O- Alkenes Flashcards
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
