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CHEM 110 > Block 4 - Functional Groups 1 > Flashcards

Block 4 - Functional Groups 1 Flashcards

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1
Q

Alkene properties

A 
Double bond - one sigma, one pi
Carbons are sp2 hybridised - flat
120°
Considered electron rich as it contains 2 electrons in the sigma bond and 2 in the pi bond
Unsaturated
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2
Q

Preparation of alkenes

A

Usually via elimination reactions

  • Acid catalysed dehydration of (removal of H2O from) alcohols (reagent: conc H2SO4)
  • Base promoted dehydrohalogenation of (removal of HX from) alkyl halides (reagent: KOH or NaOH in ethanol)
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3
Q

Saytzeff rule

A

The major product is the most substituted alkene (more Cs); the alkene with the least no of Hs directly attached to the Cs of the C=C

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4
Q

C=C acts as a ….

A

Nucleophile, as the double bond is electron rich

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5
Q

Alkenes: types of addition reactions

A

Hydrogenation

Reactions initiated by addition of an electrophile E+

  • H+ as an electrophile
  • Halogenation
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6
Q

Alkenes: hydrogenation

A

Reagent: H2/catalyst (Pt or Pd)
Sometimes called a reduction
Occurs with syn stereochemistry - both Hs add to the same side of the molecule, i.e. always cis
- H atoms absorbed onto catalyst surface

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7
Q

Alkenes: H+ as an electrophile

A

Overall, addition of HZ (Z = halogen, OH, OR) via a carbocation intermediate

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8
Q

Markovnikov’s rule

A

The more substituted product is formed
Addition of an asymmetrical reagent to an asymmetrical alkene gives the major product of the compound where the electropositive part of the reagent (usually H+) has bonded to the carbon of the C=C that is directly bonded to the greater no of H atoms

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9
Q

1° 2° and 3° carbocation intermediates

A

3° more stable than 2° more stable than 1°

More stable –> more likely to form –> major product

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10
Q

Halogenation - steps

A
  1. Halogen acts as an electrophile. C=C is e- rich, so pushes e- in Br2 to one end –> induced dipole in Br2
  2. Halide ion acts as a nucleophile

Occurs with anti stereochemistry, i.e. always trans

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11
Q

Halogenation: presence of other nucleophiles

A

Presence of other nucleophiles can compete and give diff products

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12
Q

How are alkenes often detected in the lab

A

Discharge of Br2 colour (orange to colourless)

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13
Q

Preparation of alkynes

A

Di-dehydrohalogenation of dihaloalkanes

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14
Q

Br2 addition to alkene

A

Always adds trans to an alkene

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15
Q

Why doesn’t benzene undergo reactions typical of alkenes

A

Due to resonance

Aromatic rings are more stable than normal C=C bonds

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16
Q

Resonance energy of benzene

A

Conjugated double bonds present gives it extra stability, referred to as ‘resonance energy’

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17
Q

How to tell if cyclic hydrocarbons are aromatic

A

They contain (4N + 2)pi electrons, where N is the number of rings

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18
Q

Aniline

A

Benzene ring monosubstituted with NH2

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19
Q

Nitrobenzene

A

Benzene ring monosubstituted with NO2

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20
Q

Phenol

A

Benzene ring monosubstituted with OH

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21
Q

Toulene

A

Benzene ring monosubstituted with CH3

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22
Q

Disubstituted benzene ring positions

A

Ortho (1)
Meta (2)
Para (3)

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23
Q

> Disubstituted benzene rings - numbering

A

Use lowest possible sum of numbers for substituents

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24
Q

Alkynes

A 
Triple bond - one sigma, two pi
Carbons are sp hybridised - 180°
Unsaturated
Generally nucleophiles
Generally undergo addition reactions
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25
Q

Alkyne: Hydrogenation - reagent

A

To form alkane: H2/Pt or Ht/Pd

To form Z alkene (cis): Lindlar catalyst/H2 [Pd/Pb(OAC)2]

To form E alkene (trans):

  1. Li/liq NH3
  2. H2O
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26
Q

Alkyne: Electrophilic addition of HX and X2

A

Markovnikov’s rule followed for HX addition
Reaction can be stopped after addition of one mole equivalent of reagent
Anti-stereochemistry of addition is observed

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27
Q

Alkyne: Hydration

A

Addition of only one mole equivalent of water
Products are ketones (except for ethyne)
Reagent: aq H2SO4 / HgSO4

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28
Q

Tautomeric equilibrium

A

Species involved are tautomers

Enol (unstable) and keto (more stable)

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29
Q

Formation of alkynide anions from terminal alkynes

A

The H on a terminal alkyne is weakly acidic and can be removed with a strong base (Na+NH2-)
Forms C- (nucleophile)

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30
Q

Aromatic compound reactions

A

Have extra stability of ‘resonance energy’ which prevents them from doing addition reaction chemistry
Undergo electrophilic aromatic substitution

