ALDEHYDES

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• ALDEHYDES KETONES
• (ALKANALS ALKANONES)

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ALDEHYDES KETONES (ALKANALS ALKANONES)
Aldehydes ketones both contain the carbonyl
group.

• The simplest aldehyde is formaldehyde (CH2O). It
  is the only aldehyde without an alkyl group
  attached to the carbonyl C.

All other aldehydes, such as acetaldehyde
(CH3CHO), have one alkyl group and one H attached
to the carbonyl C.

• All ketones have two alkyl groups attached to the
  carbonyl C.

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Aldehydes and Ketones are Electrophiles

• The carbonyl group has a strong dipole.
• DEN(O-C) (3.5-2.5) 1.0 (a polar bond)
• The d carbon is an electron acceptor, (an
  electrophile).
• Good nucleophiles (CH3MgBr) and even fair
  nucleophiles (NH3) will readily add to the
  carbonyl group of aldehydes and ketones.

The weak p bond breaks as the Nu- adds, so that
C remains tetravalent (? 5 bonds).

• The alkyl group and the H atom bonded to the
  carbonyl are not leaving groups. They are not
  displaced because hydride (H-) and alkanides
  (R-) are extremely strong bases.
• pKb H- -21 and pKb CH3- -40! (CH3-
  methide).

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Aldehydes and Ketones are Electrophiles

• Aldehydes and ketones are moderately reactive as
  electrophiles (electron acceptors) among the
  carboxylic acid derivatives.

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Basicity of Aldehydes and Ketones

• The d- oxygen is a weak base (pKb ca. 21)Its non
  bonded es are protonated by strong acids.

• The charge is shared with the carbonyl C by
  resonance forming a carbocation a very good E.
  
• Even weak Nu-s (like H2O and ROH) will donate
  electrons to an aldehyde or ketone in the
  presence of a strong acid catalyst, e.g., H2SO4
  or HCl.

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Acidity of Aldehydes and Ketones

• The a-carbon is the carbon bonded to the
  carbonyl, not the carbonyl carbon itself.
• Hydrogens bonded to the carbonyl carbon, the
  a-carbon, the b-carbon, etc. are not polar and
  thus are not acidic hydrogens.

• The a-hydrogens can be removed by strong bases
  because the carbanion that forms is stabilized by
  resonance with the adjacent carbonyl oxygen
  forming an enolate.

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Boiling Points and Solubility of Aldehydes and
Ketones

• The carbonyl group is strongly polar but does not
  produce hydrogen bonding (It has no polar
  hydrogens). As a result, the boiling points of
  aldehydes and ketones are higher than the
  nonpolar hydrocarbons and the alkyl halides but
  lower than those of alcohols.
• Formaldehyde is a gas at room temperature (b.p.
  -21 ?C) but heavier aldehydes are liquids.
  Acetone, the simplest ketone, is a liquid at room
  temperature (b.p. 56 ?C).
• Lower molecular weight aldehydes and ketones are
  water soluble. Acetone, formaldehyde and
  acetaldehyde are miscible in water.

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IUPAC Nomenclature of Aldehydes
Aldehydes in open chains alkaneal ? alkanal

3-bromobutanal
4-hydroxypentanal
2-phenylethanal

• The parent chain must contain the CHO- group, and
  this group is numbered as carbon 1 (because it is
  always at a chain end).

Aldehydes attached to rings ringcarbaldehyde
? ringcarbaldehyde
3-hydroxycyclopentanecarbaldehyde
benzenecarbaldehyde
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Common Names of Aldehydes

• In the common system, aldehydes are named from
  the common names of the corresponding carboxylic
  acid.
• The ic acid ending is replaced with aldehyde.
  

formic acid
formaldehyde
acetic acid
acetaldehyde
propionic acid
propionaldehyde
butyric acid
butyraldehyde
valeraldehyde
valeric acid
caproaldehyde
caproic acid

• Substituents locations are given using Greek
  letters (?, ?, ?, ?, ?, ?.) beginning with the
  carbon next to the carbonyl carbon, the a-carbon.

?-bromobutyraldehyde
?-hydroxyvaleraldehyde
?-phenylacetaldehyde
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IUPAC Nomenclature of Ketones

• Ketones in both open chains and rings
• alkaneone ? alkanone
• The parent chain must contain the CO group , and
  this chain is numbered to give the carbonyl group
  as low a number as possible. In cyclic ketones,
  the carbonyl group is assigned the number 1.

1-phenyl-1-butanone
2-chloro-4-methyl-3-pentanone
2-butanone

• Ketones are just below aldehydes in nomenclature
  priority.
• A ketone group is named as an oxo substituent
  in an aldehyde.

An olefinic ketone is named as an enone,
literally -alken--one.
4-methyl-2-cyclohexen-1-one
3-oxopentanal
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Common Names of Ketones

• The two alkyl groups attached to the carbonyl are
  named and the word ketone is added as a
  separate word. It is literally alkyl alkyl
  ketone.
• The alkyl groups are listed alphabetically or in
  order of increasing size.

