Download formic (methanoic) acid

WEEK 9
CARBOXYLIC ACIDS AND ESTERS
Organic salts, the neutralization products of carboxylic acids, function as buffers, waterconditioning agents, and anticoagulants in embalming fluids. Esters are used in
embalming fluids as perfuming or masking agents for the less-pleasant odors of alcohols
and aldehydes.
STRUCTURE OF ORGANIC ACIDS
Organic acids are compound containing one or more –COOH groups. This functional
group is called a carboxyl group because it is a combination of the carbonyl,
hydroxyl, -OH groups:
C O
and
O
C
OH
Organic acids are derivatives of the hydrocarbons in which a –CH3 group has been
replaced by a –COOH group. The general formula for the acids is usually abbreviated
RCOOH. Organic acids are classified according to the number of carboxyl groups they
contain.
MONOCARBOXYLIC ACIDS
These compounds contain one –COOH group. This class of acids is also known as fatty
acids because they occur naturally in fats and oils.
One method of naming involves changing the –e ending of the corresponding
hydrocarbon to –oic. Example: Methane to methanoic acid.
FORMIC (METHANOIC) ACID
O
HCOOH
H
C OH
Formic is the strongest of the monocarboxylic acids. It has a blistering effect on the skin.
This acid is the active irritant in the defensive secretions of ants, bees, and other insects.
ACETIC (ETHANOIC) ACID
O
CH3COOH
H3C
C OH
Acetic acid gives vinegar its characteristic sharp odor and taste. However, vinegar is a
relatively dilute solution (about 4-6%) of acetic acid. Pure (99.5%) acetic acid is called
glacial acetic, because it is sometimes frozen at room temperature into a glasslike solid.
ACIDIC NATURE OF CARBOXYLIC ACIDS
In aqueous solution they dissociate and form hydrogen ions and a negative ion. Ex:
O
O
H3C
C OH + H2O
H3O+ + H3C
C O-
In this reaction between acetic acid and water, the hydrogen ion from the acid is
transferred to water, producing the hydronium ion and the acetate ion. Because the
reaction has increased the hydrogen ion concentration (really hydronium-ion
concentration) of the water, acetic acid is an acid.
The acidic hydrogen in these acids is the one in the carboxyl group. Because of the
electronegativity of the two oxygen atoms in the carboxyl group, the electrons in the
bond between the oxygen and hydrogen atoms are pulled away from the hydrogen in the
direction of the oxygens. This unequal electron distribution between oxygen and
hydrogen makes it possible for the polar water molecule to pull the hydrogen as a
positive ion away from the carboxyl group. The other hydrogens in the molecule do not
undergo the same process because there is essentially an equal sharing of electrons
between carbon and hydrogen atoms. Carboxylic acids are weak acids. The double
arrow in the above equation signifies the partial ionization of acetic acid.
ORGANIC SALTS
Carboxylic acids undergo the typical reactions of acids. The most important is
neutralization:
carboxylic acid + base
salt + HOH
When carboxylic acids are neutralized to salts, the hydrogen atom in the –COOH group is
replaced by a metal. The name of the salt is derived by changing the –ic ending of the
acid to –ate preceded by the name of the metal that replaced the hydrogen. For example:
if formic acid is neutralized by sodium hydroxide, the salt produced is named sodium
formate:
HCOOH + NaOH
formic acid
HCOONa + HOH
sodium
formate
DICARBOXYLIC ACIDS
Organic acids containing two –COOH groups are called dicarboxylic acids. Their
structures are analogous to the dialdehydes, which are named as their reduction products.
The simplest is oxalic acid:
OH
O
O
C
C
OH
ANTICOAGULANTS
All carboxylic acids will react with inorganic bases to form salts, which are ionic
compounds. The sodium, potassium, and ammonium salts of all carboxylic acids are
water-soluble.
OH
O
O
C
C OH + 2NaOH
O
ONa
oxalic acid
O
C
C ONa
sodium oxalate
(salt)
+ 2H2O
The dicarboxylic acids are singled out for this neutralization study because of the role of
oxalates as blood anticoagulating agents or water-softening agents in embalming fluids.
The oxalates react with the ionized calcium in the blood, forming insoluble calcium
oxalate. The removal of the calcium ions prevents the initiation of the blood-clotting
process. The conversion of these soluble salts of a dicarboxylic acid to an insoluble
substance gives rise to the embalming classification of this group of substances as
Precipitant Anticoagulants.
O
ONa
C
O
C ONa + Ca
(Soluble)
++
O
O
C
+
C + 2Na
O Ca O
calcium
oxalate
(insoluble)
Calcium oxalate takes the form of cubic crystals, which are insoluble in water. Oxalates
are toxic and therefore, are no longer used as anticoagulants in embalming fluid.
