Chapter 13 - Alkenes Flashcards
alkenes contain at least?
one C=C
GF?
CnH2n
Have similar ?
Chemical and physical properties
the π bond can only form after?
σ bond is formed
π bond?
- is a region of high electron density - the bond is the reactive region of the molecule
- restricts the rotation around the C=C
in the region of the double bond, they have a ? shape
flat, are ‘planar’
shape around C=C?
Trigonal Planar - 120 bond angle
are ? reactive than alkanes?
more
uses of hydrogenation?
- Veg and animal oil/ fats have a high proportion f unsat fats
- presence of C=C means chains don’t fit well together
➡ liquids at room temp ➡ makes them difficult to use as spread? - but we can change this
how can we change it (uses of halogenation cont.)?
- by hydrogenating the fats, we can raise their b.p.
- bc sat. fats have higher bp ➡ this produces margarine
but there are some ____ of incl unsat. fats in the diet?
benefits
are there are also some ____ of eating certain sat. fats?
Negatives
transfats?
- the high temps in this process produce a trans arrangement of molecules
- called trans-fats- these are thought to contribute to raised cholesterol levels
- this can have many negative impacts on health
when can 2 products be formed ?
when a H halide is added to an unsymmetrical alkene
what are polymers?
macromolecules, made from small repeating units - monomers
which bond is broken in the monomer in polymerisation?
- the π bond
- the electrons from these bonds are used to make sigma bonds w neighbouring C atoms
steps to deduce the structure of the polymer from monomer?
1: draw monomer w the C=C as the focus
2: draw square brackets around the monomer
3: change monomer to polymer- change C=C ➡C-C and draw 2 lines extending from each C THROUGH []
4: add subscript n (left side)
steps to deduce monomer from polymer?
1: identify repeating unit
2: draw monomer w C-C as focus
3: Change C-C ➡ C=C
What are addition polymers used in?
plastics, clothing
Problem with addition polymers: crude oil?
are often made from materials derived from crude oil - a non- renewable resource - there’s a finite amount of it
Problem with addition polymers: very stable?
- Addition polymers are very stable bc they are sat. organic compounds
- plastics they make can remain chemically unchanged for a number of years
- as household waste is aroun 10% plastic, this is a problem
how polymers r dealt with?Landfil?
- rubbish and waste can be buried in large holes
- the non biodegradable nature of a lot of this waste can make it a danger for wildlife
how polymers r dealt with? combustion?
- plastics can be burnt
- this process releases: CO2 - greenhouse gas
- depending on how the plastic burns, can als release other chems inlcu: Hydrogen Chloride gas - toxic but can be removed using gas scrubbers
how polymers r dealt with? combustion- useful?
- plastics can be burnt in power stations & the energy released can be sued to generate electricity
how polymers r dealt with? reusing?
- an alt. to landfill
- some plastics can be used more than once
Bioplastics?
- are even more sustainable than biodegradable polymers
- are materials made from renewable resources that are biodegradable
- contain only biodeg. polymers
- if the plant starch is grown sustainably, then the plastic is ‘carbon neutral’
carbon neutral meaning?
the C released is offset by that taken in
issues with bioplastics?
- land is needed to grow th plants ➡ this land could be used in other ways - e.g. to grow food
- the polymer breaks down in the envir so they cannot be used in other, useful ways - e.g. electricity production
problem with photodegradable polymers?
a lack of light such as in the bottom of a landfill pit may mean that these plastics don’t break down
E/Z isomers - Z?
- stands for Zusammen - german for together
- highest priority substituents on the same side of C=C
- Add Z- to normal name
E/Z isomers -E?
- E stands for entgegen - german for opposite
- highest priority substituents on opposite sides of C=C
- E- infront of normal names
Naming E/Z isomers steps?
