Alkenes Flashcards
general formula of an alkene
CnH2n
cycloalkene formula
CnH2n-2
are alkenes saturated or unsaturated and why
unsaturated, contains a c=c double bond
in terms of bonding what does it mean to have a double bond
more bonds can be made with additional electrons in addition reactions
describe the double bonds in terms of sigma and pi bonds
the double bond is made up of sigma and pi bonds
- a pi bond is formed when two lobes of two orbitals overlap sideways
- pi bond electron density is spread out above and below the nuclei
(the electrostatic attraction between the nuclei and shared pair of electrons is weaker resulting in a relatively low bond enthalpy)
which is stronger a single or a double bond and why
the sigma single bond is stronger
- two orbitals overlap in a straight line in the same space
- gives the highest possible electron density between the two positive nuclei, resulting in a strong electrostatic attraction between the nuclei and the shared pair of electrons
resulting in a strong bond enthalpy. whereas, due to its spread out electron density, the pi bond in the double bond has a much lower bond enthalpy so is a weaker covalent bond
what is electrophilic addition
when an alkene double bond opens up and atoms are added to the carbon atoms
describe electrophilic addition
- alkene double bond has lots of electrons, easily attracted by electrophiles who lack electrons
- e.g. positively charged ions, polar molecules
what is an electrophile
electron-pair acceptors, short of electrons so are attracted to where there’s plenty (such as alkene double bond)
what is produced when alkene reactions with hydrogen
an alkane
what state must alkene be in to react with hydrogen
gas
conditions for addition reaction with hydrogen
- 150 degrees celcius
- nickel catalyst
describe how margarine is created
- its double bond is removed to raise its melting point so it becomes solid at RT
- made by hydrogenation of unsaturated vegetable oils
what type of reaction are reactions with halogens
electrophilic addition reaction
alkene + halogen ->
dihalogenoalkane
describe the test for alkenes
bromine water is used. bromine is added across the double bond to form colourless dibromoalkane
how are alcohols formed
steam hydration
what state do alkenes need to be in to be hydrated by steam
gas
conditions for steam hydration
- 300 degrees celcius
- Phosphoric (V) acid catalyst
- 60-70 atm
how are diols formed
oxidation
what is a diol
an alcohol with two -OH groups
conditions required for oxidation
- alkene react with acidified potassium manganate (VIII)
colour change when alkene reacts with acidified potassium manganate (VIII)
purple- colourless
