Alkenes Flashcards
what is the definition of an alkene ?
an unsaturated hydrocarbon
what is the general formula of alkenes ?
CnH2n
what does unsaturated mean ?
it means they contain at least 1 carbon to carbon double bond
what is meant by a double bond that is covalent ?
it means that there are 2 shared pairs of electrons
this makes them electron rich
what are the physical properties of alkenes ?
- very similar to alkanes
- insoluble in water
- mpt / bpt increase with chain length
what is the shape of alkenes ?
planar (flat) due to the double bond
what is an additional property of the shape of alkenes ?
they have restricted rotation as they have a fixed shape
what kind of isomerism do alkenes present that is specific to alkenes ?
E-Z isomerism
(geometric isomerism )
what is an example of a functional group isomer pair that includes alkenes ?
alkenes and cycloalkanes
what are alkenes in terms of reactivity and why ?
they are very reactive
- due to the carbon to carbon double bond
what do akenes + water make ?
alcohols
conditions for the reaction of alkenes + water ?
- high temp
- high pressure
- use a catalyst ( H3PO4)
what is meant by an addition reaction ?
when one molecule joins to another
what is an electrophile ?
lone pair acceptors
what are the 3 different reagents for electrophilic addition ?
- HBr (HX)
- Br2 ( any halogen )
- H2SO4 ( sulphuric acid )
what is a crabocation ?
a positively charged carbon atom
which molecule is the electrophile in the electrophilic addition of HBr ?
HBr
- because it was seeking out the negative part of the alkene = double bond
what is the major product ?
the product that forms with the most stable carbocation intermediate
state the stability of the carbocations ?
tertiary > secondary > primary >
conditions for electrophilic addition with BR2 ?
aqueous ( bromine water)
what is the test for unsaturation ?
bromine water + alkene
- shake it
what is the positive result for the bromine water test ?
orange –> colourless solution
why does the bromine water turn colourless ?
because the bromine combines with the alkene so that there is no more bromine in the water
conditions for electrophilic addition with H2SO4 ?
cold ,concentrated H2SO4
what does H2SO4 act as ?
a catalyst
