How does Reaction Mechanism in more detail?

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How does Reaction Mechanism in more detail?

Postby JaeHoCho1B » Sat Feb 25, 2017 2:36 pm

Hello! Could anyone explain to me on pg 84 of the Course Reader on how the reaction mechanism works and how to approach them? Thank you so much for your help!

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Re: How does Reaction Mechanism in more detail?

Postby Natalie_Boyd_1C » Sun Feb 26, 2017 4:54 pm

I'm just going to go over this page generally and if you have any more questions I'll try to answer them as best as I can.

Things to know:
1. A straight arrow indicates a reaction, while a curved arrow indicates the movement of electrons
a. Where the arrow starts is the source of the electrons, while where it ends is where the electrons go.
2. Because an electrophile loves electrons, it accepts an electron pair in order to bind to a nucleophile. It is any electron-deficient species. It is abbreviated E.
a. Molecules with a polarized bond can act as electrophiles.
3. A nucleophile donates electrons to an electrophile. It is any electron rich species, and is abbreviated Nu.
a. Double and triple bonds are electron rich regions (pi bonds), so they are nucleophiles.
4. Electrophiles and nucleophiles are electrostatically attracted to one another.

Now let's look at the reaction mechanism on pg 84 (I can't draw curved arrows so please refer to the course reader, I'll explain what you're seeing).

Step 1: The pi electrons in the double bond of the CH3CH=CHCH3 molecule are attracted to the positively charged H on the H-Br molecule (the H has a partial positive because the Br is more electronegative), so the CH3CH=CHCH3 molecule is the Nu and the H-Br is the E. The arrow starts at the double bond and points to the H atom to show this.

Step 2: Now the Br- is negatively charged (because it is an anion: Br has 7 valence electrons, not 8), so it is attracted to the positively charged C atom (because it is missing an electron pair that it donated also if you expanded the molecule you can see this easier
H- C - C - C - C -H
H + H H
Overall reaction: CH3CH=CHCH3 + H-Br --> CH3CH--CHCH3
Br H

This is called an electrophilic addition reaction because the H+ (E) is being added to the CH3CH=CHCH3 molecule. An electrophilic addition reaction happens when an electrophile is added to another molecule.

The next example on this page shows another electrophile addition reaction across a double bond, where the double bond on one of the bottom rings is broken when the pi electrons are donated to the H electrophile which is added onto the cholesterol molecule.

Hope this helps.

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