Structures of Aldehydes and Ketones

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Structures of Aldehydes and Ketones

• Both aldehydes and ketones contain a carbonyl
  group
• Aldehydes have at least one H attached, while
  ketones have two Cs 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

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Naming Ketones

• Parent name ends in -one
• Find longest chain containing the carbonyl group
• Number Cs 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

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Naming Aldehydes

• Parent name ends in -al
• Find longest chain containing the carbonyl group
• Number Cs 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)

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Nomenclature

• because the carbonyl group of an aldehyde can
  only be at the end of a parent chain and
  numbering must start with it as carbon-1, there
  is no need to use a number to locate the aldehyde
  group
• for unsaturated aldehydes, indicate the presence
  of a carbon-carbon double bond and an aldehyde by
  changing the ending of the parent alkane from
  -ane to -enal show the location of the
  carbon-carbon double bond by the number of its
  first carbon

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Nomenclature

• the IUPAC system retains common names for some
  aldehydes, including these three

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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

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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
• - Benedicts reagent (Fehlings reagent) (CuSO4)
  can be used to test for this type of aldehyde
  the blue Cu2 forms Cu2O, a red solid

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Oxidation

• Aldehydes are oxidized to carboxylic acids by a
  variety of oxidizing agents, including potassium
  dichromate
• liquid aldehydes are so sensitive to oxidation by
  O2 of the air that they must be protected from
  contact with air during storage

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Oxidation

• Ketones resist oxidation by most oxidizing
  agents, including potassium dichromate and
  molecular oxygen
• Tollens reagent is specific for the oxidation of
  aldehydes if done properly, silver deposits on
  the walls of the container as a silver mirror

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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

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Reduction

• The carbonyl group of an aldehyde or ketone is
  reduced to an -CHOH group by hydrogen in the
  presence of a transition-metal catalyst
• reduction of an aldehyde gives a primary alcohol
• reduction a ketone gives a secondary alcohol

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Reduction

• reduction by NaBH4 does not affect a
  carbon-carbon double bond

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Reduction

• In biological systems, the agent for the
  reduction of aldehydes and ketones is the reduced
  form of nicotinamide adenine dinucleotide,
  abbreviated NADH (this reducing agent, like
  NaBH4, delivers a hydride ion to the carbonyl
  carbon of the aldehyde or ketone
• reduction of pyruvate, the end product of
  glycolysis, by NADH gives lactate

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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

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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

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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

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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)

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Addition of Alcohols

• all steps in hemiacetal and acetal formation are
  reversible
• as with any other equilibrium, we can drive this
  one in either direction by using Le Chatelier's
  principle
• to drive it to the right, we either use a large
  excess of alcohol or remove water from the
  equilibrium mixture
• to drive it to the left, we use a large excess of
  water

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Keto-Enol Tautomerism

• A carbon atom adjacent to a carbonyl group is
  called an a-carbon, and a hydrogen atom bonded to
  it is called an a-hydrogen

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Keto-Enol Tautomerism

• A carbonyl compound that has a hydrogen on an
  a-carbon is in equilibrium with a constitutional
  isomer called an enol
• the name enol is derived from the IUPAC
  designation of it as both an alkene (-en-) and an
  alcohol (-ol)

in a keto-enol equilibrium, the keto form
generally predominates