8 Alkenes Flashcards
What are alkenes?
unsaturated hydrocarbons
What is the general formula of a ‘mono-alkene?’
CnH2n
What is the functional group of an alkene?
carbon-carbon double bond (C=C)
Describe the type of bonding in alkenes that results in ‘no rotation about the double bond.’
two sp2 orbitals overlap to form a pi-bond between two carbon atoms
two 2p-orbitals overlap to form a pi-bond between the two carbon atoms
s-orbitals in hydrogen overlap with the sp2 orbitals in carbon to form C-H bonds
the resulting shape is planar with bond angles of 120°
Draw out the orbital diagrams for the stages in the bonding of alkenes.
see document
Explain how IUPAC nomenclature is used to name alkenes.
It replaces the alk”ane” suffix with -“ene”
i.e. ethene, propene etc…
How did older nomenclature name alkenes?
replaced the corresponding alkane suffix ‘ane’ with ‘ylene’
State where the older nomenclature of alkenes tends to be used and give three examples (names and diagrams).
used in common names of industrial chemicals
i.e. ethene = ethylene, propene = propylene and trichloroethene = trichloroethylene
(see document for diagrams)
Define ‘Geometric isomerism.’
an example of stereoisomerism found in some, but not all alkenes which occurs due to the restricted rotation of C=C double bonds
What type of arrangement is geometric isomerism?
non-systematic arrangement
What two categories does geometric isomerism put alkenes into and what do they mean?
Cis = same side Trans = opposite sides
For the molecular formula, C₄H₈ draw and name the 4 isomers. Of the four identify the two that are geometric and name them as “cis/trans” and as “E/Z”
see document
What is the E/Z system better for naming?
naming complex stuctures
Define ‘E/Z isomerism.’
where you categorise molecules by ranking substituents in order of priority on each carbon
(using the list of priority from semester 1)
Put the following list in the order of preference for the principal group.
Ketone -C=O- Halogen -C-X- Esters -C=OO- Triple bond -C≡C- Alcohols -C-OH Nitriles -C≡N Acid anhydrides -C=OOO=C- Aldehydes -C=OH Ketone -C=O- Amines -NH₂/NH/N- Double bond -C=C- Acid halides -C=OX- Carboxylic acid -COOH Amides -C=ONH- Nitro -NO₂-
Carboxylic acid -COOH Acid anhydrides -C=OOO=C- Esters -C=OO- Acid halides -C=OX- Amides -C=ONH- Nitriles -C≡N Aldehydes -C=OH Ketone -C=O- Alcohols -C-OH Amines -NH₂/NH/N- Double bond -C=C- Triple bond -C≡C- Halogen -C-X- Nitro -NO₂-
(see document for how to remember)
State the order of priority for the following functional groups:
a) H, C₂H5, CH₃
b) F, H, I, Cl, C, Br
a) C₂H5 > CH₃ > H
b) I > Br > Cl > F > C > H
What two categories does E/Z isomerism put alkenes into and what do they mean?
E: higher-ranked substituents on opposite sides
Z: higher-ranked substituents on same sides
Assign the stereochemistry of the following alkenes (on the document).
a) CIP groups on the same side of the C=C double-bond; Z-isomer
b) CIP groups on opposite sides; E-isomer
c) Trick question; no assignment required due to 3 H atoms being around the C=C double bond; No differenve if the Hs move
State how alkenes are synthesised using dehydrohalogenation including:
a) reagents
b) conditions
c) what is lost
d) bonds formed
a) reagents: alcoholic sodium (or potassim) hydroxide (base to remove proton)
b) conditions: reflux in alcholic solution
c) loss of H⁺ and X- ions from adjacent carbons
d) a new pi-bond is formed
Complete the dehydrohalogenation reaction for the molecule shown on the document.
see document
Draw out the general dehydrogenation mechanism (elimination stereospecific) for the molecule shown on the document including fish-hook arrows to show movement of electrons.
see document for this
What does the product of a dehydrohalogenation depend on?
the stereoisomer you start with
State how alkenes are synthesised using dehalogenation of vicinal dibromides including:
a) the two possible reagents
b) bonds formed
a) NaI (sodium iodide) which uses the I- nucleophile to kick Br- out OR Zn/HOAc (acetic acid) for a redox reaction
b) a double (pi-) bond
What does a ‘vicinal’ dibromide mean?
