CH ENGR 104C Lecture Notes - Lecture 13: Chemical Polarity, Bond Energy, Boiling Point

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Carbonyl Chemistry – Fundamentals
Introduction
Carbonyl group:
Carbonyl functional groups:
Brief Nomenclature of Aldehydes & Ketones1
IUPAC system:
- add –al to aldehydes
- add –one to ketones
Comparing Alkenes & Carbonyls 2
Alkenes (ethylene)
Carbonyls
Double bond is nonpolar and longer
Double bond is polar and shorter
Due to polarity, double bond can be an H-bond
acceptor
Both sides of an alkene undergo nucleophilic
or electrophilic attack
Carbonyls undergo regioselective attack by
nucleophile or electrophile
The lack of a polar bond results in alkenes
having a lower boiling point than carbonyls
that are about the same size. (Fig.1)
Due to polar character (& for some carbonyls,
the ability to form H-bonds) of the carbonyl
group, carbonyls have higher boiling points.
(Fig.1)
1 http://www.elmhurst.edu/~chm/vchembook/700carbonyls.html
2 http://www.cem.msu.edu/~reusch/VirtualText/aldket1.htm
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Relative boiling points3:
(higher bp) amide > carboxylic acid > nitrile
>> ester ~ acyl chloride ~ aldehyde ~ ketone
(lower bp)
C=C bond has a bond energy of ~146
kcal/mole.
C=O bond has bond energy that varies:
H2C=O 170 kcal/mole
RCH=O 175 kcal/mole
R2C=O 180 kcal/mole
Higher bond energy of the C=O bond of carbonyls suggests that they are thermodynamically
unfavorable. How can carbonyls undergo a reaction?
The polarity of the carbonyl group and its higher basicity than alkenes lower the transition
state energy of activation and therefore result in a faster rate.
Fig.1
Reactivity of Carbonyls
- Polar double bond creates a partial positive charge on the carbonyl carbon and a partial
negative charge on the oxygen. _+ C=O _-
Three C=O Fates
Polar double bond plays a role in determining mechanism of reaction
1. Nucleophilic attack at _+ carbon
- oxygen (high electronegativity) accepts electron pair helps nucleophilic attack
- The product of nucleophilic attack is called a “tetrahedral adduct”; it is formed when an
sp2 carbon is changed to an sp3 carbon.
3 Bruice, Paula Y. “Carbonyl Compounds I.” Organic Chemistry. 4th ed. Pg. 677.
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