Chapter 13 - alkenes Flashcards
What is an alkene?
an alkene is an unsaturated hydrocarbon with the general formula CnH2n
alkenes can also be cyclical CnH2n-2
what is a sigma bond
the overlap of orbitals (usually s orbitals) between adjacent atoms to form a shared pair of electrons with an electrostatic attraction to the nuclei of the bonded atoms
what is a pi bond
the sideways overlap of P orbitals between adjacent atoms to form a shared pair of electrons with an electrostatic attraction to the nuclei of the bonded atoms
what is a double bond
a double bond is made up of a sigma bond and a pi bond
where does the pi bond occur
the pi bond occurs either above or below the sigma bond but is constantly changing
which bond is weaker and why
the pi bond is weaker than the sigma bond because its further from the nucleus and thus the electrons experience less electrostatic attraction to the nucleus.
what is a stereoisomer
“a stereoisomer is an isomer that has the same structural formula but a different arrangement in space”
this occurs because no rotation can occur around a double bond
what are the properties of a double bond
bond enthalpy is almost double the bond enthalpy of a single bond
no rotation can occur around a double bond
what is E-Z isomerism
there must be a C-_C double bond and different groups attached to the C’s of the double bond
A C
\ /
C = C A and B must be different
/ \
B D
what is an E isomer
an isomer where the two groups that are the same (or highest priority) are on opposite sides of the molecule
(comes from German entgegen (opposite))
they ‘ent on the same side
what is a Z isomer
an isomer where the two groups that are the same (or highest priority) are on the same side of the molecule
(comes from the German zusammen (together))
they are on Z same side
when is E-Z isomerism Cis-Trans isomerism
when one of the groups on each side is a hydrogen
what is the Cahn-Ingold-Prelog rule and why is it used
it is used to assign priority where any 2 of the 4 groups attatched to the carbon atoms are not the same.
the groups which are thought of as the same are the ones with the highest priority on each side
how does the Cahn-Ingold-Prelog rule work
priority is first assigned based on the atomic number of the groups immediately attached to the carbon, if their atomic number is the same, then you work off of the atomic numbers of the atoms immediately attached to them and continue until there is a difference.
REMEMBER A DOUBLE BOND DOUBLES ATOMIC NUMBER
why are alkenes far more reactive than alkanes
the pi bond can break fairly easily to attach to other atoms and molecules.
what sort of reactions do alkenes take part in
with hydrogen (and a nickel catalyst)
halogens
hydrogen halides
steam (with phosphoric acid catalyst)
hydrogenation of alkenes (occurs by electrophilic addition)
needs fairly high temperature (423K) and a nickel catalyst
if there’s more than one double bond, more than one hydrogenation can occur
H3C H H H H
\ / | | |
C = C + H2 ——> H - C - C - C - H
/ \ | | |
H H H H H
halogenation of alkenes
the same as in hydrogenation occurs but no catalyst or high temperatures are required.
this is also where the test for the bromination test comes from.
bromine attaches across the double bond and the colour disappears
addition reactions of alkenes with hydrogen halides
these can form two different products, the major product and the minor product
the major product tends to be the one where the halogen is in the middle
hydration of alkenes
alcohols are formed when alkenes react with steam in the presence of phosphoric acid catalyst
one of the H’s from the H2O attaches straight to a carbon, the OH then forms an alcohol group
what is the mechanism for addition reactions in alkenes
electrophilic addition
mechanism for hydrogen halides
1) the hydrogen halide is polar so when it approaches the double bond, the hydrogen (an electrophile) is attacked by the nucleophile double bond
2) an electron pair is donated to the hydrogen of the hydrogen halide and the hydrogen halide splits by heterolytic fission
3) you now have a carbocation and a bromide ion, these attract to each other and the bromide ion donates its electrons to the carbocation to form a haloalkane
differences in the mechanism for electrophilic addition using hydrogen or halogens rather than a hydrogen halide
the same as before but instead of naturally polar molecules, the molecules have induced dipoles formed when the molecule approaches the electron dense double bond region
what does Markownikoff’s rule state
“when a hydrogen halide reacts with an asymmetrical alkene the hydrogen of the hydrogen halide will attach itself to the carbon atom of the alkene with the greater number of hydrogens and the smaller number of carbons attached”
what are the types of carbocation
Primary - one alkyl group attached to the +ve carbon
Secondary- two alkyl groups attached to the +ve carbon
Tertiary- 3 alkyl groups attached to the +ve carbon
which carbocations form the major products
The secondary (or tertiary) carbocation will form the major product as this is the carbon with the greater number of alkyl groups attached. due to the electron ‘pushing’ ability of alkyl groups this spreads out the positive charge making the cation more energetically stable. Thus, it has a lower activation energy and forms the major product.
