What factors stabilize a C=C? 1. Bond strength – increased number of sigma bonds formed by sp2 hybrid orbitals. Sp2- sp3 is stronger and more stable that sp3-sp3 (more s character in sp2 orbitals = better overlap and stronger bonds) 2 3 one sp2-sp3 bond two sp -sp bonds

2. Increasing electron density in the area of the alkene = more stable double bond system. There are actually empty spaces around an alkene, in the form of antibonding orbitals (high energy – originally left unfilled!) Hyperconjugation is just filling in the spaces (gaps, holes, however you want to think of it) with electron density from the alkyl group(s) electron cloud(s). The more alkyl groups, the more electron density around the alkene, the stronger and more stable the alkene!

Technical definition: Hyperconjugation – “movement of electron density thru space from a filled sigma bonding molecular orbital of an alkyl group into the empty antibonding pi* molecular orbital of the C=C”. π*

H C C

H H

Electrophilic Addition Reactions – Addition of E+ to C=C via a polar type mechanism

H H-Br H Br Br-

H-Br Br Br-

Consider what must occur: Step One:

H C H H 3 H-Br H3C H

H3C H H3C H 2 sp2 sp3 sp

Pi bond is nucleophilic (e- rich). Proton (H+) of HBr is electrophilic (e- poor). Pi electrons reach out and form a new sigma bond to the proton, from one of the carbon atoms of the C=C, releasing the Br-. “Carbocation intermediate” forms.

Step Two:

H C H H H C Br H H 3 Br- 3 sp3 H3C H H3C H The Br- then donates a lone pair of electrons to the positively charged carbon atom of the intermediate, forming a new C-Br bond.

Energy Diagram:

y I

g

r

e

n

E

R P

reaction coordinate

The Intermediate: The Carbocation

• Three atoms attached to carbon (no lone pair) • Sp2 hybridized C atom planar, 120º bond angle • Perpendicular p orbital – empty! (Lewis Acid, E+)

Order of Stability for carbocations:

R R > H H R >>>>> H > CH3 R R R 2º 3º 1º

What stabilizes a carbocation? Carbocations are carboCATIONS because they are missing an electron so stability happens when electron density is added to the system. Alkyl groups are quite good at adding their electron density (what we refer to as Electron-Donating Groups, or EDG).

1. Hyperconjugation: movement of electron density thru space from a filled sigma bonding molecular orbital of an alkyl group to the empty p orbital of the carbocation. H R

R H H

Surrounding an electron-deficient carbon atom with electron density stabilizes the positive charge. 2. Inductive Effect: movement of e- density thru bonds Electrons in C-C and C-H bonds shift towards the carbon with the positive charge, delocalizing the positive charge, spreading it out over multiple atoms.

R

R R

Alkyl groups can shift e- density better than solitary H atoms (which are not polarizable – their electron clouds are not big enough to “shift”). The electrophilic addition reaction of HX:

H-Br

Br

H-Cl Cl

KI, H3PO4 I

Note that in each of the prior cases, only one product forms. In this reaction, the product that forms is very specific:

Br H-Br not Br

These are called regiospecific or regioselective reactions, which are reactions that could potentially produce two or more products but actually favor formation of only one specific product.

Markovnikov’s Rule: In an electrophilic addition reaction to an alkene, the reaction will always proceed to produce the most stable carbocation intermediate.

What does this mean? The carbocation always forms on the more substituted end of the alkene. More substituted end = more substituted carbocation = more stable carbocation. As a result, the H attaches to the end with the fewer alkyl groups and the X attaches to the end with more alkyl groups. Ex.

H-Cl Cl H

If the alkene is equally substituted and asymmetrical, a mixture of two different products occurs, which is generally considered a bad reaction – the products would have to be separated before either could be used in another reaction. H H-Cl Cl +

Cl H H

+

H 2º carbocation 2º carbocation equal in energy, equally able to form results in a mixture of both occurring in the reaction, and thus two products.

Why does the reaction favor the more substituted carbocation? The carbocation is more stable, thus the carbocation is lower in energy, has a lower activation barrier and thus allows for a faster reaction.

H-Br

KI, H3PO4

H-Cl

H-Br