Alkenes Flashcards
What are alkenes and some if their properties and uses
Ethene (C2H4), gas at room temp/pressure, is the first member of the homologous series of alkenes.
- one of the most versatile organic compounds.
- used in plastics; polythene, PVC and polystyrene.
- used to manufacture raw materials for industrial processes (ethanal, ethanol.)
- are unsaturated hydrocarbons
> at least one double bond C=C.
> general formula CnH2n
C=C double bond is a centre of high electron density
So are more reactive than alkanes.
What is EZ isomerism
Isomers are molecules with same molecular formula but diff struc formula
Eg But-1-ene and but-2-ene (structural isomers)
»_space; C=C is in a different position in each molecule - positional isomers.
..
But are two possible structures, A and B, for but-2-ene.
- To form molecule B (E) from molecule A (Z), part of the C=C must be broken.
- Bond breaking need energy; not available at room temp
- restricted rotation around the planar double bond.
Molecules A and B are examples of a type of isomerism = stereoisomerism.
Stereoisomers are molecules with the same molecular and struc formula
>but a different 3D arrangement of atoms in space.
When diff is due to the positions around double bond, the isomers are E-Z isomers.
- named either E or Z isomers due to orientation of groups attached to the C atoms of C=C
- The prefix Z says groups are on the same side of the double bond on diff carbons.
- The prefix E says groups are on opposite sides of double bond on carbons.
How to decide whether more complicated molecules are E or Z isomers
They designated as E or Z isomers by ranking groups bonded on either side of double bond.
- ranked using a set of rules - (CIP) priority rules.
- The higher the atomic number of element bonded to carbons in C=C, the higher ranking.
-
- When higher priority substituents are on same side of plane of C=C isomer is Z
- on opposite sides of plane of C=C, the isomer is E
To determine rank order, first find atomic number of atoms attached to carbons in bond.
>If these atoms are same ,consider just atomic number of atoms connected directly to that same atom
..
eg 1-bromo-2-chloro-1-fluoroethene.
- four diff groups attached to carbon atoms of the C=C.
- Consider each carbon of C=C in turn and rank the groups
The higher priority groups are on same side of plane of double bond (Z)
And if opposite sides of plane (E)
How to draw alkenes as polymers
Look at no.1
What are production and uses of polymers
Addition polymers are unreactive as are connected as unreactive alkane molecules, without a C=C double bond.
»widespread use as inert materials.
- useful as insulators, as packaging and in making containers.
-
- Two forms of poly(ethene) exist, high density (HDPE) and low density (LDPE).
•HD poly(ethene) is less flexible and is used for kitchenware
•LD poly(ethene) is more flexible , used for plastic bags and plastic bottles.
–>A catalyst is used to produce HDPE from ethene.
- Poly(chloroethene) (PVC) is a rigid plastic
used for window and door frames and drainpipes.
> may also be used for clothing/ electrical wire insulation. The difference is whether theres a plasticiser. - UPVC is unplasticised PVC; rigid and inflexible
used for window and door frames and drainpipes. - If a plasticiser molecule is added (usually diff phthalates), PVC becomes more flexible
used for clothing like wellington boots or raincoats.
Whats the polymer uses table (recycling code)
..
Polymer recycling code Uses
•Polylethene (PE)
plastic bags, bottles,
film wrapping, kitchenware
•Poly(propene) (PP)
ropes, thinsulate clothing,
carpets, crates, furniture
•polylchloroethene) (PVC)
wellington boots, raincoats,
drainpipes, window frames,
door frames, electrical wire insulation
•Poly(phenylethene) (PS)
- expanded polystyrene is
used for insulation in houses and packing, - unexpanded polystyrene is
used for toys
and containers
What are forces like between polymers
- Strong covalent bonds join atoms to each other in individual polymer molecules.
- Weak vdw intermolecular forces occur between polymer molecules.
A polymer will melt when intermolecular forces are overcome.
>
>stronger the forces, the more energy to break so higher material’s melting point is.
>As hydrocarbons are very long, the vdw forces between chains are very strong
>polymers have high melt/boiling points; chain length is variable so vdw are variable strength (melt gradually)
..
Polymers with few branches are compact; chains pack together efficiently.
