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NST Chemistry > Organic part 1 > Flashcards

Organic part 1 Flashcards

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
reaction mech NaBH4 attack ald/ket
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
26
what happens to the BH3 generated after the Nu attack of NaBH4 + ald/ket?
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
27
how do ald/ket react with organometallic reagents?
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
28
what are hydrates and how are they formed?
geminal diol - two -OHs attached to same C formed by reacting ald/ket with WATER
29
what are acetals / hemiacetals?
acetals: two -OR groups attached to one C hemiacetals: one -OR, one -OH attached to one C
30
how are acetals and hemiacetals formed from ald/ket?
by reaction with alcohols
31
identify to HOMO and LUMO in the reaction of water with ald/ket
HOMO: lone pair O sp3 LUMO: C=O pi*
32
reaction mech water with ald/ket
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
33
significant concentrations of hydrates are usually only formed from ___
aldehydes
34
why are hyrates better obtained from aldehydes than ketones?
bond angle changes from 120 to 109.5˚, so bigger R groups means greater steric clash in the product.
35
when might the formation of a hydrate from an aldehyde/ketone be favourable?
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
LUMO and HOMO reaction alcohol with ald/ket
HOMO: lone pair in O sp3 LUMO: C=O pi*
37
how do acid catalysts work to increase the rate of hydrate / hemiacetal formation ?
by making the carbonyl group more electrophilic, by protonation of the O (gains H+)
38
how do base catalysts work to increase the rate of hydrate / hemiacetal formation?
by making the nucleophile more nucleophilic, by deprotonation (of water or alcohol) so that the O has a negative charge
39
how to push acetal formation to completion
every step is reversible, so must use XS alcohol and/or remove water from the reaction mixture as it forms (eg. by distillation)
40
why can acetal formation only be catalysed by acid?
because an -OH group must be made into a good leaving group, cannot happen under basic conditions
41
mech for acetal formation from hemiacetal
-OH group protonated --> good leaving group, C=+OR formed (effectively same as protonated C=O) Another ROH attacks, repeat acid catalysed mech
42
substituions at a trigonal planar C=O group go through ___
a tetrahedral intermediate return to trigonal planar by loss of leaving group
43
the best leaving groups are...
the ones that can most easily stabilise negative charge, ie. the most stable anion
44
how does the acidity of the conjugate acid relate to the stability of the anion?
the MORE acidic, the more stable the anion eg. HCl strong, Cl- very stable
45
reaction of Grignard reagent and ketone produces... and why
a tertiary alcohol (OH generated from the C=O) but none of the alkyl chains make good leaving groups
46
eqn pH
-log[H3O+]
47
acidity constant expression
Ka = [H3O+][A-] / [HA]
48
pKa expression
pKa = -log[Ka]
49
why are strong acids able to fully dissociate but weak acids only partially?
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
the stronger the acid, the ___ the conjugate base
weaker
51
how does pKa correspond to acid strength?
LOWER pKa = stronger acid! eg. HCl has a pKa of -7
52
how does pKa correspond to leaving group ability?
the LOWER the pKa, the better the leaving group (because stronger acid = more stable anion = better leaving group)
53
give the pKa of three strong acids
HI: -10 HCl: -7 H2SO4: -3
54
pKa of ethanoic acid
weak acid, around 4.8
55
pKa of H2S
weak acid, 7.0 conjugate base HS-
56
pKa of NH4+
weak acid, 9.2
57
phenol pKa
C6H5OH pKa = 10
58
what is considered the limit for reasonable leaving groups?
ethanol, with a pKa of 15.9 CH3CH2O- isn't a very good leaving group
59
ethyne pKa
terminal H leaves 24
60
benzene pKa
43
61
methane pKa
48
62
three factors affecting anion stability
electronegative elements delocalisation of -ve charge strength of A-H bond
63
explain how electronegative atoms impact anion stability
increase e-neg of atom upon which a -ve charge sits, INCREASE stability hence F- > OH- > NH2- > CH3-
64
how does delocalisation of negative charge impact anion stability?
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
explain how the strength of the A-H bond affects anion stability
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
pKa of oxygen acids
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
define carbon acid
proton removed from C instead of O
68
how does hybridisation have an effect on pKa?
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
general reaction mech of nucleophilic substitution at C=O
Nu- attacks at C and forms bond, O gains -ve charge tetrahedral intermediate formed leaving group leaves, O- reforms C=O
70
three major factors determining whether nucleophilic substitution will occur
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
how does pKa relate to Nu- strength ?
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
three good Nucleophiles
R- (RH pKa = 50) NH2- (NH3 pKa = 33) RO- (ROH pKa = 16)
73
what two factors impact the dipole on C=O?
the inductive effect and the conjugative effect
74
what is the inductive effect and how does it impact the dipole on C=O?
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
what is the conjugative effect and how does it impact the dipole on C=O?
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
O, N, and Cl order in strength of lone pair donation
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
carboxylic acid derivatives (not incl ald/ket) order of reactivity and respective leaving groups
Acid chloride (-Cl) Anhydride (RCOO-) Ester (RO-) Amide (RRN-) Carboxylate anion corresponds w C=O shift on IR!
78
convert acid chloride to ester
add alcohol, attacks C=O Base deprotonates H+ from attached Nu, forming tetrahedral intermediate Cl- good leaving group, C=O reforms
79
convert ester to amide
add NH3, attacks C=O Base deprotonates H from +NH3, leaving NH2 attached to tetrahedral intermediate -OMe okay leaving group, C=O reforms
80

NST Chemistry (16 decks)

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