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chemistry hl > R3.4 > Flashcards

R3.4 Flashcards

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1
Q

heterolytic fission

A
  • one of the atoms is left with both bonding electrons (anion), the other receives none (cation)
  • especially in polar bonds (positive partial charge ⇒ cation)
  • often assisted with the formation of a new bond since species formed in heterolytic fission are unstable
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2
Q

rate of heterolytic fission is determined by

A

bond enthalpy: higher bond enthalpy ⇒ stronger bond ⇒ slower
bond polarity: higher polarity ⇒ faster
stability of ions formed: anions are stabilized by: high electronegativity, large atomic radius (charge is delocalized over larger volume), resonance
more stable anion ⇒ faster reaction

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3
Q

sites of attack def and types

A

places in an organic molecule that are more positive/negative as it attracts other molecules to form new bonds
types: electron-deficient and electron-rich

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4
Q

electron-deficient sites of attack

A

lower electron density
cations, atom with a positive partial charge

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5
Q

electron-rich sites of attack

A

higher electron density
anions, double/triple bonded, has nonbonding electron pairs

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6
Q

compound has multiple sites of attack ⇒

A

highly reactive, unstable

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7
Q

nucleophile

def, lewis acid/base

A

electron-rich species (negative charge) with a lone pair of electrons on a central atom or a negative charge ⇒ attracted to electron-deficient sites of attack
electron-pair donor ⇒ lewis base

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8
Q

nucleophilicity

def, depends on

A
  • =ability to donate a nonbonding electron pair
  • more available electrons ⇒ more nucleophilic (decreases across a period due to increasing electronegativity)
  • higher electron density ⇒ greater nucleophilicity
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9
Q

electrophile

def, lewis acid/base

A

= electron-deficient species (positive charge) with vacant orbitals ⇒ attracted to electron-rich sites of attack
electron-pair acceptor ⇒ lewis acid

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10
Q

preparation of alcohols with nucleophilic substitution

A

alkane under radical substitution → halogenoalkane + NaOH (nucleophilic substitution) → alcohol + NaX

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11
Q

organic alcohols + strong base

A

organic alcohols are slightly acidic ⇒ react with strong bases, creating alkoxide salts

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12
Q

alkoxide

A

RO¯ (single bond)

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13
Q

alkoxide salt

A

XRO

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14
Q

preparation of nitriles with nucleophilic substitution

A

alkane under radical substitution → halogenoalkane + KCN (reflux, n.s.) → nitrile + KX

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15
Q

elongating the carbon chain

A

by creating a nitrile

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16
Q

preparation of amines with nucleophilic substitution

A

alkane under radical substitution → halogenoalkane + NH3 (nucleophilic substitution) → amine + HX

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17
Q

preparation of ethers with nucleophilic substitution

A

alkane under radical substitution → halogenoalkane + NaOCH3 (reflux, n.s.) → ether + NaX

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18
Q

reactivity of the substrate in nucleophilic substitution is determined by

A

the leaving group: bond strength and therefore its atomic size
smaller atom ⇒ stronger bond ⇒ hardest to break ⇒ worst leaving group
largest leaving group ⇒ electrons are more dispersed ⇒ leaving group is stabilized ⇒ best leaving group

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19
Q

which halogen is the best and worst leaving group for nucleophilic substitution?

A

best is iodine (largest atom)
worst is fluorine (smallest atom)

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20
Q

nucleophile is a strong base ⇒ ?

what happens, equation

A

elimination (halogen is removed from one carbon, hydrogen is removed from the adjacent C)
halogenoalkane + Nu → alkene + HNu + X¯

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21
Q

nucleophilic substitution mechanisms

A

SN1, SN2

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22
Q

what does SN1 stand for?

A

S … substitution
N … nucleophilic
1 … unimolecular ⇒ rate of the first step depends only on one molecule (depends on only the substrate aka leaving group)

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23
Q

stability of the leaving group on the rate of n.s.

