Conjugate additions Flashcards

1
Q

How does an alkene react with a nucleophile?

A

Electrophilic addition followed by nucleophilic addition

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2
Q

What affect does having an heteroatom substituent on an alkene do for electrophilic substitution

A
  • It can enhance the nucleophilic states by using LP to mesomerically donate electron density to the alkene
  • Results in an increased rate of reaction and can react with a wider range of electrophiles
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3
Q

How do carbonyls react with nucleophiles?

A
  • electron poor sp² carbon accepts LP from Nucleophiles forming a new covalent bond
  • Nucleophilic addition
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4
Q

How does a carbonyl compound with a good leaving group react with a nucleophile

A
  • Electron poor sp² carbon accepts a LP from the nucleophile
  • This boots of the leaving groups, hence sp² hybridisation remains
  • Now a nucleophilic substitution
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5
Q

What are the two biggest factors which come into play in carbonyl chemistry?

A
  • Electronics (the influence of attached functional groups, including resonance)
  • Sterics (repulsion of reacting molecules)
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6
Q

Alkenes can be described as ……. nucleophilic

A

Weakly

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7
Q

When Alkenes react with electrophile, which orbital is used?

A

The HOMO is used to attack the electrophile

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8
Q

How can sp² hybridisation be returned when an alkene reacts with a nucleophile

A
  • A C-H bond breaks at the tertiary centre, reforming the C=C bond
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9
Q

What affect do electron withdrawing groups have on alkenes?

A

They lower the LUMO (π’) of the alkene
Which stabilises the anionic intermediate

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10
Q

Name some electron withdrawing groups

A
  • CO₂R
  • SO₂R
  • CN
  • NO₂
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11
Q

How does an alkene react with a nucleophile when it has an electron withdrawing substituent?

A
  • Attack of the sp² carbon by the nucleophile
  • Then PT from the Nu to the carbanion
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12
Q

How do the following two compounds react?

A
  • Nu attacks the sp² carbon, breaking C=C bond
  • Followed by PT
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13
Q

What is the C=C ¹³C NMR shift for an enone vs an alkene and why?

A

enone = 143-124ppm
alkene = 132-119
Due to the deshielding effect of the electron withdrawing carbonyl

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14
Q

What is the IR peaks for an enone vs and alkene and why?

A

enone = 1675 cm‾¹
alkene = 1650 cm‾¹
higher frequency peak for enone due to the electron withdrawing carbonyl

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15
Q

Enones have two electrophilic sites, which ones are they

A

2 position and the 4 position

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16
Q

How does the 1,2-addition of an enone occur?

A
  • Attack of the carbonyl carbon by Nu
  • Breaking of C=O
17
Q

What are some key features of the 1,2-addition of enones?

A
  • Generally the kinetic product (fastest)
  • However reverse reaction dominant if nucleophile is a good leaving group
18
Q

How does the 1,4-addition of an enone occur?

A
  • Attack by the Nu at the sp² alkene carbon
  • Pushes electrons onto adjacent carbon and breaks C=O bond
  • Then PT
19
Q

State key features of the 1,4-addition of enones?

A
  • Generally thermodynamic product (more stable)
  • Reversible 1,2-addition allows product of 1,4-addition to accumulate
20
Q

What is interesting about Cyanide?

A

It is a good nucleophile and a good leaving group

21
Q

Why is the thermodynamic product favoured in additions of enones, even though the activitation energy is higher than the kinetic product?

A

Because the energy of the thermodynamic product is lower and hence more stable
(if we leave this reaction long enough, only the thermodynamic product will be formed)

22
Q

How does Grignards react with a cyclic enone to form an enol (42% yield)

A
  • Nu addition of Grignards towards carbonyl carbon
  • Forms tertairy carbon intermediate, where the oxygen is protonated
23
Q

How can we affect the reactivity of the reaction of a cyclic enone with grignards?
How is the nucleophile formed? (R = Me)

A
  • Through adding copper chloride (1%)
  • Nu addition of Grignards towards Copper, breaking Cu-Cl
  • 2nd Nu addition of Grignards
24
Q

How will the nucleophile formed from Grignards and CuCl react with the cyclic enone?

A
  • Nu addition of methyl at sp² C=C
  • Electrons transfer onto adjacent carbon and C=O bond breaks
  • Then PT
  • Results in a 1,4-addition rather than 1,2-addition