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IB chemistry > org > Flashcards

org Flashcards

1
Q

why do alkanes have a low reactivity?

A
  • strength of C-C and C-H bonds
  • non polar so not susceptible to attack
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2
Q

heterolytic fission

A

results in the formation of a +ve and -ve ion

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3
Q

homolytic fission

A

results in the formation of two radicals

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4
Q

define a radical

A

reactive species which possess an unpaired electron

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5
Q

define free radical substitution

A

a type of substitution where a radical replaces an atom/group of atoms

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6
Q

why do alkanes undergo free radical substitution

A

they have a low reactivity

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7
Q

why do we use UV light for free radical substitution?

A

it is strong enough to break the Cl-Cl or Br-Br bonds but not C-C or C-H bonds

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8
Q
A
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9
Q

describe the mechanism for free radical substitution

A
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10
Q

limitations of free radical substitution

A

not effective for creating a specific halogenoalkane
- any of the hydrogens can be substituted
- multi substitutions can also occur

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11
Q

what type of reaction do alkenes undergo

A

electrophilic addition

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12
Q

why do alkenes undergo electrophilic addition

A
  • double bond has a high electron density so attracts electrophiles
  • pi bond can break to form 2 new bonds
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13
Q

define an electrophile

A

an electron deficient species that can accept an electron pair from a nucleophile

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14
Q

describe the bromine water test for alkanes and alkenes

A

alkanes remain orange
alkenes go from orange to colourless

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15
Q

give the mechanism for electrophilic addition for alkenes

A
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16
Q

alkene + hydrogen

A

nickel catalyst, 180’C
alkane

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17
Q

alkene + steam

A

phosphoric V acid
300’C
alcohol

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18
Q

alkene + halogen

A

in hydrocarbon solvent
vicinal dihalide

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19
Q

describe Markvnokov’s rule

A

in an asymmetric alkane undergoing an electrophilic addition reaction with a hydrogen halide, the H atom will add to the C atom that is bonded to the most hydrogen atoms
- in an intermediate carbocation, the positive inductive effect of the R groups pushes electrons towards the positive carbocation, stabilising it

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20
Q

when do alcohols combust and what does this produce

A

in plentiful supply of oxygen to form carbon dioxide and water

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21
Q

dehydration of alcohols

A

alcohol -> alkene + water
phosphoric acid, heat under reflux and collect the product (alkene) by distillation due to its lower boiling point

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22
Q

partial oxidation of primary alcohol

A

primary alcohol + oxygen -> aldehyde + water
- K2Cr2O7 (potassium dichromate)
- sulphuric acid
- heat and distil off the product as soon as it is formed to prevent further oxidation

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23
Q

complete oxidation of primary alcohols

A

primary alcohol + oxygen -> carboxylic acid + water
- excess K2Cr2O7 (potassium dichromate)
- sulphuric acid
- heat under reflux for at least 10 minutes then distil off the product

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24
Q

oxidation of secondary alcohols

A

secondary alcohol + oxygen -> ketone + water
heat with potassium dichromate, dilute sulphuric acid

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25
Q

potassium dichromate colour change

A

orange to green (oxidised)

26
Q

acidified potassium manganate (KMnO4) colour change

A

purple to colourless

27
Q

esterification

A

alcohol + carboxylic acid -> ester + water
- heat in water bath with concentrated sulphuric acid

28
Q

carbonyl group vs carboxyl

A

C=O vs COOH

29
Q

define a chiral carbon

A

carbon attached to 4 different atoms/groups, (optically active)

30
Q

optical isomers

A

enantiomers, or non-superimposable mirror images of each other

31
Q

optically active compound

A

a compound that can rotate the plane of plane-polarised light and be distinguished using a polarimeterr

32
Q

racemic mixture

A

contains equal amounts of both enantiomers - optically inactive as the two rotations in opposite directions cancel each other out

33
Q

physical and chemical properties of enantiomers

A

physical : identical except the ability to rotate the plane of plane polarised light in opposite directions
chemical : identical except when they interact with other optically active substances

34
Q

diastereomers

A

not mirror images of each other. contain different configurations at one or more, but not all, of the equivalent stereocenters

35
Q

define a nucleophile

A

an electron rich species which can donate a lone pair of electrons to an electrophile

36
Q

what type of mechanism do primary halogenoalkanes undergo?

