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2X Chemistry > Isomerism +Steroechemistry > Flashcards

Isomerism +Steroechemistry Flashcards

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1
Q

what are the 3 types of isomerism

A

-structural
-conformational
-stereoisomerism

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2
Q

define structural isomerism

A

molecules that have the same molecular formula but differ in the sequence in which the atoms are bonded together

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3
Q

what differs for structural isomers and why

A

-physical properties
-chemical properties if they have different functional groups

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4
Q

what’s the general rule for conformational isomers

A

the isomer with the least number of eclipsed bonds is the lowest in energy

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5
Q

how many degrees are there between the H in eclipsed conformation

A

0 degrees

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6
Q

how many degrees are there between the H in staggered conformation

A

60 degrees

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7
Q

why is the eclipsed conformation higher in energy than the staggered conformation

A

due to stereo electronic (electrons repel each other)repulsion between C-H bonds

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8
Q

how does staggered conformation minimise stereo electronic repulsion

A

-the electrons are further apart
-so not repelling one another as much
-so lower in energy

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8
Q

why is the chain conformation of cyclohexane strain free

A

-as eclipsing is completely prevented
-bond angles are very nearly tetrahedral
-there’s no stereo electronic repulsion

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9
Q

what’s the lowest energy conformer of butane+ why

A

-anti-conformation
-due to minimisation of steric hinderance (2 methyl groups are far apart) +stereo electronic repulsion

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10
Q

what’s the highest energy conformer of butane+ why

A

-eclipsed isomer
-due to steric hinderance of 2 methyl groups+ stereo electronic repulsion of all bonds

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11
Q

what are the 2 types of positions for H bonds

A

-axial (vertical, up and down)
-equatorial (horizontal, up and down)

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12
Q

how are more stable conformations made

A

when bulkier/bigger groups are put in the equatorial groups

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13
Q

what happens when a conformer ring flips

A

a H which appears equatorial in one conformer will be axial in the ring flipped conformer

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14
Q

describe the boat conformation of cyclohexane

A

-stereo electronic repulsion between the flagpole H’s
-H’s in the base of the boat

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15
Q

describe the twist boat conformation of cyclohexane

A

doesn’t have total eclipsing of H bonds at the base of the bond so relieves some of the stereo electronic repulsion

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16
Q

which conformer does cyclohexane mainly exist in + why

A

chair conformation as it’s the most stable

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17
Q

what are the 2 chair conformations of mono substituted cyclohexanes

A

equatorial and axial

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18
Q

why is the equatorial conformer more stable

A

as it doesn’t have 1,3-diaxial strain

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19
Q

what’s the general rule for mono-substituted cyclohexanes

A

the cyclohexane will adopt the conformation which has the most substituents in the equatorial position

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20
Q

what happens when there are groups on the hexane ring

A

the biggest group adopts the equatorial position

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21
Q

what are the 2 types of stereoisomerism

A

-enantiomerism
-geometric isomerism

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22
Q

when do molecules exhibit enantiomerism

A

when they lack all 3 symmetrical elements
-a centre of symmetry
-an alternating axis of symmetry
-a plane of symmetry

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23
Q

what’s a chiral molecule

A

-have an asymmetric carbon
-are non-superimposable
-compounds of enantiomers of each other

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24
Q

what are the properties of enantiomers

A

-physical properties are the same
-rotation of plane polarised light differs
-chemical properties differ

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25
Q

what’s a racemic mixture+describe a property

A

-an equimolar mixture of 2 enantiomers
-it isn’t optically active

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26
Q

what are the priority rules of Cahn-Ingold-prelog Convention

A
  1. atom with the highest atomic mass attached to the stereocentre, is the highest priority
  2. look at groups attached
  3. higher oxidised atoms(double bonds) have higher priority
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27
Q

what are the assignment rules of absolute configuration

A
  1. make sure the lowest priority group (usually H)is pointing away (on the dashed line). may have to redraw the molecule
  2. assign a sequence of priority to the atoms or groups of attached to the stereo centre
  3. draw an arrow away from the highest priority group to the lowest priority group
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28
Q

what do you assign if the arrow rotates anti-clockwise

A

S

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29
Q

what do you assign if the arrow rotates clockwise

A

R

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30
Q

what are diastereomers

A

stereoisomers which aren’t related as mirror images

31
Q

what are the properties of diastereomers

A

-differ in physical +chemical properties

32
Q

what’s diastereomeric resolution

A

converting inseparable enantiomers into diastereomers then using chromatography or recrystallisation to resolve the enantiomers

33
Q

what are the 3 steps involved in diastereomic resolution

A
  1. racemic mixture are reacted with a single enantiomer of a different chiral compound giving diastereomers
  2. new diastereomers are separated by crystallisation or chromatography
  3. separated diastereomers are then chemically converted back into the original enantiomers
34
Q

what’s a mess compound

A

molecules which contain stereogenic centres but aren’t optically active, they’re achiral or meso