the two bromine atoms are bonded to neighbouring/adjacent carbon atoms
Finish off the following dehalogenation of a vicincal dibromide (on document).
see document
What is ‘dehydration’ of a molecule and what does it require?
removal of water - requires heat and a strong acid
State how alkenes are synthesised using dehydration of primary alcohols, including:
a) reagents
b) conditions
c) bonds formed
d) the second less significant product also made
a) conc. sulfuric acid (H₂SO₄)or conc. phosphoric acid (H₃PO₄)
b) temperature of 180°C, reflux (but can depend on the structure of alcohol)
c) a double (pi-) bond
d) H₂O
State how alkenes are synthesised using dehydration of tertiary alcohols, including:
a) reagents
b) conditions
c) bonds formed
d) the second less significant product also made
a) 20% sulfuric acid (H₂SO₄) or phosphoric acid (H₃PO₄)
b) temperature of 85°C, reflux (but can depend on structure of alcohol)
c) a double (pi-) bond
d) H₂O
Which type of side reaction may you get with dehydration of alcohols for alkene synthesis?
a rearrangement reaction
Give an example of an ‘elimination’ reaction in the Krebs cycle for biosynthesis of an alkene and state the enzyme required (draw out the molecules too).
citric acid → cis- aconitic acid
aconitase
- (see document for molecules)
Give an example of a ‘dehydrogenation’ reaction in the Krebs cycle for biosynthesis of an alkene and state the enzyme and cofactor required (draw out the molecules too).
succinic acid → fumaric acid
succinate dehydrogenase
- FAD cofactor ( which becomes FADH₂ once 2 Hs are removed)
- (see document for molecules)
Give a general ‘radical’ reaction in fatty acid biosynthesis and state the enzyme and other chemicals required (draw out the molecules too).
R-C=OSCoA → R-C=C=OSCoA
(C-16-C-18 chain Desaturase
Acyl-CoA Fe(II)XY
molecule) H⁺, O₂, NADPH
-(see document for molecules)
What is the general trend in boiling point in alkenes and why?
increases up the homologous series (trends similar to those in alkanes)
due to Van Der Waals’ forces increasing
What is the general trend in melting point in alkenes and why?
increases up the homologous series
depends on the isomer - see fatty acids
What is the general trend in solubility in alkenes and why?
immiscible (don’t mix with) water or some polar solvents
miscible(do mix with)/soluble in most organic solvents
What is the general trend in density in alkenes?
less dense than water
State the composition of the double bond in alkenes using a diagram.
the double bond consists of sigma and pi components
see document for diagram
Explain, in terms of bonding in a C=C double bond, why alkenes are more reactive than alkanes using a diagram of the bonding in ethene.
pi-electrons are further from the carbon nuclei and less firmly-bound
pi-bonds are weaker than sigma-bonds and more easily broken
electrons are exposed above and below the plane - easily-accessible
Therefore, alkenes are more reactive than alkanes
(see document for diagram with points)
What type of reactions do alkenes tend to undergo due to their high reactivity level?
addition reactions
State the main 3 categories of reactions an alkene can undergo.
- addition
- reduction
- oxidation
Which 2 subcategories of addition reaction can an alkene undergo?
- ionic
- free radical
For the hydration reaction, state:
a) elements added
b) reagent
c) the product from ethene
a) H₂O
b) H₂O, acid catalyst
c) see document
For the halogenation reaction, (where X= halogen) state:
a) elements added
b) reagent
c) the product from ethene
a) X₂
b) X₂ e.g. Br₂
c) see document
For the halohydrin reaction, state:
a) elements added
b) reagent
c) the product from ethene
a) HOX
b) X₂ in water, i.e. Br₂ in water
c) see document
For the hydrohalogenation reaction, state:
a) elements added
b) reagent
c) the product from ethene
a) HX
b) HX, e.g. HBr, HCl
c) see document
For the hydroxylation reaction, state:
a) elements added
b) reagent
c) the product from ethene
a) HOOH
b) OsO₄, KMnO₄ (O₄ is key)
c) see document
For the hydrogenation (reduction) reaction, state:
a) elements added
b) reagent
c) the product from ethene
a) H₂
b) H₂, transition metal catalyst i.e. Ni, Pt
c) see document
What is the main reaction of an alkene?
the addition of an electrophile
State the 2 main stages of electrophilic addition to an alkene and draw out the mechanism using ethene as your main molecule and HBr as your electrophile.