A

Sn2

37
Q

draw out the mechanism for Sn2

A
38
Q

in Sn2, what is involved in determining the ror?

A

both the halgenoalkane and the nucleophile

39
Q

why do primary halogenoalkanes react via Sn2?

A

the small space occupied by the two hydrogen atoms allows room for the nucleophile to approach the central carbon atom

40
Q

what mechanism do tertiary halogenoalkanes undergo?

A

Sn1

41
Q

draw out the mechanism for Sn1

A
42
Q

why do tertiary halogenoalkanes undergo Sn1?

A

the positive inductive effect of the R groups pushes electrons towards the central carbon ion, stabilising it, allowing it to form

43
Q

what determines the rate of Sn1

A

the halogenoalkane

44
Q

factors affecting the rate of nucleophilic substitution

A

type of halogenoalkane
nature of halogenoalkane
type of solvent

45
Q

type of halogenoalkane

A

primary < secondary < tertiary
tertiary quickest as they undergo an ionic mechanism, so Sn1 faster than Sn2

46
Q

nature of halogenoalkane

A
  • c-cl is stronger than c-br
  • c-br is a better leaving group as the bond takes less energy to break
47
Q

type of solvent

A

Sn1: protic, polar solvents (eg water, ethanol)
- contain N/O atoms bound to H
- solvate both the carbocation and the halide ion well, stabilising them and making them more easily formed
Sn2: non-protic, polar solvents (eg propanone)
- do not contain N/O atoms
- does not solvate nucleophile well, leaving it free to attack the halogenoalkane

48
Q

describe benzene

A
  • 6 carbon atoms, 6 hydrogen atoms
  • hexagonal planar molecule
  • each c atom is sp2 hybridised and forms a sigma bond w each of the 2 neighbouring c atoms and a sigma bond with a hydrogen atom
  • 6p orbitals contain an electron each and form the delocalised pi bond above and below the plane of the molecule
49
Q

physical evidence for the structure of benzene

A
  1. bond lengths: if benzene contained both C-C and C=C bonds, it would be expected that the bonds be different lengths, some shorter than others. However, all the bonds are the same length, which is inbetween the length of a C-C and a C=C bond
  2. Enthalpy of hydrogenation: enthalpy change of hydrogenation/combustion is less exothermic than
    predicted for cyclohexa-1,3,5-triene, which accounts for the extra energy needed to overcome the resonance, or delocalisation energy of the molecule
50
Q

chemical evidence for the structure of benzene

A

benzene undergoes substitution reactions much more readily that addition reactions. If it contained c=c double bonds, it would be expected to undergo addition reactions readily. this is because the delocalisation, or resonance energy, needs to be overcome before molecules can add

51
Q

draw the mechanism for the nitration of benzene

A
52
Q

conditions needed for nitration of benzene

A

conc sulphuric acid to protonate the HNO3 to form the electrophile, No2+
50’c (at temperatures higher than this, 1,2 dinitrobenzene is formed)

53
Q

describe lithium aluminium hydride

A

LiAlH4
- stronger reducing agent (can be used for carboxylic acids)
- must initially be used in aprotic solvent (eg ether) as it reacts vigorously with water to release hydrogen
- once the intermediate product has formed, it can be reacted with water to form the product

54
Q

describe sodium borohydride

A

NaBH4
- can be used in protic solvents but is not strong enough to reduce carboxylic acids

55
Q

reduction of aldehyde

A

aldehyde -> primary alcohol

either, H+ (aq)

56
Q

reduction of ketone

A

ketone -> secondary alcohol

either, H+ (aq)

57
Q

reduction of carboxylic acid

A

carboxylic acid -> primary alcohol

LiAlH4 in ether
H+ (aq)

58
Q

describe the reduction of nitrobenzene to phenyl amine

A
  1. reduction of tin to produce C6H5NH3+
    - heat under reflux w tin, and conc HCl
  2. release of phenyl amine via an acid-base reaction (adding NaOH)
59
Q

define structural isomers

A

when the atoms are bonded in a different way

60
Q

define stereoisomers

A

have different arrangement of atoms in space, but do not differ in connectivity

61
Q

give two examples of stereoisomers

A

conformational - convert by rotation around a sigma bond
configurational - convert by breaking and reforming a bond (optical, E/Z)

IB chemistry (29 decks)

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