Chapter 13- Alkenes Flashcards
what are alkenes
unsaturated hydrocarbons where the molecule contains at least one carbon to carbon double bond in their structure
alkenes general formula
CnH2n but note cyclic alkenes and alkenes with more than one double bond do not follow this general formula
How are pi bonds made
, the sideways overlap of the 2 p-orbitals result in the formation of a pi bond.
why is rotation possible around every atom in alkanes and not alkenes?
the pi bond locks the two carbon atoms in position and prevents them from rotating around the double bond
what is the shape of an alkene
trigonal planar, 3 regions repel each other as far aprt as possible so bond angle is 120.
give one example of alkenes in the natural world
Alkene Limonene C10H16 is found naturally in the rinds of citrus fruits and is responsible for the smell and flavour of oranges and lemons
define stereoisomers
compounds that have the same structural formula but different arrangements of the atoms in space
2 types; E/Z and cis/trans
give a reason for how stereoisomerism is possible in alkenes.
The restricted rotation of groups attached to each c atom around the double bond by position of pi- bonds electron density below the plane of the sigma bond allows for this
what are the two conditions necessary for a molecule to have E/Z isomerism?
- a c to c double b
- different groups attached t oeach carbon atom of the double bond.
how is E/Z isomerism different to cis/trans isomerism ?
Cid/trans isomerism requires one of the attached groups on each c atom of the double bond to be a h atom
Using the cahn-ingold-prelog nomenclature, how are groups prioritised?
if the groups of higehr priority are on the same size then= z isomer
if grops of higher P are diagonally placed across double b then = E isomer
The higher the atomic numebr the higehr the priority
Difference between E/Z isomerism and cis/trans isomerism
E/Z - isomers with different arrangments in space and locked rotation around double bond
cis/trans - a type of E/Z isomerism that arises when there are 2 H’s and Two non-hydrogen functional groups attached to each carbon.
why are alkenes unsaturated hydrocarbons?
contains 1 or more carbon to carbon double bonds
electrophile
electron pair acceptor
addition reaction
2 or more molecules bonded to make one product
what is hydrogenation of an alkene and what are the conditions?
- hydrogen added across double bond
- over a nickel catalyst at 423K in a c
- ADDITION REACTION where alkene becomes saturated = alkane
give an example of an everyday use of alkanes and the effects of this on health?
vegetable and animal oil goes under hydrogenation t o form margarine - forming trans double bonds - trans- fats
raises cholesterol levels
hydration and the conditions
- water molecule in form of steam added across the C-C double bond - producing alcohol
- ## high pressure and a phosphoric/sulfuric acid catalyst
halogenation and conditions
- halogen molecule added across C-C double bond
- room temp .
- alkene becomes saturated and a dihaloalkane is formed .
describe the test for saturation?
mix alkene with BR2 water = bromine molecules add across double bond = change of colour from orange brwn to colourless
Through what mechanism does halogenation take place?
electrophilic addition
reaction of alkene with hydrogen halides = what and under what conditions?
- hydrogen halide is added across the C-C double bond .
- room temperature
- if alkene is a gas (ethene) then two gases are mixed and reaction takes place
- if alkene is a liquid then the hydrogen halide is bubbled through it.
- forms dihaloalkanes
what is Markownikoff’s rule?
states that when a hydrogen halide reacts with an unsymmetrical alkene , the H of the hydrogen halide attaches itself to the carbon atom of the alkene with the greater number of hydrogen atoms and smaller number of carbon atoms.
polymers
repeating units of monomers
