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IB biology > topic 10/20 > Flashcards

topic 10/20 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

describe the mechanism for free radical substitution

A
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9
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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10
Q

what type of reaction do alkenes undergo

A

electrophilic addition

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11
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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12
Q

define an electrophile

A

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

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13
Q

describe the bromine water test for alkanes and alkenes

A

alkanes remain orange
alkenes go from orange to colourless

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14
Q

give the mechanism for electrophilic addition for alkenes

A
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15
Q

alkene + hydrogen

A

nickel catalyst, 180’C
alkane

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16
Q

alkene + steam

A

phosphoric V acid
300’C
alcohol

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17
Q

alkene + halogen

A

in hydrocarbon solvent
vicinal dihalide

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18
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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19
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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20
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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21
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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22
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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23
Q

oxidation of secondary alcohols

A

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

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24
Q

potassium dichromate colour change

A

orange to green (oxidised)

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25
acidified potassium manganate (KMnO4) colour change
purple to colourless
26
esterification
alcohol + carboxylic acid -> ester + water - heat in water bath with concentrated sulphuric acid
27
carbonyl group vs carboxyl
C=O vs COOH
28
define a chiral carbon
carbon attached to 4 different atoms/groups, (optically active)
29
optical isomers
enantiomers, or non-superimposable mirror images of each other
30
optically active compound
a compound that can rotate the plane of plane-polarised light and be distinguished using a polarimeterr
31
racemic mixture
contains equal amounts of both enantiomers - optically inactive as the two rotations in opposite directions cancel each other out
32
physical and chemical properties of enantiomers
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
33
diastereomers
not mirror images of each other. contain different configurations at one or more, but not all, of the equivalent stereocenters
34
define a nucleophile
an electron rich species which can donate a lone pair of electrons to an electrophile
35
what type of mechanism do primary halogenoalkanes undergo?
Sn2
36
draw out the mechanism for Sn2
37
in Sn2, what is involved in determining the ror?
both the halgenoalkane and the nucleophile
38
why do primary halogenoalkanes react via Sn2?
the small space occupied by the two hydrogen atoms allows room for the nucleophile to approach the central carbon atom
39
what mechanism do tertiary halogenoalkanes undergo?
Sn1
40
draw out the mechanism for Sn1
41
why do tertiary halogenoalkanes undergo Sn1?
the positive inductive effect of the R groups pushes electrons towards the central carbon ion, stabilising it, allowing it to form
42
what determines the rate of Sn1
the halogenoalkane
43
factors affecting the rate of nucleophilic substitution
type of halogenoalkane nature of halogenoalkane type of solvent
44
type of halogenoalkane
primary < secondary < tertiary tertiary quickest as they undergo an ionic mechanism, so Sn1 faster than Sn2
45
nature of halogenoalkane
- c-cl is stronger than c-br - c-br is a better leaving group as the bond takes less energy to break
46
type of solvent
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
47
describe benzene
- 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
48
physical evidence for the structure of benzene
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
49
chemical evidence for the structure of benzene
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
50
draw the mechanism for the nitration of benzene
51
conditions needed for nitration of benzene
conc sulphuric acid to protonate the HNO3 to form the electrophile, No2+ 50'c (at temperatures higher than this, 1,2 dinitrobenzene is formed)
52
describe lithium aluminium hydride
LiAlH4 - stronger reducing agent (can be used for carboxylic acids) - must initially be used in parotid 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
53
describe sodium borohydride
NaBH4 - can be used in protic solvents but is not strong enough to reduce carboxylic acids
54
reduction of aldehyde
aldehyde -> primary alcohol either, H+ (aq)
55
reduction of ketone
ketone -> secondary alcohol either, H+ (aq)
56
reduction of carboxylic acid
carboxylic acid -> primary alcohol LiAlH4 in ether H+ (aq)
57
describe the reduction of nitrobenzene to phenyl amine
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)
58
define structural isomers
when the atoms are bonded in a different way
59
define stereoisomers
have different arrangement of atoms in space, but do not differ in connectivity
60
give two examples of stereoisomers
conformational - convert by rotation around a sigma bond configurational - convert by breaking and reforming a bond (optical, E/Z)

IB biology (60 decks)

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