4. Alkenes are nucleophiles and like to attack electrophiles, like protons, with positive charges:

Transcription

1 Mechanism Steps: Answer Key 1. Alcohols are nucleophilic and like to grab protons,, to form water molecules: Alkenes are nucleophiles and like to attack electrophiles, like protons, with positive charges: If the proton is part of X, then two arrows are required. The second arrow shows the bond breaking between and X, with the electrons moving to electronegative halide 8.

2 If water forms on a molecule, water can spontaneously dissociate with a single movement of electrons: The nucleophilic alkene causes X 2 to polarize and then attacks the partial positively charged halide. The second arrow is required to show the pair of electrons BETWEEN the halides moving towards the anionic end to form X -.!!- 14.!!- 15.!!- 16. Anions, like halide anions, always attack carbocations

3 In the hydration mechanism, WATER is the nucleophile and attacks the electronpoor carbocation with the oxygen lone pairs: alonium ions can be popped open on either side. With no stereochemistry, the halide anion could attack either carbon of the halonium ion. Note that both products here are the same, regardless of the direction the lines are drawn. or

4 23. Again, it does not matter, when no stereochemistry is shown, which end of the halonium ion is attacked. And again, both products shown here are exactly the same the direction the lines are drawn do not matter. or 24. With stereochemistry, the opening of the halonium ion is a bit more guided. Knowing the halonium is on the bottom face of the molecule, the halide attacking it must attack from the top face. Let the product guide you! If the product shows the topface halide in a particular position, you know which end of the halide was attacked: Water always attacks a halonium ion on the more substituted end: The water is again attacking the more substituted side. With stereochemistry, the water is attacking from the opposite face of the halonium ion. The C- bond maintains the stereochemistry of the halonium ion, while the water molecule is attached anti :

5 29. Alcohols can attack, just like water: C 3 C Sometimes to finish the formation of an alcohol, you just need to remove an extra proton, like the one on the water molecule attached to these intermediate compounds. When the proton transfer occurs, regardless of the species removing the proton, two curved arrows are always required. You must show a second arrow, moving the bonding electrons back to the more electronegative oxygen atom: Sometimes you need to remove a proton to complete the formation of a double bond. This is also a proton transfer but this proton transfer is sometimes the hardest to see.

6 In these two examples, the hydrogen that needs to be transferred has been made obvious. The electrons from the C- bond will become the pi bond of the new alkene: In the dehydration mechanism you ll have to FIND the proton that s being removed. Use the structure of the product to guide you - find the product alkene carbons in the intermediate. ne carbon of the alkene is the carbocation carbon atom (what we called the alpha carbon, in lecture). The second carbon (the beta carbon, in lecture) will have the proton that needs to be removed, and will be designated by the alkene itself (to the left, right, above, below, etc relative to the alpha carbon). Draw the necessary C- bond on the beta carbon, and provide the curved arrows. alpha carbon alpha carbon beta carbon beta carbon 37.

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