Organic chemistry - alkenes Flashcards
substituent
atom or group of atoms which could be replaced by a hydrogen atom
test for alkenes
bromine water
(addition reaction)
drawing alkene
when drawing molecules/bonds a carbon is always bonded to central C not a hydrogen e.g C=C and CH3 or H3C depending on its position
suggest why a molecule may not be named using the cis-trans naming system
-it is a larger organic compound
-has more than one atom in common
which statement about ethene is correct
it burns in excess oxygen to produce CO2 and water
identify the feature of the double bond in the E and Z isomers that causes them to be stereoisomers
restriction rotation around the pi bond
Describe the two methods for forming a haloalkane
-addition of hydrogen halides (reacts with alkenes at room temperature)
-addition of halogens added to alkenes at room temp
addition
hydrogenation –> C=C + H2
Halogenation –> C=C + Br2
state what reagents are required to make propan-2-ol
-prop-2-ene
-steam
-phosphoric acid catalyst
alkene + water (steam) and phosphoric acid catalyst =
alcohol
explain why Br-Br is attracted to the C=C of the alkene
-C=C is an electron rich region
-Br-Br becomes polarised (not polarised before due to same electronegativity) when near the alkene double bond
-this means that the partially positive Br atom becomes attracted to the C=C
suggest why an electrophile would react with ethene not ethane
ethane has no electron rich area and no pi bond
arrangement of bonds around C=C
The arrangement of bonds around the
>C=C< is planar and has the bond angle 120
C=C double covalent bond
consists of one sigma (σ)
bond and one pi (π) bond.
stereoisomers
Stereoisomers have the same structural formulae
but have a different spatial arrangement of atoms.
Alkenes can exhibit a type of isomerism
called E-Z stereoisomerism
what are alkenes
-alkene –> homologous series of hydrocarbons that have an unsaturated bond between 2 carbon atoms (CnH2n)
-alkenes can be straight chained or cyclical
single vs double bonds
-A single bond is able to rotate freely which means that the above molecules are the same and are not isomers of each other
-A double bond is not able to rotate freely. This means that the above molecules are considered isomers of each other, locked in positions
sigma bond
A single covalent bond is known as a sigma bond which are formed by the overlapping of atomic orbitals
Pi bond (single)
Pi bonds –> formed by the lateral overlapping of two lobes –> additional type of bond found in molecules with double and triple bonds
geometric isomers
-There is restricted rotation around the C=C double bond which results in the existence of geometric isomers
-When theres a double bond (or pi bond) if we wanted to rotate the atoms around it we would need to break the pi bond. However, this is not possible at room temperature because it would require energy to break the bond so free rotation is not possible around the C=C in alkenes. This forms a type fo stereoisomer
when do E/Z isomers form
-molecules have restricted rotation about a bond
-have different groups attached to the carbon atoms at the end of each bond
Z isomer
Z isomer = functional groups on the same side (cis)
E isomer
E isomer = function groups on opposite sides (trans)
trans vs cis
If an alkyl group or atom other than hydrogen is attached to each carbon then the isomers can be named cis (same side) or trans (opposite side)
Cis molecules = top or bottom are the same –> usually polar molecules with the same side groups on the same side of the double C bond
Trans isomer = same side groups placed on opposite sides of the double bond
complex organic compounds
-In more complex organic compounds where multiple hydrogens have been substituted by different group isomers, it cannot be define as cis-trans notation. Instead we use E/Z isomer notation
E/Z isomer
-isomers are defined as E or Z depending on what priority is given to the groups attached to the carbon atoms in the C=C
-E and Z isomers arise due to the double bond
-When there is more than 2 different groups around the alkene double bond we cannot have E/Z isomerism, instead it is only a geometric isomer
assigning priority
-We need to determine which groups we give priority to when there are four different groups on the carbon atoms in the double bond
-We give the highest priority to the atom with the highest atomic number
addition reactions
-Addition reactions occur with alkenes in which one molecule combined with another molecule to form a larger molecule with no other products
what do alkenes undergo addition reactions with
-hydrogen
-halogens
-hydrogen halides
-steam (H2O)
addition of hydrogen
-hydrogen can be added to the C=C double bond in alkenes to form an alkane e.g ethene + hydrogen –> ethane
-The conditions required for this is room temperature in the presence of a platinum or palladium nickel catalyst
-hydrogenation
catalytic hydrogenation
-used in the manufacture of margarine
-oily unsaturated liquids are turned into soft saturated fatty solids like margarine using a nickel catalyst
addition of halogens
-Halogens can be added to alkenes but at room temperature to form a haloalkane
-Flourine reacting with small alkenes is relativey explosive but with iodine the reaction is slow
testing for alkenes
-orange –> colourless (bromine water)
-undergoes an addition reaction to create a haloalkane which turns colourless
-unsaturated bond decolorises
-when solution becomes colourless Br2 undergoes an addition reaction
addition of hydrogen halides
-react readily with alkenes at room temperature to form a haloalkene
-hydrogen halides react with alkenes in the gas phase
addition of steam
-Alkenes react with steam in the presence of an acid catalyst (phosphoric acid catalyst or sulfuric acid) to produce alcohols
-This is a reversible reaction
Oxidation by potassium manganate (KMnO4):
-Potassium manganate can oxidise alkenes in the presence of sulfuric acid
Alkenes can be oxidized to produce cis-diols using a different type of reagent that adds atoms across the double bond via a cyclic intermediate
-during the reaction MnO4- ions are reduced to Mn2+
-colour change = purple to colourless
But-1-ene can also react with acidified potassium manganate solution. This is both an oxidation and addition reaction
Butan-1,2-ol –> OH group on 1st and 2nd carbon
