Topic 5: Alkenes pt 3

Question 1

An intro to Reaction Mechanism (Step 1)

Answer 1

1. Curved arrows are drawn only from the electron-rich site to the electron-deficient site. NEVER draw the mechanism arrow pointing toward electrons.

Question 2

An intro to Reaction Mechanism (Step 2)

Answer 2

Step 2: Atoms that get attracted by the arrows are electrophile. The atoms that do the attack are called nucleophiles.

Electrophile- Electron-deficient ions or molecules, can accept a pair of electrons.
-Electrophiles are Lewis Acids.
-Carbocations and partially positive atoms.

Nucleophile: Electron rich species, can donate a pair of electrons.
-Lewis Bases
-Polarizable nucleophile= stronger nucleophile
Nucleophiles contain a pair of electrons to be shared

Question 3

An intro to Reaction Mechanism (Step 3)

Answer 3

Step 3: Double barbed (headed) arrows represent 2 electrons moving.

Question 4

An intro to Reaction Mechanism (Step 4)

Answer 4

Step 4: Single Barbed (head) arrows represent 1 electron moving.

Question 5

Mechanisms and Arrow Pushing

Answer 5

Curved Arrows= Show how electrons move as bonds are breaking and/or forming.

There are 4 main ways that electrons move in polar reactions:
1.Nucleophilic attack
2.Loss of a leaving group
3.Proton Transfer (Acid/Base)
4.Rearrangements

Question 6

Nucleophilic Attack

Answer 6

1. Nucleophilic attack- nucleophile attacking the electrophile.

* Read
>Tail starts on the electrons (w/ - charge)
>Head, ends on a nucleus (+ charge)
>Electrons end up sharing rather then transferring

Nucleophilic attack may require more than one curved arrow.
The second arrow could be thought of as a resonance structure!!

Tip 3:
Nucleophilic attack- Pi bonds can also act as nucleophiles.

Question 7

Loss of a Leaving Group

Answer 7

Loss of a leaving group- Heterolytic bond cleavage, an atoms or group takes the electron pair.

**It is common for more than one curved arrow be necessary to show the loss of leaving group.

Question 8

Proton Transfer

Answer 8

Proton Transfer: Recall (Acids & Bases Chapter that proton transfer requires 2 curved arrows.

The deprotonation of a compound is sometimes shown with just a single arrow and “-H+” above the run arrow.

Question 9

Rearrangements

Answer 9

Rearrangments- 2 types of carbocation rearrangement are common.

** Shift can only occur from an adjacent carbon.**
Shifts only occur if a more stable carbocation results.

Question 10

Combining Patterns of Arrow Pushing

Answer 10

Multistep reaction mechanism are simply a combination of one or more of the 4 patterns covered thus far.

**Often 2 of the patterns occur in a single mechanistic step

Question 11

Adding HX to an Alkene

Answer 11

** HX= Halogen Added**

Mechanism (a depiction of how the reaction proceeds, showing the movement of electrons using arrows)

Step 1- Proton Transfer
CH3CH=CHCH3 + H-X -> CH3C+H-CHCH3 + X

Because this molecule has a positively charged carbon it is somewhat unstable and therefore only exists transiently … which is called “intermediate”

Step 2: Nucleophile Attack
CH3C+H-CHCHE-> CH3CXH-CHCH3

Question 12

Mechanism Energy Diagram

Answer 12

CH3CH=CHCH3 + H–X -> [CH3C+H-CHCH3]-> CH3CxH-CHCH3

Intermediate C exist only transiently because its carbonation is very unstable.
The instability makes C higher in energy than A & E