4.1.3 - Alkenes Flashcards
alkenes
unsaturated hydrocarbons containing a C=C bond comprising a π-bond and a σ-bond
general formula of alkenes
CnH2n
pi bond (π-bond)
sideways overlap of adjacent p-orbitals above and below the bonding C atoms, has restricted rotation
shape around a double bond
trigonal planar, 120° (3 bonding regions repel each other)
stereoisomers
compounds with the same structural formula but a different arrangement of atoms in space
E/Z isomerism
- example of stereoisomerism
- arises because rotaton about double bond is restricted
- two different groups must be attached to each carbon atom of the C=C group
cis–trans isomerism
- special case of E/Z isomerism
- two of the substituent groups attached to each carbon atom of the C=C group are the same
when there is a H on each C:
- cis isomer = Z isomer
- trans isomer = E isomer
Cahn–Ingold–Prelog (CIP) priority rules
- groups attached to C given priority based on atomic number
- higher priority groups on SAME side of double bond = Z isomer
- higher priority groups diagonally placed across double bond = E isomer
reactivity of alkenes
- much more reactive than alkanes due to presence of π-bond
- π-bond has relatively low bond enthalpy (weaker than σ-bond and broken more readily)
- alkenes undergo addition reactions relatively easily
hydrogenation of alkenes
- electrophilic addition
- alkene reacted with hydrogen
- in the presence of nickel catalyst
- hydrogen added across the double bond to form an alkane
halogenation of alkenes
- electrophilic addition
- forms dihaloalkanes
- used to test for unsaturation
- alkene reacted with bromine
- bromine water decolourised
reaction of alkenes with hydrogen halides
- electrophilic addition
- forms haloalkanes
hydration reactions of alkenes
- electrophilic addition
- alkenes react with steam (H2O)
- in the presence of a phosphoric acid (H3PO4 catalyst
- steam adds across double bond to form an alcohol
electrophile
electron pair acceptor (usually a positive ion or a molecule with δ+ charge)
Markownikoff’s rule
- the H of a HHal attaches itself to the C of the alkene with the greater number of H atoms and the smaller number of C atoms
- primary, secondary, tertiary carbocations
carbocation stability
- classified by number of alkyl groups attached to positive C atom
- tertiary carbocations = most stable
- primary carbocations = least stable
- major product forms via most stable carbocation
addition polymerisation
n monomer unit → [repeat unit] n
- produces long saturated chains containing no double bonds
how can waste polymers be processed?
- combustion for energy production
- use as an organic feedstock for the
production of plastics and other organic chemicals - removal of toxic waste products, e.g. removal of HCl formed during disposal by combustion of halogenated plastics (e.g. PVC)
biodegradable polymers
- usually made from starch or cellulose (bioplastics)
- broken down by microorganisms into water, carbon dioxide, and biological compounds
- compostable polymers degrade and leave no visible or toxic residues
photodegradable polymers
- contain bonds weakened by absorbing light to start the degradation
- light-absorbing additives also used
