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NST Chemistry > Organic part 1 > Flashcards

Organic part 1 Flashcards

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1
Q

retrosynthesis arrow

A

double arrow = “can be made from”

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2
Q

curly arrow represents

A

the movement of a pair of e- from HOMO of NU- to LUMO of electrophile

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3
Q

main considerations molecular interactions

A

HOMO-LUMO interactions
Electrostatic interactions

also:
hardness/softness - FMO or electrostatics driven
orbital coefficients
symmetry

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4
Q

all molecules with __ or ___ can act as nucleophiles

A

a free pair of e-
or at least one pi-bond

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5
Q

relative nucleophilicity

A

-ve charge > lone pair > pi-bond > sigma-bond

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6
Q

example of sigma bond Nu-

A

(BH4)-
donates a hydride to e-phile

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7
Q

an electrophile must have…

A

full or partial positive charge
OR an atom which doesn’t have a full octet of e-

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8
Q

relative electrophilicity order

A

empty orbital (eg. p-orbital) > pi* orbital > sigma* orbital

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9
Q

two mechanistic steps of nucelophilic addition

to C=O

A
  1. Nucleophilic addition to the C=O (attack)
  2. protonation of the resulting anion
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10
Q

explain why and how the Nu- attacks the C=O

A

at the C rather than O due to the large dipole, e-rich Nu attracted to ∂+ C

orbitals of C=O:
both sp2 hydbridised. O l.p. in 2 HAOs, remaining p orbitals perpendicular to the plane.
C greater contribution to pi*antibonding, hence has largest coefficient in the LUMO, Nu attacks here at 107˚ to plane

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11
Q

define alkyl, aryl, vinyl, alkynyl

A

alkyl = alkane chain
aryl = aromatic ring attached
vinyl = C=C attached
alkynyl = C triple bond C attached

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12
Q

what is an organometallic reagent?

A

any compound with a C bonded to a metal

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13
Q

what makes Li and Mg based organometallic reagents good sources of carbanions?

A

the electronegativity of Li (1.0) and Mg (1.2) vs C (2.5) means that the metal is ∂+ and the R- group is left with a ∂-

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14
Q

what is the process of making a Grignard reagent called?

A

Mg insertion

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15
Q

how is a Grignard reagent made?

A

reacting alkyl/aryl/vinyl halide with Mg turnings, in ether
Mg insertion happens at the C-X bond

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16
Q

what is the process of making an organolithium compound called?

A

Lithium halide exchange

write “Li-Hal exchange” on the arrow

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17
Q

what reagents are required to produce an organolithium compound?

A

alkyl/aryl/vinyl halide
TWO equivalents of Li
in ether

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18
Q

what are the products in making an organolithium compound?

A

one equivalent of the organolithium
one equivalent of the lithium halide salt

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19
Q

how are alkynyl organometallic reagents made?

A

deprotonate alkynes with simple alkyl/aryl/vinyl organometallics

H and Li or MgHal “swap” essentially

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20
Q

common way to deprotonate an alkyne

A

use a strong Nitrogen base, commonly sodium amide Na+ -NH2
(NH4 formed as side product)

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21
Q

what happens to organometallics in water / protic solvents?

A

organometallic carbanion is immediately protonated, destroyed

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22
Q

three types of nucleophiles that aldehydes and ketones react with`

A

Hydride (NaBH4)
organometallic reagents
water and alcohols

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23
Q

explain why Nu- attack by the hydride ion itself does not happen

A

so small with such high charge density that it only ever reacts as a base

because the filled 1s orbital is the ideal size to react with the H contribution of the sigma* orbital of H-X bond and not the LUMO of the C=O

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24
Q

HOMO of NaBH4

A

B-H sigma bond

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25
Q

reaction mech NaBH4 attack ald/ket

A

attack: e- from B-H bond attack C of C=O, charge to O, BH3 lewis acid formed

protonation: -ve charge on O reacts with H of H-X solvent (eg. MeOH), forming an alcohol

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26
Q

what happens to the BH3 generated after the Nu attack of NaBH4 + ald/ket?

A

electron deficient and sp2 hybridised, empty p orbital hence a lewis acid
reacts quickly with the oxyanion that has been generated or a molecule of solvent to product tetravalent boron anion, another H- source

can theoretically repeat to use up all 4 H’s but not necessary

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27
Q

how do ald/ket react with organometallic reagents?

