4.3 - Alkenes Flashcards
(17 cards)
Name the bonds a C=C bond comprises of
1 π-bond
1 σ-bond
What is a pi bond?
sideways overlap of adjacent p-orbitals above and below bonding C atoms
What is a σ-bond?
overlap of orbitals directly between bonding atoms
What is the bond angle and shape around each C in C=C bond?
120°
trigonal planar
Define stereoisomers
compounds with 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
What is E/Z isomerism?
stereoisomerism in terms of restricted rotation about a double bond and 2 different groups attached to each C atom of C=C group
What is cis-trans isomerism?
type of E/Z isomerism in which 2 of substituent groups attached to each C of C=C group are the same
- must be a H on each C atom of C=C group
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 π-bond enthalpy is lower
- weaker bond and more exposed
- so broken more readily
Define electrophile
electron pair acceptor
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: 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 and what are the benefits? Give one problem of disposal
- combustion for energy production
- use as an organic feedstock
- removal of toxic waste products
Benefits:
- reduced landfill waste
- less waste of resources
- save energy
Problem:
- toxic waste products may be formed e.g HCl formed during disposal by combustion of PVC
What are the environmental benefits of biodegradable and photodegradable polymers?
- reduced dependency on finite resources
- reduces accumulation of persistent plastic waste
Draw a labelled diagram to show the formation of a pi bond
1st diagram: p-orbitals
->
2nd diagram: sideways overlap with pi bond labelled
