13.4 Electrophilic Addition in Alkenes
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What is the mechanism for addition in alkenes?
Electrophilic addition
What is electrophilic addition?
ADDITION reaction first step is ATTACK by ELECTROPHILE on a region of HIGH ELECTRON DENSITY.
What is an electrophile?
species ATTRACTED to ELECTRON-RICH centre where it ACCEPTS an ELECTRON PAIR forming a NEW COVALENT BOND
What events occur during the mechanism of electrophilic addition?
(molecule being added is either polar due to different electronegativities or polarised by interacting with pi bond) • 𝛅+ nucleophile ATTRACTED to double bond, due to high electron density. • double bond breaks • COVALENT BOND forms between 𝛅+ nucleophile. • causes bond in molecule being added to break by heterolytic fission. • Carbocation + anion (from addition molecule) formed. • anion attacks 𝛅+ carbon, donating a pair of electrons + forming addition product.
Why are the addition reactions of alkenes considered electrophilic addition?
addition - adding molecule to unsaturated molecule, forming saturated. Electrophilic - first step is attack on region of high electron density (double bond).
During electrophilic addition, if the molecule being added consists of atoms with little/no difference in electronegativity, how is the electrophile attracted to the double bond?
molecule being added interacts with double bond, a region of high electron density becomes polarised/has polarity induced
What is a carbocation?
a molecule containing a CARBON with a POSITIVE CHARGE
How does carbocation stability change with degrees?
primary carbocations MOST reactive, least stable (∴ form MINOR product) tertiary carbocations LEAST reactive, most stable. (∴ form MAJOR product)
Explain the difference in stabilities of carbocations:
More immediate ALKYL groups MORE REDUCTION of CHARGE on positive carbon molecule is more neutral tertiary carbocations MORE STABLE
How is the charge on a carbocation reduced by the alkyl groups surrounding the +ve carbon?
Carbon is positive Electrons from immediate alkyl groups attracted towards positive carbon slightly reduces charge towards neutral.
What is a primary carbocation?
a carbocation with it’s positively charged carbon attached to 1 other carbon atom.
What is a secondary carbocation?
a carbocation with it’s positively charged carbon attached to 2 other carbon atoms.
What is a tertiary carbocation?
a carbocation with it’s positively charged carbon attached to 3 other carbon atoms.
What is Markownikoff’s rule?
MORE STABLE carbocations = MAJOR product