Block 4 - Functional Groups 1 Flashcards
(84 cards)
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
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)
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
4
Q
C=C acts as a ….
A
Nucleophile, as the double bond is electron rich
5
Q
Alkenes: types of addition reactions
A
Hydrogenation
Reactions initiated by addition of an electrophile E+
- H+ as an electrophile
- Halogenation
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
7
Q
Alkenes: H+ as an electrophile
A
Overall, addition of HZ (Z = halogen, OH, OR) via a carbocation intermediate
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
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
10
Q
Halogenation - steps
A
- Halogen acts as an electrophile. C=C is e- rich, so pushes e- in Br2 to one end –> induced dipole in Br2
- Halide ion acts as a nucleophile
Occurs with anti stereochemistry, i.e. always trans
11
Q
Halogenation: presence of other nucleophiles
A
Presence of other nucleophiles can compete and give diff products
12
Q
How are alkenes often detected in the lab
A
Discharge of Br2 colour (orange to colourless)
13
Q
Preparation of alkynes
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Di-dehydrohalogenation of dihaloalkanes
14
Q
Br2 addition to alkene
A
Always adds trans to an alkene
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
16
Q
Resonance energy of benzene
A
Conjugated double bonds present gives it extra stability, referred to as ‘resonance energy’
17
Q
How to tell if cyclic hydrocarbons are aromatic
A
They contain (4N + 2)pi electrons, where N is the number of rings
18
Q
Aniline
A
Benzene ring monosubstituted with NH2
19
Q
Nitrobenzene
A
Benzene ring monosubstituted with NO2
20
Q
Phenol
A
Benzene ring monosubstituted with OH
21
Q
Toulene
A
Benzene ring monosubstituted with CH3
22
Q
Disubstituted benzene ring positions
A
Ortho (1)
Meta (2)
Para (3)
23
Q
> Disubstituted benzene rings - numbering
A
Use lowest possible sum of numbers for substituents
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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Alkyne: Hydrogenation - reagent
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
26
Alkyne: Electrophilic addition of HX and X2
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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Alkyne: Hydration
Addition of only one mole equivalent of water
Products are ketones (except for ethyne)
Reagent: aq H2SO4 / HgSO4
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Tautomeric equilibrium
Species involved are tautomers
| Enol (unstable) and keto (more stable)
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Formation of alkynide anions from terminal alkynes
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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Aromatic compound reactions
Have extra stability of 'resonance energy' which prevents them from doing addition reaction chemistry
Undergo electrophilic aromatic substitution
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Wheland intermediates
Resonance stabilised cations
| Positive charges can only be ortho or para to the incoming electrophile group
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Benzene mechanism steps
Substitution: addition --> elimination
Electrophile attack - slow step
Proton loss - fast step
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How are strong electrophiles generated
```
Often formed by catalytic action
Halogenation
Nitration
Friedel-Crafts acylation
Friedel-Crafts alkylation
```
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Generation of electrophiles - halogenation
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
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Generation of electrophiles - Nitration
HNO3 + H2SO4 --> NO2+ + HSO4- + H2O
Where NO2+ is very reactive electrophile
Nitro group: NO2
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Which reactions can be introduced directly onto aromatic ring using electrophilic aromatic substitution
Br2, NO2, R (short; CH3, CH2CH3, (CH3)2CH2)
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Generation of electrophiles - Friedel-Crafts acylation
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
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Generation of electrophiles - Friedel-Crafts alkylation
Adding R to ring
RX + AlX3 (catalyst)
Only works for small alkanes
R = CH3, CH3CH2, (CH3)2CH
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Aromatic ring - reaction with Br2
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
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Diazonium ion
Benzene monosubstituted with N2+
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Benzene ring to phenol
Ring substituent changes from:
| H --- HNO3/H2SO4 ---> NO2 --- Fe/H+ ---> NH2 --- HNO2 / 0°C ---> N2+ --- H3O+ ---> OH
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Benzonitrile
Benzene monosubstituted with nitrile group (CN)
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Benzene ring to benzonitrile
Ring substituent changes from:
| H --- HNO3/H2SO4 ---> NO2 --- Fe/H+ ---> NH2 --- HNO2 / 0°C ---> N2+ --- CuCN ---> CN
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Substitution of 2nd substituent onto disubstituted benzene ring
All positions around ring are no longer equivalent
Substitution can occur ortho, meta, or para to G
G controls position of incoming electrophile
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Substitution of 2nd substituent onto disubstituted benzene ring - need to consider...
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?
