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Chemistry > properties of alkenes > Flashcards

properties of alkenes Flashcards

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1
Q

how many sigma bonds are in a carbon atom in a double bond

A

3 sigma bonds - one to the other carbon atom in the double bond and 2 to another carbon or hydrogen, leaving one electron

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2
Q

how is a pi bond formed in a double bond

A

remaining electron in carbon atom is in p orbital so double bond forms from the sideways overlap of 2 p orbitals, one from each carbon atom in the double bond

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3
Q

where is the pi electron density concentrated

A

pi electron density is concentrated above and below the line joining the nuclei of the bonding atoms

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4
Q

why cant the carbon atoms rotate around the double bond

A

the pi bond locks the 2 carbon atoms in position and prevents them from rotating around the double bond.

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5
Q

what is the shape around each of the carbon atoms in a double bond

A

trigonal planar because there are 3 regions of electron density around each of the carbon atoms. These 3 regions repel each other as much as possible forming a bond angle of 120 degrees. All of the atoms are in the same plane

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6
Q

what are stereoisomers

A

same structural formula but different arrangement of atoms in space

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7
Q

what is the difference between optical isomerism and E/Z isomerism?

A

optical isomerism also occurs in alkanes with no functional groups whereas E/Z isomerism only occurs in Carbon-carbon double bonds

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8
Q

what conditions are needed for E/Z isomerism?

A

Carbon-carbon double bond
each carbon atom in double bond must have different functional groups

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9
Q

what conditions are needed for cis-trans isomer

A

carbon-carbon double bond
each carbon atom in double bond must have a hydrogen atom

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10
Q

what is a cis isomer?

A

hydrogen atoms of both carbon atoms in same plane.

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11
Q

Is a cis isomer , a Z or E isomer

A

Z

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12
Q

what is trans isomer

A

hydrogen atoms are opposite each other

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13
Q

is a trans isomer, Z or E?

A

E

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14
Q

why does pi bond break but not sigma bonds when alkenes react?

A

pi bonds are weaker than sigma bonds so are broken more readily

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15
Q

what is the hydrogenation of alkene?

A

alkene is mixed with hydrogen over a nickel catalyst at 423 k in an addition reaction to form an alkane

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16
Q

what are the conditions for hydrogenation of alkene?

A

nicket catalyst at 424K

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17
Q

what type of reaction is for hydrogenation of alkene

A

addition

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18
Q

what is the halogenation of alkene?

A

alkenes undergo addition reaction with chlorine or bromine at room temperature

19
Q

what is the test for a double in alkenes?

A

add drops of bromine to alkene. If bromine orange colour fades away, that means it has reacted with double bond indicating a positive result

20
Q

what is the reaction of alkenes with hydrogen halides?

A

alkene reacts with gaseous hydrogen halides in an addition reaction at room temperature to form haloalkanes.

21
Q

how does the reaction of alkene with hydrogen halide occur if alkene is a gas?

A

both gases are mixed together.

22
Q

how does the reaction of alkene with hydrogen halide occur if alkene is a liquid?

A

the hydrogen halide is bubbled through it

23
Q

what is the hydration reaction of alkenes?

A

when alkenes are reacted with steam , H20 , in the presence of a phosphoric acid catalyst, H3PO4, it forms an alcohol

24
Q

what catalyst is needed for hydration of alkenes?

A

phosphoric acid, H3PO4

25
Q

why does electrophilic addition happen?

A

double bond in alkene has high electron density because of pi bonds which attract electrophiles

25
Q

what is an electrophile

A

atom or group of atoms that is attracted to an electron rich centre and accepts an electron pair.

26
Q

what charge is an electrophile

A

positive

27
Q

what is a carbocation

A

a carbocation contains a positively charged carbon atom

27
Q

what is the full mechanism for electrophilic addition in addition of but-2-ene with hydrogen bromide

A

bromide is slightly negative and hydrogen is slightly positive. The electron pair in pi bond is attracted to partially positive hydrogen atom causing the double bond to break and forming a bond between carbon atom and the hydrogen atom. H-Br bond breaks by heterolytic fission with electron pair going to bromine atom. Carbocation in hydrocarbon which reacts with negative bromine atom.

28
Q

which carbocations are most stable

A

tertiary carbocation

29
Q

which carbocations are least stable?

A

primary carbocation

30
Q

what is addition polymerisation?

A

unsaturated alkene molecules undergo addition polymerisation to produce long saturated chains

31
Q

what conditions are needed for addition polymerisation?

A

high temperature and high pressure using catalysts

32
Q

what are poly(chloroethene) used for

A

used for bottle, pipes, insulation cable due to being flexible and rigid

33
Q

what is poly(propene) used for

A

children toys, packing crates, fibre for ropes , guttering

34
Q

what is poly(phenylethene) used for

A

packaging material ,food trays and cups due to thermal insulating properties

35
Q

what is poly(tetrafluorethene) used for

A

coating for non-stick pans, permeable membrane for clothing an shoes , and cable insulation

36
Q

what are the benefits of polymers?

A

readily available, cheap, more convenient , lack of reactivity makes it suitable for storing food

37
Q

what are the disadvantages of polymers?

A

they are non-biodegradable so has negative environmental effects

38
Q

what are the ways to prevent environmental damage from polymers?

A

1) recycling polymers - chop polymers into flakes washed , dired and melted.
2) PVC recycling - recycling pvc is hazardous due to high chlorine content so releases HCL when burnt. Instead, dissolve sorted polymers using solvent and recover it by precipitation from solvent/
3) feedstock recycling - Reclaim monomers, gases and oils from polymers which can be used as raw materials for new polymers. Polymers can be mixed and unsorted.

39
Q

what is PVC recyling?

A

PVC recycling - recycling pvc is hazardous due to high chlorine content so releases HCL when burnt. Instead, sort the polymers so they arent mixed and dissolve polymer using solvent and recover it by precipitation from solvent/

40
Q

what is feedstock recycling

A

feedstock recycling - Reclaim monomers, gases and oils from polymers which can be used as raw materials for new polymers. Polymers can be mixed and unsorted.

41
Q

what is an advantage of feedstock recycling

A

polymers can be unsorted and mixed

Chemistry (18 decks)

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