1: draw the c=c as the focus
2: assign priorities
3: decide if isomer is E or Z
4: add E- or Z- in front of NORMAL NAME
E isomer =
Trans Isomer
Z isomer =
Cis Isomer
what is Zcis?
what does each line of the arrow head show?
the movement of 1 ELECTRON
- a ➡ (arrow with 2 lines) shows movement of a PAIR
what is bond fission?
breaking of chemical bonds
the fission of covalent bonds can be classified into 2 categories:?
homolytic or heterolytic
homolytic fission?
(homo = same, think homosexual)
- when the bond breaks, each electron goes to diff atom in the bond
- results in the formation of free radicals
heterolytic fission?
- when the bond breaks both electrons in the pair go to the same atom in the bond
- results in the formation of ions
bond formation?
- can be regarded as the opp. of bond fission
- bonds r formed on the collision of: 2 free radicals or oppositely charged ions
structural formula include?
the double bond
shape?
trigonal planar, 120 degrees
sigma bonds are formed when?
orbitals overlap between bonding atoms
A double bond is made up of?
a sigma and a pi bond
when does a pi bond form?
when adjacent p orbitals overlap sideways above and below the bonding C atoms
stereoisomerism must have?
diff groups on both C atoms of C=C - not diff one side and same the other
cis trans isomerism is?
there is an identical group on each end of the double bond. both C atoms have 1 of the group attached
stereoisomerism definition?
Compounds w the same structural formula and molecular formula but w a different arrangement in space
e.g.s of stereoisomersm?
- E/Z isomerism
- Optical isomerism
why does E/Z isomerism occur?
- the double bond is rigid, so the groups can stay where they are
- would have to break the c=c to rotate around it - no rotation around C=C
- if both C of the C=C have diff groups attached, the molecule has E/Z isomers
E/Z- assigning priorities?
assigned by priority of groups attached to C=C - the higher the atomic no. of the group attached to the C’s, the higher the priority (if they are the same, look at the atoms attached to those atoms)
when deciding if ` E/Z isomerism?
first draw double bond as focus
energy needed to break the sigma bond of C=C
347kJ
Energy needed to break C=C?
612kJ
Energy needed to break the Pi bond of C=C?
612 - 347 = 265kJ
explain which bond is broken when alkenes react?
the pi bond is weaker than the sigma bond so it is broken more readily - less enthalpy required to break
addition reaction =?
- alkenes usually take part in addition reactions across the double bond
- a small molecule os added across the double bond causing the Pi bond to be broken
- 2 reactant molecules form one product molecule
- unsaturated mol forms a sat mol
addition - hydrogen in the presence of a suitable catalyst e.g. Ni to form alkanes
- Hydrogen gas and gaseous alkene passed over a Ni catalyst at 150 degrees
- a.k.a hydrogenation
- used to make margarine
Addition - of halogens to form dihaloalkanes
- alkenes react rapidly w halogens at room temp
- known as halogenation
testing for alkene functional group (C=C)?
- Using bromine water gives a colour change: orange-> colourless
propene + bromine -> C3H6Br2
hydrogen halides to form haloalkanes?
- HCL, HBr, HI
- produces a halogen alkane
addition: steam in the presence of an acid catalyst to form alcohols?
- high pressure & temp, a phosphoric acid catalyst
- a.k.a hydration
what is an electrophile?
a species that accepts a pair of electrons to form a covalent bond
how many arrows in electrophilic addition?
3 - CURLY
Curly arrows go from?
where the electrons are to where they will go
as Br2 moves towards propene?
there is no permanent dipole in Br2. as the Br approaches the C=C the electrons in the double bond repel the e-s in the Br2 molecule and induce a dipole
electrophilic addition - which carbocation will form more readily?
the more stable carbocation will form more readily
electrophilic addition: Hbr and but-2-ene conditions?
room temp
addition reaction definition?
where a reactant is added to an unsaturated molecule to form a saturated molecule
curly arrow definition?
the movement of a pair of e-s
mechanism definition?
the series of steps that show how a reaction takes place
carbocation definition?
an ion that contains a + charged C atom
heterolytic fission definition?
where a covalent bond breaks and both e-s go to one atom forming a + ion and a - ion
markownikoff’s rule?
