Chem62.Chapt16

Chem62.Chapt16 - Chapter 16-1 Carbonyl Group The Carbonyl...

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Chapter 16-1 Chem 62 The Carbonyl group Carbonyl Group sp 3 hybridized, trigonal planar carbonyl carbon; partial positive C and partial negative (Lewis basic) oxygen. C O C O C O .. .. .. : : + δ δ + pi bond lone pairs : : Because of the polar C=O bond, boiling points are higher than nonpolar compounds of similar molecular weights. Aldehydes and ketones are capable of hydrogen bonding to water, alcohols and acids Spectral Properties Infrared: Ketones(C=O) 1660-1750 cm-1 shifted by 25 cm-1 if aromatic or unsaturated : PhCHO, CH 2 =CHCOCH 3 Adlehydes (C=O) 1700-1740 (C—H) 2850 1 H NMR R—C H O 9-10 ppm RC H 2 COR 2.0 - 2.6 ppm due to inductive deshielding C–O pi* is low-lying and therefore interacts well with high-lying filled-nonbonding orbitals: thus nucleophilic, not electrophilic addition reactions are charactersistic of carbonyl compounds.
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Chapter 16-2 Chem 62 The Nucleophilic Addition Reaction Nucleophilic addition reactions Addition Reactions of Aldehydes and Ketones Carbonyl group can be attacked by nucleophiles C O R R C O R R Nu sp 3 sp 2 _ : .. .. Nu: . . . . Nu: or undergo addition of reagents to the pi bond by electrophiles adding first C O R R C O R R H C O R R Nu H .. .. + . . .. .. Nu: C O R R C O R H C O H H increasing reactivity due to steric and electronic effects ketone aldehyde formaldehyde Ketones are more sterically hindered since they have two alkyl groups; aldehydes have one H and one alkyl group. Alkyl groups are electron releasing and make the C=O carbon less positive. Reaction with H 2 O: Formation of Hydrates hydrates are normally transient, unstable species which are in equilibrium with the carbonyl compound H + CH 3 C O CH 3 CH 3 C O CH 3 H O H H 2 O hydrate
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Chapter 16-3 Chem 62 The Nucleophilic Addition Reaction CH 3 C O CH 3 CH 3 C O CH 3 CH 3 C O CH 3 H H O H H CH 3 C O CH 3 H O H hydrate - H + + . . . . + . . . . . . the equilibrium constant for the formation of hydrates is dependent on the carbonyl substituents: the more sterically hindered and the more electron rich the carbonyl, the less of the hydrate that will be present, conversely, the less sterically hindered and the more electron deficient the carbonyl carbon, the more hydrate that will be present. O H H O H 3 C H O H 3 C CH 3 O F 3 C CF 3 O (H 3 C) 3 C H K hydration 41 1.8 X 10 -2 4.1 X 10 -3 2.5 X 10 -5 22,000 CH 3 C O CH 3 CH 3 C O CH 3 H O H hydrate HO Acid catalyzed mechanism Base catalyzed mechanism CH 3 C O : CH 3 O H .
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