reactivity of C=C
-the electron density is above and below the plane of the molecule
-The double bond of the alkene is an area of high electron density and is therefore an area of high negative charge
-The exposed electrons of the pi bond may be attacked by electron deficient species (those with partial positive or actual positive charges)
electrophiles
-electrophiles are molecules or ions that attack negative ions or parts of molecules which are rich in electrons with negative centres
-electrophiles attack alkene bonds or ions (positive charge or partial positive)
-Electrophiles are also described as electron acceptors bc they accept electrons from the molecule or ion they attack
explain why ethane undergoes substitution and not addition reactions
ethane has no double carbon bonds that can be shared
explain why more of isomer E than isomer F is formed in this reactioj
-different stabilities in carbocations
what happens during electrophilic addition
-When an electrophile such as H-Br, H-H or Br-Br is added to an alkene we call it electrophillic addition (always occurs with double bonds or electron dense areas)
-An alkene reacts with an electrophile to produce 1 product only
-The Pi bond breaks and the two carbons form new bonds
examples of electrophiles
H-Br + alkene –> haloalkane e.g bromoethane
Br-Br + alkene –> dihaloalkane e.g 1,2 dibromoethane
heterolytic fission
(electrophilic addition)
-When covalent bonds break they undergo heterolytic fission
-heterolytic fission –> one atom takes both of the electrons from the bond, The other atom takes none
-Heterolytic bond breaking produces ionic intermediates such as CH3 or Br. This is indicated using double headed curly arrow
curly arrow (electrophilic)
The curly arrow always goes from a bond or from a lone pair
carbocation
-During electrophilic addition reactions, the electrophiles attack the electron-rich region of the double bond (pi bond breaks as it is weaker). This forms a carbocation
Carbocation = a reactive species which contains a carbon atom which has a positive charge
nucleophile
Nucleophile = electron pair donors. They are negative ions or molecules with a lone pair of electrons, They attack positive ions or positive centers in molecules. (in the second stage of electrophilic addition the B- ion acts as a nucleophile)
symmetrical vs asymmetrical alkenes
Symmetrical alkenes = same substituent at each side of the double bond
Asymmetrical alkanes:
-When HBr attacks an alkene with three or more carbon atoms e.g propene a mix of products is formed. This is because alkenes are asymmetrical
-unequal amounts of each product are formed due to relative stabilities of the carbocation intermediates e.g more alkyl groups = more stable
stabilities of carbocations
-primary carbocations have 1 alkyl group attached to the positivley charged carbon
-secondary = 2 alkyl group
-tertiary = 3 alkyl group
-More alkyl groups = more stable so tertiary carbocations are the most stable bc there is a greater electron density than hydrogen atoms
sulfuric acid
-sulfuric acid can act as an electrophile due to the polar nature of O and H
why does the stability (likelihood to form) of carbocations increase
-alkyl groups contain a greater electron density than hydrogen atoms
-stabilising effect (positive inductive effect)
primary, secondary and tertiary carbocations
primary = 2 hydrogen atoms in structural formula
secondary = 1 hydrogen atom
tertiary = non hydrogen atoms
Explain why the products are formed in different amounts (major and minor)
-molecule formed via more stable carbocation
-major product form from teritary carbocation rather than secondary carbocation
-stability due to greater positive inductive effect from more alkyl groups
excess
bonded onto every alkene e.g HBr may be bonded twice if in excess
major vs minor
primary = minor
teritary and secondary = major
explain why NaBH4 reduces 2-methylbutanal but has no reaction with 2-methyl-but-1-ene
-H- ion is nucleophille so is attracted to positive carbon
-c=c has an electron rich area
-H- ion is repelled by c=c as only electrophille are attracted
true or false - E and Z isomers have the same boiling point
false - different boiling points
identify the feature of the double bond in the E and Z isomers that causes them to be steroisomers
restricted rotation
H2SO4 in electrophillic addition
H+
OSO2OH-
draw the structure of the hydrocarbon that is a chain isomer of methylpropene but does not exhibit stereosisomerism
CH3CH2CH=CH2
draw the structure =
structural formula
there is a very low yield of butan-1-ol from but-1-ene in this manufacturing process, Explain why
-formed from less stable carbocation
-primary rather than secondary
state a method of recycling polypropene which may produce a mixture similar to naptha
-tertiary recycling (chemical feedstock)
suggest two further pieces of information regarding the life cycle of the cups that would make any assessment of the environmental impact more reliable
-ease of recylability
-space taken up in landfill
addition polymerisation
Alkenes are able to undergo addition reactions with themselves to form a long chain polymer molecule
define a polymer
substance or material consisting of very large molecules or macromolecules composed of many repeating subunits called monomers
steps to drawing addition polymerisation
Extend the lines and polymers can only be two carbons and them repeated
During addition polymerisation the alkene double bond breaks to form an alkane single c-c bond
Steps to drawing repeating units:
-break open double bond to ensure each carbon has 4 bonds
-place square brackets around the unit
-place n (no. Of repeating units) after the brackets
efficiency of addition polymerisation
-addition polymerisation reactions can be described as highly efficient. The atom economy is 100% because no atoms are lost or gained and only one product is formed. There are no waste products
disposal of polymers
-polymers are unreactive so they are often used for storing food and chemicals safely but it is difficult to dispose of them
-they will break down and remain in landfill as polymers
how to overcome the issue of disposal
-burn polymers to be used in energy recovery
-primary recycling
-secondary recycling
-tertiary recycling (chemical feedstock)
polymers are sorted into groups then…..
