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Chemistry > 8 Alkenes > Flashcards

8 Alkenes Flashcards

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1
Q

What are alkenes?

A

unsaturated hydrocarbons

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2
Q

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

A

CnH2n

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3
Q

What is the functional group of an alkene?

A

carbon-carbon double bond (C=C)

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4
Q

Describe the type of bonding in alkenes that results in ‘no rotation about the double bond.’

A

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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5
Q

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

A

see document

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6
Q

Explain how IUPAC nomenclature is used to name alkenes.

A

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

i.e. ethene, propene etc…

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7
Q

How did older nomenclature name alkenes?

A

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

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8
Q

State where the older nomenclature of alkenes tends to be used and give three examples (names and diagrams).

A

used in common names of industrial chemicals
i.e. ethene = ethylene, propene = propylene and trichloroethene = trichloroethylene
(see document for diagrams)

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9
Q

Define ‘Geometric isomerism.’

A

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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10
Q

What type of arrangement is geometric isomerism?

A

non-systematic arrangement

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11
Q

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

A 
Cis = same side 
Trans = opposite sides
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12
Q

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”

A

see document

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13
Q

What is the E/Z system better for naming?

A

naming complex stuctures

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14
Q

Define ‘E/Z isomerism.’

A

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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15
Q

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-
A 
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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16
Q

State the order of priority for the following functional groups:

a) H, C₂H5, CH₃
b) F, H, I, Cl, C, Br

A

a) C₂H5 > CH₃ > H

b) I > Br > Cl > F > C > H

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17
Q

What two categories does E/Z isomerism put alkenes into and what do they mean?

A

E: higher-ranked substituents on opposite sides
Z: higher-ranked substituents on same sides

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18
Q

Assign the stereochemistry of the following alkenes (on the document).

A

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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19
Q

State how alkenes are synthesised using dehydrohalogenation including:

a) reagents
b) conditions
c) what is lost
d) bonds formed

A

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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20
Q

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

A

see document

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21
Q

Draw out the general dehydrogenation mechanism (elimination stereospecific) for the molecule shown on the document including fish-hook arrows to show movement of electrons.

A

see document for this

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22
Q

What does the product of a dehydrohalogenation depend on?

A

the stereoisomer you start with

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23
Q

State how alkenes are synthesised using dehalogenation of vicinal dibromides including:

a) the two possible reagents
b) bonds formed

A

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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24
Q

What does a ‘vicinal’ dibromide mean?

A

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

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25
Q

Finish off the following dehalogenation of a vicincal dibromide (on document).

A

see document

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26
Q

What is ‘dehydration’ of a molecule and what does it require?

A

removal of water - requires heat and a strong acid

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27
Q

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

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

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28
Q

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

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

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29
Q

Which type of side reaction may you get with dehydration of alcohols for alkene synthesis?

A

a rearrangement reaction

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30
Q

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).

A

citric acid → cis- aconitic acid
aconitase

  • (see document for molecules)
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31
Q

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).

A

succinic acid → fumaric acid
succinate dehydrogenase

  • FAD cofactor ( which becomes FADH₂ once 2 Hs are removed)
  • (see document for molecules)
32
Q

Give a general ‘radical’ reaction in fatty acid biosynthesis and state the enzyme and other chemicals required (draw out the molecules too).

A

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)

33
Q

What is the general trend in boiling point in alkenes and why?

A

increases up the homologous series (trends similar to those in alkanes)
due to Van Der Waals’ forces increasing

34
Q

What is the general trend in melting point in alkenes and why?

A

increases up the homologous series

depends on the isomer - see fatty acids

35
Q

What is the general trend in solubility in alkenes and why?

A

immiscible (don’t mix with) water or some polar solvents

miscible(do mix with)/soluble in most organic solvents

36
Q

What is the general trend in density in alkenes?

A

less dense than water

37
Q

State the composition of the double bond in alkenes using a diagram.

A

the double bond consists of sigma and pi components

see document for diagram

38
Q

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.

A

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)

39
Q

What type of reactions do alkenes tend to undergo due to their high reactivity level?

A

addition reactions

40
Q

State the main 3 categories of reactions an alkene can undergo.

A
  • addition
  • reduction
  • oxidation
41
Q

Which 2 subcategories of addition reaction can an alkene undergo?