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31
Q

Wheland intermediates

A

Resonance stabilised cations

Positive charges can only be ortho or para to the incoming electrophile group

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32
Q

Benzene mechanism steps

A

Substitution: addition –> elimination
Electrophile attack - slow step
Proton loss - fast step

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33
Q

How are strong electrophiles generated

A 
Often formed by catalytic action
Halogenation
Nitration
Friedel-Crafts acylation
Friedel-Crafts alkylation
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34
Q

Generation of electrophiles - halogenation

A

Catalyst: Fe used to activate the halogen and generate a very reactive electrophile
X-X + FeX3 –> X+ + FeX4-
Where X+ is the very reactive electrophile

35
Q

Generation of electrophiles - Nitration

A

HNO3 + H2SO4 –> NO2+ + HSO4- + H2O
Where NO2+ is very reactive electrophile
Nitro group: NO2

36
Q

Which reactions can be introduced directly onto aromatic ring using electrophilic aromatic substitution

A

Br2, NO2, R (short; CH3, CH2CH3, (CH3)2CH2)

37
Q

Generation of electrophiles - Friedel-Crafts acylation

A

Adding C=O to ring
Catalyst: AlX3 where X is a halogen
Forms R-C=O: very reactive electrophile, and then reacts with ring to replace a H

38
Q

Generation of electrophiles - Friedel-Crafts alkylation

A

Adding R to ring
RX + AlX3 (catalyst)
Only works for small alkanes
R = CH3, CH3CH2, (CH3)2CH

39
Q

Aromatic ring - reaction with Br2

A

Reaction only with Br2 adds 2 Br atoms trans to each other

Reaction with Br2 and FeBr3 (catalyst) substitutes an H on the ring with a Br atom

40
Q

Diazonium ion

A

Benzene monosubstituted with N2+

41
Q

Benzene ring to phenol

A

Ring substituent changes from:

H — HNO3/H2SO4 —> NO2 — Fe/H+ —> NH2 — HNO2 / 0°C —> N2+ — H3O+ —> OH

42
Q

Benzonitrile

A

Benzene monosubstituted with nitrile group (CN)

43
Q

Benzene ring to benzonitrile

A

Ring substituent changes from:

H — HNO3/H2SO4 —> NO2 — Fe/H+ —> NH2 — HNO2 / 0°C —> N2+ — CuCN —> CN

44
Q

Substitution of 2nd substituent onto disubstituted benzene ring

A

All positions around ring are no longer equivalent
Substitution can occur ortho, meta, or para to G
G controls position of incoming electrophile

45
Q

Substitution of 2nd substituent onto disubstituted benzene ring - need to consider…

A
  1. Where will substitution occur? Ortho, meta, para
  2. Will the reaction occur more or less rapidly than for the same electrophile with benzene? Is G activating / deactivating?
46
Q

Directing and activating power of substituents - categories

A

Ortho-para directors:

  • Strongly activating (-OH, -OR, -NH2, -NR2)
  • Weakly activating (-CH3 alkyl)
  • Deactivating (-X (F, Cl, Br, I))

Meta directors:

  • Strongly deactivating (-NO2)
  • Moderately deactivating (-H(or R)-C=O, OH(or OR)-C=O, CN)
47
Q

Ortho-para directors

A

Electron Donating Groups (EDG)
O, N, and halogens have unshared electron pair(s) which can be donated into Ar ring (by resonance)
Alkyl groups can donate electrons into ring (induction)

48
Q

Meta directors

A

Electron Withdrawing Groups (EWG)
All groups have a multiple bond to atom that is bonded to aromatic ring sp2 C
All groups have electropositive end of a polar bond attached to aromatic ring carbon

49
Q

Electrophilic attack when an ortho-para director is attached, e.g. phenol with E+

A

Ortho substitution: 4 resonance contributors
Meta substitutution: 3 resonance contributors; no charge de-localised by oxygen –> less favourable
Para substitution: 4 resonance contributors

50
Q

Electrophilic attack when a meta director is attached, e.g. nitrobenzene with E+

A

Ortho substitutuion: 3 resonance contributors, but only 2 reasonable (+ve charges repel –> unfavourable)
Meta substitution: 3 resonance contributors, all reasonable –> more likely to occur
Para substitution: 3 resonance contributors, but only 2 reasonable
Hence, meta wins by default - NO2 m-directing and de-activating

51
Q

Alkyl halides

A

Haloalkanes
Contain a halogen attached to an sp3 hybridised (alkyl) C
Classified as 1°, 2° and 3°

52
Q

Alcohol to alkyl halide - reagent(s)

A

SOCl2 preferred for 1° and 2° halides

HCl preferred for 3° halides

53
Q

Alkyl halide reactions - nucleophile or electrophile

A

One can replace the halogen of an alkyl halide (electrophile) with an appropriate nucleophile

54
Q

Alkyl halide reactions - nucleophiles

A

Cl-, Br-, I-
— increasing ease of substitution (faster reaction) –>
Weak bases –> less reactive –> excellent leaving groups