A

TWO steps: 1. organometallic, then 2. add H2O (bc organometallic very reactive w/ water)

e- from C-Metal bond attacks C=O, bond forms, O gains -ve charge
protonation from H-OH water to form alcohol

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28
Q

what are hydrates and how are they formed?

A

geminal diol - two -OHs attached to same C
formed by reacting ald/ket with WATER

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29
Q

what are acetals / hemiacetals?

A

acetals: two -OR groups attached to one C
hemiacetals: one -OR, one -OH attached to one C

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30
Q

how are acetals and hemiacetals formed from ald/ket?

A

by reaction with alcohols

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31
Q

identify to HOMO and LUMO in the reaction of water with ald/ket

A

HOMO: lone pair O sp3
LUMO: C=O pi*

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32
Q

reaction mech water with ald/ket

A

lp of O in H2O attacks C=O, bond formed (O attached has +ve charge) and O gains -ve charge
Another H2O molecule removes an H from -OH2 group to make -OH
Other O- is protonated w another H2O molecule to form HYDRATE

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33
Q

significant concentrations of hydrates are usually only formed from ___

A

aldehydes

34
Q

why are hyrates better obtained from aldehydes than ketones?

A

bond angle changes from 120 to 109.5˚, so bigger R groups means greater steric clash in the product.

35
Q

when might the formation of a hydrate from an aldehyde/ketone be favourable?

A

if there is a strained ring in the ald/ket, then forming the hydrate results in a release of ring strain with decreased bond angle.

36
Q

LUMO and HOMO reaction alcohol with ald/ket

A

HOMO: lone pair in O sp3
LUMO: C=O pi*

37
Q

how do acid catalysts work to increase the rate of hydrate / hemiacetal formation ?

A

by making the carbonyl group more electrophilic, by protonation of the O (gains H+)

38
Q

how do base catalysts work to increase the rate of hydrate / hemiacetal formation?

A

by making the nucleophile more nucleophilic, by deprotonation (of water or alcohol) so that the O has a negative charge

39
Q

how to push acetal formation to completion

A

every step is reversible, so must use XS alcohol and/or remove water from the reaction mixture as it forms (eg. by distillation)

40
Q

why can acetal formation only be catalysed by acid?

A

because an -OH group must be made into a good leaving group, cannot happen under basic conditions

41
Q

mech for acetal formation from hemiacetal

A

-OH group protonated –> good leaving group, C=+OR formed (effectively same as protonated C=O)
Another ROH attacks, repeat acid catalysed mech

42
Q

substituions at a trigonal planar C=O group go through ___

A

a tetrahedral intermediate
return to trigonal planar by loss of leaving group

43
Q

the best leaving groups are…

A

the ones that can most easily stabilise negative charge, ie. the most stable anion

44
Q

how does the acidity of the conjugate acid relate to the stability of the anion?

A

the MORE acidic, the more stable the anion
eg. HCl strong, Cl- very stable

45
Q

reaction of Grignard reagent and ketone produces… and why

A

a tertiary alcohol (OH generated from the C=O)
but none of the alkyl chains make good leaving groups

46
Q

eqn pH

A

-log[H3O+]

47
Q

acidity constant expression

A

Ka = [H3O+][A-] / [HA]

48
Q

pKa expression

A

pKa = -log[Ka]

49
Q

why are strong acids able to fully dissociate but weak acids only partially?

A

because of the stability of their conjugate bases
eg. Cl- not a strong enough base to deprotonate H3O+ to reverse the equilibrium, whereas the acetate ion is easily protonated by H3O+ (hence a stronger base than Cl-)

50
Q

the stronger the acid, the ___ the conjugate base

A

weaker

51
Q

how does pKa correspond to acid strength?

A

LOWER pKa = stronger acid!

eg. HCl has a pKa of -7

52
Q

how does pKa correspond to leaving group ability?

A

the LOWER the pKa, the better the leaving group

(because stronger acid = more stable anion = better leaving group)

53
Q

give the pKa of three strong acids

A

HI: -10
HCl: -7
H2SO4: -3

54
Q

pKa of ethanoic acid

A

weak acid, around 4.8

55
Q

pKa of H2S

A

weak acid, 7.0
conjugate base HS-

56
Q

pKa of NH4+

A

weak acid, 9.2

57
Q

phenol pKa

A

C6H5OH pKa = 10

58
Q

what is considered the limit for reasonable leaving groups?