Step 1: electron-density in pi-bond attacks electrophile forming a carbocation
Step 2: Nucleophile attacks carbocation giving the product
(see document for drawings)
State Markovnikoff’s rule concerning asymmetric reagents such as H-X adding to C=C.
the proton (H) adds to the carbon (in the double bond) that already has the greater number of hydrogen atoms (i.e. more alkyl groups)
For the propene molecule (on the document) draw the mechanism for the 2 different paths, state the type of carbocations formed and identify the major and minor products.
- see document
What leads to instability and how can stability be increased?
instability caused by the build-up of charge in one place
if it can be spread around or neutralised in some way, stability can be increased
What types of groups are alkyl groups and what can they do?
electron-releasing
can “push” electrons towards the carbocations which stabilise them
Order methyl, primary, secondary and tertiary carbocations in order of stability/
methyl < primary (1°) < secondary (2°) < tertiary (3°)
Rephrase Markovnikoff’s rule in terms of carbocation stability.
In an electrophilic addition, asymmetric reagents such as H-X add to the double bond so that the most stable carbocation is formed
the amjor reaction product is formed via the most stable carbocation (i.e. 2°/3°)
Define ‘biological hydrogenation.’
enzyme transfers H- (from cofactor NAD(P)H or FADH₂) and H⁺
Give an example of ‘biological hydrogenation’ and state the enzyme and cofactor involved.
biliverdin → bilirubin
biliverdin reductase
- NAD(P)H cofactor (which becomes NAD⁺ once H is removed)
- (see document for molecules)
How does ‘chemical hydrogenation’ differ and which catalyst does it require. Draw out this reaction on ethene.
uses H₂ gas
requires heterogenous TM catalyst (e.g. Pt, Pd, Ni)
State and describe the 3 types of oxidation of alkenes which can occur and show the organic products for each when done to but-2-ene.
Epioxidation - produces epioxide
Dihydoxylation - produces dihydroxy molecule
Oxidative cleavage - produces 2 ketones
What is diethylstilbestrol?
potent synthetic oestrogen
used in cancer treatment
earlier used in gynaecology - but had safety issues
Draw out the mechanism for carcinogenesis for diethylstilbestrol.
see document
Define ‘biological oxidation.’
when many enzymes, including cytochromes p450 (Fe-containing cofactors), metal-free monooxygenases (flavin peroxide cofactor) carry it out and require oxygen
What does ‘chemical oxidation’ use?
transition metal catalysts are used
Give an example of ‘chemical oxidation,’ stating its possible mediators and products.
dihydroxylation adds OH to each end of C=C
mediated by osmium tetroxide or cold alkaline KMnO₄
product is a 1,2-dialcohol or doil
What colour change does cold alkaline KMnO₄ change to when an alkene is present?
from intense purple to colourless
What is the common name given to a diol?
A glycol
State the ‘initiation’ stage of the free radical addition reaction of organic peroxide (R-O-O-R), HBr and CH₂=CH(CH₃) and the conditions required.
Inititaion: R-O-O-R → 2RO*
heat
State the 3 ‘propagation’ stages of the free radical addition reaction of organic peroxide (R-O-O-R), HBr and CH₂=CH(CH₃).
Propagation: RO* + HBr → ROH + Br*
Br* + CH₂=CH(CH₃) → CH₂Br-CH(CH₃)*
CH₂Br-CH(CH₃)* + HBr → CH₂Br-CH₂CH₃+Br*
State the main ‘termination’ stage of the free radical addition reaction of organic peroxide (R-O-O-R), HBr and CH₂=CH(CH₃).
Termination: Br* + Br* → Br₂
What is addition polymerisation also known as?
free radical polymerisation of alkenes
State how free radical polymerisation of alkenes occurs.
alkene undergoes an addition reaction with itself
all atoms in the original alkenes form part of the polymer
long hydrocarbon chains are formed
For the ethene monomer shown below, draw the addition polymer formed and name it.
n >C=C< →
—(-C-C-)— n
poly(ethene) polymer formed
What does n represent with addition polymers?
a large number
Using fish-hook arrows, draw out the propagation and termination stages of the reaction mechanism for addition polymerisation.
see document
State the conditions required for free radical polymerisation of alkenes.
Usually requires high pressure, high temperature and a catalyst.
Why is a catalyst required for free radical polymerisation of alkenes? Provide an example of such as catalyst.
a catalyst readily breaks up to form radicals which initiate a chain reaction
e.g. an organic peroxide
