Carbonyl Compounds - Aldehydes and Ketones

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Further Organic Chemistry ­ Answers
Carbonyl Compounds:
1. Carbonyl compounds contain the >C=O group. The carbon atom is bonded to the oxygen by a -bond and a -bond.
Oxygen is more electronegative than carbon, so the bonding electrons are pulled towards the oxygen atom making
it -
and the carbon +
. The polar nature of the bond makes the carbon susceptible to attack by nucleophiles. The two
types of carbonyl compounds are aldehydes and ketones.
2. Aldehydes have a hydrogen atom bonded to the carbon of the C=O group. The formulae of aldehydes can be
represented by RCHO, where R is a hydrogen atom, an alkyl group or benzene ring. Examples include:
These all have exactly the same end to the molecule. All that differs is the complexity of the other group attached.
3. Ketones do not have a hydrogen atom bonded to the carbon of the group. The formulae of ketones can be
represented as RCOR', where R and R' are alkyl or benzene ring groups. The simplest ketone is propanone (has 3 C's)
4. The carbon atom has two single bonds, one double bond and no lone pairs, so the electrons in the three bonds repel
each other and take up the position of maximum separation. This is a planar triangular shape with bond angles of
120°. This planar shape makes it easy for nucleophiles to attack the carbon atom from either above or below, and so
a single optical isomer is never obtained by addition to carbonyl compounds. The polarity of the >C=O group is not
cancelled out by the other two groups attached to the carbon atom, therefore aldehydes and ketones are polar.
5. The general increase in boiling points is due to permanent dipole-dipole forces (as they are polar molecules) and
instantaneous induced dipole-dipole forces. H bonding is not possible in carbonyl compounds because neither
aldehydes nor ketones have a hydrogen atom that is sufficiently +
, so their boiling points are lower than those of
alcohols. In comparison with alkanes and alkenes, they are not polar so their intermolecular forces are weaker and
their boiling points lower than those of aldehydes and ketones with the same number of electrons in the molecule.
6. Small aldehydes and ketones are soluble in water due to H bonds between a lone pair of the carbonyl group and
hydrogen of water. However, as size increases, the solubility decreases due to interference in H bonding by
hydrocarbon `tails' of aldehydes/ketones.
7. To test for a carbonyl group in a compound, add a solution of 2,4-dinitrophenylhydrazine (Brady's
reagent) to the suspected carbonyl compound:
Simply aldehydes and ketones give yellow precipitates
Aromatic aldehydes and ketones give orange precipitate
8. Both aldehydes and ketones react with compounds containing an H2N group. The lone pair of
electrons on the nitrogen atom acts as a nucleophile and forms a bond with the +
carbon atom in the C=O group.
However, instead of an H+ ion adding on to the O formed, the substance loses a water molecule and a C=N bond is
formed. Since a water molecule is lost, this reaction is a condensation reaction. This reaction produces an insoluble
product, which can be used to test for the presence of a carbonyl group.
>C=O + H2N >C=N-X + H2O
9. The identify of the carbonyl compound can be found by:
React the carbonyl compound with a solution of 2,4-dinitrophenylhydrazine
Filter off the precipitate
Recrystallise the precipitate using the minimum amount of hot ethanol
Dry the purified product and measure its melting temperature

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Refer to a data book and compare this melting temperature with those of 2,4-dynitrophenylhydrazine
derivatives of aldehydes and ketones.
10. Aldehydes (but not ketones) are readily oxidized. If the reaction is carried out in acid or neutral solution the product
is a carboxylic acid:
If the reaction is carried out in alkaline solution (Fehling's solution or Tollens' reagent) the product is the carboxylate
RCHO + [O] + OH- RCOO- + H2O
There are a number of suitable oxidizing agents.…read more

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These substitute into the ­CH3 group next to the C=O group, forming a CI3C=O group.…read more


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