through a transition state such as that pictured (Step 3, arrow D) gives a tertiary carbocation which undergoes deprotonation (Step 4, arrows E and F) to form the most substituted alkene (between C-2 and C-3)

Oxidation of aromatic alkanes with KMnO4 to give carboxylic acidsby James

Description: Treatment of an alkylbenzene with potassium permanganate results in oxidation to give the benzoic acid.

Notes: The position directly adjacent to an aromatic group is called the “benzylic” position.

The reaction only works if there is a hydrogen attached to the carbon.

Examples:

Notes: Note that in example 2 the extra carbons are cleaved to give the same product as in example 1. And in example 3, two benzoic acids are formed. Finally, when no hydrogens are present on the benzylic carbon, no reaction occurs (example 4).

Mechanism: For the purposes of Org 1/ Org 2 the mechanism isn’t considered that important. Manganese acts in mysterious ways. It’s thought that the first step is removal of a hydrogen by one of the oxygens on MnO4(–) in a free-radical reaction. Beyond that, it gets complicated.

Addition of NaBH4 to aldehydes to give primary alcoholsby James

Description:  Addition of sodium borohydride (NaBH4) to aldehydes gives primary alcohols (after adding acid)

Examples:

Notes: Lots of different acids can be used in the last step. It’s not important which specific acid is used, just that something is present that can form the alcohol.

Mechanism:

NaBH4 is a source of hydride (H-) and the reaction begins with the addition of hydride to the carbonyl to the aldehyde (Step 1, arrows A and B). Upon addition of acid, the oxygen is protonated (Step 2, arrows C and D) to give the neutral primary alcohol.

Notes: The choice of H2O / H2SO4 as acid isn’t crucial – this is just an example. Any source of proton (including water) will do.