Identify Carboxylic acids , amines and amino acids

Carboxylic Acids

Chemical properties

Determination of Equivalent Weight (or Neutralization Equivalent)

Molecular Weight Determination

Generally, any acid (or base) can be titrated using standard solutions of base (or acid). The neutralization equivalent obtained is usually a simple fraction of the molecular weight (1, 1/2, 1/3, etc). In the titration of an acid with standard base, the endpoint is reached when all the acid is neutralized and a drop of excess base is added. If phenolphthalein is used as the indicator, it will turn red at this instant. For a dicarboxylic acid such as malonic acid, the endpoint is reached when the last of the acid is converted to the carboxylate anion. The neutralization equivalent will be one-half its molecular weight.

CH₂ (CO₂H) ₂ + 2 NaOH ----- CH₂(CO₂Na) ₂ + 2 H₂O

Equivalent weights must be done in duplicate and the values obtained should agree within a few percent. If not, do a third determination.

The equivalent weight (neutralization equivalent) can be calculated as follows:

V (ml) x M (mmol/ml)= wt (mg)/equivalent weight (mg/mequiv); V = volume of standard base used measured accurately to at least 3 significant figures; M= molarity of standard base as long as the base is monobasic, probably listed as 0.100M NaOH or KOH (note three significant figures); wt= weight of unknown used. An equivalent weight of 120 mg/mequiv means that the molecular weight is some whole number multiple of 120; for example, 120 (if monoacid there is 1 mequiv/mmol) or 240 (if diacid there are 2 mequiv/mmol) or 360 (if triacid), etc.

For Carboxylic Acids:

Neutralization Equivalent and Equivalent Weight of Carboxylic Acid:-

Procedure:

Weigh 0.2 g (to three significant figures) of the unknown carboxylic acid, and place in a

125-mL Erlenmeyer flask. Dissolve the acid in about 50 mL of water or aqueous ethanol

(the acid need not dissolve completely, because it will dissolve as it is titrated). Titrate

the acid, using a standardized solution of NaOH of known molarity (in the range of 0.1000 M) and a phenolphthalein indicator (2 drops). Note the equivalent point (colorless to pink color).

Record the volume of NaOH used. Duplicate the run.

Calculate the neutralization equivalent (NE) from the equation:

NE = mg of carboxylic acid/molarity of NaOH x mL of NaOH used

The NE is identical to the equivalent weight of the carboxylic acid. If the acid has only

one carboxyl group, the NE and the molecular weight of the acid are identical. If the

acid has more than one carboxyl group, the NE equals the molecular weight of the acid

multiplied by the number of carboxyl groups, that is the equivalent weight. The NE can be used much like a derivative to identify a specific carboxylic acid.

Many phenols are acidic enough to behave similarly to carboxylic acids. This is especially true of those substituted with electron-withdrawing groups at the ortho and para ring positions. These phenols, however, can be eliminated by the ferric chloride test or spectroscopy.

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Derivatives of Carboxylic Acids

Chemical properties

1. Background and Properties

The important classes of organic compounds known as alcohols, phenols, ethers, amines and halides consist of alkyl and/or aryl groups bonded to hydroxyl, alkoxyl, amino and halo substituents respectively. If these same functional groups are attached to an acyl group (RCO–) their properties are substantially changed, and they are designated as carboxylic acid derivatives. Carboxylic acids have a hydroxyl group bonded to an acyl group, and their functional derivatives are prepared by replacement of the hydroxyl group with substituents, such as halo, alkoxyl, amino and acyloxy. Some examples of these functional derivatives were displayed earlier.
The following table lists some representative derivatives and their boiling points. An aldehyde and ketone of equivalent molecular weight are also listed for comparison. Boiling points are given for 760 torr (atmospheric pressure), and those listed as a range are estimated from values obtained at lower pressures. As noted earlier, the relatively high boiling point of carboxylic acids is due to extensive hydrogen bonded dimerization. Similar hydrogen bonding occurs between molecules of 1º and 2º-amides (amides having at least one N–H bond), and the first three compounds in the table serve as hydrogen bonding examples.

The last nine entries in the above table caot function as hydrogen bond donors, so hydrogen bonded dimers and aggregates are not possible. The relatively high boiling points of equivalent 3º-amides and nitriles are probably due to the high polarity of these functions. Indeed, if hydrogen bonding is not present, the boiling points of comparable sized compounds correlate reasonably well with their dipole moments.

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