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Directing and activating power of substituents - categories
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)
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Ortho-para directors
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)
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Meta directors
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
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Electrophilic attack when an ortho-para director is attached, e.g. phenol with E+
Ortho substitution: 4 resonance contributors
Meta substitutution: 3 resonance contributors; no charge de-localised by oxygen --> less favourable
Para substitution: 4 resonance contributors
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Electrophilic attack when a meta director is attached, e.g. nitrobenzene with E+
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
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Alkyl halides
Haloalkanes
Contain a halogen attached to an sp3 hybridised (alkyl) C
Classified as 1°, 2° and 3°
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Alcohol to alkyl halide - reagent(s)
SOCl2 preferred for 1° and 2° halides
| HCl preferred for 3° halides
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Alkyl halide reactions - nucleophile or electrophile
One can replace the halogen of an alkyl halide (electrophile) with an appropriate nucleophile
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Alkyl halide reactions - nucleophiles
Cl-, Br-, I-
--- increasing ease of substitution (faster reaction) -->
Weak bases --> less reactive --> excellent leaving groups
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Alkyl halide - types of substitution
SN1 (substitution nucleophilic) unimolecular mechanism
| SN2 bimolecular mechanism
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The more substituted the alkyl halide...
The more substituted the carbocation generated
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Alkyl halide: SN1 - rate
Rate ∝ [alkyl halide]
Not dependent on strength of nucleophile, but strength/basicity of it may affect course of reaction, e.g. favouring elimination
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Alkyl halide - when is SN1 favoured
When intermediate carbocation, from breaking C-X bond, is relatively stable
3° > 2° >> 1°
Some benzylic halides
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Alkyl halide: SN1 mechanism
Involves substitution by a nucleophile, with only one species involved in rate determining step (when the leaving group departs)
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Alkyl halide: SN1 and SN2 - results in...
SN1 results in loss of stereochemistry
| SN2 results in inversion of configuration
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Alkyl halide: SN1 - R and S enantiomers
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
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Alkyl halide to alcohol - reagent
H2O or hydroxide (OH-)
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Alkyl halide: SN2 - rate
Rate ∝ [alkyl halide][Nu-]
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Alkyl halide: SN2 - mechanism
Involves a transition state
Concentrated and synchronous
Substitution by a nucleophile, with 2 species involved in rate determining step
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Alkyl halide - when is SN2 favoured
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
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Alkyl halide: SN2 - chiral non-racemic 2° alkyl halide
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
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Alkyl halide: SN2 - nucleophile direction
Nucleophile must come in from opposite side of X
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Alkyl halide: When looking at stereochemistry, must think about which 3 things?
- What's happening with starting material? Already a racemic mixture?
- What's happening with product? Chiral or achiral?
- What's happening with the mechanism?
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Alkyl halide: True or false? If mechanism is SN1, you ALWAYS get a racemic mixture
False
| To get a racemic mixture, must have a chiral C - may not have chiral C
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Alkyl halide: True or false? If mechanism is SN1, you NEVER get a single enantiomer
True
| SN1 means must go through carbocation intermediate and will get 'scrambling' of stereochemical information
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Alkyl halide: True or false? If mechanism is SN2, you NEVER get a racemic mixture
False
If start with racemic mixture, produces racemic mixture
Is starting material chiral?
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Alkyl halide: True or false? If mechanism is SN2, you ALWAYS get a single enantiomer
False
Depends on whether starting compound is a racemic mixture
Depends on whether product is chiral
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Alkyl halide: E1 and SN1
E1 and SN1 can compete, leading to product mixtures
| Both proceed via a carbocation intermediate
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Alkyl halide: E1 rate
rate ∝ [alkyl halide]
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Alkyl halide: E1 favoured for...
3° > 2° >> 1°
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Alkyl halide: when is E favoured over SN1 reactions
Stronger bases, higher temperatures and a non-nucleophilic solvent favours E over SN1
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Alkyl halide: E2 bimolecular - rate
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
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Alkyl halide: what does E2 require (reagent)
Strong base RO- or HO-
| (CH3)3CO- also used because it's too bulky for SN2
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Alkyl halide: E2 favoured for...
3° > 2° >> 1°
| However, can be observed for 1° if product extends conjugation
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Alkyl halide: E1 and E2 - more than one product?
Sayzeff's rule applies when more than one alkene can be formed
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Alkyl halide: Temperature
Higher temperatures favour elimination
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Alkyl halide: sp2 carbons
A halogen bonded to a sp2 carbon (e.g. aryl halide and vinyl halide) can't undergo SN (substitution)
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Vinyl halide reactions
Can undergo elimination reactions with strong bases to form an alkyne
Can't undergo nucleophilic substitution
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Grignard reagents
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)