A

more stable leaving group ⇒ less energy required ⇒ faster reaction

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24
Q

most stable leaving group for n.s.

A

most stable is a conjugate base

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25
nucleophile effect on SN1
none
26
substrate in SN1 | which and why
**tertiary carbocations**, benzylic and allylic carbocations due to the stability of the carbocations due to **positive inductive effect** (carbon is slightly more electronegative than hydrogen ⇒ weak dipole and electrons are slightly shifted to C ⇒ adjacent alkyl groups stabilize carbocations by donating electron density, reducing the positive change on the carbon)
27
more stable carbocation ⇒ reactivity?
the more reactive it is more likely to go into the carbocation state
28
benzyllic carbocation
C+ attached to benzyl ring
29
allylic carbon
carbon atom that is directly attached to a C=C but is not part of the double bond itself
30
how are allylic and benzylic carbocations stabilized?
electron delocalization
31
solvent of SN1 | which and why
**protic solvent**⇒ able to form hydrogen bonds with its own hydrogen (contain H bonded to a highly electronegative atom), stabilizing carbocations by **counteracting the positive charge** and forming **H-bonds with the leaving group** to prevent it from bonding back to the carbocation ⇒ faster reaction, more stable transition state and reaction
32
steps in SN1
1. creation of the carbocation and leaving group separates (rate-limiting step) 2. nucleophile attacks carbocation (reaction rate is not affected by the nucleophile, fast)
33
how is the creation of the carbocation rate-limiting in SN1
better leaving group ⇒ rate of reaction is higher better leaving groups: larger atoms, leaving groups that are more stable as negative ions (halogens are good leaving groups), weak bases (strong bases are poor leaving groups), aka **good leaving groups are better able to “spread out” their negative charge ⇒ more stable**
34
SN1 energy diagram | description, activation energies + comparison
higher energy until peak 1 (intermediate of carbocation formation) => goes a bit down (local min is the carbocation intermediate) => goes a bit up to peak 2 (intermediate of nucleophile attack) => goes down Ea1 => beginning of energy to peak 1 Ea2 => carbocation is formed to peak 2 Ea1 > Ea2 => 1st step is slower
35
stereochemistry of SN1
C+ only has 3 electron domains ⇒ carbocation is trigonal planar => nucleophile attacks from the front or back => this carbon is a chiral center ⇒ optical isomers are created ⇒ product: **racemic mixture**
36
SN2 abbreviation meaning
S … substitution N … nucleophilic 2 … bimolecular ⇒ rate of the first step depends on two molecules (depends on both the substrate and nucleophile)
37
effect of nucleophile on SN2
more nucleophilic means faster reaction more available electrons (higher electron density, decreases across the period, increases down the group) ⇒ more nucleophilic
38
steps in SN2
simultaneous donation of elections to the carbon atom and electron from the covalent bond will go to the leaving group)
39
activation complex/transition state
existing bond is weakened, new bond begins to form
40
transition state vs intermediate form
transition states exist for an infinitely small period of time, while intermediate forms do not immediately form into the final product as it has some stability, and they do not represent a step of the reaction
41
arrangement of the transition state in SN2
(trigonal) planar
42
stereochemistry in SN2
nucleophile attacks at a 180° degree to the the position of the leaving group, as the leaving group creates steric hindrance, preventing the nucleophile from attacking it at the same side as the leaving group configuration is inverted ⇒ stereospecific reaction (= products formed have a specific stereochemistry, and are not a mixture of isomers)
43
substrate in SN2
