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IB Chemistry > o chem– fundamentals + isomerism + alkanes > Flashcards

o chem– fundamentals + isomerism + alkanes Flashcards

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1
Q

what is the empirical formula? (e.g. C3H6 => CH2)

A

it is the simplest formula that shows the ratio of all the different atoms in a molecule

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2
Q

what is the molecular formula?

A

it is the actual number of atoms of each element in a molecule

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3
Q

why is the structural formula useful?

A

it clearly shows how the atoms are bonded to each other

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4
Q

give me two types of aliphatic (open chains) hydrocarbons

A
  • straight-chain
  • branched
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5
Q

what class of compounds has the structure –C☰C–

A

alkyne

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6
Q

what structure do arenes have?

A

benzene ring

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7
Q

what structure do ethers have

A

R–O–R

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8
Q

what’s the structural difference between ketones and aldehydes?

A
  • ketone structure is R–C–H
    〢
    O
  • aldehyde structure is R–C–R
    〢
    O
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9
Q

is this an amide or an amine: –NH2

A

amine

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10
Q

what class of compounds is this structure?R–C–NR2
〢
O

A

amide

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11
Q

what defines a homologous series?

A
  • same functional group
  • each successive member increases by one –CH2 unit
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12
Q

homologous series have the following characteristics:
- same _______ formula
- show a gradual change in _______ properties as molar mass increases
- have similar ________ properties

A

general, physical, chemical

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13
Q

why is there resonance in benzene?

A

benzene has a delocalised π electron system

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14
Q

benzene is stable because…

A

it is resonance stabilised

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15
Q

what hybridisation does C in benzene undergo

A

sp2

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16
Q

why is benzene so stable? (4)

A
  • each C atom is sp2 hybridised
  • each C atom contains one unhybridised p orbital that lies perpendicular to the plane of the benzene ring
  • the p orbitals overlap sideways with one another -> continuous overlap of unhybridised p orbitals
  • π e- can delocalise across benzene ring -> creates delocalised π e- cloud -> resonance
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17
Q

what are the effects of resonance stability on benzene
1. ???
2. planar molecule
3. can undergo substitution rxns

A

identical C-C bond lengths

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18
Q

why does benzene have identical C-C bond lengths?

A
  • due to the partial double bond character of C–C bonds
  • as a result of delocalisation of π e- across the ring
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19
Q

why does benzene undergo substitution but not addition rxns?

A
  • sub rxns preserve the resonance stability of benzene
  • add rxns disrupt resonance stability -> energetically unfavourable
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20
Q

what are the two reactions benzene can undergo?

A
  • nitration
  • reduction
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21
Q

chain isomers have the same ______ formula and functional group, but different ________ skeleton

A

molecular, carbon

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22
Q

positional isomers have different positions of ______________

A

functional group

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23
Q

aldehydes are functional grp isomers with …

A

ketones

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24
Q

carboxylic acids are functional grp isomers with…

A

esters

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25
Q

alcohols are functional grp isomers with…

A

ethers

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26
Q

what are stereoisomers?

A

molecules w same molecular formula and same structural formula, but different spatial arrangement of its atoms

27
Q

compounds that can have E/Z isomerism must:
1. have a covalent bond with ____________
2. the two different grps bonded to ea C atom on the end of the restricted bond must be _________

A

restricted rotation, different

28
Q

in determining whether an isomer is E/Z, we need to determine which atom/molecule bonded directly to the carbon has higher ______ and thus higher _______

A

Mr, priority

29
Q

what are enantiomers?

A

stereoisomers that are mirror images of ea other and non-superimposable on ea other

30
Q

do enantiomers have identical physical, chemical and biological properties?

A
  • yes for physical
  • yes for chemical except for reaction with other chiral molecules
  • no for bio as diff spatial arrangement makes them rxt w diff enzymes
31
Q

are alkanes polar?

A

no, they r non polar

32
Q

alkanes have _______ _____ boiling points

A

relatively low

33
Q

what is LDF in secondary school terms?

A

intermolecular FOA/ Van Der Waals’ FOA

34
Q

what is the trend of boiling points down the homologous series of alkanes?