A
  • ionic

- free radical

42
Q

For the hydration reaction, state:

a) elements added
b) reagent
c) the product from ethene

A

a) H₂O
b) H₂O, acid catalyst
c) see document

43
Q

For the halogenation reaction, (where X= halogen) state:

a) elements added
b) reagent
c) the product from ethene

A

a) X₂
b) X₂ e.g. Br₂
c) see document

44
Q

For the halohydrin reaction, state:

a) elements added
b) reagent
c) the product from ethene

A

a) HOX
b) X₂ in water, i.e. Br₂ in water
c) see document

45
Q

For the hydrohalogenation reaction, state:

a) elements added
b) reagent
c) the product from ethene

A

a) HX
b) HX, e.g. HBr, HCl
c) see document

46
Q

For the hydroxylation reaction, state:

a) elements added
b) reagent
c) the product from ethene

A

a) HOOH
b) OsO₄, KMnO₄ (O₄ is key)
c) see document

47
Q

For the hydrogenation (reduction) reaction, state:

a) elements added
b) reagent
c) the product from ethene

A

a) H₂
b) H₂, transition metal catalyst i.e. Ni, Pt
c) see document

48
Q

What is the main reaction of an alkene?

A

the addition of an electrophile

49
Q

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.

A

Step 1: electron-density in pi-bond attacks electrophile forming a carbocation

Step 2: Nucleophile attacks carbocation giving the product

(see document for drawings)

50
Q

State Markovnikoff’s rule concerning asymmetric reagents such as H-X adding to C=C.

A

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)

51
Q

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.

A
  • see document
52
Q

What leads to instability and how can stability be increased?

A

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

53
Q

What types of groups are alkyl groups and what can they do?

A

electron-releasing

can “push” electrons towards the carbocations which stabilise them

54
Q

Order methyl, primary, secondary and tertiary carbocations in order of stability/

A

methyl < primary (1°) < secondary (2°) < tertiary (3°)

55
Q

Rephrase Markovnikoff’s rule in terms of carbocation stability.

A

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°)

56
Q

Define ‘biological hydrogenation.’

A

enzyme transfers H- (from cofactor NAD(P)H or FADH₂) and H⁺

57
Q

Give an example of ‘biological hydrogenation’ and state the enzyme and cofactor involved.

A

biliverdin → bilirubin
biliverdin reductase

  • NAD(P)H cofactor (which becomes NAD⁺ once H is removed)
  • (see document for molecules)
58
Q

How does ‘chemical hydrogenation’ differ and which catalyst does it require. Draw out this reaction on ethene.

A

uses H₂ gas

requires heterogenous TM catalyst (e.g. Pt, Pd, Ni)

59
Q

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.

A

Epioxidation - produces epioxide
Dihydoxylation - produces dihydroxy molecule
Oxidative cleavage - produces 2 ketones

60
Q

What is diethylstilbestrol?

A

potent synthetic oestrogen
used in cancer treatment
earlier used in gynaecology - but had safety issues

61
Q

Draw out the mechanism for carcinogenesis for diethylstilbestrol.

A

see document

62
Q

Define ‘biological oxidation.’

A

when many enzymes, including cytochromes p450 (Fe-containing cofactors), metal-free monooxygenases (flavin peroxide cofactor) carry it out and require oxygen

63
Q

What does ‘chemical oxidation’ use?

A

transition metal catalysts are used

64
Q

Give an example of ‘chemical oxidation,’ stating its possible mediators and products.

A

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

65
Q

What colour change does cold alkaline KMnO₄ change to when an alkene is present?

A

from intense purple to colourless

66
Q

What is the common name given to a diol?

A

A glycol

67
Q

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.

A

Inititaion: R-O-O-R → 2RO*

heat

68
Q

State the 3 ‘propagation’ stages of the free radical addition reaction of organic peroxide (R-O-O-R), HBr and CH₂=CH(CH₃).

A

Propagation: RO* + HBr → ROH + Br*
Br* + CH₂=CH(CH₃) → CH₂Br-CH(CH₃)*
CH₂Br-CH(CH₃)* + HBr → CH₂Br-CH₂CH₃+Br*

69
Q

State the main ‘termination’ stage of the free radical addition reaction of organic peroxide (R-O-O-R), HBr and CH₂=CH(CH₃).

A

Termination: Br* + Br* → Br₂

70
Q

What is addition polymerisation also known as?

A

free radical polymerisation of alkenes

71
Q

State how free radical polymerisation of alkenes occurs.

A

alkene undergoes an addition reaction with itself
all atoms in the original alkenes form part of the polymer
long hydrocarbon chains are formed

72
Q

For the ethene monomer shown below, draw the addition polymer formed and name it.

n >C=C< →

A

—(-C-C-)— n

poly(ethene) polymer formed

73
Q

What does n represent with addition polymers?

A

a large number

74
Q

Using fish-hook arrows, draw out the propagation and termination stages of the reaction mechanism for addition polymerisation.

A

see document

75
Q

State the conditions required for free radical polymerisation of alkenes.

A

Usually requires high pressure, high temperature and a catalyst.

76
Q

Why is a catalyst required for free radical polymerisation of alkenes? Provide an example of such as catalyst.

A

a catalyst readily breaks up to form radicals which initiate a chain reaction
e.g. an organic peroxide

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