primary halogenoalkanes, as the carbocation will be more accessible to the nucleophile
44
energy diagram of SN2 | description, activation energy
energy rises to peak (activation complex) => goes down below initial energy Ea => initial energy to peak
45
good and bad leaving group examples
good: halogens (except for F), water bad: OH, OR, NH2, F
46
solvent in SN2
reactions are accelerated in **polar aprotic solvents** (cannot form hydrogen bonds ⇒ cannot stabilize through hydrogen bonding, however the **negative charge** on the aprotic solvent **allows for the nucleophile to be free and attack**)
47
polar protic vs polar aprotic solvents in SN2
* **polar aprotic** ⇒ complex the cation through the non-bonding electron pairs on O/N in the aprotic solvent (**positive charge of cations** is attracted to **lone pairs**) to **stabilize positive ions** without stabilizing anions ⇒ nucleophile is more unstable (aka more reactive) and stable cations prevent the cations from bonding back to the nucleophile * **polar protic** ⇒ they hydrogen bond to the nucleophile ⇒ stabilizing it, strongly solvating it ⇒ making it less reactive, making it more energetically-costly to react (has to “strip off” solvent molecules first)
48
SN1 vs SN2 | substrate, solvent, nucleophile, leaving group
* substrate: SN1 (tertiary carbocation, strong effect on rate), SN2 (primary carbocation, strong effect on rate) * solvent: SN1(protic), SN2 (polar aprotic) * nucleophlie: SN1 (no effect on rate), SN2 (strong effect on rate, highly nucleophilic is preferred) * leaving group: SN1 (strong effect, favors good leaving groups), SN2 (strong effect, favors good leaving groups)
49
mechanism of electrophilc addition
1. C=C attacks partially positive atom in a covalent bond => heterolytic cleavage of that bond (electrons of covalent bond go to the electronegative element) => carbocation and anion are created 2. anion attacks carbocation positive charge => covalent bond => substituted alkane
50
side products in electrophilic addition
none
51
hydrohalogenation | equation, mechanism
alkene + HX → halogenoalkane electrophilic addition
52
in hydrohalogenation, where does the H attach to?
Markovnikov’s rule: a hydrogen halide adds to an asymmetrical alkene ⇒ halogen attaches itself to the carbon atom of the alkene with the least number of hydrogen atoms (aka hydrogen goes to hydrogen-rich sites, the other part goes to the part with the least hydrogens)
53
hydration | equation, mechanism name
alkene + H2O → alcohol electrophilic addition
54
detailed mechanism of hydration
1. requires acid to protonate water: 2H2O ⇌ H3O+ + OH¯ 1. H3O+ + alkene ⇌ carbocation (slow; H from H3O+ goes to carbon with the more hydrogens) 1. carbocation + H2O ⇌ organic compound with H2O+ attached to the carbon with the least hydrogens (fast) 1. organic compound with H2O+ attached to the carbon with the least hydrogens + H2O ⇌ alcohol + H3O+ (fast) - hydrogen from H2O+ goes to H2O
55
cracking of crude oil
long hydrocarbon → alkenes and alkanes by hydration
56
hydrogenation | equation, mechanism, role
alkene + H2 → alkane (with a **surface catalyst** (Pt or Pd)) electrophilic addition saturation of organic compounds
57
halogenation | equation, mechanism type
alkene + Br2/Cl2 → dibromo/chloroalkane electrophilic addition
58
detailed mechanism of halogenation
X2 becomes polarized as it approaches C=C (closer part becomes more positive as the electrons are repelled by the electron rich double bond) ⇒ C=C attacks the X with the positive partial charge → heterolytic fission of X—X ⇒ positive Br forms a covalent bond with one of the Cs, the other C becomes positively charged → carbocation ⇒ electrons of Br¯ attack the positive charge, forming a dative covalent bond with that C
59
bromine test for saturation
org. compound + Br2 saturated ⇒ no reaction, no color change (aka stays orange, since it would require UV/heat) unsaturated ⇒ colour change orange → transparent (reaction)