- as closely packed, the vdw forces between chains are strong
- these polymers tend to be stronger and harder, eg HDPE
-
Polymers highly branched can pack together as well like LDPE
- Since chains are not closely packed, vdws between chains are weaker
- these polymers tend to be weaker and softer.
To do when asked to circle/draw repeating unit in a polymer
OR draw monomer forming polymer
When circling repeating unit
There can be more than one possibility, with branches swtched round
» these diff repeating units come from same monomer
When drawing repeating unit,
NO brackets around it and NO n after it to indicate the repeat.
- bonds need to be shown on either side to show how join
What are electrophiles
The double bond in alkenes is an area of high electron density,
making homologous series susceptible to attack
»_space; from a class of substances = electrophiles.
Electrophiles are substances which can accept a pair of electrons.
- Polar molecules (HBr) are electrophiles. - The delta+ side of polar molecule is attracted to area of high electron density
- between C atoms of double bond.
-
Contrast this with nucleophiles
- Neutral molecules (Br2) also react with the C=C in alkenes.
- high electron density concentrated at C=C polarises neutral Br2
»_space; it becomes an electrophile.
What are two electrophilic addition reactions of alkenes
1.
C2H4 + HBr (electrophile)
–> C2H5Br
- C2H4 + Br2 (nucleophile)
–> C2H4Br2
The reactions described (by balanced chemical equations) are addition reactions
- as two molecules react to form one
- In the reaction with ethene and bromine
- the isomer 1,1-dibromoethane is not formed.
-
- The absence of this isomer is explained
by considering the mechanism of the reaction,
between electrophiles and alkenes.
Whats the mechanism with alkenes reacting with polar electrophiles
(Ad+—Bd-)
- Electrons move from high electron density area (between two carbon atoms)
to the d+ side of the electrophile. - Electrons in the A—B bond move to the Bd-.
- One C bonds to A; the other C has a positive charge.
-> A species containing +charge on a carbon atom is known as a carbocation. - B now has a -charge, shown with a lone pair of electrons.
> lone electron pair on anion is attracted to +charge on C
> one molecule is formed. - The double bond has been broken
and A-B has been added across this double bond.
>A and B are now bonded to different carbons.
..
The electrophile bond has broken unevenly.
> Both electrons the shared pair have gone to one atom. (heterolytic fission)
homolytic fission is like chlorine bond in UV light
In free radical substitution of alkanes to produce halogenoalkanes
- A—B bond has undergone heterolytic fission; each atom bonds to different carbons.
- The product, unlike reactant, is saturated
The electrophilic addition of hydrogen bromide to an alkene
Hydrogen bromide is a polar electrophile.
>The addition product is a halogenoalkane.
> This reaction will take place at fast rate
even at temps below room temp.
(Eg. ethene and HBr, but2ene and HBr)
Mechanism is no.2
Whats heterolytic fission
Heterolytic fission occurs when a
covalent bond breaks and both electrons in the bond
move to one of the atoms.
»_space;Two oppositely charged ions are formed.
The electrophilic addition of concentrated sulfuric acid to an alkene
Concentrated sulfuric acid, H,SO, is a polar electrophile.
> The addition product is an alkyl hydrogensulfate.
> The reaction will happen even at temperatures below room temperature.
Mechanism is no.3
The mechanism of the reaction of alkenes with neutral electrophiles, A-A
mechanism is similar to electrophilic addition of polar nucleophiles.
>However, theres one extra initial step, polarisation of the A-A molecule
- The A-A molecule is not polar.
When A-A molecule moves close to
double bond (area of high electron density) it becomes polarised
»_space;electrons in double bond repel ones in neutral molecules - Electrons move from area of high electron density between two carbon atoms
to the d+ side of the electrophile.
- Electrons in the A-A bond move to the Ad- - One carbon is bonded to A
and the other has a positive charge.
- A species with +charge on C atom is a carbocation.
- The other A now has a full negative charge and is shown with a lone pair of electrons. - The lone pair of electrons on anion is attracted to +charge on carbon atom
>one molecule is formed.
>
The molecule, A-A, has broken and
added across the double bond. (A and A are now bonded to different carbons.)
mechanism no 4
The electrophilic addition of bromine to an alkene
(And what test it is)
Bromine is a neutral electrophile.
