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Chemistry Module 4 > 4.1.3 Alkenes > Flashcards

4.1.3 Alkenes Flashcards

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1
Q

What are alkenes examples of

A

Unsaturated hydrocarbons

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2
Q

General formula for alkenes

A

CnH2n

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3
Q

What does the C=C Bosnia consist of

A

One sigma bond

One pi bond

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4
Q

Extra information about pi bonds

A

Exposed and have a high electron density

Vulnerable to attack electrophiles

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5
Q

Formation of sigma bonds in alkenes

A

two sp2 orbitals one from each carbon overlap and form a single C-C bond called sigma bond

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6
Q

Formation of pi bond in alkenes

A

Formed by sideways overlap of two p orbitals on each carbon forming pi bond above and below the plane of the molecule

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7
Q

Is a pi or sigma bond stronger

A

Sigma

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8
Q

Shape of alkenes

A

Trigonal planar

Angle:120

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9
Q

What type of isomerism do alkenes show

A

E/Z stereoisomerism

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10
Q

When does E/Z isomerism occur

A

Restricted rotation around C=C bond

When two different groups attached to either side of the double bond

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11
Q

How can E/Z isomerism occur in alkenes

A

Restricted rotation around the C=C bond

Pi bond

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12
Q

When is it a Z isomer

A

When priority atom/group is on the same side
Z for zusammen
On Zee Zame Zide

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13
Q

When is it an E isomer

A

If priority atom /group is on opposite side
E for entgegen
Opposite side

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14
Q

Rules for Cahn-ingold-prelog

A
  1. Compare atomic number of atoms directly bonded to carbon double bond and higher atomic number is given priority
  2. If two atoms are the same then consider atoms at distance 2 from the double bond then the highest is given priority
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15
Q

What can Z isomers also be called

A

Cis

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16
Q

What can E isomers also be called

A

Trans

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17
Q

Effect of E/z isomerism on physical properties

A

Different melting or boiling points e.g. dichloroethene
Z isomer: molecule is polar but the chlorine is on same side, one side of the molecule is more negative, intermolecular forces are London and permanent dipole dipole
E isomer: non-polar as polar c-cl bonds are on opposite sides so the dipoles cancels out, only London forces so boiling point is lower

18
Q

Reactivity of alkenes

A

Quite reactive because of the low bond enthalpy of the pi bond

19
Q

Definition of addition reaction

A

A reaction where two molecules react together to produce one

20
Q

Reaction of alkenes with hydrogen (hydrogenation)

A

Alkene-> alkane
Reagent: hydrogen
Conditions: nickel catalyst
Type of reaction: addition/reduction

21
Q

Definition of an electrophile

A

Electron pair acceptor

22
Q

Electrophilic addition of alkenes

A

Double bonds in alkenes are areas of high electron density

Attracts electrophiles and the alkenes undergo addition reactions

23
Q

Reaction of alkenes with halogens (halogenation)

A

Alkene->dihaloalkane
Reagent:diatomic halogen
Conditions: room temperature (no UV light)
Mechanism: electrophilic addition
Type of reagent: electrophile, halogen with delta plus
Type of bond fission: heterolytic

24
Q

Explanation of the mechanism of halogenation of an alkene

A

As halogen molecule approaches the alkene the pi bonds repel the electron pair in the halogen-halogen bond
Induces a dipole
Halogen is polarized and forms electrophile (delta positive halogen)
Nucleophile then attacks carbocation and adds onto the carbon

25
Q

Reaction of hydrogen halide with alkenes

A 
Alkene-> haloalkane
Reagent: HCl or HBr 
Conditions: room temperature 
Mechanism: electrophilic addition 
Type of reagent: electrophile, hydrogen delta plus
26
Q

Explain mechanism of hydrogen halide with alkene

A

Hydrogen halide is already polar so H delta plus is attracted to pi bond
Halogen nucleophile attack’s carbocation and adds across onto two carbons
Molecule is unsymmetrical so leads to formation of two products

27
Q

Why does electrophilic addition with hydrogen halide and alkene lead to two products

A

As alkene is unsymmetrical

28
Q

Markownikoffs rule

A

Atom will be added to the carbon with the least number of hydrogen attached to it

29
Q

Explain why one is a major product and one is a minor product

A

Major is formed via a more stable carbocation intermediate

30
Q

Stability of carbocations

A

Tertiary-> secondary -> primary

31
Q

Reaction of alkenes with steam (hydration)

A

Alkenes -> alcohols
Reagent: steam/H2O
Conditions: high temp 300 to 600, high pressure 70 atm and concentrated acid catalyst e.g. conc H3PO4

32
Q

Hydration reactions

A

Water is added to a molecule

33
Q

Extra information hydration

A

High pressures means not done in labs
Preferred industrially as no waste products so has a high atom economy
Separation of products is easier and cheaper to carry out

34
Q

Addition polymerisation

A

When addition polymers are formed from alkenes

35
Q

Reactivity of poly alkenes

A

Unreactive due to the strong C-C and C-H bonds

36
Q

What to remember when drawing addition polymerisation

A

n in monomer goes before
N in polymer goes after and outside brackets
Bonds in polymer must extend out of brackets
Must remove double bond to single bond

37
Q

Industrial i,portamce of alkenes

A

Formation of polymers from ethane based monomers
Manufacture margarine by catalytic hydrogenation of unsaturated vegetable oils (uses hydrogen and nickel catalyst)
Liquid vegetable oils are generally polyunsaturated alkenes
Using hydrogen and catalyst converts double bonds to single bonds
Increases melting point of oil to make it harder and more solid

38
Q

How can waste polymers be processed

A

Separation and recycling
Feedstock for cracking
Combustion for energy production

39
Q

Separation and recycling

A

Waste is sorted into each type of polymer (PTFE,PVC,PET)

Each type can be recycled by melting and remolding

40
Q

Feedstock for cracking

A

Waste polymers can be used as feedstock for cracking process allowing new production of plastics and other chemicals

41
Q

Combustion for energy production

A

Waste polymers can be incinerated and heat released can be used to generate electricity
Combustion of halogenated plastics e.g. PVC can lead to formation of toxic acidic waste products such as HCl
Chemists can minimize environmental damage by removing HCl fumes formed from combustion process

42
Q

Polymers formed from what are biodegradable

A

Isoprene (2-methyl-1,3-butadiene)
Maize
Starch

Chemistry Module 4 (8 decks)

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