Alkenes Flashcards
alkene general formula
Cn Hn2
two types of bonds from orbital overlaps
sigma
pi
sigma bond formation
overlaps from end to end across an axis
can be s - s , s - p, p - p
where are the electrons found in a sigma bond
between the nuclei of the two atoms
the electrostatic attraction between the electrons and the nuclei bonds the atoms to each other
what does hydrogen do reffering to bonds
hydrogen has just one s orbital
the s orbitals of the two hydrogen atoms will overlap to form a sigma bond
pi bond formation
pi bonds are formed from a sideways overlap of adjacent p orbitals
the two “dumbell heads” of the orbitals lie below and above the x axis
c=c bond structure in ethene
three of four e- are in sigma bonds:
two sigma bonds formed by the hydrogen atoms
one sigma bond formed with other carbon
Fourth electron occupies a p orbital on the other carbon atom to form a pi bond (overlapping sideways with other c atom):
c=c is double bond because it has one pi and one sigma bond
why are pi bonds weaker than sigma bonds
less area of overlapping between orbitals causes weaker attraction between electrons and nuclei.
What makes polymers ideal for many uses but also makes them difficult to biodegrade
Low reactivity
Ways to encourage recycling
Recycling points at landfill sites
How does the recycling of polymers reduce the use of finite resources
Lots of polymers are made from crude oil( non renewable fossil fuel)
Disadvantage of recycling polymers
Time consuming as they have to be sorted into different categories
Disadvantage of recycling polymers
Time consuming
How does the recycling of polymers reduce the use of finite materials
Lots of polymers are made up of crude oil - a fossil fuel and non renewable
Why can incineration of polymers occur
Large amount of energy stored within the polymer chains
Advantage of incineration
Process can be used to boil water and use the water vapour to turn turbines inside a power station
Process of recycling
Chopped, washed and dried then cast into pellets
Problems in recycling
Large amounts of toxic chemicals can be released
Why does incineration still cause pollution
Carbon in polymer is released as co2
Why does incineration still cause pollution
Carbon in polymer is released as co2
What is feedstock recycling
Wate polymers are broken down by chemical and thermal processes into monomers , gases and oils
These products can then be used as raw materials in the production of new polymers and other organic chemicals
Major benefit of feedstock recycling
Works with unescorted and unwashed polymers
What are chemists trying to do to make polymers better
Use chemicals that are safe and environmentally friendly
Reduce amount of chemicals used
Use renewable feedstock chemicals where possible
Reduce the energy requirements for the reaction
Improve atom economy
Consider the lifespan of the polymer
Way to remove Hcl gas from an incinerator
Wasted gas is reacted with a base or carbonate
Base reacts with the Hcl gas neutralising it\
CaO (s) + 2HCl (aq)→ CaCl2 (aq) + H2O (l)
Biodegradable polymers
Can be broken down over time by microorganisms
Example of condensation polymers that are biodegradable
Polyester and polyamide can be broken down in hydrolysis reactions
When taken to landfill they can be broken down easily and their products used for other applications
high electron density in double bond of alkenes
four electrons found on the bond
electrophiles
electron loving species
c=c are susceptable to …
electrophiles due to their high electron density
what is electrophilic addition
addition of an electrophile to a double bond
what does electrophilic addition include the addition of
C=c becomes c-c
hydrogen
steam (H2O)
hydrogen halide (HX)
Halogen
hydrogenation electrophilic addition
c2h4 +H2 —> c2h6
Steam electrophilic addition
c2h4 + H2O —> ethanol
hydrogen halides electrophilic addition
propene + HBr —> bromopropane (room temp needed)
halogens electrophilic addition
C2H4 + halogen (↓2) —> Halogenoalkane
manufacture of margerine
hydrogenation
naturally occuring veg oils are unsaturated and contain c=c
bonds react with hydrogen to become c-c
process changes veg oil from liquid to solid
oxidation of alkenes
by KMnO4 - oxidisng agent
when shaken , goes from purple to colourless (product is a diol)
means that the reaction can be used to distinguish alkanes from alkenes
heterolytic fission
breaking covelant bonds in such a way that the more electronegative atom takes both electrons from the bond to form a negative atom and takes both the electrons from the bond to form a negative ion and leaving a positive ion behind
simply put a bond is split and one of the atoms takes both the electrons in the shared pair
polarity of hydrogen halides
yes as hydrogen is positive and haliedes are negative
why does bromine have a stronger pull on e- in H-Br bond
polarity of HBr
This means Br has a partiall negative and hydrogen has a partial postive
electrophile in an addition reaction
H atom as it accepts a pair of electrons from the c-c bond in the alkene
H-Br breaks heteolytically forming a Br- ion
The mechanics of an alkene electrophilic addition (Br - Br)
alkene is reacted with a halogen
the positive atom in the halogen moves towards the alkene
the positive atom draws the electron in the c=c bond of the alkene towards the other halogen atom which is now a negative ion (opposites attract)
this leaves a halogenalkane with a positive charge
this leftover positive charge attracts the negative half of the previous halogen
negative half of halogen also joins to alkane
resultibg in dihalogenalkane
The mechanics of an alkene electrophilic addition (H - Br) or any halogen
alkene is reacted with H-Br
H has a slight positive charge while the Br has a slight negative charge
this means that the hydrogen attracts the electrons in the alkene
these electrons are passed to the bromine
if the hydrogen then bonds to the second carbon the 1 bromopropane will form
if hydrogen bonds to the first carbon then 2 bromopropane will form
thsis is after the propane is left with a positive charge which then attracts the neagtive bromine
how to tell which alkane is more likely to form
more alkyl groups on the carbon with positive charge = more stable and more likely to form
