2
Electrophilic Addition
Key definitions:
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Electrophile =A position on a molecule with a partial positive charge that will be attracted to a negatively charged region and react by accepting a pair of electrons to form a covalent bond.
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Carbocations = ion with a positively charged carbon atom.
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Reagent = substance or compound that is added to the reactants to cause a chemical reaction to occur.
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Condition = specific factors that we can change in a chemical reaction. i.e., Temperature, pressure, etc.
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Cations = positively charged ions
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Anions = negatively charged ions
What is Electrophilic addition?
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Electrophilic addition shows us the movement of electrons witha curly arrow. (In industry, these can be very useful when predicting how certain chemicals will behave and act.) Alkenes usually take part in addition reactions to form saturated compounds.
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Types of electrophilic addition reactions and conditions:
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For OCR B chemistry, you need to be aware of 4 different addition reactions and their conditions:
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Addition reaction with H-H bond
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Addition reaction with Water
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Addition reaction with Halide e.g., H-Br, H-I
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Halogen e.g Br
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Study tip:
Curly arrows are used in the reaction mechanism to show the movement of the electron pairs.
Let’s have a look at an example of this mechanism:
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Hydrogen bromide is added to ethene to form a single addition product. Show the curly arrow mechanism for this reaction.
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Step 1: Draw out your reactants:
Step 2: Add in your partial charges
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Bromine is more electronegative than Hydrogen. Due to this, The shared pair of hydrogen’s electrons are more attracted to bromine.
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This causes an uneven distribution of charge hence causing the bromine to be partially negative and for hydrogen to be partially positive.
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Partially positive and partially negative are represented using a delta sign like this: δ.
Step 3: Drawing the curly arrows:
Study tip:
When you are asked to describe a mechanism, it is easier to draw the mechanism than to write about it, even if you are provided with lines.
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The electron pair in the p bond is attracted to the partially positive hydrogen atom, causing the double bond to break.
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A bond forms between the hydrogen atom of the H-Br molecule and the carbon atom that was part of the double bond.
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The bonding pair of electrons in the hydrogen are repelled towards the δ- bromine atom.
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This causes the bond between hydrogen and bromine to break.
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A covalent bond forms between one of the carbon atoms and the δ+ Hydrogen.
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Ethene has become a positively charged ion = carbocation
The lone pair in bromine then attacks the carbocation on ethene. This is represented like this...
Step 4: Draw in your products to show the reaction Is complete.
Study tip:
Your curly arrows must be drawn correctly and precisely.
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Start the arrow at the bond or lone pair or negative charge.
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Draw the head of the arrow at the atom to which the electron pair transfers
Electrophilic addition in unsymmetrical alkenes:
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Sometimes in an exam, you make face a situation where you have an odd number of carbons in your hydrocarbon chain however the mechanism direction remains the same. All the arrows flow together in an ordered manner. The only thing that changes is the positioning of the carbocation and the name of the Alkane.
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Carbocation stability:
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Please note that this part is not in the specification but can be interesting to know about.
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There are 2 types of products that are produced:
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Major product
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Minor product
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This depends on the position of the carbocation.
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If the carbocation ion is on the 1st carbon/end of the chain = primary carbocation (least stable)
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If the carbocation ion is on the 2nd carbon= secondary carbocation
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If the carbocation ion is on the 3rd carbon = tertiary carbocation (most stable)
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The more stable the carbocation position on the molecule = more likely to be the major product.
For example:
Written by: Bansari Sanghvi