13.4 Electrophilic Addition in Alkenes Flashcards
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