- the H of a H halide or a H2O molecule will attach to the C of the C=C with the most H attached
electrophilic addition mechanism applied to when steam is added to an alkene ?
the phosphoric acid supplies H ions. The H+ ions act as an electrophile bc they have a + charge
electrophilic addition mechanism C3H6 + Br2 first step?
- the propene molecule contains a C=C so is susceptible to an electrophilic attack
- Br2 is non polar, but a dipole is induced as it approaches the propene molecule
- an e- pair from the double bond moves to the + end of the Br molecule forming a new covalent bond
- this causes heterolytic fission of the Br-Br bond
electrophilic addition mechanism C3H6 + Br2 intermediate?
- a Br- ion and a carbocation r formed
- the carbocation os a highly reactive species and the lone pair of e-s move from the bromide ion towards the + charge
- a new covalent bond is formed
how to decide which product is more stable?
- more stable = more alkyl groups attached to that carocation
what is the atom economy of addition reactions of alkenes?
- the addition reactions of alkenes have only 1 desired product. This means in terms of using all of the reactant atoms to form desired product, they are 100% efficient
- 100%
Atom economy equation?
mr of desired product/ mr of all products x 100
what is fractional distillation?
- different fractions in crude oil have different BPs
- ⬆ the temp of the system so each fraction vaporises separately from the other fractions & can be collected
NOTE: PRIORITY OF E/Z ISOMERISM?
REMEMBER: if the 2 atoms attached to a C atom in the double bond are the same, then u will need to find the first point of difference. The group which has the highest atomic number at the FIRST POINT OF DIFF is given the highest priority
- but if equivilant, the one w the more substituents of the high atomic number gets priority
but…
E/Z isomerism may not agree w Cis/ Trans isomerism e.g. 2-bromobut-2-ene
what does a triple bond contain?
- 1 sigma and 2 pi bonds
a pi bond comes from the ?
overlap of unhybridised orbitals
Sigma bond formation (detailed)?
- uses one s orbital and 3 p orbitals
- these 4 orbitals mix together and make 4 sp3 orbitals , arranged in a tetradehral shape
- bond angle 109.5
- when these sp3 orbitals overlap DIRECTLY w other orbitals, single bonds are formed called sigma bonds
double bond formation (detail)?
- when a double bond forms, 2 p orbitals mix w one s orbital forming three sp2 orbitals
- w trigonal planar, 120
- the third p orbital is at 90 degrees to the 3 sp2 orbitals
- the 3rd p orbital then overlap sideways to form a pi bond
when an alkene has 2 double bonds?
called a diene instead of an -ene
polypropene use?
plastic tubing, toys, packing crates, guttering, fibre for ropes
poly(chloroethene) use?
polyvinylchloride (PVC)
cable insulation, drain pipes, flooring
poly(but-2-ene) use?
pipes
poly(ethene) use?
bags, bottles, toys,
poly(tetrafluoroethene) uses?
PTFE - ‘Teflon’, used as coating for non-stick pans, permeable membrane for clothing and shoes, and cable insulation
poly(phenylethene) uses?
(a.k.a polysytrene)
packaging - paticularly for food such as poultry and eggs and in food trays and cups due to its thermal insulating properties
polymethylmetracrylate uses?
- used a substitute for glass in products such as shatterproof window
poly(1,1-dichloroethene) uses?
- cling film
- food packaging
describe addition polymers of alkenes?
- unsat alkene molecules undergo AP to produce long sat alkane chain called polymers
- the alkene molecule are called monomers
- the Pi bond of the alkene break and the monomers add together
- usually addition only have 1 type of monomer and the formula shows the repeat unit of the polymer
- the repeat unit is a specific arrangement of atoms in the polymer that is repeated over and over again
- its written in [] with an ‘n’ to show there is a large no. of repeats
sustainability definition?
making good use of our waste materials to prevent us from using up more of the earth’s raw materials
compostable explanation?
