13.4 Electrophilic Addition In Alkenes
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What is electrophilic addition?
The mechanism whereby alkenes take part in addition reactions to form saturated compounds.
What does the double bond in an alkene represent?
An area of high electron density, because of the presence of the Pi-electrons
What does the high electron density of Pi-electrons attract?
Electrophiles
What is an electrophile?
An atom, or group of atoms, that accept an electron pair to form a covalent bond.
Describe the stages of a reaction between ethene and hydrogen bromide.
• Bromide is more electronegative than hydrogen, so hydrogen bromide is polar, and contains the dipole Hδ+ – Brδ- • The electron pair in the Pi-bond is attracted to the partially positive hydrogen atom, causing the bond to break. • A bond forms between the hydrogen atom of the H-Br molecule, and a carbon molecule of the C=C bond. • The H-Br bond breaks by heterolytic fission, with the electron pair going to the bromine atom • A bromide ion, and a carbocation are formed • In the final step, the Br- ion reacts with the carbocation to form the addition product.
What is a carbocation?
A carbon atom that is positively charged.
How does an alkene induce polarity in a non-polar molecule? (i.e Br2)
• The Pi-electrons interact with the electrons in the Br-Br bond. • The bonding pair of electrons is equally distributed between the two Bromine atoms • As bromine approaches the Pi-bond, electrons in bromine are repelled. Then, polarity has been induced in the bromine.
What is polarisation?
When electrons are unevenly distributed in a chemical bond.
What does Markownikoff’s rule show you?
How to predict the major product of a reaction that produces several possible isomers.
What will the major product in an electrophilic reaction always be?
The more stable isomer.
What determines the stability of an isomer produced during an electrophilic reaction?
Whether it is a tertiary, secondary, or primary carbocation.
What is a primary carbocation?
A carbocation attached to one carbon atom
What is a secondary carbocation?
A carbocation attached to two carbon atoms.
What is most stable, a primary, secondary, or tertiary carbcocation?
Tertiary.