Basic Concepts and Hydrocarbons - alkenes Flashcards
Describe the bonding in alkenes
- c=c double covalent bond which has two sections involving the overlap electron clouds
- sigma bond is formed between the two carbon atoms using the direct overlap of electron clouds of the two atoms
- pie bond forms by electrons in adjacent p-orbitals overlapping above and below carbon atoms
- pie bonds are only made after sigma bonds
- the pie bond restricts rotation around the double bond making the molecule shape planar
Describe the shape of an alkene
120 degrees, and traigonal planar
- the covalent bond between the two carbon atoms are made from sigma and pie bonds this creates a high electron density by the same amount so form bond angles of 120
Basic things you need to know about alkenes
- hydrogen and carbon
- unsaturated
- CnH2n
- graduation in physical properties
- similar chemical properties
What does the C=C bond do
the C=C bond stops rotation this allows E/Z isomers to be formed
if two identical alkyl groups are attached Cis/Trans isomerism is formed
What is stereoisomerism
this is compounds that have the same structural formula but different arrangement of atoms in space
What happens in an addition reaction of alkenes
the pie bond will break first and react leaving the sigma bond between the two carbon atoms, the pie bond needs less energy to undergo homolytic fission therefore breaks more easily
- then the double bond is an area of high electron density which attracts electrophiles, for example Br2 which breaks the double bond, this makes the alkene saturated
Describe Hydrogenation
- it is an addition reaction
- Hydrogen is added across a C=C bond
- gaseous hydrogen and an alkene are mixed and passed over a nickel catalyst at 150 degrees in a reaction chamber
- saturates the alkene and products and alkane
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What is halogenation at test for? and explain
Test for saturation
- when a bromine or iodine is reacted with a saturated compound there is no reaction but with an unsaturated compound there is a reaction this changes the solution a different colour
Describe the basic way of doing halogenation
- a halogen is added across a C=C bond,
- alkene becomes saturated and a dihaloalkane is produced
Describe Halogenation
Step 1
- Bromine has non-polar molecules
- Bromine gets close to the electron rich C=C bond
- Dipole induced in Bromine
- slight positive is attracted to the high electron density of the C=C bond
Step 2
- Electrons from pie bond make a bond with a bromine ion
- this causes hetrolytic fission of the bromine molecule
- positive charge remains on a second carbon atom
- the species is very reactive and is a carbon cation
Step 3
- Two electrons from the bromide ion are shared with the carboncation
- makes a second bond and a stable product
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How does hydration work
- Addition reaction between a gaseous alkene and steam
- used to make alcohols
- condition involves high temperature (300 degrees) and pressures (65ATM) with a phosphoric acid catalyst (H3PO4)
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Describe the addition of hydrogen halides
Step 1
- large difference in electronegativity between hydrogen and bromine therefore the molecules is polar
- the slight positive hydrogen side of the HBr molecules is attracted to the high electron density of the C=C bodn
Step 2
- The electron in the pie bond makes a bond with hydrogen
- causes hetrolytic fission
- electron in the HBr molecule are given to the bromine atom - this makes a bromine ion
- a carbon cation is also made
Step 3
- the bromide ion and the carboncation bond to make a stable product
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How does the addition of hydrogen halides work
- a hydrogen halide is added across a C=C bond which produces a haloalkane
- saturated compounds based on alkane chain skeletons
- at least one hydrogen atom is replaced by a halogen atom
- gases at room temperature these reactants are bubbled through the alkene to cause a reaction
What happens when a hydrogen halide is added to an unsymmetrical alkene
- two possible products are formed
- MAJOR AND MINOR - Hydrogen binds to carbon with most hydrogen = this stablisised the carbocation as it is stalished by the presence of carbon chains either side of charged carbon
What does a Markownikoff’s rule state
helps predict the product this states that when H-X is added to an unsymmetrical alkene the hydrogen becomes attached to carbon with most hydrogen atoms to start with - carboncations who have alkyl groups attached are more stable than those with hydrogen atoms attached
small molecules such as …
monomers join together to make long macromolecules chains
How do monomers form polymers
all monomers have a C=C double bodn made up of a sigma bond and pie bond, monomers have there pie bond broken and the electrons from each pie bond makes a sigma bond with a neighbouring carbon atom on a different monomer making a long chain
why do we use repeating pattern?
useful as polymers are made up of millions of atoms so hard to use a structural formula,
square brackets show its part of a polymer, N is the number of repeating units
why cant addition polymers be broken down
they are saturated compounds so are very stable they are not attacked by microorganism or environmental conditions therefore do not change or erode
Name ways in which you can dispose of plastic
- landfill
- combustion
- combustion with electrical generator
- recycling
- using them as organic feedstock
- reusing
Describe Landfill
- Large holes dug in the ground,
- lined to stop contaminated compounds seeping into the water
- put in holes and compacted
- when full the site is caped and landscaped
What are the negative with landfill
- after being buried atmospheric conditions change to anaerobic and there is a limited supply of water
- this reduces rate of biodegradable materials so waste does not break down
Describe the process of combustion
plastics are mainly organic therefore can be burnt, this can be used to heat steam which turns a turbine and generates electricity
What are the positive and negatives of combustion
- releases carbon dioxide this can cause climate change
- HCL can be made this is removed by using gas scrubbers where a base of calcium oxide neutralises the acidic conditions
- plastics have a high calorific value so burnt in a power station and chemical energy can be used to transfer to drive turbines and generate electricity
Name a positive for reusing
can be used for some function many times
what are the positives and negative for recycling plastics
- plastics must be sorted into different types, this is expensive, labour intensive or high cost of technology
- cleared melted down and reshaped
- toxic wastes have to be removed
- not all plastics can be recycled
- no more crude oil is wasted
- quality concerns could be contaminated plastics
What is organic feedback used for
After waste has been sorted into different types a series of chemical reactions can be used to break plastic polymers to smaller organic molecules.
Names a positive for organic feedback
+ chemicals can be used in other industrial reactions
How do biodegradable polymers help the breakdown of plastics
a biodegradable polymer such as plant starch is mixed with addition polymer like poly(ethane), starch breaks down making the polymer chain smaller and biodegradable
- small pieces of addition polymer may still cause harm
What is a bioplastic
New biodegradable polymer which can be used to make bin bags
What are the pros and cons of bioplastic
- if starch is derived from sustainable farming methods then the material could be classified as carbon neutral
- plastics must decompose landfill material quickly as compost will form
- only products should be CO2, H20, inorganic compounds and biomass
- poly(lactic) acid decomposes in about 180 days and is used for cold drinks and to wrap fruit and veg
How do photodegradable polymers work
Photodegradable plastics break down chemically using energy with wavelengths similar to light
- either addition polymers with bonds within their structure are weakened by absorption light then breaks down
- May not be exposed to light in a landfill
Draw ethanol in water with hydrogne bonding
h bond from OH goes to water
