Preview Carbonyl Compounds Aldehydes and Ketones
13 important questions on Preview Carbonyl Compounds Aldehydes and Ketones
What is the major difference between the two possibilities for nucleophilic addition of Ketones or Aldehydes?
If the nulceophil is charged, H from H3O+ is taken
If the nucleophil is not charged but has an H, the H of this compound is taken for formation of the alcohol group
Which of both is more reactive: aldehydes or ketones?
Aldehydes: they have more space for attack of a nucleophil and their carbonyl is more polarized
Why are aromatic aldehydes less reactive?
because the resonance effect makes the carbonylgroup less electrophilic
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How does base-katalysed hydration of an aldehyde or ketone take place?
An hydroxide ion approaches the C, giving formation of an alkoxide ion and after protonation of the negatively charged O by water, an hydrate is formed (diol)
How does acid-katalyzed hydration of an aldehyde or ketone take place?
The O is protonised by the acid, so the remaining water can attack the C. A proton is taken from the group to form again an acid in water.
What is the key difference between acid and base katalysed hydration of an aldehyde?
Base: the C acts as a nucleophil
Acid: the C acts as an electrophil
What is a Grinard reagent?
RMgX, a compound that is strongly polarized between the C and Mg, so it acts as R- Mg+
This makes is possible to add the R- to the C of e.g. an aldehyde
How does addition of R to an aldehyde or Ketone take place using the grinard reagent?
The Mg is approached by the electrons of the O. The C becomes more electrophilic and will react with the R-
After addition of water, the Mg-O bond undergoes hydrolysis
How does an aldehyde or ketone react with H-?
H- actually does not exist, but LiAlH4 and NaBH4 act as hydride ion donors
How are imines obtained from aldehydes or ketones?
primary amines (RNHH) are used.
The N attacks the C. Proton transfer takes place between the N and the O, giving a Carbinolamine intermediate
A catalyst (acid) protonates the O, N donates electrons to the C, resulting in dehydration of the C. Water takes the H from the N, resulting in an Imine
How are enamines obtained from aldehydes or ketones?
Secondary amines (R2NH) are used.
The N attacks the C, proton transfer between N and O is carried out. The O is protonated once more using an acid and dehydration takes place. The N then forms a double bond with the C. Water can deprotonate a neighbouring C, resulting in the relocation of the double bond from N=C to C=C
How does the formation of an acetal take place?
The O is protonated.
The O of the alcohol can then attack the C. Water will remove the H of the alcohol group resulting in an hemiacetal. Again, the O is protonated, resulting in dehydration and double bond formation between the newly formed O=C. Again, an alcohol O can attack the C and deprotonated by water, resulting in an acetal.
How does formation of a cyclic acetal take place?
The same reaction is carried out as in standard acetal formation, but in this case, both reactive groups are in the same molecule
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