13.4 Electrophilic addition in alkenes
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What is electrophilic addition
A mechanism for the reaction where alkenes take part in addition reactions to form saturated compounds
The ______bond in an alkene represents a region of ______ electron density because of the presence of the __-electrons. The high electron density of the π-electrons attracts ______.
Double High π Electrophiles
What is an electrophile
An atom or a group of atoms that is attracted to an electron-rich centra and accepts an electron pair. An electrophile is usually a positive ion or a molecule containing an atom with a partial + charge (δ+)
Explain the electrophilic addition of but-2-ene and hydrogen bromide
• Bromine is more electronegative than hydrogen so hidrogen bromide is polar and contains the dipole Hδ+-Brδ- • The electron pair in the π-bond is attracted to the partially positive hydrogen atom, causing the double bond to break • A bond forms between the hydrogen atom of the H-Br molecule and a carbon atom that was part of the double bond. • The H-Br bond breaks by heterolytic fission, with the electron pair going to the bromine ion • A bromide ion and a carbocation are formed • The bromide ion reacts with the carbocation to form the addition product
What is an electrophile
An electron pair acceptor
What does a carbocation contain
Contains a positively charged carbon atom
Explain the electrophilic addition of propene with bromine
• This causes polarisation of the Br-Br bond with one having a partial positive charge and the other negative (induced dipole) • The electron pair in the π-bond is attracted to the Brδ+ end of the molecule causing the double bond to break • A bond has now been formed between one of the carbon atoms from the double bond and a bromine atom. • The Br-Br bond breaks by heterolytic fission, with the electron par going to the Brδ- end of the molecule • A bromide ion and a carbocation are formed • The Br- ion reacts with the carbocation to form the addition product of the reaction
What are the two steps in electrophilic addition
Primary - The positive charge is on a carbon atom at the end of a chain Secondary - Positive charge is on a carbon atom with two carbon chains attached
How are carbocations classified
By the number of alkyl groups attached to the positively charged carbon atom
How is an alkyl group normally represented
By the symbol -R
What are the most stable carbonations
The tertiary carbocations are the most stable
What is carbocation stability linked to
The electron-donating ability of alkyl groups Each alkyl group donates and pushes electrons towards the positive charge of the carbocation Positive charge is spread over the alkyl groups. The more alkyl groups attached to the + charged carbon atom, the more the charge is spread out making it more stable