4.1.3 Electrophilic addition in alkenes Flashcards
Define electrophile
An atom (or group of atoms) which is attracted to an electron-rich centre or atom, where it accepts a pair of electrons to form a new covalent bond
An electron pair acceptor
the clarification def. is acceptable
define electrophilic addition
An addition reaction in which the first step is attack by an electrophile on a region of high electron density
Explain the process of electrophilic addition reaction
The double bond of the alkene represents a region of high electron density because of the presence of the π-electrons.
The high electron density attracts electrophiles
An electrophile is an electron pair acceptor. It is usually a positive ion or molecule containing an atom with a partial charge (𝛿⁺)
describe the mechanism of electrophilic addition
using HBr
When an alkene undergoes electrophilic addition with HBr a curly arrow goes FROM the BOND of the C=C to slightly positive atom of the electrophile here it would be the H 𝛿⁺. The second curly arrow goes FROM the bond of the HBr (the electrophile) to the Br𝛿⁻ . The carboncation intermediate is shown as the H attached without a dipole and a + where the Br will be. The Br is show as Br⁻ and will have a curly arrow FROM the ⁻ TO the .
the + is due to the atom having 1 less electron than usual
describe the process of electrophilic addition
- if the alkene is reacted with a polar molecule such as HBr, the molecule already contains dipoles and the bond of the C=C has a curly arrow to the 𝛿⁺ atom and the bond from the reacting molecule has a curly arrow to the 𝛿⁻ i.e. H𝛿⁺ - Br𝛿⁻. If the molecule reacting with the alkene is non-polar like Br-Br then the π-electrons of the alkene which are polar will induce a dipole in the Br₂.
- The electron pair in the π-bond is attracted to the partially positive atom causing the double bond (C=C) to break
- A bond froms between the partially positive atom and one of the carbons of the double bond.
- The reacting molecule breaks by heterolytic fission with the electron going to the partially negative atom
- A negative ion is formed as well as a carbocation.
- The negative ion reacts with the carbocation to form the addition product
short notes on electrophilic addition mechanism
- Electrophile = HBr or Br₂
- the bond breaks via heterolytic fission
- carbocation: An ion that contains a positively charged carbon atom
- When the electrophile is not polar the interaction between the π-electrons in the alkene and the electrons in the electrophile bond polarise the electrophile
What does Markownikoff’s rule predict
Markownikoff’s rule predicts the major product, when an unsymmetrical alkene is reacted with HX (a hydrogen halide) resulting in a mixture of 2 products.
which product does Markownikoff’s rule predict will be the major product
The major products comes from the more stable carbocation intermediate
What makes a carbocation stable
A carbocation is more stable when it has MORE C atoms bonded to the C⁺
primary cations are the least stable and tertiary cations are the most
define carbocation
An ion that contains a positively charged carbon atom
important to note that the ion is positively charge NOT partially positive
What are the types of cations for Markownikoff’s rule
- Primary
- Secondary
- Tertiary
what is a primary carbocation
A C⁺ with one alkyl group (connected to 1 other carbon) and 2 hydrogens
least stable
alkyl groups can be represented as R - 1st = R, 2nd R’, 3rd R’’ on a single C⁺
What is a secondary carbocation
A C⁺ with two alkyl groups (connected to 2 carbons) and one hydrogen
alkyl groups can be represented as R - 1st = R, 2nd R’, 3rd R’’ on a single C⁺
what is a tertiary carbocation
A C⁺ with 3 alkyl groups (connected to 3 carbons) and no hydrogens
most stable
alkyl groups can be represented as R - 1st = R, 2nd R’, 3rd R’’ on a single C⁺