60
electrophilic addition with X2 vs X2 (aq)
X2 => only X--X is in the reaction X2 (aq) => X--X is present in the first step, OH¯ then acts as a nucleophile, haloalcohol is created
61
alkene + HX →
haloalkane, following Markovnikov's rule
62
alkene + water →
alcohol, following Markovnikov's rule
63
electrophilic substitution substrate
aromatic compounds
64
mechanism of electrophilic substitution
1. attack on electrophile ⇒ sigma complex (arenium ion): rate-determining step, aromatic nature is destroyed 2. proton (H) is lost ⇒ substitution product, aromatic nature is restored
65
significance of the destruction of aromatic nature in the first step of electrophilic substitution
⇒ unstable, requires a lot of energy (and therefore strong electrophiles) ⇒ drives to restore aromatic nature (continuing the reaction) to become more stable
66
energy diagram of electrophilic substitution | description
energy rises to peak 1 (activation complex of arenium ion) => falls a bit (minimum is the formation of the arenium ion) => rises a bit to peak 2 (activation complex of final substitution product) => falls
67
energy to break aromatic character in comparison to energy to restore it
energy to break aromatic character > to restore it
68
rate of electrophilic substitution depends on
substituents nature of donating/withdrawing electrons, aka whether they are **activators or deactivators**
69
activators | how they work, ?-directors, examples
* substituents **donate electrons to the ring** ⇒ **stabilization** of the carbocation and transition state, the ring becomes **more electron-rich** (electron-donating inductive effect) ⇒ **increased rate** of electrophilic aromatic substitution * substituent can delocalize their lone pair/electron density/hyperconjugation into the ring ⇒ stabilizing the carbocation at ortho and para positions ⇒ all activators are **ortho/para-directing** * alkyl groups (CH3, C6H5), -OH, -OR, -NH2
70
halogens as ortho/para/meta directors
halogens, despite withdrawing electrons can donate their lone pairs to the ring through resonance ⇒ ortho-para directing
71
deactivators | how they work, ?-directors, examples
* substituents withdraw electrons from the ring ⇒ destabilization of the carbocation and transition state, the ring becomes less electron-rich (electron-withdrawing inductive effect) ⇒ decreased rate of electrophilic aromatic substitution * resonance structures of deactivators place a positive charge on ortho/para positions ⇒ all deactivators are meta-directors (less unstable at meta) * -NO2, -CF3, -COOH, -CHO, -COR
72
how are aromatic compounds stabilized through resonance
through electron delocalization of pi electrons: electron withdrawal from a negatively charged centre, and e- release to a positively charged center ⇒ stabilization
73
general formula of the nitration of benzene
benzene + HNO3 →(strong acid) benzene-NO2 + H3O+
74
creation of the nitronium ion
1. HNO3 + H2SO4 ⇌ H2NO3+ + HSO4¯ 1. H2NO3+ ⇌ NO2+ + H2O
75
halogenation of benzene
benzene + X2 →(FeX3/AlX3) benzene-X + HX with a catalyst (elemental Fe, **AlCl3 or FeX3**)
76
ferric halide in electrophilic substitution
ferric halide (FeX3) ⇒ generation of electrophiles X2 + FeX3 ⇌ X+ + FeX4¯ X+ … halonium ion (electrophile) ⇒ chloronium, bromonium
77
Friedel-Crafts alkylation
benzene + RX →(AlX3/FeX3) benzene-R + HX with catalyst AlX3 or FeX3 to produce carbocation R+
78
Friedel-Crafts acylation
benzene + R-C(=O)-X →(AlX3/FeX3) benzene-C(=O)-R + HX with catalyst AlX3 or FeX3 to produce acylium ion RC+(=O) product is an acyl benzene (aromatic ketone)
79
how is the acyllium ion stabilized?
resonane form 1: R-C+=O:: form 2: R-C☰O:+
80
sulfonation of benzene
benzene + H2SO4 → benzene-SO3H + H2O electrophile … protonated SO32- requires fuming sulfuric acid (sulfuric acid saturated with SO32-) reversible

chemistry hl (13 decks)

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