35
Q

what does geometric isomerism occur from

A

due to restricted rotation about a C=C double bond

36
Q

what’s geometric isomerism

A

a diastereomer

37
Q

what are the 2 types of absolute configuration around C=C double bond

A

E(opposite side)
Z(same side)

38
Q

wha’s a stereospecific reaction

A

a reaction where a stereoisomer reacts to give a single product which is a single stereoisomer (concerted)(good)

39
Q

what’s a stereo random reaction

A

a reaction where a stereoisomer reacts to give a product which is a mixture of stereoisomers (non-concerted)(bad, as loose stereo info to make enantiomers)

40
Q

what’s a concerted reaction

A

bond formation+bond cleavage occur at the same time

41
Q

where does attack of the nucleophile take place from

A

the opposite side to the leaving group

42
Q

what’s walden inversion

A

if a chiral molecule undergoes an SN2 reaction then the absolute configuration is inverted

43
Q

when are sulfonates commonly used in chemistry

A

used as intermediates during the conversion of alcohols into other functional groups

44
Q

what’s made when 2s orbitals + 2p orbitals are hybridised on a sp3 carbon

A

4 sp orbitals, arranged in a tetrahedral shape

45
Q

what are the 3 types of p orbitals

A

px
py
pz

46
Q

what are the 2 key stages in the use of sulfonates in SN2 reactions

A
  1. formation of the tosylate or mesylate from the alcohol
  2. SN2 reaction of the nucleophile with either the tosylate or mesylate
47
Q

why are sulfonates great leaving groups

A

due to resonance stabilisation of the anion

48
Q

when do SN2 reactions of cyclohexanes take place

A

with a strong leaving group +strong nucleophile

49
Q

what are the 2 occurrences that happen during a SN2 reaction of cyclohexanes

A

-equatorial leaving group is replaced by axial nucleophile
-axial leaving group is replaced by equatorial nucelophile

50
Q

what occurs during a SN1 reaction

A

2 steps:
1. slow formation of the carbocation (rds)
2. attack of the nucleophile (fast step)

51
Q

what type of process is a SN1 reaction

A

a non-concerted, stepwise process of bond breaking +bond formation

52
Q

why is a SN1 reaction unimolecular

A

as the rate equation is dependant on only one reagent

53
Q

what type of reaction is a SN1 reaction?

A

a stereorandom reaction

54
Q

why is optical activity lost during a SN1 reaction

A

as a racemic mixture is made

55
Q

what does a planar chiral intermediate result in

A

the nucleophile can attack from both faces to give both enantiomers

56
Q

what are elimination reactions used for

A

to synthesise alkenes

57
Q

what does a E1 reaction result in

A

formations of a mixture of isomers

58
Q

what is the rate dependant on for a E2 reaction

A

the concentration of 2 reagents

59
Q

what must occur for the E2 reaction to take place

A

overlap of the sp3 orbitals on both C atoms for formation of the new pi-bond

60
Q

what are the 2 conformations that allow for overlap of the sp3 orbitals on both C atoms for an E2 reaction

A
  1. syn-coplanar
    2.anti-periplanar
61
Q

why does the anti-periplanar conformation always occur

A

-as the sun-coplanar results in eclipsing interactions
-resulting in electrostatic repulsion between the base+leaving group

62
Q

Describe the positioning of an H atom and a leaving group in an E2 reaction

A

they’re anti-periplanar

63
Q

for elimination reactions, what happens if 2 H’s are axial

A

2 products are formed

64
Q

what are the 2 types of reaction processes addition of alkenes can occur by

A

-stereospecific
-stereorandom

65
Q

when is a reaction stereospecific

A

when the reaction is concerted, bonds are broken +formed at the same time

66
Q

what are the 2 types of addition reactions

A
  1. syn-addition
  2. anti-addition
67
Q

explain a syn-addition reaction

A

as the molecule is flat, attack can take place from either face producing 2 stereoisomers

68
Q

due to a sin-addition being concerted what does this mean

A

cis-relationship of R group is retained

69
Q

explain an anti-addition reaction

A

-attack takes place from the one face+then the other
-if an intermediate is formed where C-C bond rotation is possible, then geometry of alkene can be lost

70
Q

what compound is used during dihydroxylation of alkenes

A

mainly OsO4 (osmium tetroxide) but can be done using KMnO4

71
Q

what’s OsO4 role during dihydroxylation of alkenes

A

OsO4 attacks in sun concerted fashion(both hydroxyls are added from the same face)

72
Q

what is formed in a dihydroxylation of alkenes reaction

A

a racemic mixture

73
Q

what does the sun-addition of oxygen to alkenes result in

A

the formation of epoxides

74
Q

what does the epoxidation of alkenes reaction involve

A

an alkenes+ a per acid

75
Q

why is epoxidation a useful reaction

A

as ring strained epoxides react quickly with nucleophiles to produce substituted alcohols

76
Q

what does the anti-addition of Br2 to alkenes result in

A

a racemic mixture

2X Chemistry (3 decks)

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