55
Q

Alkyl halide - types of substitution

A

SN1 (substitution nucleophilic) unimolecular mechanism

SN2 bimolecular mechanism

56
Q

The more substituted the alkyl halide…

A

The more substituted the carbocation generated

57
Q

Alkyl halide: SN1 - rate

A

Rate ∝ [alkyl halide]
Not dependent on strength of nucleophile, but strength/basicity of it may affect course of reaction, e.g. favouring elimination

58
Q

Alkyl halide - when is SN1 favoured

A

When intermediate carbocation, from breaking C-X bond, is relatively stable
3° > 2°&raquo_space; 1°
Some benzylic halides

59
Q

Alkyl halide: SN1 mechanism

A

Involves substitution by a nucleophile, with only one species involved in rate determining step (when the leaving group departs)

60
Q

Alkyl halide: SN1 and SN2 - results in…

A

SN1 results in loss of stereochemistry

SN2 results in inversion of configuration

61
Q

Alkyl halide: SN1 - R and S enantiomers

A

R-enantiomer –> R and S (racemate)
S-enantiomer –> R and S (racemate)
Therefore, either enantiomer gives a racemic mixture
Because H2O can add from above (50%) or from below (50%) where the intermediate is achiral

62
Q

Alkyl halide to alcohol - reagent

A

H2O or hydroxide (OH-)

63
Q

Alkyl halide: SN2 - rate

A

Rate ∝ [alkyl halide][Nu-]

64
Q

Alkyl halide: SN2 - mechanism

A

Involves a transition state
Concentrated and synchronous
Substitution by a nucleophile, with 2 species involved in rate determining step

65
Q

Alkyl halide - when is SN2 favoured

A

1° > 2° > 3°
From a 1° to a 2° to a 3° alkyl halide, the transition state becomes more crowded –> raises energy of transition state –> raises Ea for reaction –> less likely reaction will happen

66
Q

Alkyl halide: SN2 - chiral non-racemic 2° alkyl halide

A

When a chiral non-racemic 2° alkyl halide reacts via an SN2 pathway, a chiral non-racemic (optically active) product results from inversion of configuration
Produces one product

67
Q

Alkyl halide: SN2 - nucleophile direction

A

Nucleophile must come in from opposite side of X

68
Q

Alkyl halide: When looking at stereochemistry, must think about which 3 things?

A
  • What’s happening with starting material? Already a racemic mixture?
  • What’s happening with product? Chiral or achiral?
  • What’s happening with the mechanism?
69
Q

Alkyl halide: True or false? If mechanism is SN1, you ALWAYS get a racemic mixture

A

False

To get a racemic mixture, must have a chiral C - may not have chiral C

70
Q

Alkyl halide: True or false? If mechanism is SN1, you NEVER get a single enantiomer

A

True

SN1 means must go through carbocation intermediate and will get ‘scrambling’ of stereochemical information

71
Q

Alkyl halide: True or false? If mechanism is SN2, you NEVER get a racemic mixture

A

False
If start with racemic mixture, produces racemic mixture
Is starting material chiral?

72
Q

Alkyl halide: True or false? If mechanism is SN2, you ALWAYS get a single enantiomer

A

False
Depends on whether starting compound is a racemic mixture
Depends on whether product is chiral

73
Q

Alkyl halide: E1 and SN1

A

E1 and SN1 can compete, leading to product mixtures

Both proceed via a carbocation intermediate

74
Q

Alkyl halide: E1 rate

A

rate ∝ [alkyl halide]

75
Q

Alkyl halide: E1 favoured for…

A

3° > 2°&raquo_space; 1°

76
Q

Alkyl halide: when is E favoured over SN1 reactions

A

Stronger bases, higher temperatures and a non-nucleophilic solvent favours E over SN1

77
Q

Alkyl halide: E2 bimolecular - rate

A

Rate ∝ [alkyl halide][Nu-]

where Nu- is the base because instead of attacking the slightly positive C like in SN2, it takes a H instead

78
Q

Alkyl halide: what does E2 require (reagent)

A

Strong base RO- or HO-

(CH3)3CO- also used because it’s too bulky for SN2

79
Q

Alkyl halide: E2 favoured for…

A

3° > 2°&raquo_space; 1°

However, can be observed for 1° if product extends conjugation

80
Q

Alkyl halide: E1 and E2 - more than one product?

A

Sayzeff’s rule applies when more than one alkene can be formed

81
Q

Alkyl halide: Temperature

A

Higher temperatures favour elimination

82
Q

Alkyl halide: sp2 carbons

A

A halogen bonded to a sp2 carbon (e.g. aryl halide and vinyl halide) can’t undergo SN (substitution)

83
Q

Vinyl halide reactions

A

Can undergo elimination reactions with strong bases to form an alkyne
Can’t undergo nucleophilic substitution

84
Q

Grignard reagents

A

Alkyl, aryl and vinyl halides form Grignard reagents on treatment with Mg in dry diethyl ether (unreactive) as solvent
Magnesium is electropositive and so the attached C in Grignard reagent can be regarded as a carbanion (R-) –> acts as a carbon nucleophile (base)

CHEM 110 (6 decks)

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