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A Level Chemistry > Module 4 - Alkenes > Flashcards

Module 4 - Alkenes Flashcards

1
Q

what are alkenes

A

-unsaturated hydrocarbons
-CnH2n

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2
Q

describe how sigma bonds are formed in alkenes

A

-two sp2 orbitals (one from each carbon) overlap to form a single C-C bond called a sigma σ bond

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3
Q

describe how pi bonds are formed in alkenes

A

-rotation can occur around a sigma bond
-the π bond is formed by sideways overlap of two p orbitals on each carbon atom forming a π-bond above and below the plane of molecule

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4
Q

describe pi bonds

A

-π bonds are exposed and have high electron density
-they are therefore vulnerable to attack by electrophiles
-the π bond is weaker than the σ bond

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5
Q

describe the structure of alkenes

A

-planar around the C=C
-bond angle 120 degrees

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6
Q

what are stereoisomers

A

-they have the same structural formulae but have a different spatial arrangement of atoms

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7
Q

when does E-Z stereoisomerism arise

A
  1. there is restricted rotation around the C=C double bond
  2. there are two different groups/atoms attached both ends of the double bond
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8
Q

define a priority group

A

-the atom with the bigger atomic number is classed as the priority atom

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9
Q

describe Z isomers

A

-the priority atom is on the same side of the double bond

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10
Q

describe E isomers

A

-the priority atom is on the opposite side of the double bond

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11
Q

describe the CIP priority rules

A

-the atom of higher atomic number is given priority
-f the atoms are the same, consider the atoms at distance 2 from the double bond

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12
Q

describe cis-trans isomerism

A

-a special case of EZ isomerism in which two of the substituent groups are the same

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13
Q

define an addition reaction

A

-a reaction where two molecules react together to produce one

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14
Q

describe the conditions needed to convert an alkene to an alkane

A

-hydrogen reagent
-nickel catalyst
-addition reaction

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15
Q

describe the conditions needed to convert an alkene to a dihaloalkane

A

-bromine reagent, electrophile reagent (Brδ+)
-room temperature
-electrophilic addition

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16
Q

what is the type of bond fission when alkenes react with a halogen

A

-heterolytic

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17
Q

explain how alkenes react with a halogen (eg bromine)

A

-as the Br2 molecule
approaches the alkene, the pi bond electrons repel the electron pair in the Br-Br bond
-this induces a dipole
-Br2 becomes polar and
electrophilic (Brδ+)
-the intermediate formed, which has a positive charge on a carbon atom is called a carbocation

18
Q

describe the conditions needed for an alkene to be converted to a haloalkane

A

-HCl or HBr reagent, electrophile, HS+
-room temperature
-electrophilic addition

19
Q

explain how alkenes react with HCl/HBr

A

-HBr is a polar molecule because Br is more
electronegative than H
-he H δ +is attracted to the
electron-rich pi bond
-this reaction can lead to two
products when the alkene is
unsymmetrical

20
Q

what is markownikoff’s rule

A

-in most cases, bromine will be added to the carbon with the fewest hydrogens attached to it

21
Q

define an electrophile

A

-an electron pair acceptor

22
Q

explain what makes a carbocation intermediate more stable

A

-the more stable carbocation is stabilised because the methyl groups on either (or one) side of the positive carbon are
electron releasing and reduce the charge on the ion

23
Q

how is the major product formed in electrophilic addition

A

-via the more stable carbocation intermediate

24
Q

what is the order of stability for carbocations

A

-tertiary > secondary > primary

25
Q

describe the conditions needed for alkenes to form alcohols

A

-steam reagent
-temperature 300-600 degrees
-70 atm pressure
-catalyst of concentrated H3PO4

26
Q

describe how alkenes form alcohols

A

-they are reacted with steam in the presence of an acid catalyst
-high pressure means it is done industrially rather than in the lab

27
Q

advantages of the production of alcohols from alkenes

A

-high atom economy as no waste products
-separation of products is easier and cheaper to carry out

28
Q

define a hydration reaction

A
  • a reaction where water is added to a molecule
29
Q

what is addition polymerisation

A

-addition polymers being formed from alkenes

30
Q

reactivity of poly(alkenes)

A

-like alkanes
-unreactive
-due to strong C-C and C-H bonds

31
Q

how to draw the polymer repeating unit for any alkene

A

-draw out the monomer with groups of atoms arranged around the double bond
-then identify the repeating unit

32
Q

describe the industrial importance of alkenes

A

-forming polymer from ethene based monomers
-manufacture of margarine by catalytic hydrogenation of unsaturated vegetable oils

33
Q

what does hydrogenation of liquid vegetable oils do

A

-converts the double bonds to single bonds
-increasing the melting point of the oil so it is harder and more solid
-forming margarine

34
Q

how are waste polymers processed

A

-separation and recycling
-feedstock for cracking
-combustion for energy production

35
Q

describe how waste polymers are used for separation and recycling

A

-the waste is sorted into each different type of polymer
-then each type can be recycled by melting and remoulding

36
Q

impact of waste polymers being used as feedstock for cracking

A

-allowing for the new
production of plastics and other chemicals

37
Q

describe how waste polymers can be used in combustion

A

-can be incinerated and heat is released to generate electricity
-combustion of halogenated plastics can lead to the formation of toxic, acidic waste products such as HCl

38
Q

describe how chemists can minimise the environmental damage of combustion forming toxic products

A

-by removing the HCl fumes formed from the combustion
process

39
Q

what have chemists been developing to deal with waste polymers

A

-biodegradable polymers
-compostable polymers
-soluble polymers
-photodegradable polymers

40
Q

which polymers are biodegradable

A

-polymers formed from isoprene (2-methyl-1,3- butadiene), maize and starch

A Level Chemistry (8 decks)

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