Aldehydes and Ketones

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Chapter 17

• Aldehydes and Ketones

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• Carbonyl compound Any compound that contains a
  carbonyl group (CO).
• Carbonyl groups are strongly polarized, with a
  partial positive charge on carbon and partial
  negative charge on oxygen.

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• Polarity of carbonyl group contributes to its
  reactivity.
• Carbonyl compounds are broadly divided into two
  groups aldehydes and ketones are in one group
  and the second group contains carboxylic acids,
  esters, and amides.

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NomenclatureAldehyde Common Names

• The simplest aldehydes are known by their
  common names
• formaldehyde, HCHO
• acetaldehyde, CH3CHO
• benzaldehyde, C6H5CHO

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Common Aldehydes

• Formaldehyde, HCHO Toxic but useful. It
  kills viruses, fungi and bacteria. It is used in
  disinfecting and sterilizing equipment. The
  polymerization property of formaldehyde is
  utilized in adhesives and for binding plywood.
• Acetaldehyde (CH3CHO) Sweet smelling.
  Present in ripe fruits, especially in apples. It
  is less toxic than formaldehyde.

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• Benzaldehyde, C6H5CHO, is commonly known as
  almond oil. It is used in cooking.

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IUPAC Nomenclatureof Aldehydes

• To name an aldehyde systematically in the
  IUPAC system, the final e of the name of alkane
  is replaced by al.
• CH3CH2CH2CHO butanal
• CH3(CH2)5CHO heptanal

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Ketone Common Names

• Some ketones are also known by their common
  names. These are the names of the two alkyl
  groups (usually alphabetized) bonded to the
  carbonyl group followed by the word ketone.
• methyl ethyl ketone (MEK)
• CH3CH2COCH3
• acetone CH3COCH3

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• Acetone (CH3COCH3) It is one of the most widely
  used solvents. It can dissolve most organic
  compounds and is also miscible with water.
  Acetone is highly volatile and highly flammable.
  

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IUPAC Nomenclature of Ketones

• Ketones are named systematically by replacing
  the final e of the corresponding alkane name
  with one. The numbering of the alkane chain
  begins at the end nearest to the carbonyl group.
  The location of the carbonyl group is indicated
  by placing the number of the carbonyl carbon in
  front of the name.

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Examples

• CH3CH2CH2COCH3 2-pentanone
• (CH3)2CHCH2COCH2CH3
• 5-methyl-3-hexanone

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Physical Properties

• The polarity of the carbonyl group makes
  aldehydes and ketones moderately polar compounds
  resulting in higher boiling points than compounds
  of comparable molecular weight.
• Use CNO to determine approximate MW.

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Example

• Which has the highest BP 2-propanone or butane?

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• Aldehydes and ketones dont form hydrogen
  bonds to each other, however, they do form
  hydrogen bonds with water.
• These compounds are soluble up to a 41 carbon
  to oxygen ratio.

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Examples

• Which is more soluble in water butane or
  propanone?
• Is pentanone soluble in water?

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• Arrange the following in order of increasing
  solubility.
• hexanal
• propanal
• heptanal
• butanal

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• Aldehydes and ketones have lower boiling
  points than alcohols of similar MW, but higher
  than alkanes of similar MW.

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Examples

• Arrange the following in order of increasing
  boiling point.
• pentanal
• 2-pentanol
• ethyl isopropyl ether
• hexane

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• Arrange the following in order of increasing
  boiling point.
• hexanal
• propanal
• heptanal
• butanal

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• Aldehydes and ketones are soluble in common
  organic solvents, and those with four or less
  carbons are also soluble in water.
• Simple ketones are excellent solvents because
  they dissolve both polar and nonpolar compounds.
• The lower boiling point aldehydes and ketones are
  very flammable.

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Summary
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• Boiling point
• hydrocarbonsltethersltaldehydes and ketoneslt3
  amineslt1 and 2 amines
• ltalcoholsltammonium salts
• Solubility (C4 rule)
• hydrocarbonsltethersltaldehydes and ketoneslt3
  amineslt1 and 2 amines
• ltalcoholsltdiolsltammonium salts

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Redox Reactions
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• Since ketones cannot be oxidized, treatment
  with a mild oxidizing agent can be used as a test
  to distinguish between aldehydes and ketones.
  (Tollens test)

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Reduction of Aldehydes and Ketones

• Reduction is the reverse of an oxidation
  reaction. Reduction of a carbonyl group is the
  addition of hydrogen across the double bond to
  produce an OH group (an alcohol).
• Aldehydes are reduced to primary alcohols, and
  ketones are reduced to secondary alcohols.

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Acetals and Hemiacetals
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• OH OR
• RCOR RCOR
• 
  
• H H
• Hemiacetal
  Acetal
• OH OR
• RCOR RCOR
• R R
• Hemiketal Ketal

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• Ketals are also known as acetals and hemiketals
  are called hemiacetals.

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• Addition reaction Hemiacetal formation
• aldehyde alcohol ? hemiacetal
• ketone alcohol ? hemiketal

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• Elimination reaction Acetal formation
• hemiacetal alcohol ? acetal water
• hemiketal alcohol ? ketal water

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Hydrolysis of Acetals

• acetal water ? hemiacetal alcohol
• hemiacetal water ? aldehyde alcohol

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• --OCH3 methoxy group
• --OCH2CH3 ethoxy group

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Acetal Hydrolysis

• Acetal formation is an equilibrium reaction, the
  extent of reaction in either direction can be
  controlled by changing the reaction conditions.
• Addition of large quantity of water, in the
  presence of an acid catalyst, will drive the
  reaction back towards the aldehyde or ketone.