Chapter 14 - Alkenes
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general formula of an akene
CnH2n
general formula of a cyclic alkene
CnH2n-2
why are alkenes more reactive than alkanes
because they have a region of high electron density (the double bond) which allows them to undergo addition reactions by attracting electrophiles
whats an electrophile
electron pair acceptor
another name for double bond
pi bond
outline addition reactions with halogens (Br2)
Step 1 - The high electron density of the C=C double bond repels electrons in the bromine molecule, polarising the Br-Br bond. Step 2 - This leads to heterolytic fission of the Br-Br bond, with one bromine atom taking the bonding pair of electrons, creating a positively charged bromine which bonds to one of the carbon atoms. Step 3 - A positively charged carbocation intermediate forms. The negatively charged bromide ion then bonds to the other carbon atom. Step 4 - The final product, 1,2-dibromoethane, has a bromine atom bonded to each of the original alkene carbon atoms.
whats a carbocation
when a carbon atom is positively charged in a molecule
test for alkenes
decolorizes bromine water due to addition reaction with alkene
what conditions are used when hydrating alkenes to form alcohols
phosphoric acid 300 degrees Celsius 60 - 70 atm
what are primary secondary and tertiary carbocations
primary have one alkyl group attached, secondary have 2 and so on
why does the stability of carbocations increase with more alkyl groups
alkyl groups have a tendency to release electrons or a “positive inductive effect” which stabilizes the positive charge
what are major and minor products in electrophilic addtion
major product is made the most, it results from a more stable carbocation bonded to the electrophile
how can you get an alcohol from the products of an electrophilic addition reaction with sulfuric acid
Water and heat hydrolyse the alkyl hydrogen sulfate back to alcohols and sulfuric acid.
whats a polymer whats a monomer
monomer is one unit of a polymer polymer is long chain of monomers
why are alkenes used to make plymers
the C=C bond breaks to allow alkenes to join up and form a carbon chain
what process is joining together mnay monomers by breaking the C=C bond
addtion polymerisation
hwo does chain length affect physical properties of a plymer
Longer chains allow more surface contact between neighbouring chains. This strengthens the intermolecular forces.
hwo do branches affect physical properties of a plymer
Straight chains can pack together closely in an ordered structure, maximising intermolecular forces. Highly branched chains cannot pack as efficiently, reducing contact between chains.
what are plasticisers
plasticisers are small molecules that can be added to polymers to increase flexibility
how do plasticisers work
Plasticiser molecules position themselves between polymer chains. This pushes the chains apart, weakening intermolecular forces between polymer chains. The chains can now slide over each other more easily when bent.
why are addtion polymers unreactive
Polyalkenes contain only single covalent bonds. This makes them saturated compounds with maximum stability. They also have a carbon backbone lacking any polar groups, giving an overall non-polar molecule.