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CH10136 > Carbonyl chemistry- reduction > Flashcards

Carbonyl chemistry- reduction Flashcards

1
Q

What allows one carbonyl to react over another

A
  1. Difference in reactivity

2. Chemoselectivity

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2
Q

Would NaOH react more readily with ester or amide functional group in compound

A
  1. Ester over amide

2. Ester is more reactive as donates less back into the carbon so the carbon is more electrophilic

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3
Q

What are metal hydrides

A
  1. One of the most common reducing agents
  2. Source of ‘H’ nucleophile
  3. Have different reactivities
  4. The more reactive the less selective it is
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4
Q

Order LiAlH4, LiBH4 and NaBH4 by reactivity

A
  1. LiAlH4 is most reactive as it has a lewis acidic carbanion and a nucleophilic anion
  2. Then LiBH4
  3. Then NaBH4
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5
Q

What are Lewis acid hydrides

A
  1. Charge neutral lewis acids
  2. Only source of H when they form Lewis acid-base complex
  3. Useful for reduction of electron rich carbonyl derivatives
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6
Q

Give two examples of lewis acid hydrides

A
  1. DIBAL- forms a bridged dimer

2. BH3 (borane)- forms a bridged dimer

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7
Q

What reducing reagents react with Aldehydes

A
  1. LiAlH4- readily reacts
  2. LiBH4- readily reacts
  3. NaBH4- readily reacts
  4. BH3- slowly reacts
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8
Q

What reducing reagents react with ketones

A
  1. LiAlH4- readily reacts
  2. LiBH4- readily reacts
  3. NaBH4- readily reacts
  4. BH3- slowly reacts
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9
Q

What reducing reagents react with esters

A
  1. LiAlH4- readily reacts
  2. LiBH4- readily reacts
  3. NaBH4- slowly reacts
  4. BH3- slowly reacts
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10
Q

What reducing reagents react with amides

A
  1. LiAlH4- readily reacts
  2. LiBH4- no reaction
  3. NaBH4- no reaction
  4. BH3- readily reacts
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11
Q

What reducing reagents react with carboxylic acids

A
  1. LiAlH4- slowly reacts
  2. LiBH4- no reaction
  3. NaBH4- no reaction
  4. BH3- readily reacts
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12
Q

Describe the reactivity of LiAlH4

A
  1. Reacts with everything
  2. Best with higher reactivity carbonyls
  3. Slow reaction with carboxylic acids
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13
Q

Describe reactivity of LiBH4

A
  1. Reacts with the most reactive carbonyls

2. Doesn’t react at all with amides or carboxylic acids

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14
Q

Describe reactivity of NaBH4

A
  1. Reacts with most reactive carbonyls
  2. Slow react with esters
  3. Doesn’t react with amides or carboxylic acids
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15
Q

Describe reactivity of BH3

A
  1. Reacts best with less reactive carbonyls

2. Slow reactions with esters, ketones and aldehydes

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16
Q

What is BH3 useful for

A
  1. Good at reducing less electron rich in presence of more reactive carbonyls
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17
Q

How are aldehydes and ketones reduced to primary and secondary alcohols

A
  1. Nucleophilic addition
  2. One hydrogen equivalent
  3. Secondary alcohol produced contains a stereogenic centre
18
Q

How are esters, amides and carboxylic acids reduced

A
  1. Nucleophilic substitution first
  2. Then nucleophilic addition
  3. 2 hydrogen equivalents
19
Q

How can aldehydes and ketones be reduced to alkanes

A
  1. Clemmensen reaction

2. Wolff-Kishner reaction

20
Q

Describe the clemmensen reaction

A
  1. Zn(Hg) and concentrated HCl- very harsh conditions

2. Very harsh conditions

21
Q

Describe the Wolff- Kishner reaction

A
  1. N2H4.H2O (>10 eq)
  2. KOH (>5 eq)
  3. > 180 degrees
  4. Very harsh conditions
22
Q

