Lecture 9 Flashcards
(14 cards)
Nomenclature of Aldehydes and Ketones (R-CHO)
Naming of all organic compounds follows the IUPAC system
What about when there is more than one functional group?
Aldehydes take preference to ketones in the IUPAC system
Can Aldehydes and Ketone be a part of a Cyclic Ring
Cyclic aldehydes are not possible, because in order for the carbonyl group to
be part of the ring structure, two bonds to carbon groups would be required.
• Aldehydes may incorporate ring structures, but not be part of the ring
Properties of Aldehydes and Ketones
Good solvent for alcohols.
• Lone pair of electrons on oxygen of carbonyl can accept a hydrogen bond
from O-H or N-H.
• Acetone and acetaldehyde are miscible in water.
The carbonyl group is polar due to the greater electronegativity of oxygen
(3.5) than carbon (2.5). This influences the properties of aldehydes and
ketones, such as solubility
δ+ δ-
As size increases, solubility decreases due to interference in hydrogen
bonding by the hydrocarbon ‘tails’ of the aldehydes/ketones.
“Small” aldehydes and ketones are soluble in water due to hydrogen bonding
between a lone pair on the oxygen of the carbonyl group and the hydrog
Properties of Aldehydes and Ketones
Carbonyl group is polar (more polar than carbon-oxygen single bond), so higher
boiling point than comparable alkane or ether.
• BUT…Cannot make H-bond to each other, so lower boiling point than comparable alcohol
Reactivity of Carbonyl Group
The positive charge on the carbon atom makes it open to attack
by nucleophiles.
Although “double bonds require more energy to break than single bonds”,
compounds with double bonds tend to be more reactive as addition reactions
are possible.
Aldehydes and ketones can be reduced, forming primary and secondary
alcohols, respectively
Aldehydes may also be oxidized to carboxylic acids
Part. 3
Compounds with double bonds tend to be more reactive as addition reactions are possible.
The positive charge on the carbon atom makes it open to attack by nucleophiles. Aldehydes are
more reactive as H-atom is electron withdrawing vs. alkyl group.
For ketones - small alkyl groups bonded to the carbonyl carbon are more reactive than ketones with
large alkyl groups
Remember:
Aldehydes have less steric hindrance on the positive carbonyl carbon and only one
R group to supply electrons towards the partially positive carbonyl carbon
Preparation of Aldehydes and Ketones
An oxidation reaction increases the number of C-O, C-N, or C-X bonds in a compound (where X denotes a halogen), or it decreases the number of C-H bonds.
Oxidation of 1° R-OH
Primary alcohols are initially oxidized to aldehydes with Na2Cr2O7 and H2SO4 . The reactiondoes not stop at the aldehyde. Instead, the aldehyde is further oxidized to a carboxylic acid.
The oxidation of a primary alcohol can be easily stopped at the aldehyde if
pyridinium chlorochromate (PCC) is used as the oxidizing agent.
The reaction is carried out in an anhydrous (no water) solvent such as
dichloromethane
Part. 2
Oxidation of 2° R-OH
Secondary alcohols can be oxidized with Na2Cr2O7 and H2SO4 to ketones.
Ozonolysis
Another common way to form C = O bond (other alcohol oxidation) is via ozonolysis. This
reaction special does not simply break the C-C π bond, but additionally breaks the C-C σ bond
as well. This type of reaction is known as oxidative cleavage.
C = C bond is broken into two C=O bonds.
Ozone and the alkene undergo a concerted cycloaddition reaction—the oxygen atoms add tothe two carbons in a single step
Part. 3
How would you distinguish between aldehydes and ketones?
Using oxidation reactions – there are several tests that have been developed to determine the
presence of aldehydes, based on their oxidation to carboxylic aci
Reduction of Carbonyl Groups
A reaction that increases the number of C H bonds or decreases the number of C-O, C-N, or
C-X bonds in a compound (where X denotes a halogen), is called a reduction reaction.
NaBH4 (sodium borohydride) and LiAlH4 (lithium aluminium hydride) are reducing agents [R]
commonly used for transforming aldehydes or ketones into primary or secondary alcohol
Addition of hydride ion to an aldehyde or ketone forms an alkoxide ion. Subsequent protonation by an acid produces an alcohol. The overall reaction adds to the carbonyl grou
Isomerism of Aldehydes and Ketones
Aldehydes and Ketones that have a given number of carbon atoms are functional group isomers.
The same molecular formula C3H6O
- Positional isomers are possible for ketones (but not aldehydes, why?)
- And… skeletal isomers are possible for both
The same molecular formula C5H10O
2-pentanone, 3-pentanone
The same molecular formula C5H10O
2-pentanone, 3-methyl-2-butane
Common Aldehydes and Ketones
Vanillin - Vanilla flavouring
Benzaldehyde - Almond flavouring
Cinnamaldehyde - Cinnamon flavouring
2-heptanone - Clove flavouring
Butanedione - Butter flavouring
Carvone - Spearmint flavouring
Naturally occurring Aldehydes and Ketones
A wide variety of biologically relevant molecules possess aldehydes and/or ketone functional groups:
- Testosterone
- Progesterine
- Cortisone
- D-glucose
Sulphur containing Carbonyl groups
• The sulfur atom can either replace the oxygen or the carbon of the carbonyl group.
• When oxygen replaced by sulfur, the resulting compounds are called
thioaldehydes or thioketones, and these are generally unstable
In the second case, sulphoxides result. The best known example of a sulphoxide is dimethyl sulphoxide (DMSO),m which is also a sulphur analogue of acetone.
DMSO is an excellent polar parotid solvent; it can dissolve a wide variety of of both polar and non-polar substances
