Carboxylic acids contain the carboxyl group (COOH), which is a combination of the carbonyl group (C=O) and the hydroxyl group (-OH).
If wine is left open for a time, vinegar is produced, thus carboxylic acids are produced from alcohols.
Carboxylic acids are found in citrus fruits, crab apples, rhubarb and other foods with a sour taste.
The muscle ache we feel after exertion is caused by lactic acid (CH3CHOHCOOH).
Some substances have multiple carboxyl groups (oxalic acid – spinach and rhubarb, tartaric acid – grapes, citric acid – citrus fruits, ascorbic acid – Vitamin C, acetylsalicylic acid - Aspirin).
Naming Carboxylic Acids
Carboxylic acids are named by dropping the ending of the name of the longest chain and adding ‘-oic acid.’
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First compound: The main chain has one carbon (meth), which is singly bonded to the main chain (an) and it has a carboxyl group at the end (oic acid). The common name is formic acid, which ants use to defend themselves (they spit it into your skin after they bite you - it hurts - you leave the ants alone). Second compound: The main chain has two carbons (eth), which are singly bonded to each other (an) and it has a carboxyl group at the end (oic acid). The common name is acetic acid, which is the active ingredient in vinegar. Third compound: There is a benzene (benz) with a carboxyl group on it (oic acid). |
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| When locating substituents, always give the carbon in the carboxyl group the number 1 and continue to find the longest chain from there. In this case, there are methyl groups at the second and third carbons (2,3-dimethyl), the main chain is five carbons long (pent), all singly bonded together (an) with the carbonyl group at the end of the chain (oic acid). Notice there is no need to indicate the location of the carboxyl group because it is always at carbon 1 (similar to the aldehyde nomenclature). |
Properties of Carboxylic Acids
The properties of the carboxylic acids are based on the nature of the carboxyl group:
O
║
─C─OH
- polar, due to the C=O and –OH groups and can hydrogen bond
- they are similar in solubility to the alcohols, i.e. simple carboxylic acids are water soluble, while larger carboxylic acids become increasingly more insoluble with chain length increase
- they have the properties of acids and can react with bases in neutralization reactions
- the mp/bp of carboxylic acids are higher than those of the corresponding hydrocarbon and the addition of extra carboxyl groups increases the mp further
Preparing Carboxylic Acids
When an alcohol is oxidized, an aldehyde is produced, further oxidation of the aldehyde results in a carboxylic acid.
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| The first step is just the oxidation of a primary alcohol into an aldehyde (to which we were introduced in the last lesson). The second step is the oxidation of the aldehyde into a carboxylic acid. The active oxygen inserts itself into the bond between the C in the carbonyl group and the H. There is only one product in this step - a carboxylic acid. NameIts! Name these organic compounds. Answers at the end of the lesson. |
The active oxygen (O) is supplied by an oxidizing agent, which is itself reduced.
An interesting aside: The breathalyzer test is based on the colour change produced by the oxidizing agent when it is reduced.
CH3CH2OH + Cr6+
(orange) → CH3COOH + Cr3+ (green)
From Carboxylic Acids to Esters: Esterification
Carboxylic
acids can react with an alcohol to form an ester and water. It is known that the alcohol contributes the
–OH group to form the water molecule.
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| In the esterification reaction, the sulfuric acid pulls a water molecule from the reactants. The carboxylic acid loses the hydrogen from the carboxyl group and the alcohol loses the hydroxyl group. These two "pieces" join together to create a water molecule. The single bonded oxygen in the carboxylic acid and the carbon that used to be bonded to the hydroxyl group on the alcohol bond together, forming an ester. NameIts! Name these organic compounds. Answers at the end of the lesson. |
Esters (R-COO-R)
Esters are responsible for the odours of fruit and flowers. Synthetic esters are often added to foods and cosmetics.
Naming Esters
The name of an ester has two parts - the ester has one side from the parent alcohol and one from the parent carb acid. First the alcohol side is named, then the acid side, which includes the ‘-oate’ ending.
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| Always deal with the single bonded oxygen side of the ester first and the double bonded oxygen side second. First, number the chain outward from the single bonded oxygen. This locates the ethyl group at the second carbon (2-ethyl) of a five carbon chain (pentyl). Leave a space ( ) and contine to the other side of the compound. Second, number the chain outward from the carbon attached to the double bonded oxygen. This locates the methyl group at the fifth carbon (5-methyl) of seven carbon chain (hept), that is bonded singly (an) in an ester (oate). |
Properties of Esters
The
properties of the esters is based on this group:
O
║
─C─O─
- with the loss of the –OH group, esters are less polar than carboxylic acids and cannot H-bond
- thus they are less water soluble, have lower mp, bp and water solubility
- esters are not acidic
- it is the low molecular weight esters that have detectable scents, since they are gases at room temperature
- the larger esters are usually waxy solids
Reactions of Esters: Hydrolysis
The treatment of esters with an acid or base results in a reversal of esterification. The sodium salt of the acid (in this case, sodium propanoate) is soap and the reaction to make soap is called saponification.
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| When the ester is treated with sodium hydroxide, the bond holding the ester together breaks. The positive sodium ion is attrated to the negative oxygen, creating sodium propanoate (the salt of the carboxlic acid). The hydroxide ion bonds to the four carbon chain, creating an alcohol. NameIts! Name the ester and the alcohol in this reaction. Answers at the end of the lesson. |
NameIts!
primary alcohol = hexan-1-ol
aldehyde = hexanal
carboxylic acid = hexanoic acid
carboxlic acid = butanoic acid
alcohol = methylpropan-1-ol (no need to locate methyl group - it couldn't be anywhere else on the main chain and still have a three carbon main chain)
ester = 2-methylpropyl butanoate
ester = butyl propanoate
alcohol = butan-1-ol
primary alcohol = hexan-1-ol
aldehyde = hexanal
carboxylic acid = hexanoic acid
carboxlic acid = butanoic acid
alcohol = methylpropan-1-ol (no need to locate methyl group - it couldn't be anywhere else on the main chain and still have a three carbon main chain)
ester = 2-methylpropyl butanoate
ester = butyl propanoate
alcohol = butan-1-ol
Homework # 48-62