• As with aldehydes, substituents locations are
  given in common names using Greek letters (?, ?,
  ?, ?, ?, ?.) beginning with the a-carbon.

methyl isobutyl ketone (MIBK)
g-methoxypropyl phenyl ketone
?-chloroethyl isopropyl ketone
Some historic names persist
acetophenone
benzophenone
benzaldehyde
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Nomenclature Practice

• Name these in IUPAC and, where possible, common
  nomenclature.

(I) 1-phenyl-2-propanone
(I) 4-fluorocyclohexane-1-carbaldehyde
(c) methyl benzyl ketone

• Draw the structures of the following compounds.
• butanedial bromomethyl b-bromoethyl
  ketone 2,4-pentanedione

• And these

(I) 2-butenal
(I) 3-buten-2-one
(c) methyl vinyl ketone
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Preparation of Aldehydes (2 Methods)

1. Mild oxidation of 1 Alcohols (with anhydrous
   oxidants, PCC in CHCl2 or Collins reagent (CrO3
   in pyridine).

1,3-cyclobutanedicarbaldehyde
Dry ice (solid CO2) sublimes at 78C.

• Reduction of acid chlorides,esters, and nitriles.

acid chloride
Only 1 equivalent of very cold DIBAH is used to
avoid further reduction of the aldehyde to an
alcohol.
ester
nitrile
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Preparation of Aldehydes (2 Methods)

• Recall that 1 alcohols are readily oxidized to
  carboxylic acids by most oxidants in aqueous
  media.

• In non aqueous media, moderate to strong oxidants
  become mild, oxidizing 1 alcohols only as far as
  the aldehyde.

• Carboxylic acids can be reduced to 1 alcohols
  with LiAlH4, but no reagent has been found that
  will stop the reduction at the aldehyde.

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Preparation of Aldehydes (2 Methods)

• Carboxylic acids are difficult to reduce and any
  reducing agent strong enough to reduce them,
  e.g., LiAlH4, will not stop at the aldehyde but
  always produces the 1 alcohol.
• Several derivatives of carboxylic acids can be
  reduced to aldehydes under carefully controlled
  conditions.
• Acid chlorides, esters, and nitriles are reduced
  to aldehydes using very cold conditions (-78C)
  and only 1 equivalent of a mild reducing agent,
  diisobutylaluminum hydride DIBAH (usually in
  toluene).

• DIBAH is weaker than LiAlH4. DIBAH is neutral
  LiAlH4 is ionic.
• DIBAH is similar to AlH3 but is hindered by its
  bulky isobutyl groups.
• Only one mole of H- is released per mole of
  DIBAH.

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Preparation of Aldehydes (2 Methods)

• Study the following examples and note which
  groups are displaced by the hydride (H-) from
  DIBAH.

• Write equations showing the preparation of
• pentanal from 1-pentanol
• butanal from an ester
• benzaldehyde from a nitrile

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Preparation of Ketones (4 Methods)

1. Oxidation of 2 Alcohols with mild (anhydrous)
   oxidants, moderate, or strong oxidants, e.g.,
   H2CrO4, HNO3, KMnO4, NaOCl, etc.

4-t-butylcyclohexanone

1. Friedel Crafts Acylation of Aromatics yields
   ketones when an acid chloride is used as the
   electrophile.

1-(4-hydroxyphenyl)propanone

1. Hydration of Alkynes with Hg2 and H3O yields
   an enol, that tautomerizes to a ketone.

2-octanone
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Preparation of Ketones (4 Methods)

• Acid Chlorides Lithium Dialkyl Copper (Gilman
  Reagent) produces ketones.
• The reaction is unique to these two reagents and
  the mechanism is uncertain. As with DIBAH for
  aldehyde reductions, a low temperature (-78? C)
  solvent (ether) is used to prevent further alkyl
  addition to the ketone to form an alcohol. (Acid
  chlorides are very good electrophiles).
• Carboxylic acids, esters, anhydrides and amides
  are not reduced by diorganocopper reagents. They
  are not as reactive as acid chlorides.

2-heptanone

• Recall that a stronger reducing reagent, such as
  a Grignard (RMgBr) will also reduce an acid
  chloride to a ketone, but reduction cannot be
  stopped here. The ketone is further reduced to
  an alcohol.

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Preparation of Ketones Problems

• Write equations to show how the following
  transformations can be carried out. Show all
  reagents and intermediate products.
• 3-hexyne ? 3-hexanone
• benzene ? m-bromoacetophenone
• bromobenzene ? acetophenone
• 1-methylcyclohexene ? 2-methylcyclohexanone

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Preparation of Ketones Problems

• Recall the effects of substituents on aromatic
  rings. They affect both the reactivity of
  aromatics and the position at which Electrophilic
  Aromatic Substitution (EAS) will occur.