HYDROXY ACIDS
Citric acid is classified as a hydroxy acid. It has three carboxyl groups (-COOH) in
addition to a hydroxyl group (-OH). The salts of citric acid have anticoagulant
properties. The salts in question are:
 Sodium citrate: C3H4OH (COONa)3
 Potassium citrate: C3H4OH (COOK)3
H
H
C
COOK
OH C
COOK
C
COOK
H
C
COONa
OH C
COONa
C
COONa
H
H
H
H
Sodium citrate
H
Potassium
citrate
These substance function as anticoagulants for blood in a different manner than the
oxalates. The calcium citrate complex that results from the reaction of sodium citrate
with ionized calcium is:
H
H
C
COO Ca OOC
C
OH C
COO Ca OOC
C HO
C
COO Ca OOC
C
H
H
H
H
H
H
Sodium is a monovalent metal, but calcium is divalent. The calcium ions cross-link two
molecules of sodium citrate to form this complex. The ionized calcium is tied up and
effectively “fenced off”; hence, the term sequesterant anticoagulant. Calcium citrate is
water-soluble, but it is a nonionizable complex. In other words it suppresses the
ionization of calcium.
Citrate anticoagulants are not toxic, but are falling into disfavor. Some common bacteria
such as Staphylococcus aureus produce a series of enzymes called the coagulase factor.
The coagulase enzyme demonstrates their properties best in the presence of citrated blood
plasma. The end result is a shortcut of the blood-clotting process by converting the
soluble precursor fibrinogen into the insoluble fibrin. In any case where these bacterially
produced enzymes may be present, citrated embalming fluid actually may enhance blood
clotting instead of inhibiting it. Thus, blood drainage will be greatly impaired and the
entire process will be counter-productive.
CHELATES
Chelates have replaced the two previously discussed anticoagulants in embalming fluids.
Chelates comes from the Greek for clawlike. They are substances that bind metallic ions.
The most commonly used agent of this type in embalming fluid is
ethylenediaminetetraacetic acid (EDTA). However, as in the case of the other
anticoagulants, the sodium salt is used rather than the acid itself. The sodium salt is an
excellent sequestering agent.
LACTIC ACID
An important monocarboxylic hydroxy acid. Originally isolated from sour milk.
Produced by the action of Lactobacillus bacteria on the milk sugar lactose. Lactic acid is
formed in the body as an intermediate product in carbohydrate metabolism and is
produced by muscle metabolism. Blood-lactate levels rise after strenuous exercise and
the stiff sore feelings of muscles as the result of such activity are associated with elevated
levels of this compound. The theories attempting to explain the onset of rigor mortis
centered on the formation of lactic acid. It was originally believed that rigor was
produced by the action of lactic acid, causing the coagulation of the muscle plasma. This
was supposedly brought about by the reaction of lactic acid with a soluble protein in
muscle tissue know as myosinogen. This action converts myosinogen into myosin, which
is insoluble. With the conversion of the soluble myosinogen into the insoluble myosin,
rigor is complete.
As decomposition proceeds, myosin is readily hydrolyzed by the enzymes present in the
tissues as well as those produced by bacterial action. As the myosin is hydrolyzed with
the production of amino acids and various intermediate products, rigor passes off. The
failure of the embalmer to reestablish the desired firmness during the embalming
operation may be due to a lack of these decomposition products to coagulate when treated
with formaldehyde. This theory, however, has been largely supplanted by a more
contemporary one.
ESTERS
Esters are fragrant organic compounds formed by the reaction of an organic acid with an
alcohol. The general formula for an ester is RCOOR. The following is a general
example of their formation:
O
O
R
C OH + R OH
Acid
Alcohol
R
C
O
R
+ HOH
Ester
Esters are pleasant-smelling substances and are responsible for the fragrances of many
flowers, flavors, fruits, and perfumes. They are used in embalming fluids as perfuming
or masking agents. These perfuming substances should not be confused with deodorants,
which react chemically with odors and actually neutralize or destroy them instead of
merely covering them up. The principal ester used for this purpose is methyl salicylate,
commonly known as oil of wintergreen. This substance is produced by reacting methyl
alcohol with salicylic acid.
OH
OH
O
C
H
OH
+
C
HO
O
H
C
O C
H
H
H
+
HOH
H
Salicylic acid
Methyl alcohol
Methyl salicylate
It is also possible to react the alcohol group on salicylic acid with the carboxyl group of
acetic acid. This reaction produces aspirin.
THIOESTERS
All the previously discussed esters have been oxyesters. In thioesters, the oxygen that is
single-bonded to the carbon is replaced by sulfur.
O
R C S R
Thioester
The general formula for a thioester is RCOSR. To prepare a thioester, a thiol is reacted
with an acid.
O
O
R
C
Acid
OH +
R
S
H
Thiol
R
C
S R
+
Thioester
HOH
Water
Some thioesters are biologically important. They transfer acyl groups:
O
R C
Acyl Group
This occurs during the metabolic reactions of carbohydrates, fatty acids, and amino acids.
The most important acyl transfer agent in living organisms is acetyl coenzyme A. This
compound is the ester of acetic acid and coenzyme A, a thiol.
