O- Alkenes
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What are the bonds between carbons in an alkene?
• sigma bond- C-C • pi bond- 2nd bond (2 p-orbitals on each C atom that overlap) • sigma + pi = double bond
Why is rotation prevented in an alkene?
pi bond (e- cloud above & below single bond)
What are the bond angles in ethene?
• roughly 120 ∵ planar • H-C-H angles slightly < 120 (~118) • ∵ C=C w/ 4 e- repels ↑ strongly than groups of 2e- in C-H single bonds
What happens to the boiling point of alknens as chain length increases? (3)
• ↑ • ↑ vdW • ↑ energy required to break
How can you distinguish an alkene from an alkane from obverations in burning them?
• alkenes burn w/ smokier flames • ∵ ↑ C : H ratio • alkenes = ↑ likely to undergo incomplete combustion
How do alkenes react?
• double bond = e- rich area • attracts electrophiles (e- pair acceptor, e- deficient )
Why does Br2 react with ethene?
• induced dipole –> δ+ & δ- ends • Br-Br bond becomes polarised –> react =)
What is electrophilic addition?
• an electrophile (e- pair acceptor) • added across a double bond
What is a positively charged carbon atom in electrophilic addition called?
carbocation
Describe what happens in electrophilic addition in words.
• e- rich C=C on alkene attacks Eδ+ • E-E bond breaks –> e- pushed onto Eδ- • carbocation formed • e- from lone pair of :E- attacks +ve charged C ATTACKS !!!!!!!
How many products can be formed from an asymmetric alkene?
2- depends on which C of the double bond the 1st group binds to
What is a major product?
product formed in the majority of a reaction
What does the selectivity of an electrophilic addition reaction depend on?
stability of the carbocation intermediate
Which one would be more stable, a primary or a secondary carbocation?
2° - ↑ e- donating alkyl groups {alkyl groups- push e- density onto carbocation} - ∴ stronger positive inductive effect - ↑ stable
Describe what happens in electrophilic addition with concentrated H2SO4
• double bond attacks δ+ H on H-OSO3H • lone pair on O attacks carbocation • draw product in full
What is the Markovnikov’s rule?
H added will go to C w/ most H
Describe what happens when you add water after electrophilic addition with conc H2SO4.
• product converted into an alcohol • H2SO4 = catalyst –> reforms at the end
What happens in electrophilic addition with conc H3PO4 and water?
Alkene–>Alcohol Conditions: 1. conc. H3PO4 = catalyst 2. STEAM
How can you test if an electrophilic addition reaction has occurred ?
Bromine water - x occurred- double bond present- orange –> colourless - occurred- x double bond ∴ x change
What is a monomer ?
individual molecule that makes up a polymer