Structures of Aldehydes and Ketones
• Both aldehydes and ketones contain a carbonyl group
• Aldehydes have at least one H attached, while ketones have
two C’s attached to the carbonyl
• A carbonyl consists of a C double-bonded to an O
• Like in an alkene, the double bond consists of one sigma and
one pi bond
• The carbonyl is a very polar group
- O is more electronegative than C, so C-O bonds are polar
- Also, the carbonyl has two resonance forms
- This polarity makes carbonyls chemically reactive
O
O
H
Aldehyde
Ketone
O
+
O
O
Naming Ketones
•
•
•
•
Parent name ends in -one
Find longest chain containing the carbonyl group
Number C’s starting at end nearest carbonyl group
Locate and number substituents and give full name
- use a number to indicate position of carbonyl group
- cyclic ketones have cyclo- before the parent name;
numbering begins at the carbonyl group, going in direction
that gives substituents lowest possible numbers
- use a prefix (di-, tri-) to indicate multiple carbonyl groups in
a compound
O
O
O
O
Br
propanone
(acetone)
butanone
2-pentanone
4-bromo-3-methylcyclohexanone
Naming Aldehydes
•
•
•
•
Parent name ends in -al
Find longest chain containing the carbonyl group
Number C’s starting at end nearest carbonyl group
Locate and number substituents and give full name
- aldehydes take precedence over ketones and alcohols in naming
- ketones are called oxo as a secondary group
- alcohols are called hydroxy as a secondary group
- the smallest aldehydes are usually named with common names
- we will not name cyclic aldehydes (except benzaldehyde)
O
H
O
O
H
methanal
(formaldehyde)
H
ethanal
(acetaldehyde)
OH
H
3-methylbutanal
O
O
H
5-hydroxy-3-oxohexanal
Physical Properties of Aldehydes and Ketones
• Because the carbonyl group is polar, aldehydes and ketones
have higher boiling points than hydrocarbons
• However, they have no H attached to the O, so do not have
hydrogen bonding, and have lower boiling points than alcohols
• Like ethers, aldehydes and ketones can hydrogen bond with
water, so those with less than 5 carbons are generally soluble
in water
• Aldehydes and ketones can be flammable and/or toxic, though
generally not highly so
• They usually have strong odors, and are often used as
flavorings or scents
Oxidation of Aldehydes
• Recall that aldehydes and ketones are formed by the oxidation of
primary and secondary alcohols, respectively
• Also recall that aldehydes are readily oxidized to carboxylic
acids, but ketones are not
• Tollens’ reagent (silver nitrate plus ammonia) can be used to
distinguish between ketones and aldehydes
- with aldehydes the Ag2+ is reduced to elemental silver, which
forms a mirror-like coat on the reaction container
• Sugars (like glucose) often contain a hydroxy group adjacent to
an aldehyde
- Benedict’s reagent (CuSO4) can be used to test for this type of
aldehyde; the blue Cu2+ forms Cu2O, a red solid
Reduction of Aldehydes and Ketones
• Reduction can be defined as a loss in bonds to O or a gain in
bonds to H
• Aldehydes and ketones can be reduced to form alcohols
- Aldehydes form primary alcohols
- ketones form secondary alcohols
• Many different reducing agents can be used, including H2,
LiAlH4 (lithium aluminum hydride) and NaBH4 (sodium
borohydride)
• However, NaBH4 is usually the reagent of choice
- hydrogenation will also reduce alkenes and alkynes if present
- LiAlH4 is more reactive than NaBH4, but reacts violently with
water and explodes when heated above 120º C
O
O
NaBH4
OH
H
NaBH4
OH
Addition of Water to Aldehydes and Ketones
• H2O can add across the carbonyl of an aldehyde or a ketone,
similar to the addition of H2O to an alkene
• A partial positive H from water bonds to the partial negative
carbonyl O, and the partial negative O from water bonds to the
partial positive carbonyl C
• The product of this reversible reaction is a hydrate (a 1,1-diol)
• In general, the equilibrium favors the carbonyl compound, but for
some small aldehydes the hydrate is favored
• The reaction can be catalyzed by either acid or base
O
H3O+
H
0.1%
HO
OH
O
H3O+
HO
OH
H
99.9%
99.9%
0.1%
Mechanism of Acid-Catalyzed Hydration of Formaldehyde
• First, the carbonyl O is protonated by the acid catalyst
• Next, H2O attacks the carbonyl carbon to form a protonated
hydrate
• Finally, H2O removes the proton to form the hydrate
O
H
H
+
H
O
O
+
O
H
H
H
H
H
H
H
O
H
+
H
HO
O
H
O
H
H
H
HO
HO
O
H
+
O
H
+
H
H
H
H
OH
O
H
H
Addition of Alcohols to Aldehydes and Ketones
• Alcohols can add to aldehydes and ketones using an acid catalyst
• Addition of 2 alcohols produces an acetal (a diether)
• The reaction intermediate, after addition of one alcohol, is a
hemiacetal (not usually isolated)
• This is a reversible reaction
- removal of H2O favors acetal
- addition of H2O favors aldehyde or ketone
• Acetals are often used as protecting groups in organic synthesis
O
Acid
HO
OCH3
Acid
H3CO
OCH3
+ CH3OH
+ CH3OH
Cat.
Cat.
Hemiacetal
Acetal
Formation of Cyclic Hemiacetals
• When an aldehyde or a ketone is in the same molecule as an
alcohol, a cyclic hemiacetal can form
• These are more stable than the non-cyclic ones and can be
isolated
• Sugars, like glucose and fructose, exist primarily in the cyclic
hemiacetal form
• When an alcohol adds to a cyclic hemiacetal, a cyclic acetal is
formed (this is how sugars bond together in polysaccharides)
OH
O
Acid
O
H
Cat.
H
OH
Stereoisomers
• Recall that constitutional isomers have the same molecular
formula, but the atoms are bonded in a different order
• Stereoisomers have the same molecular formula, and the
same bonding order, but the atoms are arranged differently
in 3-D space
• There are two types of stereoisomers:
- enantiomers are non-superimposable mirror images
- diasteriomers are stereoisomers that are not mirror
images (cis-trans isomers a type of diastereomers)
OH
H
OH
CH3
Br
H 3C
Br
Enantiomers
H
H
H
CH3
H
CH3
CH3
H
CH3
Diastereomers
Chirality
• An object, or a molecule, is chiral if it has a mirror image
that is not superimposable
• The most familiar chiral objects are your hands
- the right hand is the mirror image of the left hand
- no matter how you turn them, they can’t be superimposed
• Many organic compounds are also chiral
- most biomolecules (amino acids, sugars, etc.) are chiral
and usually only one of the stereoisomers is used
• In order for a carbon in an organic compound to be chiral,
it must have 4 different groups attached (otherwise the
mirror image will be superimposable)
Fischer Projections
• Fischer projections are a simple way to represent chiral
molecules (especially sugars)
• The bonds to a chiral carbon are shown as crossed
perpendicular lines, with the chiral C at the center
- Horizontal bonds are coming towards you (like wedges)
- Vertical bonds are going away from you (like dashes)
• The D and L classification of sugars is based on the
simplest sugar, glyceraldehyde
• Compounds with more than one chiral carbon, such as
larger sugars, can also be represented as Fischer
projections
- Each place where a horizontal line crosses the vertical
line represents a carbon