-incinerated to release energy but also toxic gases
-recycled into other plastics but separation needed
-used as feedstock for cracking into shorter chains
biodegradable polymers
-the issue with most polymers is that they are not biodegradable
Biodegradable –> where microogranisms break down its own compounds
what statement about polyethene is correct
it has a higher melting point than ethene
which compound reacts with hydrogen bromide to give 2-bromo-3-methylbutane
(CH3)2CHCH=CH2
what is the IUPAC name of the major product of the reaction between 2-ethylbut-1-ene and HBr
3-bromo-3-methylpentane
diene =
2 alkenes so x 2
which has E-Z isomers
C2H2Br2
which statement is correct about poly(chloroethene)
its brittleness is reduced by plasticisers
which statement is not correct for both primary and secondary alcohols
-they are easily oxidised to carboxylic acids by acidified K2Cr2O7 solution
what compound is produced when 1-phenylethanol reacts with acidified potassium dichromate (secondary)
C6H5COCH3
structural isomer of Z-but-2-ene
cyclobutane bc cycloalkanes have same general formula as alkenes
state what you would observe if bromine water was added to poly(chloroethene) and explain
no reaction (remains orange)
Polymers are saturated (no double C bond)
give a reason why this type of stereoisomerism arises
restricted rotation around C=C double carbon bond
explain why these golf balls do not biodegrade
carbon carbon bonds cannot be hydrolysed
suggest a suitable reagent and conditions for the conversion of 2-hydroxypropanenitrile into acrylonitrile
Reagent - concentrated H2SO4
Conditions - heat
explain how this nitrogen monoxide is formed
-nitrogen and oxygen from the air react at high temperatures
which statement is correct about poly(chloroethene)
its brittleness is reduced by plasticisers
if there are 4 different groups around C=C
do according to priority
e.g Br and F
why are alkenes are more reactive
-takes less energy to break pi bond
-and has high electron density which makes alkenes highly reactive
electrophille vs nucleophille
electro = positive attracted to negative
nucleo = negative attracted to positive
hydration of alkenes
-using steam and an acid catalyst
alkene + water + phosphoric acid –> alcohol
hydrogenation of alkenes
alkenes + hydrogen –> alkane
using nickel catalyst
state the role of potassium hydroxide and propan-1-ol in the reaction
potassium hydroxide –> base
Propan-1-ol –> solvent
suggest an environmental advanatge of polymer Q over polymer P
Q is biodegradable bc it has polar C=O group so can be attacked by nucleophilles
suggest an advantage of surgical sutures made from PGA rather than from polypropene
PGA sutures are biodegradable due to polar bonds
IUPAC name for lactic acid
2-hydroxypropanoic acid
suggest one reason why PLA in landfill may take longer than 12 weeks to break down
no microorganisms/bacteria
explain what is meant by an electrophille and the term addition
electrophille - lone pair acceptor
addition - reaction which increases number of subsituents
name isomer A
Explain why A does not exhibit stereosiomerism
but-1-ene
two groups are the same on one of the C=C atoms
PVC
poly (chloroethene)
felxible + rigid due to permament dipole IMF
double bond =
planar (120)
polymers =
high melting point = many van der waals
draw polymer = include n
draw repeating unit = no bracket and no n
non-biodegrdable as they are saturated, unreactive and non-polar
detect E-Z isomer
double bond
different groups on each side of double bond
give one observation that the student made to confirm the cyclohexene was dry
drying agent turns from cloudy to clear
explain how carbon dioxide causes global warming
-C=O bonds absorb infrared radiation
-infrared radiation emitted by the earth does not leave the atmosphere
elimination reactions of halogenoalkanes to form alkenes
2 structural isomers
at least 1 E-Z isomer
ionic equation for elimination of 1-bromopropane
CH2BrCH2CH3 + OH- –> C3H6 + H2O + Br-
