3.3.4 - alkenes
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what is the general formula of an alkene
CnH2n
what elements do alkenes contain
only carbon and hydrogen atoms, making them hydrocarbons
what bonds do alkenes contain
alkenes contain at least one C=C double covalent bond
are alkenes saturated or unsaturtaed
alkenes are unsaturated
why are alkenes unsaturated
molecules with C=C double bonds are unsaturated as they can make more bonds with extra atoms in addition reactions
what feature does the C=C bond in an alkene have
as there are 2 pairs of electrons in the C=C double bond, it has a really high electron density
are alkenes reactive or unreactive?
alkenes are pretty reactive
are alkenes saturated with hydrogen?
alkenes aren’t saturated with hydrogen
how can double covalent bonds be formed?
by sharing 4 electrons between a pair of atoms
what shape do alkenes have?
alkenes have a flat planar shape
what is the reasoning for the shape of alkenes?
as there can be no rotation around the double bond, unlike with single bonds
why is there no rotation around the double bonds in alkenes?
as in addition to the single bond between the carbons, there is a p orbital (containing a single electron) on each carbon that overlap to form a pie orbital, preventing rotation of the bonds
what is sometimes called, when the rotation of the double bonds in alkenes is prevented?
this is sometimes called restricted rotation
why are alkenes reactive?
because of the double bonds which have a greater bond enthalpy, but they are an electron rich area of the molecule that can easily be attacked by positively charged electrophiles
why are alkenes more reactive than alkanes
carbon-carbon double bond gives the molecule an electron rich area that attracts positively charged electrophiles
what are electrophiles?
electron pair acceptors
electrophilic addition reaction: step one
positively charged electrophile is attracted towards double bond in the alkene
electrophilic addition reaction: step two
electrophile accepts a pair of electrons from the double bond, forming a positive ion (carbocation)
electrophilic addition reaction: step three
carbocation reacts with a negative ion to form a bond
what two types of products can from after an electrophilic addition reaction
major or minor products
what are major products
product that forms most of the time
what are minor products?
products that form less often
secondary carbocations
secondary carbocation is more stable as it has 2 alkyl groups, this carbocation will form most of the time
primary carbocations
primary carbocation is less stable as it has only 1 alkyl group, this carbocation forms less often