A
  • electron cloud size ↑
    • more polarisable
  • LDF ↑ -> energy required ↑
  • ↑ b.p.
35
Q

do straight chain or branched alkanes have a higher boiling point? (idea: ‘packing’ of the molecules)

A
  • straight chain
  • larger surface area
  • greater extent of LDF
  • requires more energy to break
36
Q

why are alkanes soluble in non-polar solvents? (hint: energy)

A

energy released from LDF btw alkanes + non-polar solvent releases enough energy to overcome LDF in alkanes and LDF in solvent

37
Q

why are alkanes insoluble in water? (hint: energy)

A

energy released from LDF btw alkanes + water is insufficient to overcome H bonding in water

38
Q

why are alkanes so unreactive? (3)

A
  1. they are non-polar
    • do not attract any species for rxn
  2. they are saturated molecules
    • no triple + double bonds to undergo addition
  3. strong C-H and C-C bonds
    • rxns have high activation energy
39
Q

what are the products of complete combustion of alkanes?

A

CO2 and H2O

40
Q

what are the products of incomplete combustion of alkanes?

A

CO, unburnt hydrocarbons, soot (C)

41
Q

what is the reaction of alkanes with halogens called?

A

free radical substituition

42
Q

in free radical sub, what reagents and conditions are needed?

A

Cl2 (g) or Br2 (l), UV light/heat

43
Q

why are free radicals so reactive? (short)

A

free radicals have an unpaired electron

44
Q

UV light is required in free radical sub as it provides ________ for the ______________ of _______ to form Cl* radicals

A

energy, homolytic fisson, Cl-Cl bond

45
Q

how is homolytic fission of the Cl-Cl bonds achieved?

A

UV light provides energy for it

46
Q

the steps of free radical sub are:
1. initiation
2. ???
3. termination

A

propagation

47
Q

in which step of FRS does the regeneration of the free radical take place?

A

propagation

48
Q

what are the products of FRS?

A
  • HX
  • X halogenoalkane
  • trace amounts of alkane w double the no. of C atoms
    (X being the halogen)
49
Q

what happens during initiation of FRS? (1)

A

UV light provides energy for the homolytic fission of Cl-Cl to form Cl radicals

50
Q

what happens during propagation of FRS? (4)

A
  • highly reactive Cl* radical reacts with alkane CH4, removing a H atom to produce HCl
  • as a result, another radical *CH3 is produced
  • methyl radical *CH3 reacts further w another Cl2 molecule to produce e final product -> CH3Cl
  • at the same time another Cl* is regenerated
51
Q

what happens during termination of FRS?

A
  • when two radicals collide -> unpaired electrons pair up to form a stable product
  • this terminates the chain rxn
  • trace amt of ethane formed
52
Q

why is only a flash of UV light necessary to initiate the rxn?

A
  • UV is only required for initiation
  • radicals can be regenerated -> allows for a chain rxn
53
Q

write out the mechanism w eqns ONLY for FRS on paper

A

-

54
Q

why is FRS NOT used to synthesise halogenalkanes?

A

it is difficult to control the
- extent of substitution
- position of subtitution

55
Q

what is a tertiary radical?

A

a radical with where the C atom w the unpaired electron is attached to three other alkyl grps

56
Q

why are tertiary radicals the most stable?

A
  • has largest no. of e- donating alkyl groups
  • can disperse the e- deficiency of the radical to the largest extent
57
Q

how does the stability of radicals affect the actual ratio of products in FRS? (compare to expected ratio)

A

actual ratio may be different than expected

58
Q

how does the rate of FRS change down the halogen group?

A

it decreases

59
Q

why does the rate of FRS decrease down the halogen grp?

A

rate of FRS is dependent on the ∆H of the propagation step

60
Q

why is incomplete combustion harmful

A
  • releases CO
  • CO binds irreversibly w haemoglobin in blood -> O2 deficiency
61
Q

what components of incomplete combustion can result in photochemical smog

A
  • unburnt hydrocarbons rxt w O2, O3, oxides of nitrogen -> in e presence of sunlight
62
Q

NO2 can form _______ (environmentally harmful) and catalyses the formation of ____ to ____, forming _______ (also env harmful)

A

acid rain, SO2, SO3, acid rain

63
Q
A

IB Chemistry (12 decks)

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