8 Alkenes Flashcards by Ruqayyah xx

What are alkenes?

unsaturated hydrocarbons

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What is the general formula of a ‘mono-alkene?’

CnH2n

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What is the functional group of an alkene?

carbon-carbon double bond (C=C)

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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°

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Draw out the orbital diagrams for the stages in the bonding of alkenes.

see document

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Explain how IUPAC nomenclature is used to name alkenes.

It replaces the alk”ane” suffix with -“ene”

i.e. ethene, propene etc…

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How did older nomenclature name alkenes?

replaced the corresponding alkane suffix ‘ane’ with ‘ylene’

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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)

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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

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What type of arrangement is geometric isomerism?

non-systematic arrangement

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What two categories does geometric isomerism put alkenes into and what do they mean?

Cis = same side 
Trans = opposite sides

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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

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What is the E/Z system better for naming?

naming complex stuctures

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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)

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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)

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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

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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

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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

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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

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Complete the dehydrohalogenation reaction for the molecule shown on the document.

see document

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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

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What does the product of a dehydrohalogenation depend on?

the stereoisomer you start with

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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

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What does a ‘vicinal’ dibromide mean?

the two bromine atoms are bonded to neighbouring/adjacent carbon atoms

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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