Chem 140C Lecture Notes Summer 2011 Chapter 19

Chem 140C Lecture Notes Summer 2011 Chapter 19 - Chemistry...

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Chemistry 140C Summer 2011 (K. Albizati) 1 Chapter 19 – Carboxylic Acids The carboxyl group is a key functional group in biochemistry and molecular biology. The oxygens are Lewis basic sites and have slight nucleophilic character and carbonyl carbon is electrophilic. Like all carbonyl groups, carboxyl carbonyls are subject to nucleophilic addition reactions: Probably a good idea to review Chapter 17. Nomenclature Carboxylic acids take precedence in naming functional groups over any functional group you have already seen. The carboxyl group is always at the end of a chain and it is numbered as C1. Order of precedence is on p 857 but is not for memorization. In naming, the suffix –oic acid replaces the –e suffix on alkane names. So…. .
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Chemistry 140C Summer 2011 (K. Albizati) 2 As usual, many of the common low molecular weight carboxylic acids have common names that have caught on over the years and persist to this day. Important ones in science and in everyday life: Important dicarboxylic acids: Nomenclature Examples: Audience Participation Time!
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Chemistry 140C Summer 2011 (K. Albizati) 3 Structure and Physical Properties The carbonyl group is sp 2 -hybridized and trigonal planar, as you would suspect, with all bond angles around it being about 120 o . R O O H 1.20 A 1.34 A However, the C-O bond lengths are not what you would expect. The carbonyl is a little longer than a typical aldehyde or ketone and the C-O single bond is a little shorter. Resonance structures can explain this: There is a lot of double bond character between the hydrogen-bearing oxygen and carbonyl carbon and a lot of single bond character between carbonyl carbon and carbonyl oxygen. Because it is such a polar functional group, carboxylic acids have relatively high MP’s and BP’s compared to the corresponding aldehydes and ketones. Also, carboxylic acids form hydrogen-bonded dimers in the solid state and in dilute non-aqueous solutions: C O O R H C O O R H hydrogen bond
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Summer 2011 (K. Albizati) 4 Low molecular weight carboxylic acids have very pungent odors. Think about vinegar, which contains a high percentage of acetic acid. Butanoic acid is formed when butter turns rancid and is responsible for its distinctive odor, hence its common name, butyric acid. Hexanoic acid – principle odor component of goats, which is similar to…… Acidity of Carboxylic Acids Carboxylic acids are the most acidic of the common organic functional groups. Much more acidic than alcohols (pK a about 17). Two reasons: - The –OH is attached to a powerful electron withdrawing carbonyl group – disperses negative charge character in the carboxylate anion and stabilizes the anion. Result is increased acidity. -
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This note was uploaded on 07/13/2011 for the course CHEM 140C taught by Professor Nefzi during the Summer '06 term at UCSD.

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Chem 140C Lecture Notes Summer 2011 Chapter 19 - Chemistry...

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