A

ethanol, with a pKa of 15.9
CH3CH2O- isn’t a very good leaving group

59
Q

ethyne pKa

A

terminal H leaves
24

60
Q

benzene pKa

A

43

61
Q

methane pKa

A

48

62
Q

three factors affecting anion stability

A

electronegative elements
delocalisation of -ve charge
strength of A-H bond

63
Q

explain how electronegative atoms impact anion stability

A

increase e-neg of atom upon which a -ve charge sits, INCREASE stability

hence F- > OH- > NH2- > CH3-

64
Q

how does delocalisation of negative charge impact anion stability?

A

the more resonance forms, the greater the stability

generally, increasing # O’s in conj base increases delocalisation due to increased number of available pi-orbitals

65
Q

explain how the strength of the A-H bond affects anion stability

A

the weaker the A-H bond, the stronger the acid (more readily donates H+) hence the better the leaving group

eg. HI > HBr > HCl > HF
(larger atomic radii = longer, weaker bonds)
could not predict from e-neg alone

66
Q

pKa of oxygen acids

A

sulfonic acids RSO2OH = 0
anion has 3 resonance forms, charge on O

carboxylic acids, RCO2H = 5
2 resonance forms, charge on O

ArOH = 10
charge delocalised on ring, charges on C, one structure w charge on O

ROH = 15
alkoxide no resonance forms, charge localised on O

67
Q

define carbon acid

A

proton removed from C instead of O

68
Q

how does hybridisation have an effect on pKa?

A

s-orbitals are closer to the nucleus than p-orbitals, more tightly held, hence more s-character = lower energy = more stable anion

sp > sp2 > sp3

69
Q

general reaction mech of nucleophilic substitution at C=O

A

Nu- attacks at C and forms bond, O gains -ve charge
tetrahedral intermediate formed
leaving group leaves, O- reforms C=O

70
Q

three major factors determining whether nucleophilic substitution will occur

A

strength of incoming Nu
reactivity of the C=O (ie. strength of ∂+ on C)
leaving group ability

(use these to determine if carboxylic derivatives will convert between each other)

71
Q

how does pKa relate to Nu- strength ?

A

the HIGHER the pKa, the better the nucleophile
bc Nu readily forms new bonds w H and C
ie. good Nu = bad leaving group

72
Q

three good Nucleophiles

A

R- (RH pKa = 50)
NH2- (NH3 pKa = 33)
RO- (ROH pKa = 16)

73
Q

what two factors impact the dipole on C=O?

A

the inductive effect and the conjugative effect

74
Q

what is the inductive effect and how does it impact the dipole on C=O?

A

relates to withdrawal of e-dens through SIGMA framework due to relative e-negativity of adjacent groups

more e-neg adj groups = greater dipole
O (3.5)
Cl (3.0)
N (3.0)

75
Q

what is the conjugative effect and how does it impact the dipole on C=O?

A

involves delocalisation of lone pair from attached group into C=O pi* system

eg. amide
lp on N interact w C=O pi* MO, lowers energy

reduces +ve on carbonyl C

76
Q

O, N, and Cl order in strength of lone pair donation

A

Cl < O < N
Cl worst donor, bc donates from 3rd shell, whereas O and N better matches w C as they are in the same O
N less e-neg than O, more willing to give lone pair

77
Q

carboxylic acid derivatives (not incl ald/ket) order of reactivity and respective leaving groups

A

Acid chloride (-Cl)
Anhydride (RCOO-)
Ester (RO-)
Amide (RRN-)
Carboxylate anion

corresponds w C=O shift on IR!

78
Q

convert acid chloride to ester

A

add alcohol, attacks C=O
Base deprotonates H+ from attached Nu, forming tetrahedral intermediate
Cl- good leaving group, C=O reforms

79
Q

convert ester to amide

A

add NH3, attacks C=O
Base deprotonates H from +NH3, leaving NH2 attached to tetrahedral intermediate
-OMe okay leaving group, C=O reforms

80
Q
A

NST Chemistry (16 decks)

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