2. REDUCTION

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2. REDUCTION

• Aldehydes are reduced to primary alcohols and
  ketones to secondary alcohols by a variety of
  reducing agents.
• Catalytic hydrogenation over a metal catalyst and
  reduction with sodium borohydride or litium
  aluminium hydride are common.

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2.0 Reduction

• Reduction
• Aldehydes are reduced to 1o R-OH ketones to 2o
  R-OH
• Reagents (a) LiAlH4 _at_ NaBH4 in ether H
• (b) Zn, H, heat
• (c) H2(g), Ni _at_ Pt, heat

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H l R-C-OH Alcohol 1º
H l R-CO aldehyde
R l R-C-OH Alcohol 2º
R l R-CO ketone
H
NaBH4
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EXAMPLE
H l CH3-CO ethanal
CH3CH2OH ethanol (alcohol 1º )
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CH3 l CH3-CO propanone
H2
CH3-CH(CH3)OH propanol (alcohol 2º)
Pt
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Bonding in the carbonyl group

• Oxygen is far more electronegative than carbon
  and so has a strong tendency to pull electrons in
  a carbon-oxygen bond towards itself.
• That makes the carbon-oxygen double bond very
  highly polar.

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• Aldehydes and ketones are especially susceptible
  to nucleophilic addition because of the
  structural features that are shown below.

aldehyde or ketone (R or R maybe H)
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• The trigonal planar arrangement of groups around
  the carbonyl carbon atom makes the carbonyl
  carbon atom is relatively open to be attacked
  from above or below.
• The positive charge on the carbonyl carbon atom
  is susceptible to attack by a nucleophile.

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• The negative charge on the carbonyl oxygen atom
  means that nucleophilic addition is susceptible
  to acid catalysis.