alkenes: content Flashcards
(26 cards)
why do stereoisomers form
restricted rotation around C=C double (π) bonds
what is a pi (π) bond and how are they formed
a type of covalent bond formed by the sideways overlap of p orbitals on adjacent atoms either side of a double bond
produces a region of high electron density above and below the molecule (and alongside the σ bond)
when do you get E/Z isomerism
When there are two different groups/atoms attached to each carbon at the ends of the double bond
how to identify E/Z isomers
use Cahn-Ingold-Prelog priority rules
how do CIP priority rules for E/Z isomerism work
- look at atoms attached to left-hand carbon
- higher atomic number= priority group
- if both are the same, consider the atoms one bond further along
- repeat for right hand carbon
- priority groups on the same side -> Z isomer
- priority groups on different sides -> E isomer
when is geometrical isomerism over a C=C bond impossible
when there is only one distinct group/species bonded to one of the double bond carbons
-eg H2C=CClH (rotation doesn’t matter)
name the four types of addition reaction (alkenes)
hydrogenation (addition of H₂)
halogenation (addition of HX)
addition of an acid
hydration (addition of water)
hydrogenation of alkenes
- catalyst
- reagents/conditions
- uses
alkene + H₂ →alkane
hydrogen added across the double bond
NICKEL CATALYST
used in the manufacture of margarine
halogenation of alkenes
- mechanism
- catalyst
- reagents/conditions
- uses
ELECTROPHILIC ADDITION
alkene + X₂ → dihaloalkene
no catalyst
used when testing for alkenes (bromine water + alkene →decolourises)
addition of acid (alkenes)
- mechanism
- catalyst
- reagents/conditions
- uses
ELECTROPHILIC ADDITION
no catalyst
alkene + [HBr or HCl] →haloalkane
used in organic synthesis when making a haloalkane
hydration of alkenes
- catalyst
- reagents/conditions
- uses
added across the double bond
alkene + H₂O →alcohol
REAGENTS: STEAM, HEAT, CONC H₂SO₄ CATALYST
used to make industrial ethanol (fuel/solvent)
outline electrophilic addition mechanism
- C=C bond is a region of high e⁻ density: attracts positive 𝛿⁺ electrophiles
- heterolytic fission: double bond e⁻ are donated to 𝛿⁺ electrophile
- C=C bond and (𝛿⁺) X-Y (𝛿⁻) bond break
- carbocation!! C⁺
- Y⁻ bonds to C⁺
how do electrophilic addition mechanisms work with halogen diatoms
electron rich area around the C=C double bond induces a 𝛿⁺ dipole in the closest atom
this repels the electrons in the other atom
the electron deficient 𝛿⁺ atom acts as the electrophile
test for alkenes
turn bromine water (yellow) colourless
(electrophilic addition)
what are the products of electrophilic addition reactions with UNSYMMETRICAL alkenes and why
major and minor product
unsymmetrical alkenes give rise to two different carbocation intermediates
when is the major product formed (unsymmetrical alkenes)
major product is formed via the more stable carbocation
carbocations: most to least stable
tertiary (C⁺-C₃) : MOST STABLE
secondary (C⁺-C₂H)
primary (C⁺-CH₂): LEAST STABLE
stability of a carbocation is increased by the presence of directly bonded alkyl groups
why are the products formed in different amounts (electrophilic addition unsymmetrical alkenes)
major product formed in greater quantities than the minor product as the more stable carbocation will form more products than the less stable
Markovnikov’s Rule
the H will bond to the carbon which is already bonded to the most hydrogens
addition polymerisation
joins alkenes (often under high temperature and pressure)
lots of monomers →long polymer chains
name the three simplest hydrocarbon polymers and give an example use
POLY(ETHENE) (=polythene)
-[-H₂C-CH₂-]-
plastic bags, milk bottles
POLY(PROPENE)
-[-CH₂-CH(CH₃)-]-
rope
POLY(BUTENE)
-[CH₂-CH(CH₂CH₃)-]-
bins
what is PVC and how is it used/altered for purpose
poly(chloroethene): -[-CHCl-CH₂-]-
- pure UPVC is hard/rigid due to strong IMFs (polar CCl bond→ permenant dipole-dipole forces)
- UPVC: doors, windows, guttering
- plasticisers are added to reduce the effectiveness of the IMFs and make the plastic more flexible
- PVC: wire covers, inflatables
- the more plasticisers added, the more flexible it becomes
- most common plasticisers are phthalates
problems with plastics and polymers
- most are non-biodegradable (cannot be quickly broken down by microbes) due to strong nonpolar C-C bonds
- landfill → takes a long time to decomposeh
how can we deal with plastics
-melted down and remoulded (breaking London forces)
- eat them
- some can be broken back into their monomers by heating (breaks the polymer bonds)p
