Introduction to Oxygen Functional Groups

James Richard Fromm


Oxygen-containing functional groups include the alcohols, ethers, aldehydes, ketones, and carboxylic acids as well as a variety of derivatives of the carboxylic acids such as the amides, esters, and acid halides.  The concept of oxidation state is useful in dealing with these classes of oxygen functional groups, but in a somewhat different way than the way in which the concept of oxidation state is used with inorganic compounds.   The oxidation state used with organic compounds is the oxidation state of the carbon to which the oxygen is attached; the other carbon atoms are ignored.  The oxidation state of the carbon is then calculated in the usual way with oxygen taken as having an oxidation state of -2 and hydrogen an oxidation state of +1.  The oxidation states of the oxygen functional groups is shown in the Table below.


Table: Oxidation States of Oxygen-Containing Functional Groups
Compound Class Name Oxidation State
CO2 Carbon Dioxide +4
RCOOH Carboxylic Acid +3
RCOX Acid Halide +3
RCOOR' Ester +3
RCONH2 Amide +3
RCHO Aldehyde +2
RC=OR' Ketone +2
ROH Alcohol +1
ROR' Ether +1

Alkynes are assigned to oxidation state +2 because loss of water from an aldehyde or ketone, which is not a reduction or oxidation reaction, leaves an alkyne.  Likewise, loss of water from an alcohol leaves alkenes and alkenes and are therefore assigned oxidation state +1.  Alkanes must then be assigned oxidation state zero.

When the substitution of a group occurs such that more than one hydrogen is replaced, or more than a single bond to the carbon chain is formed (an equivalent statement), then the naming of these compounds is not quite as straightforward.  The end or substituted carbon then constitutes both parts of the substituent group and part of the chain being substituted upon.  By tradition, the naming system includes the substituted carbon with the chain being substituted upon most of the time, but sometimes includes it as part of the substituent group.

The common oxygen-containing functional groups are shown in the Table below.


Table: Oxygen-Containing Functional (Substituent) Groups
Substituent Group Compound Class Name Change (Ending)
ROH Alcohols -ol (hydroxy)
RCOOH Carboxylic Acids -oic Acid
RCHO Aldehydes -al (aldo)
RC=OR' Ketones -one (keto)
ROR' Ethers -oxy
RCOOR' Esters -oate

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Copyright 1997 James R. Fromm