What can cyanide be used for

A
  1. Generate a new c-c bond and cyanohydrin functionality

2. It is a carbon based nucleophile

23
Q

Describe how cyanide can be used to convert an aldehyde or ketone

A
  1. Reaction is reversible
  2. Add NaCN, H2O and HCl to aldehyde or ketone
  3. Produces a alcohol with a CN group
24
Q

Why is the conversion of an aldehyde/ketone using cyanide reversible

A
  1. The CN group is a good leaving group

2. This means it is reversible

25
Q

What is formed if you add acid and water to a nitrile

A
  1. A carboxylic acid is produced

2. Need to be around 100 degrees

26
Q

How can a nitrile undergo functional group interconversion to become a carboxylic acid

A
  1. Add acid (H2SO4), Water and 100 degrees

2. Forms a carboxylic acid containing the same R group

27
Q

How can alkyl groups be added to carbonyls

A
  1. Organometallic reactions

2. Most common are either lithium or magnesium derived

28
Q

What are the properties of Li/Mg organometallics

A
  1. Not ionic but covalent
  2. Very strong nucleophiles
  3. Very strong bases- pKa of conjugate acid >40
  4. Therefore can deprotonate acid C-H bonds to form new organometallic reagents
29
Q

What is a grignard reagent

A
  1. Organomagnesium compounds with the general formula RMgX

2. X is a halide

30
Q

How is a Grignard reagent formed

A
  1. Addition of magnesium to alkyl halides (I,Br,Cl) in etheric solvents (Et2O)
  2. Addition to aryl halides (I,Br) in tetrahydrofuran (THF)
  3. Involves insertion of magnesium into carbon-halogen bond (oxidative insertion)
31
Q

Why are the particular solvents used in formation of Grignard

A
  1. Et2O used for alkyl halides- don’t need high boiling solvent to keep heat in as not a lot of heat is needed
  2. THF for aryl halides- holds heat in solution for aryl halides which need a higher heat
32
Q

How can you make organolithium reagents

A
  1. Uses an oxidative insertion process
  2. substitution of Lithium in alkyl halides (I,Br,Cl) in etheric solvents (Et2O)
  3. Substitution of lithium in aryl halides (I,Br) in tetrahydrofuran (THF)
  4. Replaces the halide completely
  5. Also can use lithium halogen exchange as another method
33
Q

What do you need to be careful about when making Mg/Li organometallic reagents

A
  1. Cannot have any acidic or carbonyl functional groups present
  2. As the reagents are very basic and reactive
  3. These need to be protected
34
Q

How are aldehydes and ketones reduced using organometallic reagents

A
  1. Nucleophilic addition

2. Form secondary and tertiary alcohols

35
Q

How is an ester reduced using an organometallic reagent

A
  1. First reduction of OR group to form ketone by nucleophilic substitution
  2. Then reduced further
  3. 2 equivalents of reducing agent
  4. Forms a tertiary alcohol
36
Q

How is an amide reduced using an organometallic reagent

A
  1. First nucleophilic substitution to form aldehyde or ketone
  2. Then react further
  3. One reducing equivalent
37
Q

How is a carboxylic acid reduced using an organometallic reagent

A
  1. It isn’t
38
Q

What is a Wittig reaction

A
  1. Use of phosphonium salts to convert aldehydes/ketones to alkenes
  2. Utilises phosphorous ylide as the key reagent which is prepared in situ from phosphonium salt
  3. Phosphonium salt is easily prepared from alkyl halide and phosphine - SN2 reaction
39
Q

How can the nature of the substituent on the ylid control E/Z geometry

A
  1. R groups= H, OR, alkyl = Z-alkene

2. R groups= alkyl = E-alkene

40
Q

Why does the difference in R-groups affect the stereochemistry

A
  1. If R group= H, OR, alkyl, the phosphorous ylid is stabilised by delocalisation of -ve charge
  2. If R group= alkyl only, the phosphorous ylid is unstabilised as the -ve charge can’t be delocalised by conjugation
41
Q

What is a ylide

A
  1. A ylide is a species which contains two oppositely charged atoms bonded directly to each other.

CH10136 (30 decks)

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