Water
AMINES AND AMIDES
Amines are organic derivatives of ammonia in which one or more of the three hydrogens
of NH3 have been replaced by alkyl or aromatic groups. As a result, like alcohols,
amines may be designated as primary secondary, or tertiary according to the position of
attachment of the nitrogen.
H N H
H
Ammonia
R N
R
H
H
Primary
amine
R
N
H
R
N
R
R
Secondary
amine
Tertiary
amine
Simple amines are named by naming the alkyl group attached to the nitrogen atom
followed by the word amine. The entire name is written as one word. Thus
methylamine:
H
H
C
N
H
H
H
The functional group for the primary amines is –NH2 and the general formula is RNH2.
Amines are characterized by a strong fishlike odor and are produced as decomposition
products of proteins and other nitrogenous animal and plant substances. These
substances in embalming terminology are referred to as IMTERMEDIATE PRODUCTS
OF DECOMPOSITION and are commonly called ptomaines. These substances are
present in spoiled food and were once thought to be responsible for the diarrhea and
vomiting associated with food poisoning or food infection. Some of the more commonly
occurring ptomaines are putrescine and cadaverine.
REACTIONS OF AMINES
Both ammonia and its derivative substances, the amines, are basic because of the
presence of an unshared pair of electrons on the nitrogen. Because of the unshared pair
of electrons on the nitrogen, ammonia reacts as a Lewis base (electron donor). Amines
also have an unshared pair of electrons on the nitrogen, and can therefore act as Lewis
bases also. Amines are considered to be the organic equivalent of a base.
AMIDES
When amines react with carboxylic acids, the products formed are water and an
ammonium salt called an amide.
O
CH2
+
H3C
H
O
N
H C
H
formic
acid
methyl amine
H
N
H
C
H
+
HOH
H
water
Amide
This is no different than other neutralization reactions that we have discussed:
Salt + Water
Acid + Base
or
Carboxylic acid + Amine
Amide + Water
Proteins are polymers of amino acids. Amines and carboxylic acids are the constituents
of amino acids.
UREA
Urea is the diamide of carbonic acid:
O
O
H2N
C
NH2
HO
Urea
C
OH
Carbonic acid
Urea is a waste product of human protein metabolism secreted into the blood by the liver
and selectively absorbed and excreted into the urine by the kidneys. In the blood, it
increases formaldehyde demand.
AROMATIC AMINES
The simplest aromatic amine is called aniline.
NH2
aniline
Aniline is commonly used as a bactericidal dye obtained from the indigo plant.
HETEROCYCLIC AMINES
If the nitrogen of an amine compound is also part of a carbon ring system, the compound
is described as heterocyclic, meaning that two different elements are in the ring. Both
five and six membered heterocyclic rings are important in biological systems.
Five- Membered Rings
Pyrrole – Pyrrole is a five-membered heterocyclic secondary amine.
N
H
A porphyin ring is composed of four pyrrole rings linked by carbon atoms.
Porphyin provides the nucleus for both hemoglobin and chlorophyll.
Indole – Indole is a heterocyclic amine that contains a pyrrole bonded to a
benzene ring. Skatole is a derivative of indole that contains a methyl (-CH3)
group attached to the pyrrole ring. Both indole and skatole are putrefactive by
products of proteins that contain the amino acid tryptophan and contribute to the
characteristic odor of feces.
CH3
N
N
H
H
Indole
Skatole
Six-Membered Rings
Pyridine – Pyridine is a six-membered ring containing one nitrogen:
N
Pyridines are the basis for many biological substances, including vitamins,
hormones, and antibacterial agents.
Pyrimidine – Pyrimidines are six-membered heterocyclic amines containing two
nitrogens in the ring.
N
N
Pyrimidines are one of two types of nitrogenous bases incorporated into the
compounds RNA and DNA.
Purine – Purines are the second type of nitrogenous base found in nucleic acids.
N
N
N
N
H
Caffeine is a purine derivative.
QUATERNARY AMMONIUM COMPOUNDS
The nitrogen in amines has an unshared pair of electrons, so it can form a fourth
additional covalent bond with a hydrogen ion, an alkyl group, or an aromatic group.
These compounds are similar to ammonium salts and are known as salts of amines.
When all four hydrogen atoms are replaced, the resultant substance is called a
QUATERNARY AMMONIUM ION. Compounds derived from this ion are called
QUATERNARY AMMONIUM SALTS.
H
H
N H
+
R-
H
Ammonium salt
R
R
N R
+
R-
R
Quarternary ammonium
salt
Some quaternary ammonium salts have detergent and disinfectant properties.
Benzalkonium chloride is a topical antiseptic used in dilute solution to cleanse and
disinfect the skin prior to surgery. It also has been employed in nasal sprays to reduce
airborne transmission of disease in the hospital environment. It is an excellent surface-
active agent that also may be used to disinfect instruments. It is routinely included as a
supplementary germicide in arterial embalming fluids. Its main drawback is that any
alkaline substance, particularly soap renders this compound useless. Reaction between
the cation of benzalkonium chloride and the negative ion of a soap molecule produces an
insoluble salt.