> The addition product is a halogenoalkane.
> Bromine gas can be mixed with/bubbled through an alkene.
The reaction will happen in absence of light and at temps below room temperature.
The reactant mixture is coloured due to the presence of bromine;
the product mixture is colourless.
» Bromine will react with any molecule which has one or more double bonds.
mechanism no5
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A variation of this reaction is used tests for unsaturation
or for double bond, C=C.
-
- Bromine is added to water to produce bromine water (orange in colour.)
- It will react with an alkene to form a colourless product.
bromine water is mixed with sample of organic substance.
When the colour changes orange to colourless without gas releasing,
- carbon-carbon double bond is confirmed
Electrophilic addition reactions to an unsymmetrical alkene
But-1-ene and but-2-ene are isomers.»_space;But-2-ene exists as two stereoisomers,
Z-but-2-ene and E-but-2-ene
-
These stereoisomers are symmetrical alkenes as groups on each C in the double bond are same.
- Each carbon of the double bond has a H and a methyl group attached.
..
- But-1-ene is an unsymmetrical alkene
Each carbon of C=C is bonded to a diff set of groups.
- One C is bonded to two H atoms while other is bonded to an ethyl group,
and H
This difference does not affect mechanism stages of electrophilic addition
– but it does lead to the possibility of two different products.
- one present in a greater proportion (the major product) than the other (the minor product).
Why do two different carbocations form, and so two different compounds
Theres the possibility of two diff carbocations forming
> depending on wch C the H from the HBr adds to.
-
- In route 1, H from HBr adds to left C,
producing +charge on C on the right,
- leading to 2-bromobutane formed.
- In Route 2 H from HBr adds to right carbon, producing +charge on left carbon - 1-bromoethane is the addition product.
..
In practice both form but approximately 90% of mixture is 2-bromobutane;
> the major product.
> due to relative stability of diff carbocations, wch may form as reaction progresses.
»_space; rel stability is ability of compound to resist changing/altering to other compounds
Carbocations are classified as, primary, secondary and tertiary
- depending on the number of alkyl groups, -R, attached
(Alkyl groups are methyl, ethyl etc)
** diagram no6**
Whats the electron donating effect in alkyl groups
Alkyl groups (methyl (-CH) or ethyl (-CHzCH3))
- have an electron donating effect like H atom.
- ‘push’ electrons away from group, towards opposite end of bond
-
- sometimes referred to as a positive inductive effect.
The electron donating effect is shown by arrow on R-C bond
OPPOSITE TO ELECTRONEGATIVITY
..
Electron donating groups decrease size of -charge on carbon
» carbocation becomes more stable.
>
>The greater number of groups the more stable ion is.
Tertiary carbocations are more stable than secondary,
which are more stable than primary.
The relative stability of carbocation come between effects of product formed.
> More stable carbocations remain for longer in reaction mixture
> have a greater chance of reacting with an anion to form a product. (major)
What did markovnikov say abt relative stability in compounds
The Russian chemist, Markovnikov, summarised the reaction by stating,
- When a compound HX (eg HBr) is added to an asymmetrical alkene
- the hydrogen becomes attached to the carbon atom of double bond
- which has the most hydrogens directly attached’.
Whats the industrial production of ethanol from ethene
Ethanol is an alcohol.
- Alcohols are a homologous series
- with the general formula C,H2+OH
- and the functional group -OH
-
Ethanol can be produced by reacting ethene with steam
>using a concentrated strong acid as a catalyst.
..
The addition of water molecule to a compound is known as hydration.
- This reaction is used industrially to manufacture ethanol from ethene
- using conc phosphoric acid as a catalyst.
-
- Ethene and steam pass over a conc phosphoric acid catalyst
absorbed on a solid silica surface.
- The reaction mixture is 60 atm and 600K.
These conditions ensure almost 100% yield but side reactions (methanol and poly(ethene)) can reduce yield a little.
Whats difference between hydration and hydrolysis
Hydration and hydrolysis are different.
Hydration is the addition of water to a substance.
Hydrolysis is the decomposition of a chemical compound by reaction with water.