- degrade and leave no visible or toxic residues, CPs based on polylactic acid are becoming more common as an alt to alkene-based polymers
polylactic acid is not an addition polymer but its molecules can be broken down when they react w water. the polymer decomposes naturally so it does not cause pollution. It can be used to make plastic drinks cups
recyling feedstock exp?
this can involve converting polymers into synthesis gas. This is a mixture of H and CO that can be reacted w hydrocarbons to make fuels. Polymers can be ‘cracked’ to make useful shorter hydrocarbons and alkenes
biodegradabe exp?
these polymers contain cellulose, starch or sugars which breaks down to form CO2 and H20 by biological processes
incineraton exp?
polymers can be burnt under controlled conditions to create heat to make electricity, but care is needed as some polymers like PVC can release toxic fumes when burnt
recycling definition?
it is possible to do this with waste polymers but it’s not w/o problems. The polymers have to be collected, sorted and processed which means that it can cost more than getting the plastic directly from crude oil. Scientists are not involved in the collecting and sorting
biodeg polymers?
- contain cellulose, starch, or sugars which breaks down to form CO2 + H2O by biological processes
- compostable polymers degrade naturally bc they have ester groups that can be hydrolysed
photodegradable polymers?
contain bonds, usually C=O that are broken down by light energy
bioplastics?
- produced from renewable plant materials like starch and plant oil, offer a renewable and sustainable alt to oil-based products. protects envir+ conserves valuable oil reserves
- as tech advances, are more likely to be used in packaging, electronics, and more fuel efficient and recyclable vehicles
outline the role of chemists in minimising envir damage caused by toxic waste from the combustion of polymers?
- chemists ensure that polymers are combusted in plentiful supply of 02 ➡ prevents incomplete combus which might produce toxic CO or unburnt C
- halogenated plastics like PVC can produce HCl when burnt ➡ the acidic gas can be removed in a neutralisation reaction w an alkali ike NaOH
Whilst branched alkenes obey the general formula, what does not?
cyclic alkenes and alkenes w more than 1 double bond
why is the C=C so rigid?
the position of the Pi bond’s electron density above and below the plane of the sigma bond
what happens when bromine reacts w an alkene?
when bromine water (an orange solution) is added dropwise to an alkene, Br adds across the double bond. The orange colour disappears
why does carbocation stability increase w the no. of alkyl groups?
- carbocation stability is linked to the e- donating ability of alkyl groups. Each alkyl group donates and pushes e-s towards the + charge of the carbocation. The + charge is spread over the alkyl groups. The more alkyl groups attached to the + charged C atom, the more the more the charge is spread out, making the ion more stable.
- thfr, tertiary carbocations are more stable than secondary which are more stable than primary
what is a repeat unit?
the specific arrangement of atoms in the polymer molecule that repeat over and over again
- always written in []
what is the benefit of using polymers?
- are readily available, cheap to purchase, more convenient for our throwaway society than alts such as glass bottles
- lack of reactivity makes them suitable for storing food and chemicals safely
recylcing polymers?
- reduces their envir impact by conserving finity fossil fuels as well as decreasing the amount of waste going to landfill.
- have to be sorted, if mixed the product is unusable
- after sorting, they’re chopped into flakes, washed, dried, melted
PVC recycling?
- the disposal and recycling of PVC is hazardous due to high Cl content and the range of additives present in the polymer.
- landill not sustainable, burning releases HCl (g) a corrosive gas & other pollutants like toxic dioxins
- new tech uses solvents to dissolve it
using waste polymers as fuel?
- some polymers are diff to recycle
- as derived from petroleum or natural gas, ahve a high stored energy value
- waste polymers can be incinerated toproduce heat, generating steam to drive a turbine producing electricity
feedstock recycling?
- describes the chemical and thermal processes that reclaim monomers, gases or oil from waste polymers.
- these materials can be used as raw materials for production of new polymers
- major + is that it’s able to handle unsorted and unwashed polymers
