Alkenes Flashcards
(12 cards)
Structure of alkenes
- unsaturated hydrocarbons=C=C bond=high electron density=attacked by electrophiles+part of addition reactions
- each carbon in C=C bond has 3 regions of electron density=arranged in trigonal planar shape=120
Reactivity of alkenes
- more reactive than alkanes due to C=C=high electron density=electrons have easy access for reactions+attracts electrophiles
Electrophillic addition of halogens (bromine)
- used to test for unsaturation
- bromine solution made —> bromine water/dissolved in CCl4
- add hydrocarbon u want to test+shake
- brown-colour disappears=alkene=Br added to alkene=x Br2 molecules or C=C bond
Why can bromine be used as an electrophile
As the Br2 molecule gets closer to C=C, its high electron density repels electrons in the Br2 molecule=induces a dipole
Electrophillic addition of HBr
Reagent: HBr
Condition: room temp
Prob w adding HBr to alkenes
Produces isomeric compounds=unsymmetrical=products x formed at the same rate
Predicting major product
Depends on type of carbocation
- alkyl groups attached to the positively charged carbon atoms are ‘electron donating groups’= inductive effect of alkyl groups=causes the carbocation to become less positively charged=charge is spread around the carbocation=energetically more stable
This means that tertiary carbocations are the most stable as they have three electron-donating alkyl groups which energetically stabilise the carbocation
Due to the positive charge on the carbon atom, carbocations are electrophiles
Electrophillic addition of H2SO4
Reagent: cold conc sulfuric acid
Condition: 0 degrees
Product: alkyl hydrogen sulfates=hydrolysed w water+heat=alcohol+sulfuric acid
Addition polymers
- made from alkenes/substituted alkenes (double bond breaks)
What affects physical properties of polyalkenes
- strength of intermolecular forces between polymer chains which depend on:
- chain length=longer chain=more surface contact between chains=stronger intermolecular forces
- branching=straight chains=more tightly packed=more surface contact between polymer chains
- long, straight polyalkenes=stronger intermolecular forces=more energy to overcome=polymer re harder+more rigid but shorter/branched polyalkenes=moe flexible
Uses of PVC=poly(chloroethene)+plasticesers
- Plasticisers=small molecules that can be added to polymers to increase flexibility=position themselves between polymer chains=pushes chains apart=weaker intermolecular forces between polymer chains=chains slide over each other more easily when bent
- how they affect PVC
Unplasticised PVC=long, closely packed chains making it rigid but brittle=used in applications eg pipes, window frames, and floor coverings.
Plasticised PVC=more flexible=used for cable insulation, shower curtains, and clothing
Why are addition polymers unreactive/inert
- due to a lack of double bonds and non-polar structure
- only have single covalent bonds=saturated compounds with maximum stability
- have a carbon backbone lacking any polar groups, giving an overall non-polar molecule.
These factors lead to very low chemical reactivity under normal conditions.