what are polymers?
polymers are extremely large molecules formed from 1000s of repeated monomer units
what is an electrophile
an electrophile is an atom or group of atoms attracted to an electron rich centre and which accepts an electron pair
what is the process of forming polymers called
addition polymerisation
what is poly(ethene) and what are HDPE and LDPE
poly(ethene) is one of the most common polymers, formed from many ethene units it is found in many places such as supermarket carrier bags.
HDPE is high density poly(ethene), this is an unbranched polymer meaning the molecules can get closer together, increasing density and bpt (because of London forces)
LDPE is low density poly(ethene), it is a branched polymer which has the opposite properties to HDPE
Poly(chloroethene) or PVC
a polymer made of monomer units which are ethene with one chlorine instead of a hydrogen
Poly(propene) and what to remember when drawing polymers
poly(propene) is fairly self explanatory
when drawing polymers make sure to draw everything attached to a carbon vertically and the C=C double bond horizontally
uses include children’s toys
polystyrene
polystyrene is like ethene monomer units but with a benzene ring on one of the carbons instead of one hydrogen
uses include packaging
poly(tetrafluoroethene)
Poly(tetrafluoroethene) is like ethene but where every hydrogen is replaced by fluorine (F)
uses include non-stick pans
overview of and benefits to recycling
reduces environmental impact by conserving fossil fuels
must however be sorted first
first chopped into flakes then washed, dried and melted down before being reused
overview and benefits of PVC recycling
normally disposing of PVC is difficult because of the high chlorine content of the polymer
Solvents are used to dissolve the polymer
high-grade PVC is then obtained through precipitation
the solvent is drained off and reused
overview and benefits of waste polymers as fuel
some polymers can’t be recycled
polymers have a high energy store because they are mostly hydrocarbons
feedstock recycling
refers to thermal and chemical processes used to reclaim monomer units from polymers
polymers don’t have to be sorted for this to occur
biodegradable polymers
usually made from cellulose or starch and broken down after disposal by microorganisms to form water, carbon dioxide and biological products
they leave no toxic residues
photodegradable polymers
similar to above but light weakens bonds
best things to write for new ways of disposing of polymers (4)
- Develop photo/biodegradable polymers
- Develop techniques for cracking polymers
- Develop use as a chemical feedstock
- Develop new ways of sorting AND recycling polymers
best things to write for other ways of disposing of waste polymers (doesn’t have to be new/future)
- separate into types and recycle
- burn them, for energy generation
- crack them
best ways of dealing with the environmental impact of PVC
- PVC recycling
- develop biodegradable alternatives
- remove HCl by reacting it with alkali (for example)
issues with disposing of PVC polymers
- non-biodegradable
- releases HCl/ toxic fumes when burnt
- can lead to chlorinated organic compounds when burnt (carcinogens)
what to remember to always include when writing about stereoisomers
NO FREE ROTATION AROUND A C=C DOUBLE BOND
what to watch out for when doing skeletal formulas of alkenes
stereoisomerism
can you obtain ethene directly through fractional distillation
no, it is obtained through the cracking of alkanes
what to watch out for when a diene undergoes electrophilic addition
some of the products may be the same despite different mechanisms
