Reactions of Alkenes - DF Flashcards
The c=c double bond has how many pairs of electrons and what does it show
2 = very electron rich (region of high electron density) which is prone to attack by an electrophile
Electrophile definition
a positive ion or molecule with a partially positive region which is attracted to a negative region in a molecule,and rectas by excepting a pair of electrons, forming a covalent bond
Addition reaction definition
where 2 molecules react to give just one product molecule
What happens when alkenes undergo electrophilic addition reactions
atoms or groups get added to c=c double bond
Reaction with bromine (Reaction of alkenes)
alkenes react with Br₂ at room temperature. This can be used as a test for alkenes: add a few drops of bromine water which is orange. Shake well. If the compound is unsaturated (contains c=c) a colour change from orange to colourless
Equation:
https://www.elevise.co.uk/uploads/9/8/0/2/98021560/screen-shot-2018-03-29-at-16-00-17_1.png
This equation shows how ethene reacts with bromine bubt all alkenes react like this with different halogens.
Electrophilic addition mechanism definition
As a Br₂ molecule approaches the electron rich alkenes double bond, the bonding electrons in Br₂ start being repelled away. So the bonding electrons are no longer evenly shared: the bottom BR becomes slightly negative (delta minus) and the top Br becomes slightly positive (delta plus). the bond has become polar
Look at diagram of this mechanism
https://s3.eu-west-2.amazonaws.com/elements.cognitoedu.org/6244d431-2858-473b-acd0-f9a1495593b1/electrophilic-addition-bromine-ethene.png
Evidence of electrophilic addition mechanism
If the reaction is done in the presence on another anion (e.g. Cl⁻ ions), then a mixture of 2 products are given. The Cl⁻ ions is initially repelled away from the alkene so the first step is as before: the Br₂ attacks the c=c double bond giving the carbocation intermediate. But once it has formed the carbocation intermediate can react either with the Br⁻ or with the added anion (Cl⁻)
Reaction with hydrogen bromide (Reaction of alkenes)
Alkenes react at room temperature with (aq) or (g) HBr or another other hydrogen halogen.
See equation
https://i.ytimg.com/vi/Alb4d0FRHLY/sddefault.jpg
Mechanism of reaction with hydrogen bromide (Reaction of alkenes)
electrophilic addition
What happens with the reaction with hydrogen bromide if the alkene is unsymmetrical (e.g. propene) (Reaction of alkenes)
there will be 2 products
See equation (for hydrogen iodine but works the same)
https://images.nagwa.com/figures/explainers/268186586415/10.svg
Reaction with water (Reaction of alkenes)
at room temperature alkenes do not react with water (liquid alkenes would just from a layer floating on the top). But under different conditions this reaction is used in the industrial manufacture of ethanol as product is an alcohol
See equation
https://encryptedtbn0.gstatic.com/imagesq=tbn:ANd9GcQbpIBHMvCSatYAfgFw1j-GV-PWdaUuXEru-A&s
Reagents and conditions of reaction with water (Reaction of alkenes)
water (steam), phosphoric acid catalyst, high temp and high pressure
Reaction with hydrogen and name of reaction (Reaction with alkenes)
This is still an addition reaction, but it is not electrophilic as it takes place on the surface of a catalyst
See equation - product = alkane
https://images.nagwa.com/figures/explainers/268186586415/1.svg
What type of catalyst is needed for a reaction with hydrogen (Reaction with alkenes)
heterogeneous
Reagents and conditions of reaction with hydrogen (Reaction with alkenes)
hydrogen gas, platinum catalyst at room temp and pressure
or
hydrogen gas, nickel catalyst at high temp and pressure
