addition of alkenes, chapter 8, exam 4

what is addition

opposite of elimination

The broken pi bond can then act as a nucleophile or a base

what is the bond acting as a base

bond breaking creates a hydrogen proton, H, added on the left side

what is the bond acting as nucleophile

Bond breaking creates an electrophile(typically has + charge, E, added on the left side

Addition and elimination rxns are in equilibrium, what determines which side the equilibrium will favor?

temperature

...........

Low temperature favors Addition

High temperature favors Elimination

…….Why?

entropy, addition decreases entropy(two things, one product), elimination increases entropy (1 reactant, two products)

At high temps reactions favor high entropy, so elimination…at low temperatures entropy isn’t as important, so addition

What must we consider for each reaction?

Regiochemistry- which carbon gets "x" and which gets "Y"?

Markovnikov (the more substituted carbon gets the bigger group (like OH or X) (The less substituted carbon gets the H) 

Why?

because a carbocation forms…and the carbocations rearrange to become more stable

vs anti-Markovnikov (The less substituted carbon gets the OH or X) (The more substituted carbon gets the H)

Stereochemistry- if we form a chiral center, what is the stereochemical outcome?

racemic (forms a carbocation)

vs syn (do they add from the same side?) (reaction is concerted (ex: BH3))

vs anti(do they add from opposite sides?) (the rxn forms a three membered ring)

vs inversion

Mechanism- do "X" and "Y" approach the pi bond from the same side or different side (syn vs anti)? What are the intermediates in the rxn?

Carbocation Mechanism

Markovnikov, possible rearrangements, racemic mixture

3 membered Ring mechanism

Hydrohalogenation

-Reaction results in the addition of the H-X to the alkene (HCl, HBr, or HI).

-Check for rearrangement in the second step (carbocation intermediate)