4.3 - Alkenes Flashcards
Bonds carbons have in alkenes
each has 3 sigma bonds and 1 pi bond
What is a pi bond?
formed by sideways overlap of 2 p-orbitals above and below bonded atoms
What is the bond angle and shape around C=C bond?
120°
trigonal planar
Define stereoisomers
same structural formula but different arrangement of atoms in space
How does stereoisomerism arise?
- rotation around C=C bond is restricted
- groups attached to each C are fixed relative to each other
Requirement for E/Z isomerism
C=C bond does not rotate
different groups attached to each C atom
Requirement for cis-trans isomerism
type of E/Z isomerism with 2 different groups on each carbon with 1 group the same on both
How to determine priority groups in stereoisomerism
highest priority = highest Mr of atoms directly attached to carbons
(if same atom, go along chain until 1st difference)
Discuss the reactivity of alkenes
- more reactive than alkanes
- because pi bond enthalpy is lower
- so weaker bond and more exposed (due to being on top and bottom)
- so broken more readily
Define electrophile
electron pair acceptor
Draw the mechanism for electrophilic addition of alkene with H-Br and Br2
Rule for carbocation stability and major product formation
tertiary = most stable
primary = least stable
major product: H attaches to C with most Hs directly attached
Properties of polymers
- lack of reactivity = hard to dispose of since most are non-biodegradable
- disposal/recycling PVC hazardous due to high Cl content and range additives. When burnt release HCl and other toxic gases
Test for C=C bond
bromine water
positive = decolourised
negative = stays orange
How can waste polymers be processed in a sustainable way? Give one problem of disposal
- combustion for energy production
- use as an organic feedstock
- recycling
Problem:
- toxic waste products may be formed e.g HCl formed during disposal by combustion of PVC
